Keywords: aboveground-belowground, functional equivalence, functional redundancy, functional similarity, litter decomposition, local adaptation, microbial community structure, plant-soil interactions

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

1 Title: Functional breadth and home-field advantage generate functional differences among soil microbial decomposers

List of authors: Nicolas Fanin 1,2 , Nathalie Fromin 3,4 , Isabelle Bertrand 1,5,6 .

Postal addresses: 1 isabelle.bertrand@supagro.inra.fr E-mail addresses: nicolas.fanin@slu.se, nathalie.fromin@cefe.cnrs.fr, isabelle.bertrand@supagro.inra.fr

Running title: Functional ability of soil microbes

Abstract. (350 words)

In addition to the effect of litter quality (LQ) on decomposition, increasing evidence is demonstrating that carbon mineralisation can be influenced by the past resource history, mainly through following two processes: (i) decomposer communities from recalcitrant litter environments may have a wider functional ability to decompose a wide range of litter

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 2 species than those originating from richer environments, i.e. the functional breadth (FB) hypothesis; and/or (ii) decomposer communities may be specialized towards the litter they most frequently encounter, i.e. the home-field advantage (HFA) hypothesis. Nevertheless, the functional dissimilarities among contrasting microbial communities, which are generated by the FB and the HFA, have rarely been simultaneously quantified in the same experiment, and their relative contributions over time have never been assessed. To test these hypotheses, we conducted a reciprocal transplant decomposition experiment under controlled conditions using litter and soil originating from four ecosystems along a land-use gradient (forest, plantation, grassland and cropland) and one additional treatment using 13 C labelled flax litter allowing us to assess the priming effect (PE) in each ecosystem. We found substantial effects of LQ on carbon mineralisation (more than two-thirds of the explained variance), whereas the contribution of the soil type was fairly low (less than one-tenth), suggesting that the contrasting soil microbial communities play only a minor role in regulating decomposition rates. Although the results on PE showed that we overestimated litter-derived CO 2 fluxes, litter-microbe interactions contributed significantly to the unexplained variance observed in carbon mineralisation models.

The magnitudes of FB and HFA were relatively similar, but the directions of these mechanisms were sometimes opposite depending on the litter and soil types. FB and HFA estimates calculated on parietal sugar mass loss were positively correlated with those calculated on enzymatic activity, confirming the idea that the interaction between litter quality and microbial community structure may modify the trajectory of carbon mineralisation via enzymatic synthesis.

We conclude that although litter quality was the predominant factor controlling litter mineralisation, the local microbial communities and interactions with their substrates can explain a small (< 5%) but noticeable portion of carbon fluxes.

Introduction

The decomposition of plant litter is one of the most important processes influencing the global carbon cycle, the mineralisation of nutrients, soil fertility, and the formation of soil organic matter in terrestrial ecosystems. Almost 40 years ago, [START_REF] Meentemeyer | Macroclimate and lignin control of litter decomposition rates[END_REF] proposed that climate and litter quality are the main factors explaining plant litter decomposition at the global scale. In the decades since, many studies have confirmed this pattern at the continental scale (e.g., [START_REF] Aerts | Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship[END_REF][START_REF] Gholz | Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition[END_REF][START_REF] Trofymow | Rates of litter decomposition over 6 years in Canadian forests: influence of litter quality and climate[END_REF][START_REF] Powers | Decomposition in tropical forests: a pan-tropical study of the effects of litter type, litter placement and mesofaunal exclusion across a precipitation gradient[END_REF]), but the factors contributing to the unexplained variation in local decomposition processes and their relative contributions remain uncertain [START_REF] Bradford | Climate fails to predict wood decomposition at regional scales[END_REF]. A variety of drivers modifying the trajectory of decomposition rates have been proposed, including diversity effects [START_REF] Heemsbergen | Biodiversity effects on soil processes explained by interspecific functional dissimilarity[END_REF][START_REF] Gessner | Diversity meets decomposition[END_REF], solar radiation [START_REF] Austin | Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation[END_REF], the temporal concordance between tree phenology and soil decomposers [START_REF] Pearse | The phenology-substrate-match hypothesis explains decomposition rates of evergreen and deciduous oak leaves[END_REF], and the structure of the soil fauna and microbial communities [START_REF] Wall | Global[END_REF], Strickland et al. 2009a). It has also been suggested that interactions between the litter quality and decomposer communities may locally influence decomposition rates due to decomposers' different perceptions and/or specialisations toward specific substrates [START_REF] Hunt | Nitrogen limitation of production and decomposition in prairie, mountain meadow, and pine forest[END_REF][START_REF] Gholz | Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition[END_REF], Strickland et al. 2009b). Nonetheless, important uncertainties remain regarding the magnitude and direction of

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 4 these interactions [START_REF] Ayres | Do plant species encourage soil biota that specialise in the rapid decomposition of their litter?[END_REF], Chapman and Koch 2007[START_REF] Freschet | Multiple mechanisms for trait effects on litter decomposition: moving beyond home-field advantage with a new hypothesis[END_REF]. In particular, little is known about the kinetics of litter-decomposer interactions over time (but see Ayres et al. 2009a), and positive or negative effects may depend on the interactions between the initial litter quality, the stage of decomposition, and the local decomposer community (Strickland et al. 2009a, Wickings et al. 2012[START_REF] Bell | High-throughput fluorometric measurement of potential soil extracellular enzyme activities[END_REF]. In this context of multiple biotic and abiotic factors regulating the decomposition rate at a local scale, we focused on a set of hypotheses addressing the effects of plant-soil interactions on carbon mineralisation during the decomposition of plant litter in a variety of contrasting ecosystems (Fig. 1). The selected landuse gradient, which ranges from cropland, plantation, and grassland to forest on a common soil substratum and varies in vegetation cover and disturbance frequency, offers a great opportunity to accurately assess the relative importance of substrate quality and microbial community on litter decomposition.

First, the 'litter quality' (LQ) hypothesis suggests that the intrinsic physical and chemical characteristics of leaf litter, commonly measured as the C:N and lignin:N ratios or the quality of various carbon forms, are the main predictors influencing the decomposition rate [START_REF] Aber | Predicting long-term patterns of mass loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems[END_REF][START_REF] Aerts | Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship[END_REF][START_REF] Cornwell | Plant species traits are the predominant control on litter decomposition rates within biomes worldwide[END_REF]. In a reciprocal litter transplant study using 16 different species across four different biomes, [START_REF] Makkonen | Highly consistent effects of plant litter identity and functional traits on decomposition across a latitudinal gradient[END_REF] showed that a small subset of litter traits explained most of the variation in litter mass loss, suggesting that the decomposition process is largely independent of the local decomposer communities. Because of these 'litter trait controls', differences in carbon mineralisation rates during litter decomposition should result from the higher consumption of easily degradable litters (e.g., rich in soluble and holocellulose contents) and the persistence of recalcitrant litters that are more resistant to decay (e.g., rich in lignin and secondary metabolites). For example, along a land-use gradient, grass litter, which

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 5 represents an energy-rich and labile substrate, should decompose faster in all types of ecosystems compared to the poorer and more recalcitrant forest litter (Fig. 1a). Support for this hypothesis is derived from a multitude of decomposition studies in which a range of litter types were allowed to decompose across a variety of ecosystems (e.g., [START_REF] Gholz | Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition[END_REF], Hobbie et al. 2006[START_REF] Makkonen | Highly consistent effects of plant litter identity and functional traits on decomposition across a latitudinal gradient[END_REF].

Second, the 'functional breadth' (FB) hypothesis stipulates that decomposer communities from recalcitrant litter environments have a wider functional ability than those originating from richer environments, meaning that they more efficiently decompose various litter types that vary widely in their chemical and physical characteristics [START_REF] Van Der Heijden | The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems[END_REF][START_REF] Keiser | The effect of resource history on the functioning of soil microbial communities is maintained across time[END_REF]. Because of the past resource history that may influence whether microbial communities are functionally 'broad' or 'narrow' [START_REF] Keiser | Disentangling the mechanisms underlying functional differences among decomposer communities[END_REF], differences in the carbon mineralisation rates during litter decomposition should result from different catabolic capacities among contrasted decomposer communities. Thus, all litter species should decompose faster in the forest soil than in any other ecosystem because of a higher capability of the forest soil microbial community to decompose a wide range of litter substrates (Fig. 1b). Consistent with this hypothesis, Strickland et al. (2009b) demonstrated that poor and recalcitrant litter was decomposed more efficiently by decomposer communities originating from forested habitats, whereas easily degradable litter was decomposed equivalently by all communities of decomposers from different types of ecosystems. These results indicate that microbial communities may be functionally dissimilar for the degradation of recalcitrant litter substrates and consequently that changes in ecosystem processes may also be strongly influenced by the functional ability of distinct decomposer communities (Strickland et al. 2009a).

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

6

Finally, the 'home-field advantage' (HFA) hypothesis suggests that litter decomposes faster in an area dominated by the plant species from which it derives (i.e., at home) rather than in an area dominated by another plant species (i.e., away) [START_REF] Gholz | Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition[END_REF]. Because of this 'local adaptation' of the decomposer community, differences in carbon mineralisation rates during litter decomposition should result from the match between a litter type and its own environment. For example, forest litter should decompose faster in forest soil, whereas wheat litter should decompose faster in cropland soil (Fig. 1c). In agreement with this hypothesis, numerous studies have documented that plant litters decompose fastest in their local environment (e.g., [START_REF] De Toledo Castanho | Relative effect of litter quality, forest type and their interaction on leaf decomposition in south-east Brazilian forests[END_REF][START_REF] Vivanco | Tree species identity alters forest litter decomposition through long-term plant and soil interactions in Patagonia, Argentina[END_REF], Strickland et al. 2009a, Wallenstein et al. 2010). Although the HFA hypothesis has generated great attention, several empirical studies did not find any evidence of such affinity between litter and microbes at home [START_REF] Ayres | Do plant species encourage soil biota that specialise in the rapid decomposition of their litter?[END_REF], Gieβelmann et al. 2011, St John et al. 2011, Veen et al. 2015), sparking an important debate about the generalizability of this mechanism (see [START_REF] Austin | There's no place like home? An exploration of the mechanisms behind plant litter-decomposer affinity in terrestrial ecosystems[END_REF] for a review). This 'affinity effect' between decomposers and their substrates has been recently amended with the 'substrate-matrix interaction' hypothesis [START_REF] Freschet | Multiple mechanisms for trait effects on litter decomposition: moving beyond home-field advantage with a new hypothesis[END_REF], which states that the composition of the decomposer community should reflect the average quality of the litter matrix rather than species-specific home litter effects, in particular in biomes dominated by multiple and chemically contrasted plant species. Despite these recent advances, few studies have addressed carbon dynamics over time in relation to simultaneous changes in litter quality, the local microbial community, and their functional abilities, although such an effort is essential for assessing the unexplained variation in decomposition rates caused by litter-microbe interactions at the local scale.

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

7

Here, we tested these three non-exclusive hypotheses by examining the plant littermicrobe interactions in a reciprocal transplant experiment of contrasting litters and their associated soils originating from a land-use gradient (forest, plantation, grassland and cropland) across a one-hectare plot. Because little is known about whether the previously described mechanisms are transient or persistent over time, we specifically followed the kinetics of carbon mineralisation over 202 days and 21 dates to better grasp the variability of responses generated by the interactions between soil microbial communities and litter types. In order to ensure that the mineralisation of 'older' organic matter via 'fresh' litter addition did not alter our conclusions on litter-microbe interactions, we also measured the mineralisation of soil organic matter following the addition of 13 C labelled flax litter (Linum usitatissimum L.) for assessing the contribution of priming effect (PE) in each ecosystem. To investigate the triangular relationships involving litter quality, the decomposer community and enzymatic activities for explaining the observed affinity effects, we analysed at three time points (at 27, 97, and 202 days) the mass loss rates of the main sugars within the cell wall, the microbial community structure via twenty-four selected PLFA markers, and the activities of four hydrolytic enzymes. We employed a new model recently developed by [START_REF] Keiser | Disentangling the mechanisms underlying functional differences among decomposer communities[END_REF] that allowed us to simultaneously test our three main hypotheses by calculating the estimated parameters for LQ, FB and HFA and their effects on carbon mineralisation. Finally, by assessing the effects of these parameters on the mass loss of parietal sugars and on enzymatic activities, we addressed for the first time whether the effects of FB and HFA on carbon mineralisation depend on the functional ability of the soil decomposer communities.

Methods

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016) (Robinia pseudoacacia L.) were the dominant plant species in each respective ecosystem. In the forest and grassland ecosystems, which present greater plant diversity than the monospecific wheat culture and black locust plantations, we selected sampling areas within nearly pure stands to prevent litter mixing and difficulties in interpreting the results due to potential diversity effects. Forest and plantation litters were collected as recently fallen leaf litter, grass litter was collected as standing-dead material, and cropland litter was collected just after the wheat harvest in October 2013. In the laboratory, all of the litter samples were sorted to remove the fruits, seeds, and roots, cut into approximately 1 cm pieces using scissors, homogenised, and dried at 40°C. Litter was sterilised by autoclaving (121°C, 20 min) twice in succession to maximise the chance that the decomposer communities were introduced only via the soil and not by the litter material, but we acknowledge that autoclaving may have altered the initial litter chemistry. The litter chemical quality was determined using standard methods after sterilisation (see Appendix

A for a detailed description of the litter chemical parameters and determination methods).

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

9

Mineral soil was collected from three different areas in each ecosystem by digging 25 × 25 cm 2 holes to a depth of 10 cm, representing three independent samples series per land-use type. The soil was passed through a 2-mm sieve, homogenised, and then stored at 4°C until use (see Appendix B for a detailed description of the soil characteristics).

The soil samples (100 g dry mass) were mixed with the litter samples at a rate equivalent to 10 g C kg -1 dry soil (between 2.05 and 2.22 g of dry mass) in 850 ml glass jars and incubated

for 202 days at 20°C. Based on preliminary tests and previous incubation experiments (e.g., [START_REF] Bertrand | Carbon and nitrogen mineralization in acidic, limed and calcareous agricultural soils: Apparent and actual effects[END_REF][START_REF] Amin | Impact of fine litter chemistry on lignocellulolytic enzyme efficiency during decomposition of maize leaf and root in soil[END_REF]), 2 g of litter was considered as the best compromise between the need to retrieve enough material for our chemical analyses after 202 days of decomposition and the quantity necessary for optimizing the stimulation of soil microbial communities. The mixture was adjusted to and maintained at a potential of -80 kPa, which corresponds to 221-230 g kg -1 of air-dried-soil water content (approximately 50% water-holding capacity) and favours microbial activity [START_REF] Bertrand | Carbon and nitrogen mineralization in acidic, limed and calcareous agricultural soils: Apparent and actual effects[END_REF]). The soil moisture was maintained throughout the incubation period by weighing the microcosms twice per month and readjusting with deionised water when necessary. We used a reciprocal transplant design (i.e., all of the litter types crossed with all of the soil types) plus additional 'no-litter' soil samples. In total, 9 replicates per treatment were constructed, permitting the destructive harvest of replicates on incubation days 27, 97, and 202 for chemical, enzymatic and microbial community analyses. On each sampling date and for each microcosm, a subsample of the remaining litter material was carefully retrieved, cleaned of soil particles, and dried at 37°C for parietal sugar and lignin analyses. Two additional subsamples of soil and litter mixtures were immediately frozen at -20°C for further PLFA analyses and enzymatic assays.

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 10

Carbon mineralisation

Carbon mineralisation was measured in triplicate for each soil × litter type combination in the presence of a CO 2 trap (15 ml of 1 M NaOH) at [3, 6, 9, 13, 17, 21, and 27], [34, 41, 51, 62, 73, 85, and 97], and [112, 126, 140, 154, 168, 182, and 202] days after the start of incubation.

Specifically, the microcosms harvested on days 27, 97, or 202 were the same microcosms from which we measured the carbon mineralisation rates during the three periods, i.e., 3-27, 28-97, or 98-202 days of incubation. This measurement trend resulted in 21 sampling dates divided into three incubation periods composed equally of 7 sampling dates. The carbon dioxide production rates (mg C kg -1 soil day -1 ) and cumulative carbon mineralisation (mg C kg -1 soil) were corrected for the contribution of the corresponding soil type by subtracting the carbon mineralisation of the 'no-litter' soil (except when 'raw data' are mentioned in the text). It should be noted that the carbon mineralisation was analysed for all of the microcosms (i.e., 3 sets of 3 replicates) on day 27, whereas only 2 sets and 1 set of microcosms were collected on days 97 and 202, respectively, since the first and second sets of microcosms had already been destructively harvested. The concentrations of CO 2 trapped in the NaOH solution were measured by continuous flow colourimetry using an auto-analyser (TRAACS 2000, Bran & Luebbe, Norderstedt, Germany).

Priming effect

Labelled 13 C flax stem were obtained from growing flax seeds (Linum usitatissimum L.)

in an airtight growth chamber in which conditions (air moisture, air temperature and photoperiod) were monitored to mimic spring conditions. The plants were grown on rockwool

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 11 plugs under hydroponic conditions using nutrient solutions as described by [START_REF] Devienne | Nitrate transport in intact wheat roots: II. Longterm effects of NO3-concentration in the nutrient solution on NO 3 -unidirectional fluxes and distribution within the tissues[END_REF]. The plants were continuously labelled with 13 C by using the same 13 CO 2 bottles throughout the entire growth cycle. Flax stem was collected at maturity (δ 13 C = 2870‰, atom% excess = 3.056), homogenised, incubated at a rate equivalent to 10 g C kg -1 dry soil (2.13 g of dry mass) and carbon mineralisation was measured as described above. Flax litter represented a mid-range quality, characterized by intermediate content of nitrogen, lignin, and soluble compounds and showed cumulative carbon mineralisation comprised between Gramineae and woody plant species with an average of 5232 mg C kg -1 soil (see Appendix C for more details).

The 13 C-CO 2 / 12 C-CO 2 ratio was determined by adding 10 ml of 0.5 M BaCl 2 to NaOH solutions and the resulting precipitate of BaCO 3 was separated by vacuum filtration (glass fiber filter) and dried at 80°C for 8 hours. The BaCO 3 precipitate was weighed in a tin cap with the addition of a catalyst (PbO 2 ), and the isotope ratio 13 C/ 12 C of the precipitate was measured with an elemental analyzer (Eurovector, Milan, Italy) coupled to a mass spectrometer (Delta S Advantage, Thermo Fisher Scientific, Bremen, Germany). The priming effect (PE) was calculated with the following equation:

PE = [ 12+13 CO 2 -amended -13 CO 2 -amended] -12 CO 2 ctrl
where 12+13 CO 2 -amended and 12 CO 2 ctrl are the cumulative amounts (mg kg -1 ) of mineralized carbon in the soil with and without litters, respectively, and 13 CO 2 -amended represents the litterderived carbon.

Litter chemical analyses

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

12

To characterise the parietal sugars initially and at days 27, 97, and 202, the plant litter samples were subjected to a cell wall preparation process that consisted of extracting the neutral detergent fibre (NDF) fraction as described by [START_REF] Goering | Forage fiber analyses (apparatus, reagents, procedures, and some applications[END_REF]. The soluble fraction was removed by boiling between 0.7 to 1 g of litter in deionised water at 100°C for 30 min and then performing an extraction with a neutral detergent solution at 100°C for 60 min to remove the cytoplasmic components and obtain the NDF fraction. All residues from the cell wall preparations were dried for one week at 30°C and ground to 80 µm. The parietal sugars were then analysed as previously described [START_REF] Machinet | Impact of plant cell wall network on biodegradation in soil: Role of lignin composition and phenolic acids in roots from 16 maize genotypes[END_REF]. Briefly, 10 mg of the NDF residue was allowed to swell in 125 µl of 12 M H 2 SO 4 for 2 h at 20°C and was then subjected to acid The lignin content of the litter subsamples was approximated as the acid-nonhydrolysable residue remaining after sulphuric acid hydrolysis according to the Klason lignin determination method [START_REF] Monties | Dosage de la lignine insoluble en milieu acide: influence du prétraitement par hydrolyse acide sur la lignine klason de bois et paille[END_REF]. Briefly, 300 mg of litter was suspended in 3 ml of 12 M H 2 SO 4 for 2 h at room temperature. The suspensions were then diluted to 1 M with deionised water, heated at 100°C for 3 h and filtered. The remaining litter was dried at 105°C and the ash

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 13 content was determined after 4 h of combustion at 500°C. The lignin content was calculated as the mass difference before and after combustion.

Enzymatic activities

At days 27, 97, and 202, we used a method by [START_REF] Bell | High-throughput fluorometric measurement of potential soil extracellular enzyme activities[END_REF] to measure the potential activity of four hydrolytic soil enzymes, which degrade a range of substrates that are common constituents of organic matter. The following enzymes were selected to represent the degradation of C-rich substrates: β-1,4-glucosidase, β-D-cellobiosidase, α-glucosidase, and β-xylosidase (see Appendix D for details regarding the initial enzymatic activity of each system). In brief, the assays were conducted by homogenising 2.75 g of the soil-litter mixture in 91 ml of 50 mM TRIS buffer (pH 7.8 for forest or grassland, pH 8.1 for cropland and plantation) in a blender for 1 min. The soil slurries were then added to a 96-deep-well microplate using an eight-channel repeat pipettor. Additional quench control replicates of the soil slurry and 4-methylumbelliferone (MUB) standard curves (0-100 µM concentrations) were included with each sample. The soil slurries were incubated with fluorometric substrates for 3 h at 25°C. After the incubation period, the plates were centrifuged for 20 min at 3000 rpm, after which 250 µl of supernatant was transferred from each well into a black, flat-bottomed 96-well plate that was scanned using a microplate reader (SpectraMax Gemini, Molecular Devices, CA, USA) with an excitation wavelength of 365 nm and an emission wavelength of 450 nm. All enzymatic activities were first calculated in nmol g -1 soil h -1 and were then corrected for the contribution of the corresponding soil type by subtracting the enzyme activities of the 'no-litter' soil. These values were subsequently integrated over time (mmol kg -1 soil) following [START_REF] Sinsabaugh | Allocation of extracellular enzymatic activity in relation to litter composition, N deposition, and mass loss[END_REF].

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 ( 4 PLFAs as indicators of the microbial community structure: branched and saturated PLFAs i15:0, a15:0, i16:0, i17:0, and a17:0 (Gram-positive bacteria); mono-unsaturated and cyclopropyl PLFAs 16:1ω7c, cy17:0, 18:1ω7c, and cy19:0 (Gram-negative bacteria) 18:1ω9, 18:2ω6,9

(fungi), 10me-16:0 and 10me-18:0 (Actinobacteria); and i14:0, 14:0, 15:0, 16:0, 17:0, i18:0, 18:0, 18:1ω5c, C18:3ω3, 20:0 and 20:4ω6 (general indicators).

Data analysis

To examine the effects of the soil and litter types on the cumulative carbon mineralisation, we performed a two-way ANOVA using the raw (total cumulative carbon

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 15 mineralisation) or net data (cumulative carbon mineralisation corrected using 'no-litter' soils).

We then individually analysed the net mineralisation rates for each litter type using linear mixed models in which the soil type and time were treated as fixed effects and were allowed to interact.

The microcosm identity was included as a random effect to account for the repeated sampling of the microcosms across time. Using Bray-Curtis dissimilarity matrices, we ran non-metric multidimensional scaling (NMDS) to visualise the differences in structure and catabolic ability of the microbial community (as relative proportions of PLFAs and of the four enzymatic activities) and then performed permutational multivariate ANOVAs to test the significance of the litter type, soil type, and time for explaining the observed variations.

The relative contribution of the litter quality, functional breadth, and home-field advantage in explaining the carbon mineralisation was estimated using the model developed by [START_REF] Keiser | Disentangling the mechanisms underlying functional differences among decomposer communities[END_REF]. This model states that carbon mineralisation (Y i ) is equal to litter ability (β l ) plus soil ability (γ s ) plus a home interaction term (η h ):

where Yi is the carbon mineralisation for observation i, β l is the ability of litter species l (from species 1 to N), γ s is the ability of the soil community s (from community 1 to M), η h is the HFA

Results

Litter quality and carbon mineralisation

Multiple litter traits varied significantly among the four litter types. For instance, the C:N ratio varied by almost eightfold, ranging from 18.4 to 140.9 between the plantation (R. 17 plantation litters, as was the lignocellulose index, ranging from 0.22 and 0.24 for cropland and grass litters to 0.65 and 0.67 for forest and plantation, respectively.

The cumulative carbon mineralisation over 202 days varied according to the litter types, ranging from 3463 to 6649 mg C kg -1 of soil for the forest and grassland litters, respectively (Fig. 2A). The variation in the average cumulative carbon mineralisation between the soil types was, however, less important than was that observed between the litter species, with a minimum of 4237 mg C kg -1 for the cropland soil and a maximum of 6287 mg C kg -1 for the forest soil (Fig. 2B, see also Appendix E for details on the control soils). Priming effect (PE) varied over time with a maximum of 232 mg C kg -1 on average after 202 days (Fig. 2C,Appendix C). After a mineralisation flush during the first 7 days, PE increased steadily for cropland and plantation soils while it was negative for grassland and forest soils, meaning that the SOM mineralisation was higher in the control than in the 13 C litter-amended soils. Although PE increased in the forest soil to similar levels than in cropland and plantation soils after 97 days of decomposition, grassland soil always showed lower PE, with a difference of 78 mg C kg -1 on average compared with the three other ecosystems (Fig. 2C). Overall, PE corresponded to 3.6 to 6.6% of net carbon mineralisation between 27 and 202 days.

Using raw data for the total cumulative carbon mineralisation measured in the four ecosystems, we observed that the average percentage of variance explained by the soil type increased during the course of the incubation, ranging from of 9.6% at the beginning to 32.1% at the end of the experiment. By contrast, the average contribution of the litter type decreased from 89.3 to 66.9% during the same time (Table 1). However, when considering the net data (cumulative carbon mineralisation corrected by 'no-litter' soils), the soil type explained only 1.9 to 4.0% of the variation in the net carbon mineralisation over 202 days, suggesting that this effect

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 18 is more subtle when the initial soil variation among ecosystem types is considered (Table 1). In this latter model, the litter species explained 94% of the average variance of carbon mineralisation independently of the sampling date (Table 1). Among the different litter traits, the lignin:N and C:N ratios were poor predictors of the mineralisation rates (p > 0.05, Appendix F).

By contrast, the lignocellulose index (LCI) showed strong negative correlations with the carbon mineralisation rates for each litter type individually but also globally using the entire dataset (r 2 = 0.47, p < 0.05, Appendix F). Finally, when the effect of each litter type was investigated separately over the course of the incubation, the dynamics of the net carbon mineralisation rates of the cropland, forest and plantation litter types were influenced by the soil type and by a soil type × time interaction that decreased from 0-27 to 98-202 days (Table 2). The soil net carbon mineralisation rates for the grass litter showed only an effect of the soil type for the period 28-97 days and significant soil type × time interactions at 0-27 and 98-202 days (Table 2, see also

Appendix G for the general model).

Microbial community structure and enzymatic production

Most of the PLFA markers showed significant differences among the soil and litter type combinations, indicating that the soil microbial community structure was affected by multiple parameters (Fig. 3A, Table 3). Over the course of the incubation, the soil type and the identity of the decomposing leaf litter were the most significant factors, explaining an average of 36% and 18%, respectively, of the variability in the community structure (Table 3). To a lesser extent, time (12%) and its interaction with the soil type (9%) or litter type (4%) also explained a significant part of the observed variation. Consistent with the reported differences in the

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 19 microbial community structure, the ordination plot from NMDS clearly distinguished the communities from the four soil types (Fig. 3A). The microbial communities were plotted according to a pattern in which the first axis depicted the land-use gradient from cropland to forest and the second axis separated the agro-systems (cropland and plantation) from the less intensively managed ecosystems (forest and grassland).

Similar to what was observed for the microbial community structure, we found that the soil type was the most important driver, explaining 28% of the variation in the enzymatic activity (Fig. 3B, Table 3). The soil type was visually confirmed as a main driver of enzymatic activity when the NMDS plot was observed, although this result was clearer from the PLFA data. In the NMDS plot, a clear segregation occurred along the first two axes between the plantation and cropland soils on the one hand (down left), and the forest and grassland soils on the other hand (top right) (Fig. 3B). The litter types and their interaction with time or soil only slightly influenced the patterns of enzyme activities in the different soil-litter type combinations (Table 3B). In contrast, time and its interaction with the soil type explained 25% and 19% of the variability in enzymatic patterns, respectively.

Major controls of litter carbon mineralisation and their relations to enzymatic activity

When estimating the β l (LQ), γ s (FB), and η h (HFA) metrics from the net carbon mineralisation data, we found that β l was the metric that showed the largest variation (Fig. 4), with values ranging from -1725 to +1426 mg C mineralised kg -1 soil on average across the soil and litter type combinations. The grass and cropland litters showed the highest LQ index compared to the plantation and forest litters (Fig. 4A), confirming that more labile litter types

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 20 degraded fastest across all of the ecosystems (Fig. 2A). Overall, we observed an initial and rapid increase or decrease in β l for the grass and forest litters, followed by a slow decline in carbon mineralisation over the incubation period (Fig. 4A). Meanwhile, a transition period occurred for the cropland and plantation litters between 0 and 27 days, followed by a regular increase or decrease in β l , suggesting that the notion of litter quality for the soil decomposers may vary from the short to the long term (Fig. 4A). When estimating the functional ability γ s that varied from -249 to + 364 mg C mineralised kg -1 soil, we observed that the 'ability' of decomposer communities was highest and increased during the experiment for the forest soil (Fig. 4B). To a lesser extent, the microbial community from the grassland soil also displayed positive γ s values, with an increasing functional ability after a time lapse of approximately 100 days. In contrast, the plantation and cropland soil types presented negative γ s values over the course of the incubation experiment, indicating that these communities were unable to degrade a wide range of qualitatively contrasting litter types (Fig. 4B). Finally, when evaluating whether a litter type decomposed faster in an area dominated by the plant species from which it derived, we observed an increasing affinity effect η h when the plantation soil microbial community matched its own litter (Fig. 4C). Similarly, we found a positive η h between the grass litter and its own environment, but this effect was transient over time and decreased after 140 days. No significant HFA was found for the forest (p > 0.05 for the whole incubation period), and a disadvantage to decomposing at home was found for the cropland litter, indicating that this litter type decomposed more rapidly elsewhere than in its own environment (Fig. 4C).

When calculating the parameter estimates corresponding to the effects of FB (γ s ) and HFA (η h ) on the parietal sugar mass loss and net integrated enzymes, we observed positive correlations between these two parameters across the various systems, indicating that a higher

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

21

'functional ability' or 'affinity effect' on the decomposition process was related to a higher enzymatic activity (Fig. 5, see also Appendix H for more details). In agreement with the carbon mineralisation results, we observed positive γ s estimates for both the enzymatic activity and sugar mass loss in the forest ecosystem, whereas the η h estimates were highest in the plantation and grassland (Fig. 5). Specifically, the positive effects that FB and HFA had on parietal sugars were mainly related to xylose and glucose dynamics during the early phase of decomposition, whereas after 100 days of decomposition, lignin appeared to increase in importance as a factor explaining the FB and HFA effects (Appendix I).

Discussion

Little is known regarding how the triangular interactions involving litter quality, the decomposer community, and enzymatic activities can modify the trajectory of carbon dynamics within a land-use gradient. To quantify the relative contributions of litter quality (LQ), functional breadth (FB), and home field advantage (HFA) (Fig. 1), we implemented a reciprocal litter-soil transplant decomposition experiment over 202 days. Despite the relatively similar decomposition patterns across all soil-litter combinations, we found a twofold variation in the magnitude of carbon mineralisation among the most chemically contrasting litter types at the end of the incubation period (Fig. 2A), and distinct mineralisation rates for a given litter type incubated with different soil inoculums (Table 2). Our results confirm that litter decomposition is mainly influenced by litter quality, but indicate that the local microbial communities and interactions with their substrates also explained a subtle (< 5%) but significant part of the variation in carbon mineralisation (Table 1). Neither the amplitude nor the ranking of the priming effect (PE)

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 ( 4

Litter quality as a major control of carbon mineralisation

In agreement with our first hypothesis, the litter quality consistently explained more than two-thirds of the carbon mineralisation, confirming that litter traits are the best predictors of carbon dynamics [START_REF] Makkonen | Highly consistent effects of plant litter identity and functional traits on decomposition across a latitudinal gradient[END_REF][START_REF] Cleveland | Litter quality versus soil microbial community controls over decomposition: a quantitative analysis[END_REF]. In contrast to the often assumed relevance of the bulk nitrogen concentration or the lignin:N ratio of leaf litter, we found that the variability in the carbon mineralisation rates among the different litter species was primarily explained by the degree of recalcitrance of plant cell walls (Appendix F). These results suggest that the complexity of the cell wall network can retard or enable the access of microorganisms to easily degradable litter compounds and that simple estimators of the cell wall structure such as the lignocellulose index (LCI) are useful indicators for assessing mineralisation rates using predictive models at a local scale (Moorhead andSinsabaugh 2006, Moorhead et al. 2014).

Testing the functional importance of microbial communities

Whether contrasting microbial communities are functionally equivalent regarding ecosystem processes remains an essential issue for our prediction of the carbon balance along a

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 23 land-use gradient. Overall, the soil type explained 9 to 32% of the variation in carbon mineralisation during the course of the experiment (Table 1). However, when accounting for the initial variability among the different ecosystems, the soil type explained only 2 to 4% of the variation, suggesting that the response induced by the litter-microbe interactions was smaller in our study than in other reports (Ayres et al. 2009b). The PE increased on average from 57 to 232 mg C kg -1 between 27 to 202 days, meaning that we overestimated litter-derived CO 2 fluxes by 3.6 to 6.6% in our models. In addition, PE was lower in grassland soils over the course of the incubation experiment (Fig. 3C), indicating that caution must be taken when considering the variation of litter-microbe interactions between grassland and the other soil types. However, the relative consistency among soil types (CV = 27%) explaining 0.03 to 1.18% in the variation in net carbon mineralisation suggest that other factors besides PE also significantly influenced the variation in decomposition rates.

Among the potential mechanisms to explain the responses induced by plant-soil interactions, microorganisms may differ in their abilities to degrade contrasting litter species, i.e., in their FB [START_REF] Keiser | Disentangling the mechanisms underlying functional differences among decomposer communities[END_REF]. In agreement with this second hypothesis, all of the tested litter types decomposed more rapidly in the forest soil (Fig. 4B), confirming that microbial communities originating from forested habitats display a greater ability to degrade various substrates compared to those originating from more labile litter environments. Although we cannot substantiate the notion that litter quality is 'in the eye of the beholder' (Strickland et al. 2009b), our results confirm that contrasting microbial communities may differ in their functional abilities for decomposing various litter species (Strickland et al. 2009a). More importantly, investigating the temporal dynamics of carbon mineralisation allowed us to determine that the capacities of soil microbes are not set in stone. For example, the FB of the grassland microbial

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 24 community, which had not faced complex substrates in their recent history, became positive and increased steadily after 100 days (Fig. 4B). This result is particularly surprising given that the fingerprint of the past resource is often invoked to explain plant-microbe interactions, suggesting that additional mechanisms may influence the ability of a community to be functionally 'broad'

or 'narrow'. Although we cannot judge with certainty, the gene and protein expression, the physicochemical characteristics of the surrounding environment, the diversity of soil decomposers, and/or the presence of specific groups of decomposers may all explain the functional ability dynamics. An alternative explanation would be that although we consciously selected one species in almost pure stands for the ecosystems of grassland and forest, microbial communities from these two ecosystems encountered a wider variety of chemical substrates than did those originating from 'agro-systems', which likely influenced the contrasting functional abilities observed among the four different soil types.

Another way to address functional dissimilarity between contrasting microbial communities is the HFA hypothesis, which investigates whether microbes decompose litter that is derived from a 'home' site more efficiently than litter that is derived from a 'away' site [START_REF] Gholz | Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition[END_REF]. Here, although the forest litter decomposed fastest in its own environment, we did not find any evidence for an HFA effect in the forest soil because all of the litters decomposed proportionally more rapidly in this ecosystem. Although the interpretation of HFA results should be treated with caution since their variability is greater than that of LQ and FB, we found a small positive HFA when the plantation and grassland litters matched their own environment, indicating that soil microbial communities originating from these two environments were 'preadapted' to decomposing substrates they were accustomed to encountering (Fig. 4C).

Counterintuitively, we demonstrated here that HFA is not systematically greater with increasing

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 25 litter recalcitrance and that 'affinity effects' between microorganisms and their resources can also be observed on more labile substrates. This idea has been recently supported by a metaanalysis of 125 reciprocal litter transplants, in which Veen et al. (2014) concluded that HFA effects were not related to particular litter types but were stronger when the litter quality between 'home' and 'away' sites was more dissimilar.

Microbial community structure and enzymatic activities: how can we explain the functional dissimilarity?

To date, functional dissimilarities of soil microbial communities generated by FB and HFA have not been separately quantified (but see [START_REF] Keiser | Disentangling the mechanisms underlying functional differences among decomposer communities[END_REF], and our study is one of the first to determine their relative contribution over time. Here, we found that the magnitude of these two mechanisms was similar but that the directions of these effects were, in some cases, opposite depending on both the litter and soil types (Fig. 4). The functional dissimilarity generated by the effects of FB and HFA on parietal sugar mass loss were closely linked to soil enzymes (Fig. 5), suggesting that the extracellular enzymatic activities are the main driver of the interactions between litter quality and the decomposer communities. Surprisingly, despite the large variations in microbial groups among the different ecosystems (Fig. 3A), we did not find [START_REF] Treseder | Integrating microbial ecology into ecosystem models: challenges and priorities[END_REF]. However, although we excluded all the major groups of macrofauna by sieving, we cannot ignore that the variability in micro-and mesofauna diversity may have also contributed to the plant-soil interactions. Because higher trophic levels have been shown to largely contribute to the interaction between litter and decomposer communities, particularly in the poorest sites (e.g., [START_REF] Milcu | All size classes of soil fauna and litter quality control the acceleration of litter decay in its home environment[END_REF]Manning 2011, Perez et al. 2013), increasing the diversity of decomposers in the field will likely reveal a more complex situation than that observed in our microcosm experiment.

Conclusions

Our fully factorial microcosm experiment provides new insight into the relative effects of litter quality and microbial community on decomposition rates. The effect of soil type on carbon mineralisation suggests that microbial communities have a subtle but significant role in regulating decomposition rates. In particular, the mass loss of various parietal compounds in a

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

27

given litter type can vary strongly between microbial communities from contrasting soil types.

Whether microbial diversity or soil physico-chemical properties are directly responsible for the local variability observed in enzymatic activities nevertheless remains a complex question. In other words, if microbial diversity is critical to enzymatic synthesis, then the absence of specific groups due to ecosystem disturbance might have substantial impacts on the local carbon fate. If not, enzymatic activity across various lineages of soil organisms should converge to optimise resource acquisition and thus the functional traits of soil microbes would be more relevant than their identity for predicting the ecosystem processes [START_REF] Talbot | Endemism and functional convergence across the North American soil mycobiome[END_REF]. However, the relatively small amount of variation that was explained by the litter-microbe interactions compared to that which was explained by litter quality suggests that the composition of the microbial community plays only a minor role in regulating the decomposition rates along our land use gradient. This result questions the relevance of increasing efforts to understand the role of resource-consumer interactions during litter decomposition in various ecosystems. Although this issue is challenging, more attention should be paid to determining whether fundamental knowledge about microbial diversity will considerably improve our predictions of carbon mineralisation and enzymatic activities.

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1

28

Lashermes for the fruitful discussions; and the reviewers for their constructive comments that helped to improve the manuscript. We also thank the 'Office National des Forêts' (ONF) for access to the black locust plantation and the 'Ferme Expérimentale de Grignon' for authorising the sampling of the different 'AgroParisTech' sites.

Ecological Archives

Appendices A-I are available online:

Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016) decomposer communities are locally adapted to their own litter, with each litter type decomposing fastest in an area dominated by the plant species from which it was derived than in an area dominated by another plant species. Based on the degree of recalcitrance of plant cell walls that varied strongly between Gramineae and woody plant species, and on the nitrogen content that was almost threefold higher in the grassland litter compared with the cropland litter, we defined the relative ranking of litter quality as following: F. arundinacea, T. aestivum, R. pseudoacacia and F. sylvatica. The + sign indicates that the carbon mineralisation rate is higher for the plant-soil interactions, which are illustrated by arrows in the different panels. All + signs represent patterns of variation rather than absolute values. HFA estimates the interaction between the litter decomposition and the decomposer communities in each ecosystem.

  hydrolysis with 1 M H 2 SO 4 for 2 h at 100°C. The monosaccharides released by the acid hydrolysis were separated by high-performance anion-exchange chromatography (HPAEC) on a CarboPac PA-1 column (Dionex ICS 5000+, Thermo-Scientific, Sunnyvale, CA, USA). The monosaccharide composition was analysed, and nine sugars were quantified using 2-deoxy-Dribose as an internal standard and standard solutions of the following neutral carbohydrates: Larabinose, D-glucose, D-xylose, D-galactose, L-rhamnose, D-mannose, L-fucose, D-galacturonic acid and D-glucuronic acid. The parietal sugars were calculated as the sum of all of the monosaccharides.

  ), 1023-1037. , DOI : 10.1890/15-1263.1 14Microbial community structureThe microbial community structure was determined at days 27, 97, and 202 by an analysis of the group-specific phospholipid fatty acids (PLFAs) according to the protocol described by[START_REF] Fanin | Litter fingerprint on microbial biomass, activity, and community structure in the underlying soil[END_REF] (Appendix D for details regarding the initial community composition of each system). From each microcosm, a 10 g frozen soil subsample was used for total lipid extraction in a single-phase mixture of chloroform:methanol:phosphate buffer at pH 7.4 (1:2:0.8 by vol.), eluted selectively, and subjected to a mild methanolysis. The resulting PLFAs were analysed on a gas chromatograph equipped with a flame ionisation detector (Agilent 6890, Agilent Technologies, Palo Alto, USA) with a 60 m × 0.25 mm × 0.15 μm DB-23 column (Agilent Technologies, Palo Alto, USA) and by gas chromatography-mass spectrometry (Shimadzu QP2010 plus, Shimadzu Corporation, Suzhou, China). We retained the following 24

  of h (from home combinations 1 to K), and Home h = Litter l * Soil s when l and s are home-field pairings. The parameters to be estimated are β l , γ s and η h . The intercept term is defined by α and represents the average carbon mineralisation rate for all observations in the dataset after controlling for litter, soil and home-field pairings. Negative parameter estimates indicate lower carbon mineralisation than the average rate observed across all samples in the device. The error term is defined by ε. Using this model, we calculated a quality index (LQ), the ability of soil Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI : 10.1890/15-1263.1 16 microbial communities (FB), and the interactions between litter and soil (HFA) on the net cumulative carbon mineralisation. Keiser's model was also used to determine the relative contributions of FB (γ s ) and HFA (η h ) in explaining the sugar mass loss (computed for the sum of all parietal sugars) and enzymatic activities (computed for the sum of the four net integrated enzymatic activities). We then explored the correlations between the parameter estimates for FB and HFA on the parietal sugar mass loss and on the net integrated enzymes to visualise potential relationships between these two sets of variables. All of the statistical tests were performed using R software (version 3.1.1, The R Foundation for Statistical Computing, Vienna, Austria) and SAS software (version 9.0, SAS institute, NC, USA).

  pseudoacacia) and cropland (T. aestivum) litters, respectively, whereas the bulk litter lignin concentration varied by a factor of 2.5 between the forest (F. sylvatica) and grass (F. arundinacea) litters (39.3% and 15.5%, respectively; see Appendix A for more details). The variability of the litter chemistry was particularly apparent when considering parietal sugars, especially between forest and cropland litter types, with more than threefold variation in the glucose and xylose concentrations. The polyphenol contents were the highest in the forest and Comment citer ce document : Fanin, N., Fromin, N.,Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :10.1890/15-1263.1

  conclusions on FB and HFA results, strengthening the idea that the past resource history can play a significant role during litter decomposition. In light of the new advances provided by our incubation experiment, we propose a new theoretical concept based on enzymatic activities for addressing the issue of functional dissimilarity of contrasting microbial communities originating from a variety of ecosystems.

  any direct relationship between the PLFA-based community structure and the hydrolytic enzyme production. This result begs the question as to whether the relationship between the composition of soil communities and the enzymatic synthesis is causal at the local scale. Similarly,[START_REF] Purahong | Uncoupling of microbial community structure and function in decomposing litter across beech forest ecosystems in Central Europe[END_REF] recently demonstrated a decoupling between the microbial community structure and hydrolytic, but not oxidative, enzymatic activities. At minimum, these data indicate that the Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI : 10.1890/15-1263.1 26 enzymes necessary for degrading complex molecules are under the direct control of specific groups of decomposers (i.e. fungi), and thus that any effect of community structure on decomposition should be accentuated when one is interested in more recalcitrant leaf litter compounds (Wallenstein et al. 2013). In agreement with this theory, we found that in most of the litters presenting positive plant-soil interactions, lignin generally exhibited greater FB and HFA effects compared to more labile monosaccharides after an approximately 100-day delay (see Appendix I). Consequently, because contrasting microbial communities can change the course of the decomposition process (Wickings et al. 2012), we believe that models predicting carbon mineralisation should include parameters that explicitly represent the variability in the functional abilities of decomposers

FiguresFig. 1 .

 1 Figures

Fig. 2 .Fig. 4 .

 24 Fig. 2. Raw cumulative carbon mineralisation obtained for each (A) litter type and (B) soil type.

Fig. 5 .42Figure 1 .43Figure 2 .44Figure 3 .45Figure 4 .

 51234 Fig. 5. Correlations between the parameter estimates of sugar mass loss as a function of the net

  INRA, UMR 614 Fractionnement des AgroRessources et Environnement, 2 esplanade Roland Garros, F-51100 Reims, France; 2 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden,

3 Centre d'Ecologie Fonctionnelle et Evolutive (CEFE), CNRS -Université de Montpellier -Université Paul Valéry Montpellier -EPHE, 1919 Route de Mende, F-34293 Montpellier Cedex 5, France, 4 PROMES -CNRS, 7 rue du Four Solaire, F66120 Odeillo, France, 5 INRA, UMR Eco&Sols, 2 Place Viala, 34060 Montpellier Cedex 1, France, 6 Corresponding author: Isabelle Bertrand,

  The microcosm experiment, chemical analyses and enzymatic assays were performed at the laboratory 'Fractionnement AgroRessource et Environnement' in Reims, and the PLFA analyses were performed at the 'Plate-Forme d'Analyses Chimiques en Ecologie', PACE, technical facility of the Laboratoire d'Excellence 'Centre Méditerranéen de l'Environnement et de la Biodiversité' in Montpellier. This study was financially supported by the National Institute of Agronomic Research (INRA), the region Champagne Ardennes and the EPRC project.

	experiment shows soil animal impacts on decomposition are climate-dependent. Global
	Change Biology 14:2661-2677.
	Wallenstein, M. D., M. L. Haddix, E. Ayres, H. Steltzer, K. A. Magrini-Bair, and E. A. Paul.
	2013. Litter chemistry changes more rapidly when decomposed at home but converges
	during decomposition-transformation. Soil Biology and Biochemistry 57:311-319.
	Wallenstein, M. D., A. M. Hess, M. R. Lewis, H. Steltzer, and E. Ayres. 2010. Decomposition of
	aspen leaf litter results in unique metabolomes when decomposed under different tree
	species. Soil Biology and Biochemistry 42:484-490.
	35

Comment citer ce document : Fanin, N., Fromin, N.,

Bertrand, I. (2016)

. Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI : 10.1890/15-1263.1 Wickings, K., A. S. Grandy, S. C. Reed, and C. C. Cleveland. 2012. The origin of litter chemical complexity during decomposition. Ecology letters 15:1180-1188.

Table 1 .

 1 ANOVA results with the percentage sums of squares explained (%SS) for the effects of 789 soil, litter type and their interactions on the cumulative carbon mineralisation. Raw data or net 790

	791	data (corrected by control soils) were used.

. Functional breadth and home-field advantage generate functional differences among soil microbial decomposers.

Ecology, 97 (4), 1023-1037

. , DOI : 10.1890/15-1263.1 36 Tables 788 Comment citer ce document : Fanin, N., Fromin, N., Bertrand, I. (2016). Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI : 10.1890/15-1263.1 37

Table 2 .

 2 Results of linear mixed models to test for the effects of soil type and time on the net 793

	794	carbon mineralisation rates.

NS

Comment citer ce document : Fanin, N., Fromin, N.,

Bertrand, I. (2016)

. Functional breadth and home-field advantage generate functional differences among soil microbial decomposers. Ecology, 97 (4), 1023-1037. , DOI :

10.1890/15-1263.1 38

Table 3 .

 3 PERMANOVA evaluating the effects of soil type, litter type and time on (A) the 797 microbial community structure and (B) enzymatic activities. 798 Note: Permutational multivariate ANOVA were performed to examine the relative influences of 799 soil type, litter type, time, and the interactions between these three explanatory variables on the 800 variation in the microbial community PLFA-based structure and enzymatic activities over the 801

	A Microbial community structure	df	SS	MS	F.model	P-value	r 2
	Soil type		3	0.14	0.05	71.6	< 0.0001	0.36
	Litter type	3	0.07	0.02	36.8	< 0.0001	0.18
	Time		2	0.05	0.24	36.9	< 0.0001	0.12
	Litter type × Time	6	0.04	0.01	9.2	< 0.0001	0.09
	Soil type × Time	6	0.02	0.00	4.1	< 0.0001	0.04
	Soil type × Litter type	9	0.00	0.00	0.7	0.94 NS	0.01
	Residuals	114	0.07	0.00			0.19
	Total		143	0.39
	B Enzymes	df	SS	MS	F.model	P-value	r 2
	Soil type		3	0.11	0.037	66.3	< 0.0001	0.28
	Litter type	3	0.02	0.005	9.4	< 0.0001	0.04
	Time		2	0.10	0.050	88.5	< 0.0001	0.25
	Litter type × Time	6	0.01	0.001	2.7	0.012	0.02
	Soil type × Time	6	0.08	0.013	22.6	< 0.0001	0.19
	Soil type × Litter type	9	0.02	0.002	4.2	< 0.0001	0.05
	Residuals	114	0.06	0.001			0.16
	Total		143	0.40
	802	course of the incubation experiment.

α-glucosidase, and β-xylosidase) after correction by the 'no-litter treatment' soil.

Acknowledgments

We thank Gonzague Alavoine for the management of the incubation experiment; Pascal Thiebeau for his help with litter sampling; Sylvie Millon, Manon Gaddi, Antoine Portelette, and Jorge Lebrato Mejijas for their highly appreciated laboratory assistance; Olivier Delfosse and Marie Sauvadet for their help during microcosm destruction; Sylvie Recous and Gwenaëlle