J F Ponge

Ecological study

Observed on a 5 x 5 cm small surface of litter in a 35-years-old Scots pine stand with bracken and the moss Pseudoscleropodium purum, the F 1 layer is extensively invaded by a mycelial mat made of several mycorrhizal fungi. Observations under a light microscope gave circumstantial evidence of the role of these fungi in advanced stages of decomposition: they seem to protect the partly decayed plant material and the faeces deposited by soil animals from subsequent attack by soil bacteria. Penetration of pine needles and cadavers of soil arthropods is prominent in so far as animals have previously made entries by tunnelling into the substrates. In addition the black mycorrhizal fungus Cenococcum geophilum was observed to penetrate bracken epidermal cells by its own means and to make lysis zones in dead arthropod cuticles. Consequences for forest soil ecology and tree nutrition are discussed in view of existing literature.

Introduction

A small volume of soil in a Scots pine (Pinus sylvestris L.) stand was observed and described from an ecological point of view [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit[END_REF](PONGE , 1985a(PONGE , 1985(PONGE b, 1988)). This micromorphological study focussed on interrelationships between soil animals and microorganisms during litter decomposition and root development. Since in the top first centimeters the bulk of living and dead organisms was made of mycorrhizal fungi and fine roots of pine, it was judged necessary to publish a separate paper on this topic. Our aim was to ascertain the importance of the root system of trees and its associate mycelia in the decomposition processes.

Field studies on the occurrence of fungi during pine needle decomposition have been made both by direct observation [START_REF] Kendrick | Biological aspects of the decay of Pinus silvestris leaf litter[END_REF][START_REF] Berg | Research notes on decomposition. IV Electron microscopic studies on decomposing needle litter from Pinus silvestris. A preliminary study[END_REF]MITCHELL and MILLAR 1978;MITCHELL et al. 1978) and isolation methods [START_REF] Gremmen | Microfungi decomposing organic remains of pines[END_REF][START_REF] Kendrick | Micro-fungi in pine litter[END_REF][START_REF] Kendrick | Biological aspects of the decay of Pinus silvestris leaf litter[END_REF]KENDRICK 1963;2 SARBHOY 1964;HAYES 1965aHAYES , 1965b;;[START_REF] Brandsberg | Fungi isolated from decomposing conifer litter[END_REF][START_REF] Watson | Fungal succession on loblolly pine and upland hardwood foliage and litter in North Mississippi[END_REF]GREMMEN 1977;[START_REF] Black | Colonization of Scots pine litter by soil fungi[END_REF][START_REF] Soma | Ecological studies of soil organisms with references to the decomposition of pine needles. I. Soil macrofaunal and mycofloral surveys in coastal pine plantations[END_REF]. Other studies on coniferous species may be added for comparison such as the work of [START_REF] Gourbiere | Vie et décomposition des aiguilles de sapin (Abies alba Mill.). Etude des microflores fongiques associées[END_REF][START_REF] Gourbiere | Vie et décomposition des aiguilles de sapin (Abies alba Mill.). Etude des microflores fongiques associées[END_REF][START_REF] Gourbiere | Champignons des aiguilles de sapin (Abies alba Mill.). X. Observation en chambre humide[END_REF] and [START_REF] Gourbiere | Champignons des aiguilles de sapin (Abies alba Mill.). X. Observation en chambre humide[END_REF] on Abies alba Mill. No occurrence of mycorrhizal fungi inside or at the periphery of pine and fir needles was noticed, although [START_REF] Soma | Ecological studies of soil organisms with references to the decomposition of pine needles. I. Soil macrofaunal and mycofloral surveys in coastal pine plantations[END_REF] indicated the importance of mycelial spread of ectomycorrhizal fungi in the Ao (litter) layer and presumed that they could be involved in decomposition processes. Reasons for this lack of information may be found in the methods used (isolation on agar plates or observation of fructifying fungi in damp chambers) and in the stage of decomposition at which the studies were conducted.

The importance of decaying wood in the establishment of mycorrhizal root systems has nevertheless been stressed by [START_REF] Boullard | Etude des mycorrhizes ectotrophes constituées par les essences forestières en Margeride. Importance de ces associations pour le reboisement des landes de la Lozère[END_REF], MEYER (1979), [START_REF] Kondas | An observation on the occurrence of pine mycorrhiza-like structures in association with bracken rhizomes[END_REF], [START_REF] Kropp | Fungi from decayed wood as ectomycorrhizal symbionts of western hemlock[END_REF], [START_REF] Maser | The fallen tree. A source of diversity[END_REF].

The presence of different materials such as pine needles, wood debris, bracken leaflets, faecal pellets and the pine root system in the same small volume of soil enabled us to identify relationships which generally escaped observers working on specific substrates, often in isolation from their natural environments.

The interest of such morphological studies for forest ecology and management emerges from the following points: assessment of the health of the absorbing root system of trees and its degree of mycotrophy, knowledge of the origin of organic matter that accumulates in the humus part of the soil (here mainly chitin from fungal walls and arthropod cuticles), knowledge of the organisms that take place in decomposition processes for each component of the litter. These last two points must be highlighted, because direct observation gives more information on what happens in the soil at a given time than current methods in microbiology and soil organic matter chemistry.

Material and methods

A small surface (ca. 5  5 cm) of moder humus was sampled in a 35-years-old Pinus sylvestris plantation (Orleans forest, France) on 11/VIII/81 and microstratified in the field into L 1 (brown needles), L 2 (black needles), F 1 (broken needles + pine roots and widespread mycelial mat) and other layers not described here.

Observation of the underlying layers showed that they accumulate faecal material from soil fauna, arthropod cuticles and fungal cell walls, mainly from dematiaceous fungi. Biological activity mainly takes place in the F 1 layer where young feeding roots of pines are actively growing together with their associate fungi. Consequently the results presented here will concern, for the main part, the F 1 layer, although a lot of information was derived from previous investigation of the L layers. Fixation was made immediately, before any transport, in 95% ethylalcohol. A more complete description of the stand has been given in [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit[END_REF] and ARPIN et al. (1986).

Observations were made under a light microscope at 400  magnification after sorting the plant and animal material under a dissecting microscope. Pine needles, wood and bark and other dense fragments were cut in 7.5 µm sections and mounted in chloral-lactophenol (25 cc lactic acid + 50 g chloral hydrate + 25 cc phenol).

Phase contrast allowed one to discern between dead and living cells at the time of fixation. Presence of cytoplasm is easily discernable by the opacity of cell contents, provided the fungal walls are not melanized [START_REF] Frankland | Importance of phase-contrast microscopy for estimation of total fungal biomass by the agar-film technique[END_REF]. When necessary (e.g. for dematiaceous fungi), methyl blue was used in lactophenol as a staining agent for cell contents. Animals were either dissected (oribatid mites) or observed after clearing (other groups) in order to analyse their gut contents. The faecal material was embedded and cut like the plant material.

Recognition of the fragments by external characters before cutting them proved to give a more reliable interpretation than direct soil sectioning. The nature of the mycelial mat and its connection to the fungus mantle of the mycorrhizae and to the internal colonizers of plant debris and cadavers were also ascertained by direct observation at a low magnification in addition to the microscopic characters observed in sections.

Results

Mycorrhizal types and their associate mycelia

The black mycorrhizal ascomycete Cenococcum geophilum (= Cenococcum graniforme) forms typical mycorrhizae on short roots which may be dichotomous (Fig. 1) or not (Fig. 2). Identification of the fungus was made by observing the structure of the mantle surface [START_REF] Ferdinandsen | Cenococcum Fr. A monographic study[END_REF][START_REF] Hatch | A jet-black mycelium forming ectotrophic mycorrhizae[END_REF]TRAPPE 1971). Not only short roots may be colonized by this fungus, but also long roots. Beyond the elongation zone, where root hairs occur on nonmycorrhizal roots, this fungus often forms a collar (Figs. 1 and2), with characteristic mantle and penetration of hyphae between the cortical cells (Hartig net), as on short roots. Some long root apices are also transformed into mycorrhiza-like structures. In this case, the mantle is present but the meristem tissues remain active, with numerous mitoses, and no penetration by hyphae is visible: only elongation of the root seems to be impeded. These observations agree with the findings of WILCOX (1968b), who demonstrated that in growing roots, growth of the fungus was not at the same rate as the root, thus protecting the apex from being colonized, when the root grows well. It may be postulated that long roots with mycorrhizal apices stopped their growth for some obscure reason. These roots could not have been dormant because of the lack of metacutized layer (WILCOX 1968a), which made possible colonization of the apex by the fungus. The extra-matricial mycelium of Cenococcum geophilum is clearly visible: It is made of brown (melanized walls) hyphae that sprout at a right angle from the mantle of the mycorrhizae (Fig. 1). This feature was also encountered at the periphery of its sclerotia (Fig. 3) or when the fungus had developed inside of faecal material, e.g. faeces of lumbricid worms. It must be noticed that the presence of hyphae attached to sclerotia is not a constant feature (MEYER, personal communication, see also TRAPPE 1969). Nevertheless, in the present study, all sclerotia were observed in the immediate vicinity of the roots and connected by hyphae to mycorrhizal mantles.

Another common mycorrhizal fungus is a basidiomycete tentatively identified as Hyphodontia (BOIDIN, personal communication), more precisely belonging to a group of three strongly related species [H. pallidula (Bres.) John Erikss., H. alutaria (Burt) John Erikss., H. arguta (Fr.) John Erikss.] that possess lagenocystidia [START_REF] Eriksson | The Corticiaceae of North Europe[END_REF]. These cystidia are present not only in the hymenium of the carpophores but also on the vegetative hyphae and this typical character was found on the specimen studied. Connection of this fungus to a type of mycorrhiza was characterized because the hyphae cannot be detached from the orange-brown mycorrhizae which are often present on the same long root as the former species (C. geophilum, Fig. 2) and from hyphal characters (hyphal width, clamp connections, mode of branching). The influence of this fungus on the root system of pine seems to be more pronounced than this of C. geophilum: repeated branching of the short roots (Fig. 2), thickness of the Hartig net (1 to 2 cell layers, compared with only 1 for the ascomycete), intensity of the defence reaction of the host (2 layers of tannin cells, against 1 only for C. geophilum), penetration of the endodermis in the long roots, prime invasion of the stele once long roots have died (PONGE 1988).

In contrast, Hyphodontia seems not to have a strong affinity towards growing parts of the roots, since there is an absence of mantle around apices of short roots and absence of development of this fungus just beyond the elongation zone, unlike C. geophilum (PONGE 1988). Hyphodontia seems to be responsible for the main part of the mycelial mat that spreads throughout the F layer in the small surface under study. Around the orangebrown mycorrhizae, there appears to be a profuse cottony white mass embedding the root (on Figure 2 the mycelium connected to the mycorrhiza was partly stripped off in order to show the root). We may wonder whether this fungus is a true mycorrhizal fungus and not a parasite of dying roots. Two points may prove it to be mycorrhizal: its constant association with the same type of root colour and branching, and the fact that it has been found around and in the youngest roots, i.e. in the L 2 layer (PONGE 1985a). Penetration of the endodermis was observed only in long roots, never in short roots (feeder roots).

Less common in the small volume studied is a pale pinky type of mycorrhiza with a smooth mantle (Fig. 4). The colour of this mycorrhiza is perhaps not exactly the same as in the living state, since alcohol had probably dissolved some pigment. Nevertheless its smooth aspect makes it readily recognizable. Connection to a given mycelium is consequently difficult to determine. Analogy between mantle hyphae (PONGE 1988), other hyphae present in the mycelial mat and hyphae of a rhizomorph with characteristic asterocystidia makes it belongs possibly to Resinicium bicolor (Fr.) Parm. (= Odontia bicolor). But this identification is far from reliable, contrary to the two former cases.

Mycelial mat

The general aspect of the mycelial mat is that of a dense network of hyphae where Hyphodontia is largely dominant. It can be seen embedding animal faeces or any other material or be free of organic remains.

Detailed examination of the hyphae revealed the basidiomycetous (dikaryotic) nature of the second (orange-brown mycorrhizae) and third (pale pink mycorrhizae) type of mycorrhizal fungi. The production of calcium oxalate crystals is of very common occurrence in the mycelium of Hyphodontia, where they often entirely coat the hyphae, the size of the crystals varying strongly from one place to another (Fig. 5).

The hyphae of Cenococcum geophilum, in the mycelial mat, may vary greatly in appearance, with walls either being covered by wart-like protuberances (Fig. 6) or smooth (Fig. 7), the latter being of more common occurrence. The degree of melanization of the walls may also vary greatly (Fig. 8), but generally lack of melanization is associated with penetration of plant material (see later). The results concerning the mycelial characters of this fungus and its polymorphism are in fairly good agreement with the cultural observations of [START_REF] Mikola | On the physiology and ecology of Cenococcum graniforme especially as a mycorrhizal fungus of birch[END_REF]. Differences in the stage of development may be invoked to explain the degree of melanization and of roughness of the fungal walls (MEYER, personal communication). Compared to the other two fungi, the hyphae of C. geophilum are also characterized by the thickness of their cell walls (at least in melanized parts), which protect them from grazing by some small animal groups, even when they are specialized fungal feeders (Collembola, some oribatid mites, PONGE 1988). For that reason, the mycelia of the two basidiomycetes seem to be more heavily and selectively consumed by members of the soil fauna. In addition, incrustation of the walls by phenolic pigments (melanin, FLAIG 1972) make them difficult to digest. Oribatid mites of the camisiid species, however, proved to be able to digest these walls (PONGE 1988). The association of pigments to chitin and protein parts [START_REF] Mangin | Chemical composition of hyphal walls of the ectomycorrhizal fungus Cenococcum geophilum[END_REF]) confers to the walls of this fungus strong similarities to arthropod cuticles [START_REF] Peter | Structural studies on sclerotized insect cuticle[END_REF]), with which it shares the same resistance to degradation. It must be noticed that other dematiaceous fungi develop similar mycelia, such as Phialocephala [START_REF] Wang | New species of ectendomycorrhizal and pseudomycorrhizal fungi: Phialophora finlandia, Chloridium paucisporum, and Phialocephala fortinii[END_REF], only connection to mycorrhizal roots enabling us to identify C. geophilum.

Penetration of the plant material by mycorrhizal fungi

Pine needles

Penetration of pine needles by Cenococcum geophilum in the F layer was considered as a particular stage in their decomposition process (PONGE 1988). This stage follows invasion by other fungi, whose remains are visible in the mesophyll and the stele, but no development of Verticicladium trifidum Preuss (the commonest internal colonizer in the L layer) was recorded before the entry of the mycorrhizal fungus. This was interpreted as a difference in the seasonal development of these two fungi (PONGE 1988). The observed fungus develops a dense felt of intermingled hyphae, whose aspect and mode of branching strongly differs from that observed in the aerial mycelium of C. geophilum. Presumption that it is the same fungus comes from zones where the two forms are co-existing (Fig. 9). MIKOLA (1948) observed the same sudden changes in his cultures. Here also identification by mean of mycelial characters proves to be impossible. In the present case, no hyphal connection was observed with pine roots, thus keeping unsolved the question of what species this fungus belongs to, really.

Growth of the fungus is restricted to the mesophyll part of the needles, just under the hypodermis. Since no penetration of cell walls was observed, we may postulate that the fungus grew freely, splitting the plant tissues, thus covering the inside of the needle by its own prosenchym-like tissue. Although these hyphae appear to be dead, we did not observe any bacterial development in the needles invaded by this fungus, contrary to our observations in the L layer where fungal colonies were dead or senescent. This suggests some antibiotic properties of the walls of this fungus (Cenococcum geophilum is known to produce a substance strongly active against Gram-positive bacteria, [START_REF] Krywolap | An antibiotic produced by the mycorrhizal fungus Cenococcum graniforme[END_REF]. The need to use lipid dissolving solvents to extract this substance (op. cit.) probably indicates that it is produced in the walls, thus deterring bacteria even after the death of the fungus. The needles penetrated by this dematiaceous mycelium seem very palatable to soil animals, especially oribatid mites (phthiracarid and eu-phthiracarid species, PONGE 1988), although the fungal tissue itself is not consumed. Consequently; a lot of needles colonized internally by this fungus are tunnelled by mites, which allows mycelia of mycorrhizal fungi (Hyphodontia and aerial form of Cenococcum) to freely enter the needles (only the cavities, never the cell structures).

Pine wood

Observations on a young fallen branch (5 years, included in the L layer and sampled with the other debris, PONGE 1985a) showed intense development of a basidiomycetous fungus with numerous lagenocystidia. This characteristic feature enabled us to identify this fungus as Hyphodontia, i.e. the same genus forming the orangebrown mycorrhizae (see lagenocystidia on aerial parts of the extra-matricial mycelium, Fig. 10). Hyphodontia spp. are only known as wood fungi, causing white rot [START_REF] Eriksson | The Corticiaceae of North Europe[END_REF], and not as mycorrhizal ones (this genera is absent from a comprehensive review by TRAPPE 1962, although other Corticiacae are noticed). The present observations suggest that this fungus could live both saprophytically and symbiotically, although existence of a hyphal connection between the wood substrates and roots has not been demonstrated here.

Pine bark

Pine bark was studied on various fragments from the fallen branch previously described, several pine twigs and free fragments probably detached from the tree stem. Cenococcum geophilum was widespread on the surface of all these fragments, mainly as a hyphal mat (Fig. 7), but in some cases it formed a pseudo-parenchymatous tissue (Fig. 11). Penetration of hyphae to the inside of bark pieces was never observed, although bark from pine twigs was, in the F layer, a site of intense activity of soil animals and bacteria (PONGE 1988).

Pine roots

In this paper we consider that dead roots are incorporated into the litter compartment of forest soil. Thus the fate of mycorrhizal fungi after death or senescence of the root is in the scope of this study.

Cenococcum mycorrhizae were not observed to decay in the studied layer. Resistance of this fungus to summer or adverse sites dryness is well-known [START_REF] Wright | Ectotrophic mycorrhizae on pine seedlings in Oregon[END_REF][START_REF] Saleh-Rastin | Salt tolerance of the mycorrhizal fungus Cenococcum graniforme (Sow.) Ferd[END_REF]MEYER 1987) and is used, among other characteristics such as tolerance to acidity [START_REF] Mikola | On the physiology and ecology of Cenococcum graniforme especially as a mycorrhizal fungus of birch[END_REF], to explain its widespread occurrence in raw humus.

On the contrary, decay features were commonly observed in the orange-brown mycorrhizae. They became fairly brown in colour and their most conspicuous characteristics was the contrast between the mantle, which was rapidly invaded by bacteria and algae (compared to C. geophilum, see below) and the inside of the roots. The Hartig net (fungus) and the tannin cells (tree) remained intact probably a long time after the root began to senesce.

The most pronounced features of decomposition were observed on long roots. Of observed phenomena, the penetration of the stele by Hyphodontia was the main characteristic of this early stage of decomposition.

Sometimes this fungus was accompanied by Cenococcum geophilum (typical form). No penetration of the cortical cells by mycorrhizal fungi was observed, unlike observations of [START_REF] Mikola | On the physiology and ecology of Cenococcum graniforme especially as a mycorrhizal fungus of birch[END_REF] on Cenococcum mycorrhizae of birch.

Other plant material

The most pronounced features of fungal penetration by mycorrhizal fungi were observed in the epidermis of bracken fern [Pteridium aquilinum (L.)] leaflets. Figures 12 and13 show Cenococcum hyphae penetrating plant cell walls by means of haustoria. Some penetrating hyphae are melanized (Fig. 12), some others not (Fig. 13), perhaps according to their developmental stage. In the cases here described, where the material was directly mounted without being cut, the physical connection between these internal hyphae and the mycelium spread on the surface of the leaves may be easily followed by varying the focusing plane.

Penetration by mycorrhizal fungi (both Cenococcum geophilum, Hyphodontia and the fungus supposed to make pink mycorrhizae) was observed inside bracken petioles after their invasion by soil animals, and in some other substrates such as moss stems which also contained holes made by fauna.

Animal corpses

The inside of arthropod corpses (mainly oribatid mites whose cuticle is resistant to degradation) was invaded by other animals such as enchytreid worms that deposit their faeces (PONGE 1988), but also by numerous fungi that thrive well on this N-rich substrate. Among them, the mycorrhizal Hyphodontia colonizes essentially the remains of phthiracarid or eu-phthiracarid mites after they have lost their prostomium (which allows the fungus to enter freely the bubble-shaped abdomen). The most interesting feature is the attack of the chitinous part of the cuticle of arthropod remains by Cenococcum geophilum (Fig. 14): a hollow zone is clearly visible around the dark hyphae that penetrate the chitinous part of the cuticle.

Discussion

The present findings first give strength to the idea that mycorrhizal fungi are not purely dependent on the tree for their nutrition but may also benefit from nutrients produced during the decay of organic materials. For most species the problem has so far been unsolved as to whether these fungi rather feed on the tree or on the litter, and whether nutrition of the tree benefits from them or not. Our study cannot answer these disputed points, which requires experiments to discern between the causes and the effects. Nevertheless, we can try to analyse our results in the light of existing knowledge.

The fact that mycorrhizae and their associate mycelia develop well in pure organic substrates was ascertained by study of the vertical distribution of short roots of pine in soils with raw humus [START_REF] Mikola | Mycorrhizal relations in the raw humus layer of northern spruce forest[END_REF][START_REF] Bowen | Root distribution of Pinus radiata[END_REF][START_REF] Harvey | Distribution of ectomycorrhizae in a mature Douglasfir/larch forest soil in western Montana[END_REF]). The same holds true for beech [START_REF] Meyer | The role of the fungus Cenococcum graniforme (Sow.) Ferd. et Winge in the formation of mor[END_REF][START_REF] Meyer | Distribution of root tips and tender roots of beech[END_REF] and spruce [START_REF] Mikola | Mycorrhizal relations in the raw humus layer of northern spruce forest[END_REF]. Wood and charcoal were also noticed as good media for mycorrhizal root development [START_REF] Boullard | Etude des mycorrhizes ectotrophes constituées par les essences forestières en Margeride. Importance de ces associations pour le reboisement des landes de la Lozère[END_REF][START_REF] Harvey | Distribution of ectomycorrhizae in a mature Douglasfir/larch forest soil in western Montana[END_REF]), probably through their role as nitrogen acceptor (MEYER 1985).

From our results it may be thought that Hyphodontia and Cenococcum exploit the F layer in a nonrandom way: Each organic fragment, either from moss, bracken or pine origin, is embedded and, when passages have been made through it by the activity of soil fauna, these fungi may gain access. Faecal masses, mainly from lumbricid and enchytreid worms (with a high production of mucus), are undoubtedly preferred in view of the mycelial mat embedding and spreading throughout them, as is the case for cadavers. The question which arises is: does it matter for tree nutrition? In poor soils, where most mycorrhizal root tips are located in the Ao hoizon, many nutrients are in an organic form and confined to the litter and humus layers. The poor surface/volume ratio of tree roots and the failure of root hairs to take water and nutrients when the pores are not of the capillary size (or the channels often interrupted) make necessary the association with a fungus. We may hypothesize that when nutrients are confined to decaying organic fragments mycorrhizal fungi are an obligate partner in tree nutrition.

From the point of view of forest strategies, MEYER'S idea (personal communication) is that trees with obligate ectomycorrhizae compete better in soils with a holorganic humus layer, unlike pioneer trees without obligate mycotrophy which are more successful in mineral soils.

Whether mycorrhizal fungi are direct agents of plant litter decomposition has been debated for a long time. Some species of mycorrhizal fungi are able to decompose litter, holocelluloses or lignocelluloses even in pure culture: Laccaria laccata (MIKOLA 1956), Tricholoma spp. (NORKRANS 1950;[START_REF] Lundeberg | Utilization of various nitrogen sources, in particular bound soil nitrogen, by mycorrhizal fungi[END_REF][START_REF] Trojanowski | Decomposition of 14 C-labelled lignin, holocellulose and lignocellulose by mycorrhizal fungi[END_REF], Suillus spp. [START_REF] Dahm | Cellulolytic and pectolytic activity of mycorrhizal fungi, bacteria and actinomycetes associated with the roots of Pinus sylvestris[END_REF], Xerocomus subtomentosus [START_REF] Lindeberg | On the occurrence of polyphenol oxidases in soil-inhabiting basidiomycetes[END_REF][START_REF] Lundeberg | Utilization of various nitrogen sources, in particular bound soil nitrogen, by mycorrhizal fungi[END_REF]), Lactarius spp. (LINDEBERG 1948;[START_REF] Lundeberg | Utilization of various nitrogen sources, in particular bound soil nitrogen, by mycorrhizal fungi[END_REF][START_REF] Giltrap | Production of polyphenol oxidases by ectomycorrhizal fungi with special reference to Lactarius spp[END_REF], Clitopilus prunulus [START_REF] Lundeberg | Utilization of various nitrogen sources, in particular bound soil nitrogen, by mycorrhizal fungi[END_REF], Cenococcum geophilum, Amanita muscaria, Rhizopogon spp. [START_REF] Trojanowski | Decomposition of 14 C-labelled lignin, holocellulose and lignocellulose by mycorrhizal fungi[END_REF]). In our observations, Cenococcum geophilum seems to be able to penetrate the epidermic cells of Pteridium aquilinum, but no lysis of the plant cell wall was observed on a wide scale, contrary to Verticicladium trifidum and Marasmius androsaceus in pine needles (PONGE 1985a). Hyphodontia, although very active on pine wood, was not observed to degrade pine needles, but rather to benefit from the activity and autolysis of other groups of soil fungi, permeating and embedding the dead plant and faecal material. That mycorrhizal fungi can absorb and metabolize complex organic substances produced during plant and animal decomposition has been proved: this is for instance the case for proteins with Cenococcum geophilum [START_REF] Mikola | On the physiology and ecology of Cenococcum graniforme especially as a mycorrhizal fungus of birch[END_REF][START_REF] Botton | Induction of extra cellular proteases in the ascomycete Cenococcum geophilum[END_REF], Suillus bovinus, Rhizopogon roseolus and Pisolithus tinctorius (ABU ZINADAH and READ 1986), Paxillus involutus [START_REF] Read | In support of Frank's organic nitrogen theory[END_REF] and for fulvic acids with Pisolithus tinctorius [START_REF] Tan | Fulvic acid and the growth of the ectomycorrhizal fungus, Pisolithus tinctorius[END_REF]. From that literature and what we observed to occur in a humus sample, we can hypothesize that with differing opportunities in their environment, mycorrhizal fungi may or may not express their lytic potentialities. Perhaps this is the main point that separates them from true decomposer fungi, although strain differentiation may also be invoked [START_REF] Lundeberg | Utilization of various nitrogen sources, in particular bound soil nitrogen, by mycorrhizal fungi[END_REF].

The last point we want to discuss is the antagonism between mycorrhizal fungi and other soil microorganisms. We observed an intense bacterial (and algal) development on the inside as well as on the periphery of pine needles in the L layer (PONGE 1985a), i.e. in the absence of a dense mycorrhizal mycelial mat.

In the F layer, contrary to what was expected, bacteria were much less numerous and lysis of bacterial colonies was observed inside some pine needles (PONGE 1988). The only places where bacteria seemed to be in an active state were some micro-sites in faecal pellets, generally near the surface, but these colonies were made of a few cells and probably in a quiescent stage. Antibiotic properties of mycorrhizal fungi against soil bacteria were stated [START_REF] Krywolap | An antibiotic produced by the mycorrhizal fungus Cenococcum graniforme[END_REF][START_REF] Sasek | Cultivation and antibiotic activity of mycorrhizal basidiomycetes[END_REF]MUSILEK 1967, 1968;MARX 1968a, b;[START_REF] Grand | An antibiotic detected in conifer foliage and its relation to Cenococcum graniforme mycorrhizae[END_REF]MARX 1972), but these properties are shared by many other basidiomycetes (WILKINS 1948). It is therefore difficult to have a clear view of these antagonisms from the existing literature. Concerning the relationships between mycorrhizal and other fungi, we observed that plant fragments with an active saprophytic fungal flora were not colonized by Cenococcum geophilum or Hyphodontia sp. This suggests that there exists two groups of fungal colonizers which are not coexisting during the decomposition process of pine needles: the one is able to penetrate entire needles by its own means and digest lignocelluloses (Verticicladium trifidum, Marasmius androsaceus, PONGE 1985a), the other can penetrate needles only after tunnelling by soil animals and probably benefits from their previous digestion (mycorrhizal fungi, such as, here, Cenococcum and Hyphodontia). A depressive effect of mycorrhizal fungi against true decomposer fungi was ascertained by the experiments of GADGIL and GADGIL (1975), confirmed on the field by their trenching and clearfelling experiment [START_REF] Gadgil | Mycorrhiza and litter decomposition[END_REF]GADGIL 1971, 1978). Unfortunately their results were not supported by other studies: trenching of pine roots [START_REF] Berg | Is litter decomposition retarded in the presence of mycorrhizal roots in forest soil? Swedish Coniferous Forest Project[END_REF], observations in pure cultures on Cenococcum geophilum [START_REF] Mikola | On the physiology and ecology of Cenococcum graniforme especially as a mycorrhizal fungus of birch[END_REF]. The work of [START_REF] Dighton | Interactions between tree roots, mycorrhizas, a saprotrophic fungus and the decomposition of organic substrates in a microcosm[END_REF] even concludes the opposite, pine roots stimulating cellulose decomposition even in sterile conditions, this effect being enhanced in the presence of the mycorrhizal Suillus luteus or suppressed in the presence of the decomposer Mycena galopus, the latter being alone or associated with Suillus. Since the same experiments made with another mycorrhizal basidiomycete, Hebeloma crustuliniforme, gave rise to reduced decomposition by this fungus (as compared to roots alone), we may tentatively conclude that the nature of the fungus is quite decisive and that no generalization can be made concerning these antagonisms.

From the points discussed above we may wonder whether certain mycorrhizal fungi are useful in forests from the angle of tree nutrition (apart from auxin and antibiotic production, that might stimulate growth of the trees and protect them against pathogens). The fate of the melanized walls of Cenococcum geophilum is worthy to note in respect to this problem, since they seem to be the main products (with cuticles from oribatid mites) that strongly resist degradation in our humus sample. Faecal pellets of numerous animal groups contain the walls of this fungus, which are rarely digested to some extent (PONGE 1988). Then the question arises of an indirect role of Cenococcum geophilum in the non-accessibility of nitrogen (and probably also phosphorus) in moder humus types, although their organic layers are richer in this element [START_REF] Delecour | Etude de la matière organique dans une bio-toposéquence de sols forestiers ardennais. I. Distribution du carbone et de l'azote dans les fractions humiques[END_REF].

Moreover this fungus may be responsible by itself for the formation of a moder or a mor humus [START_REF] Meyer | The role of the fungus Cenococcum graniforme (Sow.) Ferd. et Winge in the formation of mor[END_REF] if it inhibits by its development some major animal species. This is purely conjectural but requires further study, given the importance of mycorrhizal fungi in the diet of the animals we studied [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit[END_REF](PONGE , 1985a, b;, b;PONGE 1988). Although some works indicate a benefit of this fungus on tree nutrition and growth [START_REF] Boullard | Etude des mycorrhizes ectotrophes constituées par les essences forestières en Margeride. Importance de ces associations pour le reboisement des landes de la Lozère[END_REF][START_REF] Lamb | Effect of mycorrhizal fungi on the growth and nutrient status of slash and radiata pine seedlings[END_REF][START_REF] Sihanonth | Transfer of nutrients from ectomycorrhizal fungi to plant roots[END_REF], Cenococcum geophilum seems to be less effective than other fungi in stimulating plant growth [START_REF] Marx | Growth and ectomycorrhizal development of loblolly pine seedlings in fumigated and nonfumigated nursery soil infested with different fungal symbionts[END_REF] or even deleterious as in the experiments of [START_REF] Lundeberg | Utilization of various nitrogen sources, in particular bound soil nitrogen, by mycorrhizal fungi[END_REF] with raw humus as a substrate. In the latter work the author proved that the depressive effect on seedling development was caused by net immobilization of nitrogen by this fungus. In addition, our own observations show that the extent of penetration of the root is less with this fungus than with the basidiomycete Hyphodontia. We think then that environmental conditions which encourage the development of Cenococcum geophilum, as compared to other mycorrhizal fungi, must be ameliorated as far as possible, unless resistance to adverse conditions (summer drought, waterlogging) is required. Unfortunately, the effects of some major factors of importance to forestry practice, such as light, are contradictory [START_REF] Mikola | On the physiology and ecology of Cenococcum graniforme especially as a mycorrhizal fungus of birch[END_REF][START_REF] Wright | Ectotrophic mycorrhizae on pine seedlings in Oregon[END_REF], thus, at the present stage of our knowledge, it is difficult to select an appropriate method to influence the composition of the mycorrhizal fungal flora once the young tree has been planted in the field. mycorrhizal hyphae do not penetrate the decaying fragments by their own means, but rather benefit from tunnels and cavities previously made by soil fauna. Nevertheless in particular substrates the two species studied behave as true decomposers: Cenococcum geophilum can penetrate bracken epidermis cells by the means of haustoria and Hyphodontia sp. develops itself as a white-rot fungus in pine wood. The bacterial and algal development which was starting in the lower part of the L layer seems to be hindered in the F layer, probably under the influence of the antibiotic activity of mycorrhizal fungi. Consequences for forest soil ecology and tree nutrition are discussed in view of existing literature, and the role of Cenococcum geophilum which probably acts as a sink for some nutrients (N, P) in humus types where this fungus is too much abundant was pointed out.

Résumé

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .Fig. 11 .Fig. 12 .Fig. 13 .Fig. 14 .

 123678911121314 Fig. 1. Colonization of a long root and two short roots of pine by the mycorrhizal fungus Cenococcum geophilum. F 1 layer

Figs.1-8

Acknowledgements

The author was greatly indebted to several specialists who were asked for advice and criticisms and revising of the English language, namely Prs F.H. MEYER (FRG), F. LE TACON (France), H.E. WILCOX (USA) and Dr. J. DIGHTON (England).

Summary

This study was conducted in order to better understand the interrelationships between the fine root system of pine, the abundant mycelial mass spreading throughout the litter, and the decomposing plant and animal material. Fungi were determined to be mainly mycorrhizal, by observing their connection to roots. The two main species are the ascomycete Cenococcum geophilum (with dematiaceous walls) and the basidiomycete Hyphodontia sp. (with hyaline walls). They were observed to embed and penetrate plant and animal remains.

Preferences were for fragments from an animal origin which were richer in nitrogen: faeces of oligochetes (lumbricid and enchytreid worms) and cadavers, mainly of oribatid mites. It seems that in most cases


G A J M Jagers Op Akkerhuis

F De Ley

H J C Zwetsloot

J.-F Ponge

L Brussaard

Soil microarthropods (Acari and Collembola) in two crop rotations on a heavy marine clay soil BY

Keywords: soil mesofauna, microarthropods, Acari, Collembola, agro-ecosystem, crop rotation, correspondence analysis, soil disinfection

an inventory was made of the mites and springtails in crops of a six-year and a three-year rotation. Of the three-year rotation a disinfected and a non-disinfected half were sampled. The distribution of the mesofauna was examined by means of correspondence analysis and frequency tables. The mesofauna was indicative of factors related to year and climate, depth, crop, rotation and disinfection of the soil.

INTRODUCTION

During 15 years of crop rotation experiments at the experimental farm "De Schreef", yield losses up to 10% were recorded in potato crops of a three-year rotation as compared with a six-year rotation (Anon., 1984). 1 Corresponding author.

Data processing

The species composition of the soil fauna as related to depth, crop, rotation or year was examined by means of correspondence analysis. For a detailed explanation of correspondence analysis see GREENACRE (1984). The unit of measurement used in correspondence analysis was the number of animals of a certain taxon per sample (66 cm 3 ). Of the total of 35 taxa identified, 12 taxa occurring in less than fifteen samples were omitted from the analysis. These rare taxa are enumerated at the end of the appendix. Also samples with less than four species were omitted from correspondence analysis, resulting in a total of 268 samples representing 23 taxa. All 295 samples (5 samples were lost) were used for the analysis of quantitative aspects of the mesofauna.

Interactions between mesofauna totals and depth, crop, rotation or soil disinfection were examined with the twoway test of independance using the "Chi-square" statistic.

The weather

The years 1983 and 1984 were quite different with respect to temperature and rainfall (Fig. 1). The weather of 1983 was relatively warm and wet. The temperature of the soil at 10cm depth mostly stayed above 3°C in winter, while in July temperatures higher than 20°C were common. Heavy and prolonged rainfall was recorded between March and the end of June. For a long time the soil was water-logged and in many places in the fields puddles were formed. Due to these wet conditions, sowing or planting was delayed until late May (Fig. 1). In 1984, however, the weather was relatively cold with a dry spring. Soil temperature (at 10cm depth) frequently fell below 3°C during winter and in summer temperatures over 20°C were seldom measured. The growing season started with a dry spring followed by relatively much precipitation during the summer. All crops were sown or planted before May.

II. -RESULTS

A) Crop effects

The number of mites and springtails captured in a field during the year varied in close relation to the crop grown (Fig. 2 and Tab. II). In cereals (spring barley and winter wheat) the mesofauna was relatively numerous, especially pyemotid mites (Pygmephorus blumentritti) and springtails not belonging to the Tullbergia krausbaueri group. Also predatory mites (Veigaia planicola, rest-group predatory mites) appeared in high numbers. The mesofauna in cereals peaked at the end of the summer when the cereals ripened (Tab. III). The following species were especially abundant in the last two samplings: V. planicola, Arctoseius cetratus, P. blumentritti, Pygmephorus selnicki, Tarsonemus talpae, Nanorchestes sp., T. krausbaueri, Ceratophysella denticulata, Isotoma notabilis, Folsomia candida and Megalothorax minimus.

Fewer animals were found in flax and peas. These crops grew very poorly in 1983 and the growth of a green manure crop in these crops probably affected the soil mesofauna more than the main crops flax and peas themselves.

In the root and tuber crops (potato and sugar beet) very few animals were captured in 1983, springtails of the T. krausbaueri group and cryptostigmatid mites being the most abundant.

In 1984 the potato crop of the three-year rotations (R3 and R3*) harboured more mites than in 1983. This did not hold for the potato crop of the six-year rotation.

B) Depth

For each taxon the vertical distribution of mites and springtails in the soil, summarized for 1983 and 1984 for all crops, is shown in table IV. Mites and springtails show significant differences in depth distribution [d. f. = 4; P(χ 2 >556) « 0,01].

On average, mites were most numerous at depths of 0-5 cm, whereas springtails were most numerous at depths of 2.5-17.5 cm (Tab. IV). In cereals the number of mites and springtails inhabiting the soil near the surface was large in comparison to all other crops. This was especially true for springtails. In the root and tuber crops the number of mites and springtails in the surface layer was small, especially in potato. The potato crops planted as a second rotation after cereals showed higher numbers of mites and springtails at greater depth. Some species of mites and springtails were particularly abundant near the soil surface (A. cetratus, Tectocepheus velatus, P. selnicki, T. talpae, Eupodes sp., Nanorchestes sp., Isotoma viridis) while other species were found most frequently in the deeper soil layers (Veigaia agilis, O. minus, T. krausbaueri, M. minimus, F. candida and Folsomia sp.). All other species were distributed according to the mean depth distribution (Tab. V).

C) Effects of crop rotation

Effects of crop rotation on the soil mesofauna should be visible as marked shifts in total numbers and abundance of certain taxa either in all crops of the rotations compared or in a corresponding crop that had the same previous crop(s) in all rotations. In 1983 larger numbers of mites appeared in crops of the six year rotation than in corresponding crops of the three-year rotation (Tab. II, IV, Fig. 2a). This effect was mainly accounted for by the cryptostigmatid mite T. velatus. The reverse was true for the potato crop of 1984.

The relation between rotational and year related effects was examined in the 1983 and 1984 potato crops. For mites, the effect of the year on the extracted numbers differed highly significantly for the six-year and the not-disinfected three-year potato crops [d. f. = 4; P(χ 2 >123) « 0.01]. In the six-year rotation more animals were caught in 1983 than in 1984, in the three-year rotation a reverse effect was observed.

For springtails, no association was found between year and rotation when comparing rotation R6 and

R3.

In all rotations spring barley had the same preceding crop. Expression of effects of the rotation on the mesofauna was, however, hampered by the presence of Italian ryegrass (Lolium multiflorum) undersown in the three-year rotation spring barley only. Therefore, no unequivocal explanation exists for the selective abundance of V. nemorensis, T. velatus, Oppia nova and F. candida in spring barley of the six-year rotation and of V. planicola, Hypoaspis similisetae, A. cetratus, P. blumentritti, Nanorchestes sp. and C. denticulata in the threeyear rotation of this crop.

D) Effects of soil disinfection

For the mesofauna in the corresponding potato, beet and barley crops a significant association existed between the effects of soil disinfection (R3, R3*) and the cultivated crops (mites [d. f. = 2; P(χ 2 > 49) « 0.01], springtails [d. f. = 2; P(χ2>26) « 0.01]).

The mesofauna totals in potato and beet of the disinfected rotation R3 were consistently lower than in the non-disinfected rotation R3*. In barley, mites and springtails reacted differently, the largest mesofauna numbers being found in barley of the disinfected rotation.

Since in both years mites were less abundant in the R3* th an in the R3 potato crop, no significant association was found for mites of soil disinfection and year.

Springtails

III. CORRESPONDENCE ANALYSIS

The first four axes resulting from correspondence analysis could be given a meaningful interpretation.

Axes 1 to 4 accounted for respectively 12.0, 9.0, 8.6 and 5.7 percent of the total variance. Plots of axis 1 versus 3, 2 versus 3, again 2 versus 3 and 2 versus 4 are represented in figures 3, 4, 5 and 6.

Axis 1 (Fig. 3) Along the first axis samples from near the soil surface of the six-year rotation oppose those from the deeper soil layers of the three year rotation especially in beet and potato. This ordination of the samples along the first axis is in many ways characterized by T. velatus (TVE) and its nymph (NTV). T. velatus (TVE) was captured in largest numbers near the soil surface. In the six-year rotation T. velatus (TVE) was much more abundant than in the three-year rotation, in particular when compared with the disinfected half of the plot (Tab. II). Accordingly, samples and species from near the soil surface of the abovementioned crops, mostly of the six-year rotation are at the top of the first axis.

The species at the bottom of Figure 3 were abundant in the deeper soil layers and/or were mostly found in the crops of the three-year rotation. Species such as F. sp. (FSP), O. minus (OMI) and its nymph (NOM), T. krausbaueri (TKR), unidentified Heterostigmata (HET), Nanorchestes sp. (NAN), Cryptopygus bipunctatuc (CBI), A. cetratus (ACE), C. denticulata (CDE), the male of a Pygmephorus sp. (MPS), F. candida (FCA), P. selnicki (PSE) and P. blumentritti (PBL) belong to this group.

As a consequence of the combination of factors related to depth and rotation, surface-dwelling species that were most abundant in the three-layer rotation such as A. cetratus (ACE), P. sellnicki (PSE), Nanorchestes sp. (NAN) and T. talpae (TTA), have affinity along the first axis with the deeper soil layers.

Axis 2 (Fig. 4, 5 and6)

Along axis 2 the potato crop of 1984 is opposed by all crops of 1983 (Fig. 4).

Apparently the soil fauna of both years was greatly influenced by some year-related factor(s). Because the potato crops were the only crops sampled in 1984, interpretation of the year effect along axis 2 is meaningful for the potato crops only.

Correspondence analysis demonstrated the relative importance of T. krausbaueri in characterizing the soil community of the potato crops in both years. This importance follows from the fact that in the 1983 potato crop T. krausbaueri (TKR) was about 5 times more numerous than in the potato crop of the following year.

Together with T. velatus (TVE), T. krausbaueri was the only species abundant in potato in 1983. In 1984 the potato crop was inhabited by many different species. Some of these species were typically associated with this particular 1984 potato crop [Astigmata except Anoetidae (AST), Anoetidae (ANO), V. agilis (VAG), Entomobryinae (ENE), M. Minimus (MMI)]. Unlike the position of V. agilis, the Anoetidae and the Astigmata, the position of F. candida (FCA) is due to a highly clustered distribution in the field. The extreme position of this species along axis two is therefore of little importance in regard to the effects along this axis.

Other species lived in large numbers in the preceding gramineous crops (in 1983 Italian ryegrass was sown in the six-year rotation potato cr op after the cultivation of peas had failed) and were found in 1984 in small numbers in the potato crops [I. notabilis (INO), C. denticulata (CDE), P. blumentritti (PBL), unidentified Heterostigmata (HET), P. selnicki (PSE) and V. planicola (VPL)].

Axis 3 (Fig. 3, 4 and5) Ordination of the data along axis 3 showed that the species composition of the soil mesofauna strongly depended on the crops and associated agricultural practice (Fig. 5a andb).

Firstly, ignoring for a moment the results of 1984, a great difference existed between the species composition in cereals and the root and tuber crops. Data from the root and tuber crops are at the left of axis 3 (Fig. 5a). These crops were characterized by a relative abundance of species such as O. minus (OMI) and its nymph (NOM), T. krausbaueri (TKR), M. minimus (MMI), Astigmata except Anoetidae (AST), O. nova (ONO), Eupodina except Eupodes sp. (AEU), and T. velatus (TVE). Scattered along axis 3 are the core samples for flax and peas. Both these crops grew very poorly in 1983 and the soil mesofauna probably was influenced more by the green manure than by the crops themselves. For these reasons, flax and peas will not be mentioned hereafter.

At the right of axis 3 are the data for cereals. The cereals, especially spring barley of the three-year rotation (undersown with Italian ryegrass), were characterized by species such as Nanorchestes sp. (NAN), P. blumentritti (PBL), the male of a Pygmephorus sp. (MPS), A. cetratus (ACE), I. viridis (IVI), Pseudosinella alba (PAL) and T. talpae (TTA).

Secondly, the effect of spring barley on the mesofauna of the following potato crop varied with the year (Fig. 4). The potato crops of the three-year rotation had the same previous crop and were subject to the same farm practices in 1983 and 1984. Therefore the year-related differences in species composition found in these crops were most likely caused by weather conditions or by the dates of sowing and planting, which in turn were also influenced by the weather.

Thirdly, the effect of spring barley on the species composition of the following potato crop was found to be most pronounced in spring or early summer, after which it slowly faded away (Fig. 5 b). This effect is demonstrated by the fact that the three-year potato samples from the first two sampling dates both in 1983 and 1984 are more at the cereal-associated side of axis 3 than those from the last two sampling dates. Species associated with this effect were I. notabilis (INO), C. denticulata (CDE), P. blumentritti (PBL), Nanorchestes sp.

(NAN), unidentified Reterostigmata (RET), Alliphis halleri (ARA), H. similisetae (RSI), P. selnicki (PSE) and V. planicola (VPL). The mite A. halleri (ARA) shared all characteristics of the group of cereal-associated species mentioned above, except that it was extremely rare in the preceding spring barley crop. Its abundance in the 1984 potato crops of the three-year rotation therefore did not result from a large population in spring barley.

In combination with axis 1 (Fig. 3), axis 3 shows that in 1983 the composition of the mesofauna in cereals had a more equal vertical distribution than in root and tuber crops. In the latter crops, surface and deeper soil layers were inhabited by quite different species. In the 1984 potato crop, however, this vertical difference was almost absent.

Axis 4 (Fig. 6) Besides the similarity in soil mesofauna of the cereals and the 1984 potato crop along axis 3 (Fig. 3), the 1984 potato crop also showed a characteristic fauna of its own, as illustrated by the opposite ordination of the data from the 1983 cereals and 1984 potato crop along axis 4 (Fig. 6). The potato crop of 1984 typically was inhabited by unidentified Heterostigmata (HET), V. agilis (VAG), A. halleri (AHA), Entomobryinae (ENE), O.

IV. -DISCUSSION

The soil mesofauna provided information on the soil system as a whole, and in relation with the observed yield losses. This information will be discussed in relation to year, crop and rotation, followed by a paragraph on yield depression.

Year

Judging from the mesofauna in the 1983 and 1984 potato crop, the extremely different weather conditions in these years had a profound influence on the soil system.

Indicative of this influence was the massive appearance in the 1983 potato crop of springtails of the T. krausbaueri group when all other springtails occurred in small numbers only. The small size and worm-like body of T. krausbaueri and its adaptations to living at depth are most likely an advantage over other species when colonizing compact soils.

The high relative abundance of these springtails suggests that the wet and relatively warm conditions during winter and spring of 1983 resulted in a soil that had many small pores and a low content of fresh organic material (see Appendix). The mesofauna of the potato crop in 1984 differed substantially from that of the preceding year (Fig. 4). During the spring of 1984 in particular, many more species were present that were associated with the preceding spring barley crop than in 1983 (Fig. 5b). Partly this may be a consequence of the early sampling in 1984 but, more likely, it points to a slow decomposition of the barley straw in the relatively dry and cold winter of 1984. Apart from the cereal-associated species, the following taxa were typically found in the 1984 potato samples: F. candida, Astigmata, V. agilis, Anoetidae, Entomobryidae and M. minimus. Most of these species have a comparatively large body size and are indicative of a loose soil structure when found in the deeper soil layers. The abundance of anoetid mites in spring implies moist soil with abundant food (protozoa and bacteria).

Crop

Numbers as well as species composition of the mesofauna indicated large differences between the soil conditions under cereals compared with those under root and tuber crops.

Cereals had a very abundant mesodauna particularly I. notabilis, C. denticulata, Lepidocyrtus spp. and the group of rare or unidentified springtails, and the pyemotid mites P. blumentritti, P. selnicki, T. talpae. Since pyemotids and this group of springtails depend mainly on fungi for their nutrition (see Appendix), the abundance of these species suggests a rich fungus flora in soils under cereals. More specific information about the cereal soil system can be obtained by examining the biology of those species that were selectively associated with cereals (axes 3 and 4). These species can be divided into two groups.

The first group consists of species, such as A. cetratus, I. viridis, P. selnicki and T. talpae, which were present only in cereals. These species were mostly found near the soil surface. They belong to the fauna inhabiting the litter layer (see Appendix). Their high abundance is indicative of organic material in an early stage of decomposition (A. cetratus), the presence of algae (I. viridis), a rich fungal growth (P. selnicki and T. talpae) and humid conditions at the soil surface.

The species of the second group were found in large numbers in cereals, but not typically near the soil surface. They were also found in the following crop (potato) where their abundance diminished slowly (axis 3) (I. notabilis, C. denticulata, Nanorchestes sp., P. blumentritti, H. similisetae and V. planicola). These species live on fungi (I. notabilis, C. denticulata), nematodes (H. similisetae) or springtails (V. planicola) and are an indication of fresh decomposing organic matter. Accordingly, they were most abundant when the roots had died during the ripening of the cereals and in the deeper soil layers of the 1984 three-year potato crops, where undecomposed organic material from the preceding spring barley stubble that had been plowed down was still present.

A. halleri increased in numbers from a few specimens in spring barley to a high abundance on the first two sampling dates in the subsequent three-year potato crop. This pattern of increase was similar to that observed by BÜHLMANN (1984) in a potato crop that was planted after plowing up temporary grassland.

A. halleri is strictly nematophagous (see Appendix) and accordingly a rise in the population of A.

halleri indicates a rise in the nematode population. The nematodes could have profited from the plowed-down stubble of spring barley or from the rotting seed potatoes.

The mesofauna of the root and tuber crops in 1983 consisted mainly of springtails of the T. krausbaueri group and cryptostigmatid mites, especially T. velatus. T. krausbaueri lives in the deeper soil layers and survives well, even when only small pores and decomposed organic matter are available. T. velatus is known to occur in a wide range of habitats; often habitats where other species are lacking (see Appendix). Species which require fresh organic matter or a constant microclimate in the topsoil, such as exists under a crop coyer, were absent from the bare soil between the rows of the root and tuber crops.

Soil compaction in the root and tuber crops, as indicated by large numbers of T. krausbaueri, may be the result of more severe tillage operations for seedbed preparation than in other crops. Soil compaction will have been most severe in 1983 when the soil was still wet when tilled for seedbed preparation.

Rotation

A crop rotation effect on the mesofauna is indicated by the relatively low abundance of most taxa of mites in the three-year rotation in 1983. T. velatus in particular was negatively affected by the short rotation period and soil disinfection. T. velatus has a thick cuticle, resists desiccation well, feeds on fungi and is sensitive to many pesticides. However, even when this species was harmed by the application of metamsodium in the disinfected half of the three-year rotation, this does not explain its low abundance in the non disinfected half.

Other factors, such as soil compaction or a different quality or quantity of the organic matter or fungal flora in the three-year rotation, may be involved.

The results presented here show that, apart from the factors weather and depth (which cannot be influenced by the farmer), the kind of crop and related crop management practices strongly affect the mesofauna of the agro-ecosystem investigated. Relations between the soil mesofauna and vegetation are best known from oligotrophic ecosystems such as blanket bog (BLACKITH, 1974) or steppe (STEBAEVA, 1963). Several factors may have led to the marked plant-mesofauna interaction that was found in the nutrient-rich agro-ecosystem investigated. High microbial activity of the calcic soil may have caused a rapid decomposition of the organic material, especially when the mean annual temperature and the humidity of the soil were high (as was the case in 1983). Under such conditions, species associated with organic material in an early stage of decomposition depend on the direct supply of organic material by a crop. The root system of cereals apparently added much suitable fresh organic material to the soil, as is demonstrated by the large number of species associated with such organic material that were recorded in cereals. In potato or sugar beet only species with low ecological specificity or species associated with the late stages of decomposition and small pores were abundant. In combination with a very low abundance of species associated with freshly decomposing organic material, the mesofauna indicated that the supply of organic matter in the root and tuber crops was low. It is therefore likely that rotation effects on the mesofauna are the result of a high percentage of root and tuber crops in the three-year rotation (66%) in comparison with the six-year rotation (33%).

In previous work correspondence analysis or related techniques have been used for analysing data of the soil mesofauna [START_REF] Ponge | Application de l'analyse factorielle des correspondances à l'étude des variations annuelles dans les populations de microarthropodes[END_REF][START_REF] Ponge | Les biocénoses des Collemboles de la Forêt de Sénart[END_REF][START_REF] Ponge | Les Collemboles, indicateurs du type d'humus en milieu forestier[END_REF]PARR, 1978;[START_REF] Curry | Relationships between microarthropod communities and soil vegetational types[END_REF]. During winter GERS (1982) studied the collembolan fauna of fields previously sown to wheat or maize and demonstrated that the species composition was influenced by factors related to soil depth and the preceding crop, and factors related to plowing, surface soil cultivation or no tillage. [START_REF] Andrén | Soil fauna (microarthropods, enchytraeids, nematodes) in Swedish agricultural cropping systems[END_REF] concluded from PCA (Principal Component Analysis) ordination of data on the soil fauna in Swedish cropping systems that "... the main factors are site dependent (climate, soil type, etc.) or time dependent (within year and between year variation)". They also found effects of soil tillage and of fertilization.

Yield depression

The indication by the mesofauna of small pores and decomposed organic material coincides with lower yields in the three year rotation potato crop. However, the yields in the metamsodium treated three year rotation potato crop were higher even though in this part the mesofauna was less abundant and comprised less species than in the not treated three year rotation. Therefore negative effects of the three year rotation on the soil system as indicated by the mesofauna caused less damage to the crop than unknown negative factors of which the influence is diminished by metamsodium treatment of the soil.

It is unlikely that after three years the N-flush which normally occurs following soil disinfection will have attributed to higher yields of potato in the three year rotation. BEUTE and BENSON (1979) discussed the relation between small soil fauna and plant disease. Of the mesofauna, certain acarid mites (Rhizoglyphus sp., Rh. echinopus, Rhizophagus sp. and Caloglyphus sp.), pyemotid mites (Stenotarsonemus laticeps, Siteroptes spp.) and certain springtails were reported to aggravate infections of plant tissue with pathogenic fungi which showed very limited pathogenicity in absence of these animals. Of the above mentioned taxa only acarid mites have been reported as secondary parasites in association with damage to potato [START_REF] Baker | Relationships of fungi, mites, and the potato-rot nematode[END_REF][START_REF] Karg | Untersuchungen über die Veranderungen und Wechselbeziehungen der Mikroarthropoden in Kartoffelnematoden verseuchten Flächen[END_REF]. Acarid mites have strong chelicerae which are capable of cutting fun gal material or plant tissue. They feed on a wide range of foods such as fungal material, fresh or decaying plant material, yeast or nematodes [START_REF] Woodring | The nutrition and biology of saprotrophic Sarcoptiformes[END_REF]MURAOKA & ISHIBASHI, 1976;[START_REF] Sturhan | Pflanzenparasitische Nematoden als Beute der Wurzelmilbe Rhizoglyphus echinopus[END_REF]. The number of acarids captured in the potato crops in this study was rather small (Tab. II).

However, larger numbers of acarids were found in both years in the three-year than in the six-year rotation potato crop. They may have caused selectively more root damage in the three-year rotation potato crop, which would account for part of the yield reduction. This possibility needs further study.

SUMMARY

In 1983 and 1984 an inventory was made of the edaphic mites and springtails in a six-year rotation, a three-year rotation and a three-year rotation in which the soil was disinfected with metamsodium after the potato crop was harvested. The aim was to find possible direct or indirect biological factors related to the yield loss observed in the three-year rotation potato crop. The test site was situated on a heavy marine clay soil. Samples were taken four times in the course of the growing season. The mesofauna was extracted by means of Macfadyen high-gradient extraction. The following depths were extracted: 0-2,5; 2.5-5; 7.5-10; 15-17. 5 and 25-27. 5 cm.

The results were examined by analytical and synthetical procedures.

The mesofauna was much more abundant in cereals than in root and tuber crops. Most mites and springtails were found near the surface of the soil except in potato when it was preceded by a cereal crop that was plowed down. In 1983, a year with relatively heavy and prolonged rainfall, more mites were found in corresponding crops of the six-year rotation than of the three-year rotation. The short rotation reduced the abundance of Tectocepheus velatus, whereas the percentage of the Tullbergia krausbaueri group and the number of Heterostigmata were increased. In 1984, a year with colder and drier weather, more mites and springtails were found in the potato crop of the three-year rotation than in the potato crops of the six-year rotation.

Correspondence analysis resulted in four axes that could be given a meaningful interpretation.

Ordination of the samples and species along the first axis was mainly due to the presence of Tectocepheus velatus and was related with depth, rotation and climatic factors. The second axis showed a year-effect for the 1983 and 1984 potato crops. The species composition of the mesofauna in many ways reflected crop effects as shown by the ordination of the samples and species along axes 3 and 4.

There was a suggestion of a possible contribution of the Acaridae to the yield loss. This has to be subjected to further study. The mesofauna was indicative of a low content of fresh decomposing organic matter and a compacted soil structure in the root and tuber crops. The high percentage (66%) of root and tuber crops is the most probable cause of the low abundance and species richness of the mesofauna in the three-year rotation.

Lower yields in the three year rotation potato crop coincided with the indication by the mesofauna of a more compact soil structure and less fresh decomposing organic matter, which was even more so in the disinfected three year rotation. Still, soil treatment with metamsodium partially reduced the yield depression.

RESUME

Les microarthropodes du sol (acariens et collemboles) de deux assolements culturaux sur sol argileux lourd d'origine marine

Un inventaire des acariens du sol et des collemboles a été réalisé en 1983 et 1984, dans des cultures avec assolement sur 6 années, 3 années sans désinfection du sol et 3 années avec désinfection au metamsodium après la culture des pommes de terre. Le but de cette étude est de trouver des indications biologiques directes ou indirectes relatives au déficit de production observé chez les pommes de terre de l'assolement triennal. Des échantillons ont été prélevés à quatre dates différentes pendant la période de croissance végétale. La mésofaune a été extraite à l'aide du système à haut gradient de Macfadyen. Les profondeurs suivantes ont été échantillonnées: 0-2,5cm; 2,5-5cm; 7,5-10cm; 15-17,5cm; 25-27,5cm. Les résultats ont été analysés à l'aide de techniques analytiques et synthétiques (analyse des correspondances).

La mésofaune est beaucoup plus abondante dans les céréales que dans les cultures de racines ou de tubercules. La plupart des acariens et des collemboles se rencontrent près de la surface du sol sauf dans le cas où la pomme de terre a été précédée par une céréale, enfouie après la culture. En 1983, année où les chutes de pluie ont été abondantes et de longue durée, les acariens ont été récoltés en plus grand nombre dans les cultures de l'assolement sur 6 années par rapport aux cultures correspondantes de l'assolement triennal. Les rotations rapides affectent négativement l'abondance de Tectocepheus velatus tandis que Tullbergia groupe krausbaueri et les Heterostigmata sont au contraire favorisés. En 1984, année où le temps a été plus froid et plus sec, les acariens et les collemboles ont été trouvés en plus grande abondance dans les cultures de pommes de terre de l'assolement triennal que dans celles de l'assolement sur 8 ans.

Acari

Mesostigmata

ACE -Arctoseius cetratus (Sellnick, 1940)

A. cetratus is phoretic on sciarid flies and feeds on sciarid eggs or larvae, on caecid larvae, on Tyrophagus myceliophagus and on rhabditid nematodes [START_REF] Binns | Drei auf Champignonbeeten vorkommende rauberische Milben[END_REF], on T. krausbaueri, Folsomia fimetaria, nymphs of oribatid mites and on Tyrophagus spp. [START_REF] Karg | Ökologischen Untersuchungen von edaphischen Gamasiden (Acarina, Parasitiformes)[END_REF]. SARDAR (1980) observed that A. cetratus consumed Liebstadia similis, but that Tyrophagus putrescentiae, collembola, tribolium eggs and larvae of Ceratozetes were not eaten. It was found in high numbers after application of 550 t cattle slurry per ha [START_REF] Bolger | Effects of cattle slurry on soil arthropods in grassland[END_REF] and was more numerous in two-or four-year ley than in permanent pasture [START_REF] Whelan | An analysis of the acarine communities in the herbage and soil of three adjoining grassland sites in Co. Kildare, Ireland[END_REF].

ARA -Alliphis halleri G. and R. Canestrini, 1981. (= Alliphis siculus, Oudemans, 1905) This species is one of the early colonizers of substrate [START_REF] Lagerlöf | Succession and activity of microarthropods and enchytraeids du ring barley straw decomposition[END_REF] and was found in the soil, but also in the herbage of grassland [START_REF] Whelan | An analysis of the acarine communities in the herbage and soil of three adjoining grassland sites in Co. Kildare, Ireland[END_REF]. It is phoretic on Geotrupus spp. [START_REF] Götz | Düngerbewohnende parasitiforme Milben und die Gattung Macrocheles Latrelle 1829[END_REF] and possibly also Copris hispanus [START_REF] Costa | The mesostigmatic mites associated with Copris hispanus (L.) (Coleoptera: Scarabaeidae) in Israël[END_REF] and feeds selectively on nematodes [START_REF] Karg | Ökologischen Untersuchungen von edaphischen Gamasiden (Acarina, Parasitiformes)[END_REF][START_REF] Karg | Raubmilben als Antagonisten von Nematoden[END_REF][START_REF] Sardar | The abundance and trophic habits of Mesostigmata (Acari) of the soil of grazed grassland[END_REF]. In potato it was distributed throughout the soil profile. In winter wheat it lived near the soil surface. Its abundance depended more on application of manure than on the kind of crop (BÜHLMANN, 1984).

A female A. halleri lives about 90 days and produces in that period about 110 eggs (at 15°C). When fed with Nematospiroides dubuis or Pelodera strongyloides, reproduction and survival of A. halleri was much better than when fed with Globodera rostochiensis larvae [START_REF] Sardar | The abundance and trophic habits of Mesostigmata (Acari) of the soil of grazed grassland[END_REF]. HSI -Hypoaspis similisetae (Karg, 1965) After sewage sludge application this species became numerous (HÖLLER, 1959) and was considered indicative of the early rotting stage. According to [START_REF] Karg | Acari, Milben. Unterordnung Anactinochaeta (Parasitiformes). Freilebenden Gamasina (Gamasides), Raubmilben, Die Tierwelt Deutschlands[END_REF], H. similisetae is normally found in moist soil in humus between roots. It is associated with nematodes (VAN DE BUND, pers. comm.). RPR -Unidentified nymphal stages or adults of mesostigmatid mites VAG -Veigaia agilis Berlese. This species feeds on collembola [START_REF] Karg | Der Einfluss verschiedener Fruchtfolgen, insbesondere mit mehrjarigem Kleegras, auf schädliche und nützliche Mikroarthropoden im Boden[END_REF]. In two-or four-year ley it was less frequently found than in permanent pasture [START_REF] Whelan | An analysis of the acarine communities in the herbage and soil of three adjoining grassland sites in Co. Kildare, Ireland[END_REF].

VPL -Veigaia planicola (Berlese, 1892)

Little is known about this species except that it is more common in arable land than in woodland soil [START_REF] Karg | Acari, Milben. Unterordnung Anactinochaeta (Parasitiformes). Freilebenden Gamasina (Gamasides), Raubmilben, Die Tierwelt Deutschlands[END_REF].

Astigmata

ANO -Anoetidae

These mites feed on bacteria and protozoa that are filtered out of the matrix water (HUGHES, 1959).

They were the first inhabitants of fresh sewage sludge (HÖLLER, 1959) and survive submersed situations very well (pers. obs.). The adults and nymphs are slow-moving creatures that are sensitive to desiccation. Under dry conditions the deutonymph can develop to the phoretic and drought resistant hypopus stage. It takes 7-12 days for Anoetidae to complete their life cycle [START_REF] Woodring | The nutrition and biology of saprotrophic Sarcoptiformes[END_REF]. Anoetids were also found in the herbage or on grasses [START_REF] Whelan | An analysis of the acarine communities in the herbage and soil of three adjoining grassland sites in Co. Kildare, Ireland[END_REF].

AST -All Astigmata except the Anoetidae. Nanorchestidae were found abundantly in manured fields [START_REF] Karg | Der Einfluss verschiedener Fruchtfolgen, insbesondere mit mehrjarigem Kleegras, auf schädliche und nützliche Mikroarthropoden im Boden[END_REF][START_REF]Die Abhangigkeit der bodenbewohnenden Collembolen von Düngung und anderen Standortsfactoren unter Dikopshofer Verhältnisse[END_REF] or in fields heavily infested with potato cyst nematodes [START_REF] Karg | Untersuchungen über die Veranderungen und Wechselbeziehungen der Mikroarthropoden in Kartoffelnematoden verseuchten Flächen[END_REF]. PBL -Pygmephorus blumentritti n. sp. Krczal PSE -P. selnicki (Krczal, 1958).

Pyemotidae

In general, the Pyemotidae feed on fungi. They pierce the fungal hyphae with their sticking mouthparts and suck up the contents. Except for Siteroptes graminis no specific information was found about the Pyemotidae captured in this study. Siteroptes graminis has been found in association with fungal infections of grasses [START_REF] Krczal | Systematik und Okologie der Pyemotiden[END_REF]. Adults of some Pyemotidae can practice phoresy on insects or in sect larva.

SCU -Scutacarus spp.

TTA -Tarsonemus talpae n. sp. Schaarsmidt.

Cryptostigmata

OMI -Oppia minus (Paoli, 1908). NOM = nymph of O. minus [START_REF] Alejnikova | Changes in the structure of animal populations in soil under influence of farm crops[END_REF] found this small species in large numbers in continuous rye and wheat. It is found more in the deeper soil layers than near the soil surface [START_REF] Karg | Die Bedeutung der Microorganismen für die Entwicklung und für die Fruchtbarkeit des Bodens[END_REF][START_REF] Vannier | Réactions des microarthropodes aux variations de l'état hydrique du sol. Techniques relatives à l'extraction des arthropodes du sol[END_REF] and, like Oppia nova, it was less abundant in nematode infested soil of a potato field [START_REF] Karg | Die Bedeutung der Microorganismen für die Entwicklung und für die Fruchtbarkeit des Bodens[END_REF].

ONO -Oppia nova (Oudemans, 1970).

NON = nymph of O. nova.
Oppia nova has been reared on lichens or mushrooms. It feeds in all its nymphal stages and in the adult stage, it is parthenogenetic and lives at 25°C about 30 days [START_REF] Woodring | The nutrition and biology of saprotrophic Sarcoptiformes[END_REF]. HARTENSTEIN (1962) concluded that this species is strictly fungivorous, but according to LUXTON (1981) O. nova is a microbial feeder. In permanent grassland it was more abundant than in two-or four-year ley [START_REF] Whelan | An analysis of the acarine communities in the herbage and soil of three adjoining grassland sites in Co. Kildare, Ireland[END_REF] and it was less abundant in potato soil heavily infested with cyst nematodes than in soil that was almost free of infection [START_REF] Karg | Untersuchungen über die Veranderungen und Wechselbeziehungen der Mikroarthropoden in Kartoffelnematoden verseuchten Flächen[END_REF]. TVE -Adult of Tectocepheus velatus (Michael, 1880).

NTV = nymph of T. velatus.

This large species was reared on "skeletonized leaves from the litter layer of broadleaved woodland", is parthenogenetic, has small eggs and produces two generations a year (MURPHY & JALIL 1963[START_REF] Schenker | Population dynamics of oribatid mites (Acari: Oribatei) in a forest soil ecosystem[END_REF]. It feeds on fungi (MITTMANN, 1983). It lives near the soil surface [START_REF] Weis-Fogh (t | Ecological investigations of mites and collemboles in the soil[END_REF][START_REF] Karg | Die Bedeutung der Microorganismen für die Entwicklung und für die Fruchtbarkeit des Bodens[END_REF] and is sensitive to application of urea in woodland soil (MARSHALL, 1977) and to the pesticides endosulfan, lindane, methomyl, aldicarb, oxamyl and DBCP (HEUNGENS & [START_REF] Heungens | Toxicity of insecticides and nematicides on soil mites and Collembola in pine litter substrate[END_REF]. It resists desiccation better than O. nova [START_REF] Riha | Zur Ökologie der Oribatiden in Kalksteinböden[END_REF], was found in soils with a wide range of pH (CHISTYAKOV, 1972) and is sensitive to frost [START_REF] Lebrun (ph | Quelques aspects de la phénologie des populations d'Oribatides (Acari, Oribatei) dans le sol forestier en Moyenne-Belgique[END_REF]. [START_REF] Schalk | Zur Bodenfauna von Wiesen-und Luzernebestanden unterschiedlicher Standorte unter besonderer Berücksichtigung der Oribatiden[END_REF] states that this species survives a broad spectrum of ecological conditions and will be found where other species are lacking. Collembola CBI -Cryptopygus bipunctatus (AXELSON, 1903). CDE -Ceratophysella denticulata (BAGNALL, 1941). [START_REF] Bolger | Effects of cattle slurry on soil arthropods in grassland[END_REF] always found increased numbers after application of cattle slurry on grassland. It profits strongly from fresh organic material (PIART and DUVIART, 1985) and is an early species in decomposition (BECKMANN, pers. comm., HÖLLER-LAND, 1959). It was most abundant in the top 5 cm of the soil, and suffered from plowing [START_REF] Gers | Incidence de la simplification du travail du sol sur la microfaune édaphique hivernale: données préliminaires[END_REF]. When reared in microcosms in competition with Onychiurus gr. armatus it was less successful at 16°C (constant temperature) but it had a better development at 23°C (fluctuating temperature) (LONGSTAFF, 1976). COL -All other springtails.

ENE -Entomobryinae

FCA -Folsomia candida (Willem, 1902). This parthenogenetic species feeds on fungi such as Rhizoctonia, Pythium, Fusarium or the mycorrhizal fungi of leek (Allium porrum) (KARG, 1969, WARNOCK and[START_REF] Warnock | The influence of a collembola, Folsomia candida, on the mycorrhizal association of leek, Allum porrum, and the vesicular-arbuscular endophyte Glomus fasciculatus[END_REF] of which it prefers newly grown hyphae (LEONARD and BRADBURY, 1984), but can be reared also on yeast [START_REF] Snider (r | The life history of Folsomia candida (Willem) (Collembola; Isotomidae) relative to temperature[END_REF]. At 21°C (constant temperature) a female F. candida produced on average 63 eggs during her life, "l" was 20.3 and r. day -1 was 0.15 [START_REF] Snider (r | The intrinsic rate of increase: Its interest to ecology and its applications to various species of collembola. In: Soil organisms as components of ecosystems[END_REF]. At temperatures of 15, 21 or 26°C representants of this species lived respectively 240, 136 or 72 days producing in that period on average 1344, 1011 or 130 eggs [START_REF] Snider (r | The life history of Folsomia candida (Willem) (Collembola; Isotomidae) relative to temperature[END_REF]. [START_REF] Gers | Incidence de la simplification du travail du sol sur la microfaune édaphique hivernale: données préliminaires[END_REF] found this species associated with deeper soil layers and with plowing, especially when maize rather than wheat was the previous crop.

Legends of figures

  showed a significant association of rotation and year [d. f. = 1; P(χ 2 >39) « 0.01] with largest counts in 1983 in the R3 potato crop and in 1984 in the R3* potato crop. The vertical distribution of the mesofauna in R3 and R3* was significantly associated only with soil disinfection in 1983 in beet (mites [d. f. = 4; P(χ 2 >44) « 0.01], springtails [d. f. = 4; P(χ 2 >10) « 0.05]) and barley (mites [d. f. = 4; P(χ 2 >49) « 0.01], springtails [d. f. = 4; P(χ 2 >11) < 0.025]) and in 1984 in potato (mites [d. f. = 4; P(χ 2 >38) « 0.01], springtails [d. f.=4; P(χ 2 >25) « 0.01]).

  minus (OMI) and its nymph (NOM), I. notabilis (INO), Anoetidae (ANO) and M. minimus (MMI). Conditions in 1983 must have been less favourable for the development of a species-rich fauna typical of potato since the potato crop of that year occupied an intermediary position. The cereals winter wheat and spring barley were characterized by the following taxa; T. talpae (TTA), I. viridis (IVI), P. selnicki (PSE), P. alba (PAL), Nanorchestes sp. (NAN), A. cetratus (ACE) and F. sp. (FSP).

  Schreef", and Mr. C.F.VAN DE BUND of the Research Institute for Nature Management (RIN) at Arnhem for advice concerning the identifications. C.N. BELJAARS and H.P. DE ROOS are thanked for aid with the identifications and/or practical assistance.

L

  'analyse des correspondances a fourni 4 axes interprétables. Le classement des relevés et des espèces le long du premier axe correspond essentiellement à la présence de Tectocepheus velatus et est lié à la profondeur, au type d'assolement et aux facteurs climatiques. Le second axe montre un effet de l'année de culture pour ce qui concerne la pomme de terre. Le classement des relevés et des espèces le long des axes 3 et 4 indique de différentes manières l'influence du type de culture. Les résultats suggèrent une contribution possible des Acaridiés au déficit de production. Ceci doit faire l'objet d'une prochaine étude. La mésofaune s'avère indicatrice de la faible teneur en matière organique fraiche et de la compacité du sol sous les cultures de racines et de tubercules. Le fort pourcentage (66 %) occupé par ces cultures dans l'assolement triennal est probablement responsable de la faible abondance et du faible nombre d'espèces de la mésofaune qui le caractérisent. APPENDIX SPECIES LIST AND ECOLOGICAL DATA (In alphabetic order)

  Prostigmata AEU -All Eupodina except Eupodes sp. and Nanorchestes sp. EUP -Eupodes sp. HET -All unidentified Heterostigmata. MPS -Male of an unidentified Pygmephorus sp. probably P. blumentritti. NAN -Nanorchestes sp.

  Rare species: Veigaia cerva (KRAMER, 1876) (VCE), Veigaia exigua (BERLESE, 1917) (VEX), Saprosecans baloghi Karg 1964 (SBA), Antennoseius sp. (ASP), Prozercon triighardi (HALBERT, 1923) (PTR), Pygmephorus ursulae n. sp. Krczal (PUR).

FIG. 1

 1 FIG. 1. -Soil temperature at 10cm depth and rainfall during 1982, 1983 and 1984, mean values for 10-day periods. Arrows (↓) indicate sampling dates.

  Fig. 1

Fig. 3

 3 Fig. 3

  Fig. 4

Fig. 6

 6 Fig. 6

Distribution of the most numerous mites and springtails in the different rotations as affected by the crops

  ).1 The sampling on 1-XI took place after harvest of the potato crop and after plowing. 2 Flax was undersown with white clover (Trifolium repens).3 The sampling on 1-XI took place after harvest of the beets.4 Spring barley in R3 and R3* was undersown with Italian ryegrass (Lolium multiflorum). . (Approximation of the number of animals per m 2 to a depth of 27.5 cm on a given day can be obtained by multiplication of the data by 100.)

	TAB. II. -Crop TAB. III. -Seasonal	year:	Potato		WW FL		1983 Sugarbeet		Spring barley	PE		1984 Potato
	Year Taxon:	R6			R3	1983 R3*			R6	R3	R3* R6	R3	R3*		R6	R3 1984	R3*
	Sampling dates Acari					15-VI											26-IV
	Mesostigmata V. planicola Restgroup H. similisetae	1 3 7			3 4 12	18-VII 23-IX 0 44 1-XI 1 10 0 0	41 8 5	2 6 2	4 0 0	4 7 1	23 46 1	30 36 5	40 23 15	26 7 9	26 7 5	14-VI 6 7-VIII 21 48 32 18-IX 5 6
	Rotation A. halleri	0			10	CROP 3	0	0	1	0	1	1	0	1	0	3	6	18
	6 year (R6) Cryptostigmata 3 year (R3) T. velatus (nymph+adult) 212 3 year (R3*) (disinfected) O. minus (nymph+adult) 16 O. nova (nymph+adult) 17			39 9 10	PO 1 PO 1 39 PO 1 3 6	15 5 33	WW -49 -36 15	FL 2 BE 3 -BE 3 165 75 18 -BE 3 22 19 33 11 12 13	BA BA 4 190 10 BA 4 4 16 32 9	2 8 7	PE -98 -20 1	19 16 14	PO 64 PO 30 PO 15	20 17 9
	Soil labour practices Prostigmata
	PO Eupodes sp Nanorchestes sp P. blumentritti P. selnicki	6 0 3 11	R6 R3 R3* 10 0 24 1	12 OH, Harvest 25 0 0 OH, Harvest 5 26 OH, Harvest, FTC, Metamsodium treatment 44 4 19 14 38 31 0 0 0 1 1 43 2 5 3 6 205 237 365 44 54 1 94 3 3 11 9 24 69 16	8 1 7 4	17 6 14 1	17 11 26 8	26 9 28 10
	WW Restgroup	0	R6	1	FTC 1	17	0	13	1	0	33	4	64	2	4	27	50
	FL T. talpae BE Astigmata Acaridae	1 1	R6 R6	2 8	1 Plowing, Leveling (2 times) 84 8 1 5 Plowing, OH (2 times), Harvest 2 2 1 50 15 3 1	15 19	7 35	16 2	14 3	0 2	0 108 28 0
	Anoetidae	2	R3	3	FTC, Plowing, OH (2 times), Harvest 3 0 2 5 3 3 19	6	3	51	38	32	19
	Collembola BA T. krausbaueri C. denticulata I. notabilis	142 0 0	R3* R6 277 149 TFC, Plowing, OH (2 times), Harvest 224 151 199 198 120 132 123 170 OH R3 1 0 14 0 2 0 1 4 33 42 OH 13 4 49 0 11 0 0 174 20 82	102 34 0 1 1 37	32 29 80	40 32 182
	F. candida	0	R3* 1	OH 1	2	0	0	1	9	35	9	0	6	3	47	0
	PE Lepidocyrtus sp. P. alba FTC = Fixed-tine cultivator. 3 4 Folsomia sp. 0 OH = Oscillating harrow. M. minimus 2	R6	5 0 1 1	1 Plowing, OH 22 25 1 22 8 1 5 0 2 5 15	4 5 6 10	5 9 0 8	3 5 8 12	13 18 24 10	5 14 19 20	6 18 7 0	15 5 2 12	3 1 0 41	5 0 0 15	2 1 0 7
	TOTAL ACARI	289			147 106	388	304 275 163 120	735 587 725	279 199 451 353
	TOTAL COLLEMBOLA	150			304 158	373	234 246 228 161	440 292 354	149 127 228 270

variation in the abundance of mesofauna species in cereal and potato crops. Values given are sums of the first two samplings (spring) or the last two samplings (late summer) for cereals (winter wheat and three spring barley crops) and potato (two thirds of the summarized data of the six potato crops

  ). (Approximation of the number of animals per m 2 to a depth of 27.5 cm on a given day can be obtained by multiplication of the data by 50. Abbreviations of species names are explained in the appendix.)

	TAB. IV. -Vertical
			Cereal	Potato				Cereal		Potato
			late	late			late		late
		spring summer spring summer		spring summer spring summer
	Taxon:					Taxons:
	Mesostigmata					Astigmata
	VPL	26	111	31	9	AST	12	45	29	71
	RPR	20	95	41	15	ANO	10	17	40	25
	HSI	9	12	15	2	Prostigmata (excl. Heterostigmata)
	AHA	0	2	24	5	EUP	82	64	20	39
	VAG	0	0	8	19	NAN	0	98	14	3
	ACE	4	21	2	1
	VNE	6	13	1	4
	Cryptostigmata					Collembola
	NTV	65	112	118	48	TKR	175	476	97	353
	TVE	7	33	15	11	INO	52	273	189	19
	NOM	2	1	1	15	CDE	20	73	39	4
	OMI	9	21	30	17	MMI	8	27	27	18
	NON	20	6	8	11	COL	11	33	5	1
	ONO	30	25	15	14	FCA	2	44	1	41
	Heterostigmata					LSP	24	24	8	6
	PBL	301	532	53	16	PAL	23	49	2	2
	PSE	1	206	12	11	FSP	6	49	0	1
	HET	79	39	49	7	IVI	14	24	1	0
	TTA	20	102	2	4	CBI	18	8	6	0
	MPS	41	29	4	4	ENE	0	0	2	10
	SCU	18	8	8	7

distribution of mites and springtails in relation to crop and rotation. Solid lines separate crops which are not in sequence in the crop rotation. Dashed lines separate crops which are in sequence in the crop rotation but were sampled on different fields in the same year. No lines are drawn between crops that were grown on the same field in two successive years

  .(Approximation of the number of animals per m 2 for each layer on a given day can be obtained by multiplication of the data by 50.) Vertical distribution of mesofauna, summarized for all samples. (Approximation of the number of animals per m 2 at a given day and for each layer can be obtained by multiplication of the values with 3. Abbreviations of species names are explained in appendix.)

	TAB. V. -Depth (cm) and numbers				Depth (cm) and numbers
	Taxon	0-2.5 2.5-5 7.5-10 15-17.5 25-27.5 Taxon	0-2.5 2.5-5 7.5-10 15-17.5 25-27.5
	Mesostigmata VPL	80	74		59	31	Mites	25	Astigmata AST	54	39			Springtails 46 123	15
	RPR		55	37		42	1983 51		53	1984 ANO	82	46	1983 19	27	1984 14
	RSI ARA Rot. depth R6 0-2.5 VAG ACE	15 11 4 20	18 5 6 3	PO 46	16 13 8 3	BE 10 10 110 10 4	BA 4 6 351 10 3	PO Prostigmata EUP 38 NAN	PO 135 4 72	BE (exc . Heterostigmata) BA 103 39 27 19 94 24 12 4	PO 21 15 25
	VNE	2.5-5	5	8	116	8	98	4	187 3	66	16			44	148	26
	7.5-10 Cryptostigmata 15-17.5 NTV 518 25-27.5 TVE 66 NOM 1	245 55 1	42 56 30	42 15 7	18 30 46 18 3 11		97 79 19 21 4 21	25 Collembola 39 TSL 31 INO CDE	60 38 252 33 128 46	349 274 41		73 58 52	574 150 26	77 86 525 35 73 37	17 391 34 28 26 9
	R3 Heterostigmata 0-2.5 OMI 2.5-5 NON 7.5-10 ONO	4 13 19	11 20 38	27 48 19	19 20 30	96 73 29 13 5 33	340 107 81 2 42 15	85 MMI 63 COL 80 FCA LSP	21 26 25 69 112 5 51	18 15 10 32		23 43 56	41 24 8 15	112 37 65 10 42 88 8	38 18 9 35 13 42 10
	PBL	15-17.5 408	233	30	108	13 124	68 82	120 PAL	100 39	38		50	11	24	14	85 9
	PSE R3* RET TTA MPS	25-27.5 158 0-2.5 15 2.5-5 121 26	42 32 28 33	25 14 38	24 69 2 19	20 27 39 81 18 2 4	56 14 333 20 160 3 4	103 FSP 41 IVI 58 CBI ENE	48 2 5 31 8 8 5	4 7 5 5		56 18 23	25 3 11 4	49 135 12 1 95 11 3	29 49 2 25 8 51 2
	SCU	7.5-10	13	16	21	12	33 12	118 10	87 Total mites	70 1977	1186	61	685	46 784	90 487
		15-17.5		26		17			66	133 Total collembola	47 658	801		32	891	44 817	78 547
		25-27.5		8		13			48	35	30			27	34	27

ACKNOWLEDGEMENTS

The authors wish to thank Ir. O. HOEKSTRA and Ir. J.G. LAMERS of the Research Station for Arable Farming and Field Production of Vegetables (PAGV) at Lelystad for technical assistance and discussions at "de

FSP -Folsomia sp. INO -Isotoma notabilis Schaffer, 1896. This parthenogenetic species (PETERSEN, 1971) lives in the litter layer or in the soil just beneath it (KARG, 1962) and was also found on snow by PIART and DUVIART (1985). The juveniles penetrate the soil more deeply and are found in the soil surrounding the roots (MÜLLER, 1959; ULBER, 1978). It was found as early colonizer of decomposing organic matter (HÖLLER-LAND, 1959). GERS (1982) found relatively many I. notabilis in the deeper soil layers. Soil tillage affected this species relatively little.

IVI -Isotoma viridis

Bourlet, 1839. This large species lives at the soil surface or just in the soil [especially the juveniles (KARG, 1961)]. It produced twice as much eggs when fed with algae instead of "Tetramin". LSP -Lepidocyrtus spp. MMI -Megalothorax minimus Willem, 1900. M. minimus is a late successional species (PARR, 1978) which shows little vertical movement (HÖLLER-LAND, 1962). PAL -Pseudosinella alba (Packard, 1873).

P. alba lives in neutral or alkaline soils (PONGE, 1980). It eats specifically from fungi, of which it likes the spores the best, followed by conidiophores or hyphae (ARPIN et al., 1980; PONGE and CHARPENTIÉ, 1981). It is an early species in succession (PARR, 1978).

TKR -Tullbergia krausbaueri s.l. Börner, 1901. [A group of species until recently not separated in literature (Rusek, 1979)]. This parthenogenetic species (PETERSEN, 1971) probably has only one generation per year (LEINAAS, 1976). T. krausbaueri inhabits small pores (HAARLÖV, 1955) of deeper soil layers (KARG, 1961;GERS, 1982) and feeds most probably on fungi (KARG, 1969). They are not bound to rhizosphere soil or to a particular crop (MÜLLER, 1959; ULBER, 1978). As one of the very few springtails, they survive continuo us fallow very well (VAN DE BUND 1970). Furthermore, they are associated with late stages of decomposition of organic material