A,D Romain Tramoy
email: romain.tramoy@gmail.com.

Mathieu Sebilo

B Thanh

Thuy Nguyen

Johann Schnyder

Carbon and nitrogen dynamics in decaying wood: paleoenvironmental implications

Environmental context. Carbon and nitrogen isotopes of terrestrial organic matter are widely used for reconstructing past environments, but organic matter is exposed to degradation as soon as it is deposited during what is called early diagenesis. This study explores the effects of this process on organic carbon and nitrogen isotopes, and concludes that it homogenises an environmental signal by integrating all their components. Thus, early diagenesis may not preclude paleoenvironmental reconstructions.

Introduction

The carbon and nitrogen isotopic composition (δ [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C and δ 15 N) of terrestrial organic matter is widely used for reconstructing past environments for time periods from hundreds to million years. [START_REF] Talbot | A high-resolution palaeoclimatic record for the last 27 500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter[END_REF][START_REF] Pride | Nitrogen isotopic variations in the Gulf of California since the last deglaciation: response to global climate change[END_REF][START_REF] Jenkyns | Nitrogen isotope evidence for water mass denitrification during the Early Toarcian (Jurassic) oceanic anoxic event[END_REF][START_REF] Nguyen Tu | Isotope reconstruction of plant palaeoecology. Case study of Cenomanian floras from Bohemia[END_REF][START_REF] Andersson | Elemental and isotopic carbon and nitrogen records of organic matter accumulation in a Holocene permafrost peat sequence in the east European Russian Arctic[END_REF][START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF][START_REF] Wan | Stable Carbon and Nitrogen Isotopic Studies of Devonian Land Plants -An Indicator of Paleoclimate and Paleoenvironmental Changes[END_REF][START_REF] Andreeva | Stable isotope (δ 13 C, δ 15 N, δ 18 O) record of soils in Buryatia, southern Siberia: implications for biogeochemical and paleoclimatic interpretations[END_REF] However, organic matter is exposed to degradation as soon as it is transported, integrated into soils or deposited in sediments. Degradation processes of organic matter constitute the earliest stage of diagenesis, which leads in turn to the geological archive as diagenesis stages advance. [START_REF] Knicker | Stabilization of N compounds in soil and organic-matter-rich sedimentswhat is the difference?[END_REF][START_REF] Vandenbroucke | Kerogen origin, evolution and structure[END_REF] In this context, paleoenvironmental investigations require consideration of the potential alteration of the primary isotopic signal during early diagenesis because most of the organic matter is degraded during this stage. [START_REF] Tyson | Sedimentary Organic Matter[END_REF][START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Baudin | Géologie de la Matière Organique[END_REF] Changes of δ [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C values during early diagenesis of terrestrial organic matter (e. g. plant or litter material) are well known and mainly attributed to a preferential loss of [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-enriched carbohydrates. [14- 18] In contrast, nitrogen dynamics during terrestrial organic decay are more complex and consequences on δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values are not well constrained. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Melillo | Factors controlling mass loss and nitrogen dynamics of plant litter decaying in northern streams[END_REF][START_REF] Freudenthal | Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes[END_REF][START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF] Indeed, they involve complex interactions of biotic and abiotic factors: microbial processing vs N sources and environmental conditions. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Bunnell | Microbial respiration and substrate weight loss -I[END_REF][START_REF] Benner | Diagenesis of belowground biomass of Spartina alterniflora in salt-marsh sediments[END_REF][START_REF] Connin | Isotopic discrimination during long-term decomposition in an arid land ecosystem[END_REF][START_REF] Kramer | N isotope fractionation and measures of organic matter alteration during decomposition[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF][START_REF] Austin | Dual role of lignin in plant litter decomposition in terrestrial ecosystems[END_REF][START_REF] Lerch | Variations in microbial isotopic fractionation during soil organic matter decomposition[END_REF] For example, the effect of microbial processing on δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values depends on microbial communities (biotic), themselves dependent on the oxygenation conditions of the system. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF] However, changes in δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values during early diagenesis of terrestrial organic matter in a depositional context that allows preservation at the geological time scale (immersed environments) have not been assessed yet because most studies have dealt with litterbag experiments on field or soil incubation. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Melillo | Factors controlling mass loss and nitrogen dynamics of plant litter decaying in northern streams[END_REF][START_REF] Benner | Diagenesis of belowground biomass of Spartina alterniflora in salt-marsh sediments[END_REF][START_REF] Connin | Isotopic discrimination during long-term decomposition in an arid land ecosystem[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF][START_REF] Lerch | Variations in microbial isotopic fractionation during soil organic matter decomposition[END_REF][START_REF] Steart | The chemical constraints upon leaf decay rates: taphonomic implications among leaf species in Australian terrestrial and aquatic environments[END_REF][START_REF] Bragazza | Seasonal variation in nitrogen isotopic composition of bog plant litter during 3 years of field decomposition[END_REF] In addition, considering the importance of coarse woody debris in terrestrial organic matter [START_REF] Tyson | Sedimentary Organic Matter[END_REF][START_REF] Fukasawa | Accumulation and decay dynamics of coarse woody debris in a Japanese old-growth subalpine coniferous forest[END_REF] and of immersed environments for the geological archive (sediments), it is essential to assess the effect of early diagenesis on potential paleoenvironmental proxies, such as δ [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C or δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N, for this material and in these kinds of environments. Because fungi are the most important decomposers of woody material, [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF][START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] we designed a laboratory experiment that mainly focussed on biotic factors involved during early diagenesis of woody material, by limiting the action of abiotic factors (by use of constant temperature, oxygenation, pH and absence of light).

Centimetric pieces of wood were incubated in water in order to assess the effects of early diagenesis on δ [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C and δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values of woody terrestrial organic matter in immersed environments.

Wood samples were immersed either in (i) distilled water (DW) or (ii) river water (RW). Incubations conducted in DW were used as a control treatment, because microbial activity was considered to be minimal. Mass-loss rates of decaying wood and carbon dynamics (C content and δ 13 C values) were determined to characterise the biotic processes involved during degradation of woody debris. Then, their consequences on δ 15 N values are discussed to evaluate the preservation potential of the initial δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N signal in fossil and terrestrial sedimentary organic matter.

Experimental

Samples

A treetop branch of Sciadopitys verticillata (Cupressaceae, conifer) was collected in December 2012 from a garden in Tokyo, Japan, after having lain for 6 months on the ground to ensure the natural presence of decay organisms. We chose this endemic specimen because it has been chemically related to the morphogenus conifer Xenoxylon Gothan, which was widespread during Jurassic and Cretaceous times and is of paleoclimatic interest nowadays. [START_REF] Philippe | Distribution and palaeoecology of the Mesozoic wood genus Xenoxylon: palaeoclimatological implications for the Jurassic of western Europe[END_REF][START_REF] Marynowski | Systematic relationships of the Mesozoic wood genus Xenoxylon: an integrative biomolecular and palaeobotanical approach[END_REF] Both extremities of the branch and the bark were removed to clean the branch and avoid heterogeneities that were too large between its different parts (Fig. 1). The branch (~60 cm long; diameter ~3 cm) was then sliced in ~2-mm-thick pieces, which were air-dried and weighed.

Experimental design

Samples were submitted to degradation in two different types of water: (i) DW, and (ii) RW from La Marette (Essonne, France), sampled in October 2013. The pH of the water samples was checked with pH paper before each sampling. It remained close to neutral and constant for both types of waters during the whole experiment. RW was filtered with a Whatman filter (22 µm; Prolabo, Paris) before filling Erlenmeyers. Samples were placed individually into Erlenmeyers, with 75 mL of DW and RW (Fig. 1). Erlenmeyers were covered with finely perforated aluminium foil to allow gas exchange between water and the atmosphere and avoid external contamination by dust. They were then continuously shaken in the dark in ambient air (room temperature conditioned at 22 °C) to keep the water oxygenated and inhibit the development of photosynthetic microorganisms that could interfere with degrading heterotroph microorganisms. An oxygenated environment has been preferred to promote relatively efficient degradation when compared with oxygen-depleted environments and to prevent denitrification, which leads to high loss of [START_REF] Spiker | The effects of early diagenesis on the chemical and stable carbon isotopic composition of wood[END_REF] N.

The incubation experiment was carried out for 73 weeks and wood pieces were collected after 2, 4, 8, 16, 32, 52 and 73 weeks, corresponding to t1 to t7 respectively. Initial RW was stored in a 10-L canister in the dark and water was bubbled in order to keep it oxygenated. Filtered RW or DW was added to Erlenmeyers at t3, t5 and t6 to counterbalance evaporation in Erlenmeyers.

After sampling, pieces of wood were rinsed with distilled water to remove superficial microorganisms. They were then frozen, freeze-dried, weighed, scanned (with EPSON scanner) and ground (<40 µm) for analysis with an ultracentrifugal ZM-200 mill.

Analyses

Carbon content and δ 13 C values were determined by isotope-ratio mass spectrometry using a Vario Pyro Cube analyser, coupled with an IsoPrime Micromass mass spectrometer connected to a Micromass dilution system. Nitrogen content and δ 15 N values were measured with a Flash 2000 analyser coupled with a Thermo Scientific Delta V Plus mass spectrometer connected to a ConFlo IV dilution system. Systems precision and accuracy were monitored using tyrosine (N = 7.73 %, δ 15 N = 10.01 ‰ and C = 59.66 %, δ 13 C = -23.20 ‰,) as internal laboratory standard that was calibrated with international standards IAEA-N-1 (δ 15 N = 0.4 ‰), IAEA-N-2 (δ 15 N = 20.3 ‰) and IAEA-NO-3 (δ 15 N = 4.7 ‰) and expressed relative to N2-AIR for nitrogen and relative to Vienna Pee Dee Belemnite (VPDB) for carbon. Nitrogen analyses were systematically performed in duplicate (n = 51), whereas for carbon analyses, samples were analysed in duplicate (n = 12) every 10 samples, precision being usually better for carbon isotopes. The overall precision (2σ) was <0.002 and <0.6 % for N and C content respectively, and <0.2 ‰ for N and C isotopes.

Initial variability of the branch and sampling strategy

Before beginning the experiment, 11 pieces of wood, randomly distributed along the branch, were selected to evaluate the initial variability of %C, δ 13 C, %N and δ 15 N. Carbon content and isotopes were on average 47.4 ± 0.5 and -24.1 ± 0.1 ‰ respectively, whereas nitrogen content and isotopes were on average 0.08 ± 0.01 % and -0.2 ± 0.5 ‰ respectively (Table 1). In agreement with literature, initial variability of δ 15 N values was much higher than for carbon. [START_REF] Poulson | Nitrogen isotope variation of tree rings as a potential indicator of environmental change[END_REF][START_REF] Bukata | Carbon and nitrogen isotope variations in tree-rings as records of perturbations in regional carbon and nitrogen cycles[END_REF][START_REF] Leavitt | Tree-ring C-H-O isotope variability and sampling[END_REF] In addition, the δ 15 N values were randomly distributed along the branch and showed extreme values from -0.8 to +1.1 ‰.

Considering the initial variability of δ 15 N values, experiments were run in triplicate for each type of water, meaning that six wood pieces were sampled (three per type of water). Within triplicates, individual wood pieces were chosen according to their position along the branch, with a maximum distance between them to be representative of the overall initial variability along the branch.

Water chemistry control

Water chemistry (PO4 3-, NO3 -, NO 2-and NH4 + ) was controlled in each type of water at t0, t2, t3, t5, t6 and t7 by spectrocolorimetry using a Gallery system. As expected, DW contained no or trace amounts of these chemical compounds, with no forms of nitrogen (always <0.2 ppm). In contrast, RW did contain significant amount of nitrates at t0 (~20 ppm), which disappeared after t2 (<0.1 ppm remained), whereas other nutrients remained in trace amounts.

Statistical treatment and data sets

Significance of the linear regressions and of the mean comparisons (e.g. differences in N content between samples in RW and DW) were tested using Pearson correlations and Student's tests respectively, with the programing language R (version 3.2.2, R Foundation for Statistical Computing, Vienna, Austria). Hypotheses were tested on the basis of a 5 % significance level (P value), meaning that a relation or comparison is significant when P< 0.05 for n observations. Original data sets are given in the Supplementary material, whereas condensed results used for figures are reported in Table 2.

Results

Macroscopic observations and mass-loss patterns

At the initial stage, all wood pieces showed multimillimetre-sized zones with reddish-brown colour, which may be attributed to black-rot or soft-rot fungi (Fig. 2a andb, feature i). [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF] Fungi were probably already within the wood before branch sampling because decay starts as soon as a tree is cut, [START_REF] Fengel | Aging and fossilization of wood and its components[END_REF] or even before because wood of living conifers is known for hosting fungi. [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF] Alternatively, the branch may have been colonised while it was on the ground. Translucent and whitish fungi developed within 2 weeks on wood pieces in DW, whereas no visible change occurred in RW. After 8 weeks (t3), fungi turned brown-black on wood pieces in DW, and became widespread after 16 weeks (t4; Fig. 2a, features i and ii). At this time, the RW in the Erlenmeyers became cloudy and yellowish, with millimetre-sized floating particles (Fig. 2b, feature iii). In addition, white-rot fungi developed on wood pieces in RW as indicated by their mottled appearance, their whitish-yellow coloration and their brittle and spongy traits as well as their loss of color contrast (Fig. 2b, features iv and v). [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF] Those features were not observed on samples in DW, suggesting that no or less white-rot fungi developed.

The degradation features mentioned became increasingly marked at the end of the experiment (Fig. 2c andd). Growth-rings of wood pieces in DW became increasingly dark, while they were progressively erased in RW, with increasing numbers of floating woody particles, suggesting two different microbial communities.

Consequently, wood pieces in RW seemed to have been degraded faster than those in DW. The higher decay rate of samples in RW is confirmed by a weight loss of ~30 % after 73 weeks (t7), whereas it reached less than 10 % for samples in DW (Fig. 3). Considering that the amount of inorganic material is negligible in wood, the mass loss is attributed to organic matter loss. In addition, mass loss is linearly linked to time (t) in both waters with a slope three times steeper for wood pieces in RW (-0.36t) than in DW (-0.11t), where t is expressed in weeks (Fig. 3).

Carbon patterns

The carbon content of wood pieces in both DW and RW increased between t0 and t1 (2 weeks), from 47.4 to 48.5 and 48.9 % respectively (Fig. 4a). After 32 weeks (t5), they progressively tended towards initial values. Although a small increase in δ [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C values occurred (-24.1 ‰ at t0 to -23.9 ‰ at t4), it was in the range of the analytical precision (±0.2 ‰; Fig. 4b). In addition, the floating woody particles were sampled at t5 in an Erlenmeyer containing RW. They significantly differed from the original piece of wood with (i) carbon content of 35 v. 50 % in the original piece of wood (Fig. 4a), and (ii) δ 13 C value of -22.6 v. -24.0 ‰ (Fig. 4b).

Nitrogen patterns

In contrast to carbon content, the nitrogen content of wood pieces depicted two different patterns according to the water type (Fig. 4c). The average %N of wood pieces in DW was 0.07 % for the whole duration of the degradation, significantly lower than the initial value (0.08 %; n = 21; P< 0.001), whereas the average %N of wood pieces in RW was significantly higher (0.09 %; n = 21; P< 0.02). In addition, N content of wood pieces in DW remained close to 0.07 % after a rapid decrease (t0 to t3) from the initial value (0.08 %). In contrast, the N content of samples in RW almost linearly increased from the initial value to 0.12 % at t7 (Fig. 4c). This reflects an accumulation of nitrogen in the remaining wood, called the 'immobilisation phase'. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF] As a consequence, the atomic C/N ratio decreased linearly from ~650 to ~400 in RW wood as degradation advanced (Fig. 5). In the same time, the C/N ratio of wood pieces in DW increased to >800 until t3, and then remained stable until the end of incubation. Those very high C/N ratios of wood pieces are characteristic of fresh wood. [START_REF] Tyson | Sedimentary Organic Matter[END_REF] In contrast, floating woody particles sampled at t5 in RW showed a C/N ratio of 30 (Fig. 5), typical of substantially decayed wood. [START_REF] Tyson | Sedimentary Organic Matter[END_REF] The immobilisation phase in RW is associated with significantly higher δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values (+1.7 ‰ on average) of the remaining wood, when compared with the initial value (-0.2 ‰; n = 21; P<< 0.001), whereas δ 15 N values of wood pieces in DW remained almost constant at approximately -0.5 ‰ (Fig. 4d). In addition, the floating woody particles (in RW) sampled at t5 showed a higher nitrogen content than the original piece of wood (1.37 v. 0.09 %) and δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N value (2.7 v. 1.0 ‰; Fig. 4d). The very high nitrogen content suggests that wood particles were invaded by N-rich microorganisms.

Discussion

Decomposers and degradation rate

Basidiomycetes fungi have been the major decomposers of wood since at least the upper Devonian. [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF] They are classified as white-rot or black-rot fungi, based on macroscopic aspects. [START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] In contrast, soft-rot fungi comprise Ascomycetes and Deuteromycetes, which are less efficient than Basidiomycetes for degrading wood. [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF][START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] Black-rot and soft-rot fungi have the same morphological characteristics, but black-rot fungi do not support very wet conditions, whereas soft-rot fungi do. [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF] Consequently, based on morphological characteristics of the samples after incubation, soft-rot fungi are the most probable decomposers in DW, whereas white-rot fungi are more likely in RW (Fig. 2). [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF][START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] White-rot fungi are able to selectively degrade lignin and hemicellulose before celluloseespecially in gymnosperms, which is the case in the present studyor to simultaneously degrade those molecules. [START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] In contrast, soft-rot fungi preferentially decompose cellulose and hemicellulose, while lignin is only slightly degraded by those fungi, leading to less brittle traits when compared with white-rot fungi. [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF][START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] The low efficiency of soft-rot fungi for degrading wood when compared with Basidiomycetes could thus explain the lower mass loss of wood pieces in DW than in RW.

The mass-loss rate of wood pieces in RW (-0.36t; Fig. 3) is close to that described during the first degradation phase of litterbags containing red pine litter, with a slope of -0.45t with t in weeks. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] In the study mentioned, this first degradation phase was described as a constant mass loss (linear) dominated by the loss of carbohydrates, and lasted 44 months to reach 80 % of mass loss. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] Then, the second phase was characterised by very slow or negligible mass loss during 33 additional months.

Those two phases were also reported in different wood specifies species after artificial aging by thermal treatment. [START_REF] Schleser | Carbon isotope behaviour in wood and cellulose during artificial aging[END_REF] However, the second phase was not observed in wood samples of the present study, suggesting that the first phase of degradation was not entirely completed (Fig. 3). Extrapolating results to 80 % of mass loss for wood pieces in RW, the first phase of degradation would have been complete in ~220 weeks, which is ~50 weeks longer than reported in soils. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] Thus, the degradation rate of wood pieces in RW is slightly lower than measured in soils, [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] and in contradiction with another study [START_REF] Steart | The chemical constraints upon leaf decay rates: taphonomic implications among leaf species in Australian terrestrial and aquatic environments[END_REF] that showed significantly higher degradation rates in aerobic water than in soils. This discrepancy could be due to the difference in the plant material used in both studies, which dealt with needles [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] and green leaves from angiosperms. [START_REF] Steart | The chemical constraints upon leaf decay rates: taphonomic implications among leaf species in Australian terrestrial and aquatic environments[END_REF] Nevertheless, the mass loss of wood pieces obtained here confirms that this parameter is a good indicator of the state of degradation. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Steart | The chemical constraints upon leaf decay rates: taphonomic implications among leaf species in Australian terrestrial and aquatic environments[END_REF]

Carbon dynamics

Natural decomposing and artificial aging of plant material has often shown C enrichment and [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C depletion in the remaining organic matter, mainly attributed to the preferential loss of carbohydrates (cellulose and hemicellulose) during early diagenesis. [START_REF] Spiker | The effects of early diagenesis on the chemical and stable carbon isotopic composition of wood[END_REF][START_REF] Bates | Chemical changes and carbon isotope variations in a cross-section of a large Miocene gymnospermous log[END_REF][START_REF] Schleser | Carbon isotope behaviour in wood and cellulose during artificial aging[END_REF] In the present study, the increase in carbon content of the samples (remaining wood) incubated in both types of water probably reflects the increasing relative predominance of C-rich compounds resulting from loss of carbohydrates.

Considering the fungi type deduced above from macroscopic observations, this would imply that softrot fungi and white-rot fungi preferentially degraded cellulose and hemicellulose. [START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] However, δ 13 C values did not significantly change. The absence of expected [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C depletion in the remaining wood after carbohydrate loss may be linked to the low amount of material affected by degradation, with a maximum mass loss of ~30 % for samples in RW and ~10 % in DW. Alternatively, the loss of [START_REF] Baudin | Géologie de la Matière Organique[END_REF] Cenriched carbohydrates may have been counterbalanced by the loss of hydrolysable [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-depleted compounds [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] or by the simultaneous loss of lignin and carbohydrates, in agreement with decay abilities of Basidiomycetes in RW. [START_REF] -L. Eriksson | Microbial and Enzymatic Degradation of Wood and Wood Components[END_REF][START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] Leaching of hydrolysable compounds may have occurred as soon as samples were immersed, but its influence is hard to discuss here because of the lack of sampling data during the first days of the incubation.

If carbohydrates were effectively degraded, their C would have been reused by microbes, which would be integrated in the floating woody particles or in the remaining wood. In RW, the woody particles were analysed at t5. Their high %N (1.37 v. 0.09 %) and low atomic C/N ratio (30 v. 769) relative to the remaining wood suggest a high content of microbial biomass because it is concentrated in N. [START_REF] Gobat | Le Sol Vivant: Bases de Pédologie, Biologie des Sols 2010[END_REF] In addition, the lower %C (35 v. 49 %) and higher δ [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C values (-22.6 v. -24.0 ‰) in those floating particles, when compared with the remaining wood (Fig. 4a andb), support the idea that carbonaceous molecules mainly originate from C-depleted and [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-enriched carbohydrates of the remaining wood. [START_REF] Spiker | The effects of early diagenesis on the chemical and stable carbon isotopic composition of wood[END_REF] Additional loss of [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF] C might be associated with microbial respiration. [START_REF] Lerch | Variations in microbial isotopic fractionation during soil organic matter decomposition[END_REF] The isotopic effect of preferential degradation and microbial respiration during the incubation is probably not sufficient to be recorded in the slightly degraded remaining wood or it is counterbalanced by concomitant loss of [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-enriched and [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-depleted compounds. The latter hypothesis may be favoured, at least in RW, because Basidiomycetes (white-rot fungi) are able to simultaneously degrade [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-depleted lignin and [START_REF] Baudin | Géologie de la Matière Organique[END_REF] C-enriched carbohydrates. [START_REF] Martínez | Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin[END_REF] Nitrogen dynamics Nitrogen dynamics are much more complex either in DW or RW. The C/N ratio tracks changes in chemical composition of samples and thus reflects degradation processes. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF] Considering similar behaviours in C content of samples in both types of waters (Fig. 4a), C/N variations likely reflect changes in N content (Fig. 5). Consequently, the rapid decrease in N content of samples in DW (Fig. 4c), and the resulting increase of C/N ratio (Fig. 5), are most likely due to N release. In contrast, the opposite pattern was observed in samples incubated in RW, with an increase in N content (Fig. 4C) leading to a decrease of C/N (Fig. 5). This points to N accumulation in the remaining wood. In both cases, the microenvironment of samples should be considered and its implications are discussed below. It should also be noted that substantial isotopic fractionation processes like denitrification can be ruled out as it only occurs under anoxic conditions [START_REF] Thamdrup | New pathways and processes in the global nitrogen cycle[END_REF] and that N2 fixation, if any, has a very small fractionation factor. [START_REF] Michener | Stable Isotopes in Ecology and Environmental Science[END_REF] Distilled water N loss of the remaining wood in DW was associated with significant lower δ 15 N values (-0.5 ‰ on average), compared with the initial one (-0.2 ‰; n = 21; P< 0.05; Fig. 4d), suggesting the preservation of a 15 N-depleted pool and/or preferential degradation of a 15 N-enriched pool. Such N loss has already been observed during the first 120 days of incubation of Spartina plants. [START_REF] Benner | Diagenesis of belowground biomass of Spartina alterniflora in salt-marsh sediments[END_REF] However, it was attributed to leaching without [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N fractionation, which is not consistent with the 15 N depletion measured in samples incubated in DW (Fig. 4d). Consequently, leaching is probably not appropriate to solely explain N dynamics in DW.

Alternatively, the extraction of N by fungi is suspected because field incubations showed they are able to extract N from the colonised wood by gaining access to proteinaceous material, which results in concentrating N in their mycelium. [START_REF] Watkinson | The Role of Wood Decay Fungi in the Carbon and Nitrogen Dynamics of the Forest Floor[END_REF][START_REF] Bebber | Simulated nitrogen deposition affects wood decomposition by cord-forming fungi[END_REF] Thus, rapid N release from wood pieces in DW is probably associated with the development of mycelium that grew partly within the wood and partly in the water (Fig. 2a). Fungi probably used nitrogen from the wood for their metabolism, leaving it depleted in N. [START_REF] Bebber | Simulated nitrogen deposition affects wood decomposition by cord-forming fungi[END_REF] This is supported by the absence of an external N source because DW is nutrient-free. Because N depletion occurred rapidly (within 2 weeks) and did not substantially change during the rest of the incubation (e.g. stable C/N ratio; Fig. 5), microbial communities may have reached an equilibrium within 2 weeks, in response to the low nutrient availability. The 15 N depletion of the remaining wood is not currently well understood, but preferential degradation of a 15 N-enriched pool is suspected. In particular, a large range of positive δ 15 N values (2-8 ‰) in individual amino acids of a spatterdock plant has been reported [START_REF] Fogel | Transformation of plant biochemicals to geological macromolecules during early diagenesis[END_REF] and it has also been demonstrated that proteins were 3 ‰-enriched relative to cells. [START_REF] Macko | Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms[END_REF] Proteins and amino acids may therefore correspond to the 15 N-enriched compounds preferentially extracted by fungi as a nutrient source, [START_REF] Laiho | Decay and nutrient dynamics of coarse woody debris in northern coniferous forests: a synthesis[END_REF] all the more because their preservation potential is low. [START_REF] Vandenbroucke | Kerogen origin, evolution and structure[END_REF]

River water

Nitrogen dynamics in RW samples showed an opposite pattern compared with samples in DW, with N accumulation in the remaining wood (Fig. 4c) associated with significantly higher δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values (1.5 ‰ on average v. -0.2 ‰ at t0; Fig. 4d). N immobilisation has been commonly reported during decomposition of leaf-litter, [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF][START_REF] Fogel | Transformation of plant biochemicals to geological macromolecules during early diagenesis[END_REF] sphagnum litter [START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF] and woody litter [START_REF] Laiho | Decay and nutrient dynamics of coarse woody debris in northern coniferous forests: a synthesis[END_REF] or in decaying wood of Fagus sylvatica inoculated with basidiomycete fungi. [START_REF] Bebber | Simulated nitrogen deposition affects wood decomposition by cord-forming fungi[END_REF] According to those authors, it reflects accumulation of nitrogen during an immobilisation phase of nutrients. Two main processes are able to supply nitrogen in those decaying materials: (i) N2 fixation by specialised microorganisms, [START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF][START_REF] Laiho | Decay and nutrient dynamics of coarse woody debris in northern coniferous forests: a synthesis[END_REF] and (ii) uptake by cord-forming fungi from the surrounding environment. [START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF][START_REF] Bebber | Simulated nitrogen deposition affects wood decomposition by cord-forming fungi[END_REF][START_REF] Laiho | Decay and nutrient dynamics of coarse woody debris in northern coniferous forests: a synthesis[END_REF] However, rates of N2 fixation are too low to explain N accumulation in decaying wood, [START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF][START_REF] Laiho | Decay and nutrient dynamics of coarse woody debris in northern coniferous forests: a synthesis[END_REF] because fungi are not able to fix N2 given N2-fixing organisms are limited to prokaryotes. [START_REF] Shridhar | Review: nitrogen-fixing microorganisms[END_REF] Hence, N2 fixation is probably not the major process involved in N accumulation. Alternatively, cord-forming fungi have the ability to scavenge N from the surrounding environment, and further import it into the decaying material via a translocating network of mycelium that interconnects discontinuous nutrient sources. [START_REF] Berg | Litter Decomposition: a Guide to Carbon and Nutrient Turnover[END_REF][START_REF] Bebber | Simulated nitrogen deposition affects wood decomposition by cord-forming fungi[END_REF][START_REF] Laiho | Decay and nutrient dynamics of coarse woody debris in northern coniferous forests: a synthesis[END_REF] Thus, wood-decaying fungi are likely the mediators of N accumulation into decaying wood in RW. In the present study, the source of exogenous N is RW and the cord-forming fungi are most probably whiterot fungi (Basidiomycetes), their activity being the cause of the continuous decrease in C/N ratio (Fig. 5).

The δ 15 N variations in wood samples are likely due to the isotopic composition of the N imported by fungi (N source) and/or influence of metabolism effects leading to internal fractionation of [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N during N incorporation. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Lerch | Variations in microbial isotopic fractionation during soil organic matter decomposition[END_REF][START_REF] Zeller | 13 C and 15 N isotopic fractionation in trees, soils and fungi in a natural forest stand and a Norway spruce plantation[END_REF][START_REF] Dijkstra | 15 N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function[END_REF][START_REF] Hobbie | Fungal functioning in a pine forest: evidence from a 15 N-labeled global change experiment[END_REF] However, metabolism effects in fungi are thought to be low because no or little change in δ 15 N values was detected after N transfer to their host. [START_REF] Zeller | 13 C and 15 N isotopic fractionation in trees, soils and fungi in a natural forest stand and a Norway spruce plantation[END_REF] Thus, N sources must be seriously considered here. The only form of inorganic N detected in RW was NO3 -. The rapid decrease in NO3 -concentration in RW from ~20 ppm to 0 (within 8 weeks; Fig. 4c) supports its incorporation into the wood by fungi. After t3 (8 weeks), no NO3 -in RW was detected at t5, t6 and t7, although original RW was added at t3, t5 and t6. It probably reflects the complete consumption of NO3 - by microorganisms because (i) water sampling was performed before water addition in the Erlenmeyers, and (ii) time largely exceeded 8 weeks between each sampling. Two lines of evidence support the incorporation of N-NO3 -by fungi within the wood. First, δ 15 N values of the remaining wood were positively correlated with N content (Fig. 6a), attesting that nitrate was [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N-enriched relative to the wood. Second, the y intercept of a Keeling plot of the data enabled estimating the isotopic signature of the source of N, [START_REF] Keeling | The concentrations and isotopic abundances of atmospheric carbon dioxide in rural and marine air[END_REF] thus suggesting a δ 15 N-NO3 -equal to 6.6 ‰ (Fig. 6b), which is in agreement with the typical δ 15 N-NO3 -values measured in rivers of the region (+4 to +13 ‰). [START_REF] Sebilo | Isotopic composition of nitrate-nitrogen as a marker of riparian and benthic denitrification at the scale of the whole Seine River system[END_REF] However, according to Miller and Tans, [START_REF] Miller | Calculating isotopic fractionation from atmospheric measurements at various scales[END_REF] this value is only valid when considering a stable geochemical background of the wood, which is the case here because 'background' refers to its initial δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N and %N values. Because N recycling also occurs during microbial decomposition, [START_REF] Fogel | Transformation of plant biochemicals to geological macromolecules during early diagenesis[END_REF] part of the N within wood could have been 'replaced' by exogenous N. In this case, the δ 15 N-NO3 -value can be lower than 6.6 ‰.

Organic nitrogen in RW is also a valuable external N source. [START_REF] Houlton | A climate-driven switch in plant nitrogen acquisition within tropical forest communities[END_REF] However, nitrate is likely the main source of N for microorganisms because nitrates are precursors of dissolved organic nitrogen, leading to a similar δ 15 N signature for dissolved organic matter and nitrates. [START_REF] Kienast | Unchanged nitrogen isotopic composition of organic matter in the South China Sea during the last climatic cycle: global implications[END_REF][START_REF] Robinson | A review of nitrogen isotopic alteration in marine sediments[END_REF] Alternatively, other microbial processes mediated by bacteria can account for the observed increasing δ 15 N values of wood pieces in RW. Microbial processes in general and bacterial processes in particular are often considered as the main drivers of δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N values under natural conditions (modulated by the oxygenation conditions), over the importation of exogenous N. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Connin | Isotopic discrimination during long-term decomposition in an arid land ecosystem[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF][START_REF] Lerch | Variations in microbial isotopic fractionation during soil organic matter decomposition[END_REF][START_REF] Gälman | Decadal diagenetic effects on δ 13 C and δ 15 N studied in varved lake sediment[END_REF] In this context, it has been proposed that increasing δ 15 N values of the remaining organic material under oxic conditions was the result of the liberation of [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N-depleted compounds during microbial (meaning bacterial) processing, leading to an increasing C/N ratio. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF] In the same way, other authors have suggested that [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N enrichment of their incubated litter was best explained by 'microbial-mediated' isotopic fractionation. [START_REF] Connin | Isotopic discrimination during long-term decomposition in an arid land ecosystem[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF] In all cases, N loss was invoked, which is not observed here. Even though bacterial growth within the remaining wood is in agreement with decreasing C/N values, their main nitrogen source must be the river water. However, we cannot completely exclude bacterial-mediated isotopic fractionation in the present study. Besides, bacterial-mediated isotopic fractionation could be one of the sources of the nonexplained variance (43 %) in the relationship between the δ 15 N values and the %N of samples in RW (Fig. 6a), which is best explained by the incorporation of exogenous N. [START_REF] Benner | Diagenesis of belowground biomass of Spartina alterniflora in salt-marsh sediments[END_REF][START_REF] Bragazza | Seasonal variation in nitrogen isotopic composition of bog plant litter during 3 years of field decomposition[END_REF] The heterogeneous distribution of fungi on samples may also be a major source of that non-explained variance and of the large variability in wood masses (Fig. 3).

Paleoenvironmental implications

Previous studies with litterbags concluded that δ 15 N values are more variable than δ 13 C values during early diagenesis, because of nutrient availability (exogenous N) and microbial processing. [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF][START_REF] Connin | Isotopic discrimination during long-term decomposition in an arid land ecosystem[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF][START_REF] Fogel | Transformation of plant biochemicals to geological macromolecules during early diagenesis[END_REF] The same conclusions were drawn for lacustrine sediments. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Gälman | Decadal diagenetic effects on δ 13 C and δ 15 N studied in varved lake sediment[END_REF] According to those authors, selective preservation of refractory compounds (e.g. lignin), and preferential loss of labile or biodegradable compounds, should not drastically modify δ 13 C values, whereas microbial processing and subsequent [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N discrimination result in large modifications of δ 15 N values. The present incubation of wood samples does agree with those assumptions, δ 13 C values being indistinguishable from the initial value (Fig. 4b), whereas δ 15 N values were highly variable (Fig. 4d).

Fogel and Tuross [START_REF] Fogel | Transformation of plant biochemicals to geological macromolecules during early diagenesis[END_REF] concluded that changes in δ 15 N values during early diagenesis 'may preclude the use of δ 15 N in plant fossil material or sedimentary organic matter for reconstructing paleoenvironments'. By extrapolating the linear regression between δ 15 N values and N content of samples in RW (Fig. 6a) to typical values of lignites rich in organic matter (~1 % in Paleocene-Eocene lignites) [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF] and recently peatified organic matter (~2 %), [START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF] the expected δ 15 N values (~45 and ~94 ‰) would be unrealistic, when compared with those recorded elsewhere (-6 to +7 ‰). [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF] This discrepancy suggests that early diagenesis, leading to the 'geological' value, is not linear whether due to incorporating exogenous N [START_REF] Benner | Diagenesis of belowground biomass of Spartina alterniflora in salt-marsh sediments[END_REF] or microbial processing. [START_REF] Connin | Isotopic discrimination during long-term decomposition in an arid land ecosystem[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF][START_REF] Fogel | Transformation of plant biochemicals to geological macromolecules during early diagenesis[END_REF] Thus, the effect of early diagenesis on δ 15 N values is very fast and can be considered as instantaneous at the geological time scale. A similar non-linear relationship was reported in peatified sphagnum, where δ 15 N values increased from -6 to 0 ‰ during the first 40 years, whereas it increased by +1 ‰ during the next 180 years of burial. [START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF] Based on the associated decrease in C/N ratio along the peat core, those authors attributed the increase in δ 15 N values partly to microbially mediated isotopic fractionation and partly to retention of [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N-enriched substrates. Similarly, the incorporation of N in the present wood samples and exportation of N from them are likely driven by microbial community activity (e. g. fungi): N is incorporated when available in the surrounding environment (i.e. in RW) and exported when nonavailable (i.e. in DW). The incorporation (exportation) of N in (from) wood illustrates the rapid response of microbial processes to environmental conditions. [START_REF] Dijkstra | 15 N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function[END_REF] In that case, early diagenesis homogenises initial δ 15 N values of individuals (i.e. wood pieces) and δ 15 N values of the surrounding environment (i.e. NO3 -in water). Therefore, when utilising organic nitrogen isotopes (δ 15 Norg) for paleoenvironmental reconstructions, the so called 'primary signal' of paleoenvironmental conditions is unlikely to be recorded. However, the smoothed environmental signal (in space and time), such millennial or multimillennial-scale climatic change may be preserved thanks to early diagenesis homogenisation as suggested by previous paleoenvironmental studies. [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF] It could explain why the range of terrestrial δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] Norg values in prequaternary sediments are so limited, with most values between -1 and +7 ‰ [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF][START_REF] Boudou | Organic nitrogen chemistry during low-grade metamorphism[END_REF] compared with the range of δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] Norg values recorded worldwide in modern soils and plants (-8 to +22 ‰). [START_REF] Craine | Ecological interpretations of nitrogen isotope ratios of terrestrial plants and soils[END_REF] Changes in oxygenation conditions during burial may also influence δ [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] Norg values in an opposite way, with decreasing values in oxic environments and increasing values in anoxic environments, due to the presence of different microbial communities. [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF][START_REF] Asada | Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in sphagnum litter[END_REF] Thus, evaluating the effect of early diagenesis under oxic and anoxic conditions is of interest, but may be challenging because anoxic conditions strongly slow down degradation processes. [START_REF] Jenkyns | Nitrogen isotope evidence for water mass denitrification during the Early Toarcian (Jurassic) oceanic anoxic event[END_REF][START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF] Fig. 1. Outlines of the method used to evaluate nitrogen and carbon dynamics during wood decay of the initial coniferous branch. DW, distilled water. RW, river water; t, time. of samples after 73 weeks (t7). DW, distilled water; RW, river water; i, reddish-brown colour characterising black-rot or soft-rot; ii, black fungi floating from the tree-ring limits; iii, floating woody particles; iv, white spots characterising white-rot fungi; v, increasing uniformity of the coloration. Because woody particles were analysed at t5 without replicates, the error bar is the average error of analyticial devices: ±0.6 and ±0.002 % for C and N content respectively, and ±0.2 ‰ for carbon and nitrogen isotopes. NO3 -in river water (RW) remained at trace levels after t2.

Fig. 2 .

 2 Fig. 2. (a and b) Morphology of samples before and after 16 weeks (t4) of degradation. (c and d) Morphology

Fig. 3 .

 3 Fig. 3. Mass loss of samples v. time. Error bars are the standard deviation (2σ), based on triplicates. 100 % corresponds to the initial value. DW, distilled water. RW, river water; t, time.

Fig. 4 .

 4 Fig. 4. Carbon and nitrogen dynamics of wood samples v. incubation time. Error bars correspond to 2σ.

Fig. 5 .

 5 Fig. 5. Atomic C/N ratio of wood pieces. Only the correlation between C/N and time in river water (RW) is significant. The red star correspond to the floating particles sampled at t5 in in river water (RW). Error bars (2σ) are within the symbols. DW, distilled water.

Fig. 6 .

 6 Fig. 6. (a) δ 15 N values relative to N content of wood samples incubated in river water (RW). (b) δ 15 N values relative to the inverse of the N content of the same eight groups of samples: 10 samples for the initial values and 3 samples for each grey point. Both linear regression are significant with P < 0.05.

Acknowledgement

We thank Julien Legrand, who collected the branch used. We are grateful to Véronique Vaury (Institute of Ecology and Environmental Sciences of ParisEES-UPMC) for analyses. This study was supported by the EMERGENCE project from UPMC. We also thank Sylvie Derenne for access to experimental facilities.

Conclusion

This original incubation experiment was conducted with wood pieces immersed in river water and distilled water in order to evaluate the effects of early diagenesis on carbon and nitrogen isotopes. The significant development of fungi in DW was not expected and enabled the comparison of degradation rates, and carbon and nitrogen dynamics of decayed wood between soft-rot and white-rot fungi. It also highlighted the incorporation of prediagenesis fungal material. In all samples, δ 13 C values did not significantly change, in agreement with low modification of δ 13 C values generally reported during early diagenesis of organic matter.

Nitrogen dynamics of wood samples in DW and RW showed two different patterns: N release, associated with slight [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N depletion (-0.3 ‰), was in evidence in samples immersed in distilled water.

It was attributed to the extraction of a 15 N-enriched pool (i.e. proteins) by degrading fungi.

Conversely, N accumulation, associated with [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N enrichment of the remaining wood, was seen in samples immersed in RW. This was mostly attributed to the importation of [START_REF] Melillo | Carbon and nitrogen dynamics along the decay continuum: plant litter to soil organic matter[END_REF] N-enriched nitrate of the RW into the remaining wood. The primary signal of organic nitrogen isotopes may not be recorded in the fossil archive, but early diagenesis seems to average an environmental signal by integrating individual signals (i.e. wood, fungi, NO3 -) and microbially mediated isotopic fractionation. This integration could be almost instantaneous at the geological time scale, hence potentially enabling paleoenvironmental reconstructions. However, because the marine setting and oxygen-depleted environments represent large areas and conditions favourable to terrestrial organic matter accumulation, this kind of incubation experiment should also be performed in salty water and under anoxic conditions, although the latter conditions can be a limit of the methodology used, because of the long time needed when considering the low degradation rate in anoxic environments.

Supplementary material

Content and isotopic data of carbon and nitrogen used for statistical tests xxx are available from the journal online (see http://www.publish.csiro.au/?act=view_file&file_id = EN16049_AC.pdf).


Djamal Rabli
email: djamal.rabli@gmail.com

Ronald Mccarroll
email: ronald_mccarroll@upcm.fr

A revised study of the Li + 2 alkali-dimer using a model potential approach

Keywords: model potential, Lithium cation, ab-initio, adiabatic energies, molecular spectroscopic constant

 but attractive when ab-initio techniques are employed.

.

Introduction

The alkali-dimer lithium cation Li + 2 is a simple diatomic molecular ion, that may be considered as a prototype for molecular systems involving a single active electron. It has been the subject of both theoretical and experimental studies and several accurate adiabatic potential energy curves and molecular spectroscopic constants, based on different approaches, have been reported in the literature during the last decade [1,2,3,4,5,6].

From the experimental side, several works mainly focusing on the X 2 Σ + g ground state were realized. To estimate the ground state dissociation energy, Mathur et al. [7] used an ionization process of the lithium dimer by exploiting two different lines of the Ar + ion laser; going from the lithium dimer X 1 Σ + g ground state trough its B 1 Π u state, to finally reach the Li + 2 cation ground state. On the other hand measurements accomplished by McGeoch et al. [8] also enabled the knowledge of molecular spectroscopic constants of the X 2 Σ + g ground state, while Bernheim et al. [9,10] provided the same quantities, by investigating a Rydberg series of excited electronic states of the lithium dimer using an optical-optical double resonance spectroscopy.

From the theoretical point of view, a restricted Hartree-Fock-Roothaan approach was developed by Henderson et al. [11], while Müller et al. [12] carrying out calculations with frozen atomic core, generated adiabatic potential energy curves of states dissociating up to the Li + (1s 2 )+Li(3d) limit. In a study by Konowalow et al. [13], the first eight lowest-lying electronic states were obtained by valence configuration calculations, whereas Schmidt-Mink et al. [14] employed ab-initio self-consistent-field (SCF) and valence CI techniques combined with a core polarization potential for a better description of the inter-shell correlation effects.

In addition to pseudo-potential method used by Fuentealba et al. [15] and local potential approach developed by D. Hasman [16], Jasik et al. [3], adopted an ab-initio approach where calculations were based on the SCF configuration interaction scheme. Similarly, Bouzouita et al. [2] adopted an ab-initio approach, based on a non-empirical pseudo-potential for the Li(1s 2 ) core where core-core and core-valence correlations have been added to generate the adiabatic energy curves of states dissociating up to Li + +Li(3d) asymptotic limit. More recently Musial et al. [4] used an ab-initio calculation with all inner shell electrons correlated to study the first 18 electronic states.

If ab-initio methods provided most available data describing the Li + 2 cation, model potential techniques also were used by Henriet et al. [17] in a study focusing on the ground state. Similarly, Cerjan [18] adopted a model potential approach to determine molecular properties, of the lowest 2 Σ + g and 2 Π u states. In a more detailed investigation, based on model potential techniques, Bottcher and Dalgarno [19] reported the potential energy curves of the first low-lying, four Σ, two Π and one ∆ states. But to our best knowledge, the most extensive work exploiting model potential techniques remains the one by Magnier et al. [1] who used a potential

Level

Exp [START_REF] Bashkin | Atomic Energy Levels and Grotrian Diagrams[END_REF] This work of Klapisch [20] to report a complete set of 58 adiabatic states for internuclear distances in the range 4 to 40 a.u.

[1] [2] [3] [4] Li(1s
Having one single active electron, certainly makes the lithium cation a relatively simple system to investigate. However when comparing results obtained by different approaches, agreement is far from being guaranteed for all states. While different methods give results which are in reasonable agreement for the ground state, for several excited states, an important disagreement is observed when analyzing data resulting from a model potential approach [1] and those based on ab-initio methods [2,3,4]. The states 3 2 Σ + u and 4 2 Σ + u for example have been found to be repulsive by model potential techniques [1], but are found to be attractive when ab-initio approaches are employed. It is unclear from Magnier et al. [1] whether this disagreement is limited by some conceptual defect of the model potential technique or whether there is some other source of error.

Since ab-initio methods predict for the states showing disagreement, large values of equilibrium distances (R e > 18 a.u), where model potential techniques in principle should give an accurate description of the system, in the present work we propose to re-investigate the Li +

2 system using a model potential of Klapisch [20]. We aim to sort out whether these disagreements were physical effects or numerical artifacts of particular implementation of the model potential method.

Except where stated, atomic units will be used throughout.

Theory

The interaction of an electron with a closed shell lithium core is described by the model potential of Klapisch [20] where r is the radial distance of the electron form the lithium nucleus. The eigenvalues of the corresponding model atomic Hamiltonian

V(r) = - 1 r - 2e -7.90875r + 10.321re -3.90006r r , (1) e
- s i E B( Li + ) A( Li + ) u u r A r B R
H = T + V(r), ( 2
)
where T denotes the electronic kinetic energy operator, are compared with the experimental energy values of the neutral Lithium atom in Table 1. The precision of the computed energy levels is adequate for the particular applications of this work.

Taking the lithium nucleus at A and B (see Fig. 1), the Li + 2 molecular cation is then treated as a set of two closed shell Li + (1s 2 ) cores and a single active electron, whose position is described in the Born-Oppenheimer approximation [21] by the effective Hamiltonian

H e = T + V(r a ) + V(r b ) + U core , ( 3
)
where T is the electronic kinetic energy operator, r a and r b the radial distances of the active electron from nuclei A and B respectively. U core may be mainly taken as the nuclei repulsion modified by a polarization term, namely:

U core = q 1 q 2 R - α 2R 4 , (4)
where q 1 =q 2 = 1 is the ionic core charge and α=0.19 a.u. the polarizability of the lithium atom [22].

The spectrum of the effective Hamiltonian H e defined in Eq (3) is determined by conventional variational techniques, using a basis set of Slater-type orbitals f sto , expressed in prolate spheroidal coordinates

λ ∈ [1, ∞], µ ∈ [-1, 1] and , φ ∈ [0, 2π] where λ = 1 2R (r A + r B ), µ = 1 2R (r A -r B ) ( 5
)
and φ is the azimuthal angle.

The basis set of Slater-type orbitals take the following form:

f sto (λ.µ, φ) = N[(λ 2 -1)(1 -µ 2 )] Λ 2 λ p µ q e -R 2 ζ(λ+ǫµ) e imφ , (6)
where N denotes a normalization factor, ǫ a parameter that takes the values ±1 depending on whether the orbital is centered on nucleus A or nucleus B, (p,q) integers, ζ a variational parameter describing the decay of the orbital, m the projection of the electronic angular momentum on the z-axis (coinciding with the internuclear axis R) and Λ =| m |=0,1,2,. . . for Σ, Π and ∆ molecular states respectively.

Results

We have computed the adiabatic potential energies of the attractive ground state dissociating to a ground state lithium atom Li(1s 2 ,2s) and Li + and of the manifold of excited states correlated in the asymptotic region to Li + +Li(1s 2 ,nl), where nl=2s,2p,3s,3p and 3d. Graphics showing our adiabatic energies versus the internuclear separation R, (R ≤ 50 Å), for Σ, Π and ∆ states are presented in Fig. 3, Fig. 4 and Fig. 5 respectively while comparison of the molecular constants obtained in the present work and selected theoretical and experimental data, is shown in Table 2, Table 3 andTable 4.

Σ states

A basis set of 345 vectors were used to generate the Σ states for different internuclear separation. For the 1 2 Σ + g ground state, present calculations yield a dissociation energy D e =10494 cm -1 and an equilibrium distance R e =3.083 Å. These values may be compared to the experimental ones of Bernheim et al. [9,10], D e =10464 cm -1 and R e =3.11 Å. Our results also show small differences when they are compared to the ab-initio ones of Jasik et al. + Ground and first excited states

1 2 Σ g + 1 2 Σ u + Li + +Li(2s) Figure 2: Li + 2 :
The adiabatic energies of the ground 2 Σ + g and the first excited 2 Σ + u states. The red curve corresponds to Schmidt-Mink et al. [14] results, blue curve to Konowalow et al. [25] results, while the black curve indicates the present results.

As mentioned in Table 2, for the 1 2 Σ + g ground state, it is interesting to notice that our vibration constants ω e =262.771 cm -1 and ω e χ e =1.645 cm -1 , compare well to the experimental values [9,10], ω e =262 cm -1 and ω e χ e =1.7 cm -1 .

Present calculations also show that the first excited state 1 2 Σ + u correlated to Li(1s 2 ,2s)+Li + in the asymptotic limit, has a repulsive character at short internuclear distances, however it possesses a weak potential minimum of 89 cm -1 occurring at an equilibrium internuclear distance R e =9.929 Å. These values may be compared to the ab-initio ones of Bouzouita et al. [2] D e =88 cm -1 and R e =9.911 Å and to those of Jasik et al. [3] D e =89 cm -1 and R e =9.942 Å. The present results also perfectly agree with the ones by Musial et al. [4] R e = 9.954 Å and D e =88 cm -1 . On the other hand, our vibration constants ω e =16.312 cm -1 and ω e χ e =0.750 cm -1 , related to the shape of the potential energy curve, compare well to those of Bouzouita et al. [2] resulting from an ab-initio approach, ω e =15.81 cm -1 and ω e χ e =0.74 cm -1 .

For the 2 2 Σ + g state correlated to Li + +Li(2p) in the asymptotic limit, the present study shows that the potential energy curve has a well pronounced minimum of 2522 cm -1 occurring at an equilibrium distance R e = 6.823 Å. These results are in good agreement with those of Musial et al. [4] R e = 6.826 Å and D e =2529 cm -1 . An excellent agreement is also observed when comparing our results to the abinitio ones of Jasik et al. [3], giving a dissociation energy D e =2517 cm -1 and an equilibrium distance R e =6.819 Å.

The analysis of the present results shows that the second excited state 2 2 Σ + u dissociating in the asymptotic limit to Li + +Li(2p), exhibits a relatively weak minimum of 127 cm -1 occurring at an equilibrium distance R e = 13.203 Å, with vibration constants ω e =13.069 cm -1 and ω e χ e =0.335 cm -1 . As shown in Table 2, these results compare favorably to other values resulting from different approaches [1,2,3,4].

For the 3 2 Σ + g state, our study shows that the corresponding potential energy curve has an energy minimum occurring at 11.059 Å, with a corresponding dissociation energy of 3139 cm -1 . Those values are compared to the ab-initio calculations of Bouzouita et al. [2] D e =3093 cm -1 and R e =11.08 Å and to those of Musial et al. [4] D e =3119 cm -1 and R e =11.071 Å. For this state, we also report that our calculated vibration constants, ω e =55.449 cm -1 and ω e χ e = 0.245 cm -1 , are in good agreement with those of of Musial et al. [4], using an ab-initio approach.

As previously mentioned, ab-initio studies [2,3,4] have shown that the 3 2 Σ + u state has an attractive character, while Magnier et al. [1] exploiting model potential techniques, described a such state as completely repulsive. The present study based on a model potential approach, shows that the potential energy curve of the 3 2 Σ + u state is exhibiting an energy minimum occurring at 19.413 Å, with a corresponding dissociation energy D e =165 cm -1 , vibration constants ω e = 11.131 cm -1 , ω e χ e = 0.188 cm -1 and a value of 37498 cm -1 for T e . Those new results agree with the ab-initio ones of Jasik et al. [3] and with those of Bouzouita et al. [2] as well. A very good agreement is also obtained when comparison is made with those of Musial et al. [4] giving R e =19.475 Å, D e =168 cm -1 , ω e =11.06 cm -1 , ω e χ e = 0.20 cm -1 and T e =37457 cm -1 .

As shown in Table 2, there is a good agreement of our 4 2 Σ + g state spectroscopic constants and those resulting from previous works. The present work yields a dissociation energy D e =1678 cm -1 and an internuclear equilibrium distance R e =16.334 Å. Those may be compared to the ab-initio values and R e =16.472 Å. As previously mentioned, there is a serious disagreement between the ab-initio results [2,3,4] and those based on model potential approach [1] for the 4 2 Σ + u state. Contrary to results of Magnier et al. [1], describing this state as repulsive, our potential energy curve for the 4 2 Σ + u state, clearly shows the existence of an energy minimum at an equilibrium distance R e =22.631 Å, with a corresponding dissociation energy D e = 409 cm -1 . The analysis of the potential energy curve relative to this state, gives vibration constants ω e =11.471 cm -1 , ω e χ e =0.080 cm -1 and a value of 40942 cm -1 for T e . As shown in Table 2, it is easy to notice that these new results exploiting model potential techniques, compare favorably with all ab-initio results [2,3,4].

Present work repulsive

Theory [1] repulsive Theory [2] repulsive Theory [3] repulsive Theory [4]

Present work repulsive

Theory [1] repulsive Theory [2] repulsive Theory [3] repulsive Theory [4] repulsive

Π states

A basis set of 311 vectors were used to generate the Π states. For the 1 2 Π u first excited state in the Π u symmetry, correlated to Li + +Li(2p) in the asymptotic limit, present calculations yield a dissociation energy D e =2107 cm -1 and an equilibrium distance R e = 3.964 Å. It is clear from Table 3, that present results compare favorably to the previous ones based on ab-initio [2,3,4] and to those exploiting model potential methods [1] as well. Our vibration constants ω e = 105.146 cm -1 and ω e χ e =1.311 cm -1 , shown in Table 3, agree well when comparison is made with other values resulting from different approaches. For the 1 2 Π g first excited state in the Π g symmetry, also correlated to Li + + Li(2p) in the asymptotic limit, our results indicate that a such state has a repulsive character since the corresponding potential energy curve does not exhibit any energy minimum.

For the 2 2 Π u state, present study shows that the potential energy curve has an energy minimum occurring at an equilibrium distance R e = 9.587 Å and a dissociation energy D e =3312 cm -1 . The analysis of the potential energy curve relative to this state gives for vibration constants ω e = 50.220 cm -1 and ω e χ e =0.190 cm -1 and 38067 cm -1 for T e . As mentioned in Table 3, it is easy to notice that these results agree with the ones resulting from different approaches [1,2,3,4].

On the other hand, as predicted by previous studies based on different approaches [1,3,2,4], the analysis of the present results, shows that both the 3 2 Π u and 3 2 Π g states are completely repulsive, since their potential energy curves do not possess any energy minimum.

The 2 2 Π g state has been the subject of a disagreement when comparing exist-ing ab-initio [2,3,4] and model potential results of Magnier [1]. For this state, present calculations indicate the existence of a well pronounced energy minimum corresponding to an energy dissociation D e = 401 cm -1 , occurring at an equilibrium distance R e = 18.677 Å, as expected by ab-initio results [2], giving for dissociation energy and equilibrium radius the values D e = 411 cm -1 , R e = 18.65 Å, respectively. A good agreement is also observed when comparison is made between present results and those of Musial et al. [4] giving D e =425 cm -1 and R e =18.799 Å. Other spectroscopic constants ω e =13.301 cm -1 , ω e χ e = 0.110 cm -1 and T e = 40978 cm -1 compare favorably to values resulting from other approaches as shown in Table 3. Theory [1] repulsive Theory [2] repulsive Theory [3] repulsive Theory [4] repulsive

∆ states

A basis set of 300 vectors were used to generate the ∆ states adiabatic energies. The obtained results are summarized in Table 4 and the corresponding energy curves are plotted in Fig. 5 for different internuclear distances up to 50 Å.

The 1 2 ∆ g state, correlated in the asymptotic limit to Li + +Li(3d), exhibits an energy minimum at an equilibrium distance R e =9.502 Å, with a corresponding dissociation energy D e = 330 cm -1 . These values, as shown in Table 4, are in good agreement with those of Magnier et al. [1], but they cannot be compared to the abinitio [2,3] results, describing a such state as completely repulsive. A reasonable agreement is obtained when the present results are compared to those based on an ab-initio approach of Musial et al. [4], giving R e =9.394 Å and D e =174 cm -1 .

From Table 4, it is also easy to see that our frequencies ω e =27.004 cm -1 and ω e χ e =0.552 cm -1 , compare favorably to the ab-initio results [4] ω e =26.64 cm -1 and ω e χ e =0.56 cm -1 . On the other hand, our calculations are giving a value of 41453 cm -1 for T e , which may be compared to T e =41528 cm -1 of Musial e al. [4], exploiting an ab-initio approach.

For the excited state 1 2 ∆ u , the present study shows that the corresponding potential energy curve does not have a minimum, therefore the 1 2 ∆ u is a completely repulsive state, as predicted by previous studies of Magnier et al. [1] and Musial et al. [4]. However, it is important to mention that the 1 2 ∆ u state remains a subject of disagreement when comparison is made with ab-initio results of Bouzouita et al. [2], describing a such state as attractive with an energy minimum occurring at an equilibrium distance around 21 Å and a very weak dissociation energy of 52 cm -1 .

Conclusion

We have used a model potential approach to compute the adiabatic potential energies and the molecular spectroscopic constants of the Li + 2 molecular ion, dissociating up to Li + +Li(1s 2 ,3d) limit. A good agreement between our calculations and those based on ab initio methods has been obtained in the present work. This confirms the accuracy of model potential techniques in studying molecular systems with a single active electron.

It is also interesting to note that the previous disagreement between ab-initio [2,3,4] and model potential [1] results for the excited states 3 2 Σ + u , 4 2 Σ + u and 2 2 Π g , has been sorted out in the present work. Contrary to the claims of Magnier et al. [1] that these states are repulsive, our model potential results has shown that they are attractive as predicted by other studies exploiting ab-initio approach.

Although there are still a few unexplained features of the computed potential energy curves for ∆ states. Certainly, an experimental study in the near future of such states would be of great interest.

Finally, it should be remarked that the model potential method used in this work can be applied to the calculation of more highly excited states, if required. Obviously, a larger basis set may be needed, but it is to be expected that the precision of the adiabatic states of Li + 2 , correlated to atomic states Li(1s 2 ,nl), for which l=n-1, should be of the same order as for the states correlated to Li(1s 2 ,2p) or Li(1s 2 ,3d). For l < n-1, the precision is expected to be of the same order as for the states correlated to Li(1s 2 ,3s) or Li(1s 2 ,3p). The interaction potential for the higher excited states of Li + 2 should also be given to the same level of precision.

Figure 1 :

 1 Figure 1: Relative coordinates for the active electron.

  [3] R e =3.093 Å and D e =10498 cm -1 , to those of Bouzouita et al.[2], R e =3.095 Å and D e =10475 cm -1 and to the recent ones of Musial et al.[4] R e =3.103 and D e =10439 cm -1 .

Figure 3 :

 3 Figure 3: Li + 2 potential energy curves of the 2 Σ + states. Solid lines correspond to 2 Σ + g states, dashed lines to 2 Σ + u states.

Figure 4 :

 4 Figure 4: Li + 2 potential energy curves of the 2 Π states. Solid lines correspond to 2 Π g states, dashed lines to 2 Π u states.

Figure 5 :

 5 Figure 5: Li + 2 potential energy curves of the 2 ∆ states. Solid lines correspond to 2 ∆ g states, dashed lines to 2 ∆ u states.

Table 1 :

 1 Comparison of calculated and experimental energy levels in cm -1 of the lithium atom.

Table 2 :

 2 Spectroscopic constants for 2 Σ + states, R e in Å, D e , ω e , ω e χ e , B e and T e in cm -1 .

	State	Reference	R e	D e	ω e	ω e χ e	B e	T e
	1 2 Σ + g	Present work	3.083	10494 262.771 1.645	0.505	0
		Theory [1]	3.122	10466 263.08 1.477	0.4945	0
		Theory [2]	3.095	10475	264	1.94	0.506	0
		Theory [3]	3.093	10498 263.39			0
		Theory [4]	3.103	10439 262.58	1.58		0
		Exp [7]		10276				0
		Exp [8]	3.032	10807 263.45	1.35		0
		Exp [9, 10]	3.11	10464 262(2)	1.7	0.496	0
		Theory [14]	3.099	10441 263.76 1.646	0.5006	0
		Theory [24]	3.096	10444	265.5	1.89	0.501	0
		Theory [25]	3.127	10324				0
	2 2 Σ + g	Present work	6.823	2522	78.602 0.612	0.103	22874
		Theory [1]	6.879	2525	84.16	0.684	0.1154	22844
		Theory [2]	6.741	2424	79.23	0.49	0.106	22955
		Theory [3]	6.819	2517	79.04			22800
		Theory [4]	6.826	2529	78.67	0.86		22825
		Theory [14]	6.654	2390	82.94	0.623	0.1085	22987
	3 2 Σ + g	Present work 11.059	3139	55.449 0.245	0.039	34524
		Theory [1]	11.113	3143	56.62	0.212	0.0405	34496
		Theory [2]	11.08	3093	54.56	0.16	0.039	34593
		Theory [3]	10.947	3058	57.9			34533
		Theory [4]	11.071	3119	55.58	0.20		34506
	4 2 Σ + g	Present work 16.334	1678	22.014 0.072	0.018	39673
		Theory [1]	16.404	1724	22.14	0.095	0.0178	39644
		Theory [2]	16.499	1674	25.27	0.10	0.017	39714
		Theory [3]	16.6	1512	24.03			39768
		Theory [4]	16.472	1678	21.47	0.12		39683
	5 2 Σ + g

Table 3 :

 3 Spectroscopic constants for 2 Π states, R e in Å, D e , ω e , ω e χ e , B e and T e in cm -1 .

	State	Reference	R e	D e	ω e	ω e χ e	B e	T e
	1 2 Π u Present work	3.964	2107 105.146 1.311	0.306	23291
		Theory [1]	4.022	2100 108.26 1.246	0.3108	23270
		Theory [2]	3.952	1995	110.5	1.19 0.310921 23388
		Theory [3]	3.981	2133 105.25			23197
		Theory [4]	3.978	2102 105.38	1.03		23251
		Theory [14]	3.976	2103 105.58 0.922	0.3040	23277
		Theory [25]	4.014	1852
	2 2 Π u Present work	9.587	3312 50.220 0.190	0.052	38067
		Theory [1]	9.631	3330	50.79	0.186	0.0527	38039
		Theory [2]	9.99	2979	45.74	0.08 0.048655 38565
		Theory [3]	9.107	3008	60.06			38285
		Theory [4]	9.516	3283	52.20	0.21		38078
	3 2 Π u Present work repulsive
		Theory [1] repulsive
		Theory [2] repulsive
		Theory [3] repulsive
		Theory [4] repulsive
	1 2 Π g Present work repulsive
		Theory [1] repulsive
		Theory [2] repulsive
		Theory [3] repulsive
		Theory [4] repulsive
	2 2 Π g Present work 18.677	401	13.301 0.110	0.014	40978

Table 4 :

 4 Spectroscopic constants for 2 ∆ states, R e in Å, D e , ω e , ω e χ e , B e and T e in cm -1 .

	State Reference	R e	D e	ω e	ω e χ e	B e	T e
	1 2 ∆ g This work	9.502	330 27.004 0.552	0.053	41453
	Theory [1]	9.578	324 28.14 0.431	0.0556	41425
	Theory [2] repulsive
	Theory [3] repulsive
	Theory [4]	9.394	174 26.64	0.56		41528
	1 2 ∆ u This work repulsive
	Theory [1] repulsive
	Theory [2]	21.52	52	20.52	0.38 0.037447 41877
	Theory [3] repulsive
	Theory [4] repulsive