The composition of vegetation cover in Arctic environments is evolving due to climate change. This study focused on estimating carbon and nitrogen stocks in vegetation and soil for different Arctic habitats (i.e., sub-ecosystem environments) with contrasted plant communities and soil profiles, to better understand the spatial variation of these stocks and the relationships between aboveground vegetation and soil characteristics in these environments. Vegetation composition and soil characteristics were measured in different habitats in a mire and a heathland ecosystem in northern Sweden. Multivariate analyses were used to define dominant vegetation and soil profile types, and carbon and nitrogen stocks were computed and compared across these types. Shrub-dominated and graminoid-dominated vegetation types presented similar carbon stocks but different nitrogen stocks, and shrub-dominated environments presented significantly higher proportions of their stocks distributed in the long-lived (i.e., woody) compartment. Soil stocks varied considerably between ecosystems. However, no clear relationships could be established between current vegetation community composition and soil carbon and nitrogen stocks across environments. The increase in shrub or graminoid abundance may affect carbon and nitrogen cycles most through biomass distribution between plant compartments rather than through the absolute stocks they represent. Ultimately, in arctic and subarctic ecosystems experiencing climate change, the interactions between plant biomass composition and soil characteristics could have contrasted consequences for ecological functioning and C and N cycles.

Intensified agricultural practices, particularly the increased use of nitrogen fertilizers, are fueling the rise of nitrous oxide (N2O) in groundwater. This gas is produced through the nitrification/denitrification of inorganic nitrogen in the groundwater. Based on combined chemical and isotopic analyses of nitrate (NO3-) and chemical analysis of N2O, we present the first-ever evidence for widespread denitrification-driven N2O accumulation in Indian groundwater that often exceeds 25 times the saturation level. This phenomenon is particularly concerning in regions where low precipitation is received with intensive groundwater irrigation, leading to an inadvertent accumulation of N2O in the groundwater. The emission factor for groundwater (EF5g) from the agricultural soils of India is significantly lower (0.00067) than the global mean values (0.008), suggesting that indirect fluxes of N2O from the Indian soils are lower than the global mean. Implementing sustainable agricultural practices and maximum feasible control measures could further reduce indirect N2O emissions from agricultural soils and their associated environmental consequences.

Land use changes are known to alter terrestrial silicon cycling and the export of dissolved silicon from soil to fluvial systems, but the impact of such changes on groundwater systems remain unclear. In order to identify the processes responsible for groundwater geochemistry and to assess the impact of agricultural processes, we examined multiple isotopic tracers (delta Si-30, oxygen (delta O-18) and hydrogen (delta H-2) isotopes) in groundwater, soil porewater and surface water from two contrasted watersheds having the same gneissic lithology, one forested (Mule Hole) and one intensely cultivated (Berambadi) in the Kabini basin in South India. In the cultivated watershed, groundwater exhibits high Cl- and NO3- concentrations indicative of fertilizer inputs and solute enrichment from evapotranspiration due to multiple groundwater pumping/recharge cycles. The DSi concentration in groundwater is significantly higher in the cultivated watershed (980 +/- 313 mu M) than in the forested one (711 +/- 154 mu M), indicating more intense evapotranspiration due to irrigation cycles. The groundwater delta Si-30 values ranged from 0.6 parts per thousand to 3.4 parts per thousand and exhibit no significant differences between cultivated (1.2 +/- 0.5 parts per thousand) and forested (1.0 +/- 0.2 parts per thousand) watersheds, indicating limited impact of land use and land cover. Groundwater also shows no significant seasonal differences in DSi and delta Si-30 within watersheds, indicating a buffer to seasonal recharge during wet season. The delta Si-30 of a majority of groundwater samples fits a steady-state open flow through system, with an isotopic fractionation factor ((30)epsilon) between precipitating phase and groundwater ranging from -1.0 parts per thousand and - 2.0 parts per thousand, consistent with precipitation of kaolinite-type clays, dominant in the study area. The steady-state flow through system in groundwater can be interpreted as a continuous DSi input from mineral weathering reactions with a dynamic equilibrium between Si supply and precipitation of secondary phases. We also observe, in both watersheds, similar DSi and delta Si-30 values in local surface water that includes small streams and a river (406 +/- 194 mu M, 1.6 +/- 0.3 parts per thousand) and in soil porewater (514 +/- 119 mu M, 1.6 +/- 0.2 parts per thousand). Compared to soil porewater, groundwater exhibits significantly lower delta Si-30 signatures and higher DSi, reflecting the contribution of an isotopically light silicon source, resulting from water-rock interaction during percolation through the unsaturated zone. We assign this steady input of DSi to the weathering of primary silicate minerals in the regolith, such as Na-plagioclase, biotite and chlorite, with formation of kaolinite and smectites type clays. A simple isotopic mass balance suggests that deep regolith weathering can contribute to almost half of the DSi in groundwater. We conclude that silicon cycling in soil porewaters, and surface waters are directly impacted by land use, while the isotopic composition of groundwater remains unaffected. Our results indicate that Si isotopic signatures of weathering, adsorption, and plant uptake occurring in the shallow soil and saprolite horizons are partly overprinted and homogenized by the regolith weathering in the deep critical zone, irrespective of land use and seasonality.

Water stable isotope analysis using Cavity Ring-Down Spectroscopy (CRDS) has a strong between-sample memory effect. The classic approach to correct this memory effect is to inject the sample at least 6 times and ignore the first two to three injections. The average of the remaining injections is then used as measured value. This is in many cases insufficient to completely compensate the memory effect. We propose a simple approach to correct this memory effect by predicting the asymptote of consecutive repeated injections instead of averaging over them. The asymptote is predicted by fitting a y=ax+b relation to the sample repetitions and keeping b as measured value. This allows to save analysis time by doing less injections while gaining precision. We provide a Python program applying this method and describe the steps necessary to implement this method in any other programming language. We also show validation data comparing this method to the classical method of averaging over the last couple of injections. The validation suggests a gain in time of a factor two while gaining in precision at the same time. The method does not have any specific requirements for the order of analysis and can therefore also be applied to an existing set of analyzes in retrospect.•We fit a simple y=ax+b relation to the sample repetitions of Picarro L2130-i isotopic water analyzer, in order to keep the asymptote (b) as measured value instead of using the average over the last couple of measurements.•This allows a higher precision in the measured value with less repetitions of the injection saving precious time during analysis.•We provide a sample code using Python, but generally this method is easy to implement in any automated data treatment protocol.

In present permafrost thawing context, dissolved organic matter (DOM) is a key component that controls organic and inorganic material transfer from soil to hydrographic systems. In terrestrial environments, vegetation is the main source of DOM, before degradation by microorganisms. DOM stoichiometry, aromaticity, composition or quantity control its fate, and referential data characterizing the initial DOM originating from plant biomass leaching are scarce. To better understand its dynamic, this study focuses on the characterization of water extractable organic matter (“WEOM”: a proxy of DOM) of main plant species belonging to different plant functional types typical of the subarctic region (lichen, willow, birch, and Eriophorum). Dissolved organic carbon (C) and dissolved nitrogen (N) contents of WEOM samples were measured, as well as organic C and total N contents of ground plant leaf samples (“bulk” samples). C/N ratio of bulk samples and WEOM fractions were compared to evaluate the potential extractability of C and N. The composition of both WEOM and bulk samples were characterized through solid state 13C Nuclear Magnetic Resonance (NMR) and compared. Absorbance and 3D fluorescence measurements were also performed on WEOM samples to characterize their optical properties. WEOM is significantly more extractable in vascular plants compared to non-vascular ones. Moreover, N is more extractable than C in all lichen species and Eriophorum, whereas C is as extractable as N in Salix and Betula pubescens samples. Betula nana is the only species with C more extractable than N. The solid state 13C NMR spectra of bulk sample are very similar to the spectra of corresponding WEOM, except for Eriophorum. For this species, carbonyl C contributes to 5% of bulk sample spectrum, compared to 14% of the WEOM spectrum. Based on absorbance measurements, optical index were calculated: E2/E3 is significantly higher for non-vascular plants, whereas E2/E4, E3/E4 and slope ratio (SR) do not show significant difference between plant functional types. In 3D fluorescence spectra, the contribution of “Protein-like” peak is lower for vascular plants compared to lichens, and is maximum for Eriophorum. Our results highlighted the influence of plant species on the quantity and quality of produced DOM: WEOM production process is different between vegetation species due to the quality, especially hydrophobicity and extractability of bulk OM components. The high contribution of C-N bonds in WEOM of Eriophorum might be especially important for potential complexation between DOM and trace elements like cadmium (Nigam et al., 2000). Likewise, aromatic C observed only in vascular plant WEOM samples are known to bond have a good affinity with many elements like iron, vanadium and chromium (Gangloff et al., 2014). Under climate change, vegetation cover of the Arctic region is evolving with the moving of the treeline northward and a local increase of the proportion of shrubs (Berner et al., 2013). Accordingly, significant change of DOM composition are expected with potential influence on organic and inorganic material dynamics. Berner et al., (2013). Glob. Chang. Biol. 19:3449-3462 Gangloff et al., (2014). Geochim. Cosmochim. Ac. 130:21-41 Nigam et al., (2000). Chem. Speciation Bioavailability 12:125-132

Environmental context Specific inhibitors of biological reactions in the nitrogen cycle can be used to determine the origin of reactive nitrogen species; these nitrogen species potentially degrade water quality or influence climate. However, inhibitors can potentially interfere with methods for the analysis of stable isotope ratios and concentrations of ammonium, nitrite and nitrate. The effect of this interference on several commonly used methods was investigated. These findings should help avoid the use of inappropriate analytical methods and improve data quality in studies of the nitrogen cycle. Abstract Characterisation of the reaction steps involved in nitrification can help determine the processes that produce potentially harmful environmental pollutants such as nitrite, nitrate and nitrous oxide (N2O). The use of nitrification inhibitors can uncouple the reactions and therefore assist in their mechanistic and isotopic characterisation. However, nitrification inhibitors can interfere with the methods for determining the concentrations and stable isotope ratios of ammonium, nitrite and nitrate. The interference of allylthiourea, hydrazine or sodium chlorate in colorimetric methods and stable isotope measurements were assessed. Ammonium concentrations were measured with the salicylate method. Nitrite and nitrate were measured with the Griess reaction, with nitrate first being reduced to nitrite with vanadium (III) chloride. For the stable isotope analysis, nitrite was reduced to N2O in a 1 : 1 sodium azide and acetic acid buffer solution; preceded, when necessary, by ammonium oxidation to nitrite by hypobromite or nitrate reduction to nitrite on an activated cadmium column. Sodium chlorate did not interfere with any of the analyses and none of the inhibitors interfered with the stable isotope ratios determination of nitrate. Allylthiourea interfered with ammonium and nitrate quantification. Both allylthiourea and hydrazine also clearly interfered in the determination of the nitrogen stable isotope ratio of ammonium, while only allylthiourea interfered in the determination of nitrogen and oxygen stable isotope ratios of nitrite. Although we suggest methods to overcome some of these interferences, our study demonstrated that the analytical methods used in combination with allylthiourea or hydrazine as nitrification inhibitors should be considered with caution when designing experiments.

Intense exploitation of groundwater resources associated with increasing agricultural activity along with extensive utilization of the fertilizers can lead to significant alterations in the hydrological and biogeochemical cycles in shallow aquifers. In the current study we use geochemical and isotopic tracers to understand the origin and mixing of groundwater in aquifers and chemical weathering under contrasting land use: the intensely cultivated Berambadi and the pristine forested Mule Hole catchments. The Berambadi region experiences tropical semi-arid climatic conditions, with depleting water table and salinization of groundwater while Mule Hole is situated inside Bandipur National Park, thus ideal to understand the hydrogeochemical cycles in undisturbed conditions. Both Mule Hole and Berambadi aquifer recharge occurs through surface water percolation and to a lesser extent from lateral flow. Groundwater samples were collected during the summer (dry) season in March and South-West monsoon (wet) season in August 2019 from both watersheds. Silicon isotope fractionation in natural waters is majorly controlled by soil-water interaction leading to dissolution of primary minerals and formation of secondary minerals and also from biogenic sources and uptake. The δ 30 Si of groundwater samples from Berambadi and Mule Hole (average value of 1.05± 0.51‰) were heavier than gneissic bedrock composition (-0

Excessive inputs of sediment and acceleration of primary production have been observed worldwide in a large number of water bodies. Human-environment interactions were recognized as one of the main drivers of this evolution during the 20th century with the occurrence of major landscape changes and a greater use of agricultural inputs. In this study, we used paleo-production proxies such as chlorophyll-a, organic matter properties (TOC and TN concentrations, delta C-13 and delta N-15) measured in sediment cores dated with fallout Pb-210(ex) and Cs-137 activities for reconstructing changes in accumulation rates and sources of organic matter during the recent period of agri- cultural intensification (1920-2020). In order to record these changes at the regional scale, sediment cores were collected at the outlet of several headwater catchments (n = 9), covering a wide range of land covers / land uses across the Loire River basin (117,000 km(2)), France. The rates of sedimentary organic matter deposition in the studied water bodies accelerated from 1950 onwards (+ 48 %). Between 1950 and 1970, the signature of sedimentary organic matter indicates a dominant contribution of soil-derived inputs. This period corresponds to major landscape modifications across the basin (land consolidation, stream re-design, implementation of file drains) driving a general acceleration of erosion rates. Then, from 1960 onwards, chlorophyll-a and C/N proxies indicate an increase in primary production coupled with a decrease of terrigenous supply in agricultural catchments. These proxies were strongly correlated to the agricultural inputs during the 1955-1990 period (e.g., r = 0.9 between chlorophyll-a content and N inputs), suggesting a progressive eutrophication of these reservoirs driven by increasing fertilizer use. During these 35 years, sedimentary organic matter deposition rates increased on average by 30 %. During the 1990s, despite a slight reduction in fertilizer use (- 13 %), the paleo-production proxies (e.g. chlorophyll-a) still indicate a positive trend suggesting the contribution of another driving factor such as climate warming or again nutrient release from soils and sediments. In the absence of long term geochemical and sediment input monitoring, paleolimnological reconstructions provide a powerful tool to reconstruct past agricultural pressures in rural environments. This study illustrates the impact of intensive farming on water body siltation driven by varying sources of organic material during the 20th century. In addition, these results suggest that eutrophication processes of these reservoirs with contrasting land uses started during the 1960-1970 period and are still ongoing nowadays.

Intense irrigation along with extensive use of fertilizers significantly effects the hydrological and biogeochemical cycles in shallow aquifers. Land use changes associated with human activities are known to be a major controlling factor of the terrestrial silicon cycle, altering silicon fluxes to surface and groundwater. In the present study we determined dissolved silicon concentration (DSi) and δ30Si of shallow groundwater samples collected from bore wells and piezometers of two watersheds in Southern India under contrasting land use: one intensely cultivated (Berambadi) and one forested (Mule Hole). Intense groundwater irrigation in the Berambadi region leads to water table depletion, progressive salinization and occurrence of nitrate hotspots in groundwater. We collected groundwater samples during two periods, during the summer (dry) season in March and during the South-West monsoon season in August from both watersheds. DSi values ranged from 410 µM to 1487 µM, with a lower value during August sampling indicating dilution effects caused by monsoon precipitation. Mule Hole and Berambadi aquifer recharge mostly occurs through surface water percolation or from lateral flow. Groundwater composition thus exhibits seasonal variation depending on precipitation which can be traced using water isotopes (δ18O and δ2H). The depleted values in Berambadi groundwater (average δ18O of -2.99 ‰ and δ2H of -15.86 ‰) compared to forested watershed in Mule Hole indicate higher contribution from meteoric water likely due to quicker turnover resulting from continuous irrigation. Silicon isotope fractionation in natural waters is majorly controlled by soil-water interaction consisting in dissolution of primary minerals and formation of secondary minerals and also from biogenic sources and uptake. Preliminary results show no significant differences in δ30Si signatures in groundwater from the two watersheds (1.1 ± 0.3 ‰) in dry season despite higher and more variable DSi concentration in cultivated watershed (1100 ± 260 µM vs. 790 ± 120 µM for the forest). Assuming similar discharge, higher DSi concentration in Berambadi during both seasons indicates increased export/mobilization of Si into aquifer when compared to forested landscape. We will further refine our understanding of Si biogeochemistry in groundwater and the changes associated with land use by comparing the water and silicon isotopes with the germanium/silicon ratio and major element compositions in comparison with surface water data.

The conversion of the natural grasslands of the Pampa biome (Southern Brazil) into cropland may lead to an increase in soil erosion rates and sediment delivery to the rivers. Grasslands represent a significant sink of carbon, and according to the literature, 59% of the soil organic carbon (SOC) is lost when pastures are converted into cropland. It makes soils even more vulnerable to water and land degradation. This study aims to evaluate the impact of land use change on the river sediment composition by calculating the sediment contribution of each potential sediment source using organic matter composition, ultra-violet and visible (UV-VIS) spectra derived parameters and fallout radionuclide activities, as potential tracers in a sediment fingerprinting approach. The study site (Ibirapuitã river basin – 5,942 km²) is located in the Pampa biome, Southern Brazil, were sandy and shallow soils predominate, occupied mainly by native grasslands that are gradually being converted to cropfields, especially soybean. Potential sediment sources were sampled, which include croplands (n=36), native grasslands (n=31), unpaved roads (n=31) and subsurface sources (channel banks (n=18) and gullies (n=16)). Samples were taken from the soil surface layer of croplands and grasslands, as well as from the top layer of exposed sites of gullies, channel banks and unpaved roads. Samples were oven dried (50 °C), gently disaggregated and dry sieved to 63 mm to avoid particle size effects prior to further analysis. Suspended sediment samples were collected using time integrated samplers deployed in the bottom of the river, and during rainfall runoff events at the outlet of the catchment. Organic matter parameters (total organic carbon - TOC, total nitrogen - TN, δ13C and δ15N) were measured using a continuous flow isotope ratio mass spectrometry (EA-IRMS). Diffuse reflectance spectra in the UV-VIS wavelengths was measured using a Cary 5000 UV-VIS-NIR spectrophotometer, and 33 parameters were derived from the spectra. Fallout radionuclide (137Cs and 210Pbxs) activities were measured by gamma spectrometry using low-background high-purity germanium detectors. Tracers were selected following a three step procedure, including: (i) a conservative range test, (ii) a Kruskal–Wallis H-test, and (iii) a linear discriminant function analysis. The selected tracers were introduced into a mass balance mixing model to estimate the source contributions to in-stream sediment by minimizing the sum of square residuals. TOC and TN show significant differences between cropland and native grassland, while the isotopes δ13C and δ15N, presented a lower discrimination potential. TOC and UV-VIS derived parameters did not present a good discriminant potential when they were tested in isolation, although they increased the source discrimination when combined with organic matter parameters. Fallout radionuclides have a good discriminant potential between surface and subsurface sources, but also between native grasslands and croplands. Croplands are the main sediment source in the Ibirapuitã river catchment (36%), followed by the native grasslands (33%). However, the area occupied by croplands is approximately eight times smaller, demonstrating that erosion processes have been intensified by the conversion of native grasslands into croplands and/or croplands are better connected to the river network.

Urban environments have been shown to have profound, yet still poorly understood effects on key ecosystem processes such as carbon (C) and nitrogen (N) cycling, as well as for key aspects of organism ecology, such as community assembly and network interactions. The study of natural abundances of C and N stable isotopes, 13C and 15N, can help infer mechanistic hypotheses on ecosystem processes at play in urban environments. No existing study has investigated whether a visible urban effect on soil and plant C and N biogeochemistry is being transferred to the animal compartment of urban ecosystems through trophic interactions. Here, we report 13C and 15N natural abundance values measured on three pollinating species of wild bees (Lasioglossum laticeps, Lasioglossum morio, and Lasioglossum nitidulum) sampled in 12 locations along an urbanization gradient in the Ile-de-France region (Paris area, France). Our objective was to test whether already recorded isotopic patterns in urban soils and plants in the Paris area would also be visible in pollinators, and to reflect on whether this would be indicative of an urban isotopic signal being transferred to pollinators. Results indicate a significant effect of urbanization on the δ13C and δ15N for all three bee species, with enrichment for both 13C and 15N linked to increased urbanization. This could be linked to an urban imprint on the diverse plants on which pollinating insects forage on in the Paris area, or to other factors linked to the physiology and foraging strategies of insects. A better understanding of the drivers influencing pollinator δ13C and δ15N could help better understand urban biogeochemistry and trophic chains, as well as elucidate the origin of specimens and the migratory capacities of bee species, an important yet still elusive question considering the increase of habitat fragmentation.

Nitrous oxide (N2O) emissions from a nitrifying biofilm reactor were investigated with N2O isotopocules. The nitrogen isotopomer site preference of N2O (15N-SP) indicated the contribution of producing and consuming pathways in response to changes in oxygenation level (from 0 % to 21 % O2 in the gas mix), temperature (from 13.5 to 22.3 ∘C) and ammonium concentrations (from 6.2 to 62.1 mg N L−1). Nitrite reduction, either nitrifier denitrification or heterotrophic denitrification, was the main N2O-producing pathway under the tested conditions. Difference between oxidative and reductive rates of nitrite consumption was discussed in relation to NO2- concentrations and N2O emissions. Hence, nitrite oxidation rates seem to decrease as compared to ammonium oxidation rates at temperatures above 20 ∘C and under oxygen-depleted atmosphere, increasing N2O production by the nitrite reduction pathway. Below 20 ∘C, a difference in temperature sensitivity between hydroxylamine and ammonium oxidation rates is most likely responsible for an increase in N2O production via the hydroxylamine oxidation pathway (nitrification). A negative correlation between the reaction kinetics and the apparent isotope fractionation was additionally shown from the variations of δ15N and δ18O values of N2O produced from ammonium. The approach and results obtained here, for a nitrifying biofilm reactor under variable environmental conditions, should allow for application and extrapolation of N2O emissions from other systems such as lakes, soils and sediments.

The microbial reduction of nitrate, via nitrite into gaseous di-nitrogen (denitrification) plays a major role in nitrogen removal from aquatic ecosystems. Natural abundance stable isotope measurements can reveal insights into the dynamics of production and consumption of nitrite during denitrification. In this study, batch experiments with environmental bacterial communities were used to investigate variations of concentrations and isotope compositions of both nitrite and nitrate under anoxic conditions. To this end, denitrification experiments were carried out with nitrite or nitrate as sole electron acceptors at two substrate levels respectively. For experiments with nitrate as substrate, where the intermediate compound nitrite is both substrate and product of denitrification, calculations of the extent of isotope fractionation were conducted using a non-steady state model capable of tracing chemical and isotope kinetics during denitrification. This study showed that nitrogen isotope fractionation was lower during the use of nitrite as substrate (ε = −4.2 and −4.5‰ for both treatments) as compared to experiments where nitrite was produced as an intermediate during nitrate reduction (ε = −10 and −15‰ for both treatments). This discrepancy might be due to isotopic fractionation within the membrane of denitrifiers. Moreover, our results confirmed previously observed rapid biotic oxygen isotope exchange between nitrite and water.

The microbial reduction of nitrate, via nitrite into gaseous di-nitrogen (denitrification) plays a major role in nitrogen removal from aquatic ecosystems. Natural abundance stable isotope measurements can reveal insights into the dynamics of production and consumption of nitrite during denitrification. In this study, batch experiments with environmental bacterial communities were used to investigate variations of concentrations and isotope compositions of both nitrite and nitrate under anoxic conditions. To this end, denitrification experiments were carried out with nitrite or nitrate as sole electron acceptors at two substrate levels respectively. For experiments with nitrate as substrate, where the intermediate compound nitrite is both substrate and product of denitrification, calculations of the extent of isotope fractionation were conducted using a non-steady state model capable of tracing chemical and isotope kinetics during denitrification. This study showed that nitrogen isotope fractionation was lower during the use of nitrite as substrate (ε = −4.2 and −4.5‰ for both treatments) as compared to experiments where nitrite was produced as an intermediate during nitrate reduction (ε = −10 and −15‰ for both treatments). This discrepancy might be due to isotopic fractionation within the membrane of denitrifiers. Moreover, our results confirmed previously observed rapid biotic oxygen isotope exchange between nitrite and water.

Nitrate content of surface waters results from complex mixing of multiple sources, whose signatures can be modified through N reactions occurring within the different compartments of the whole catchment. Despite this complexity, the determination of nitrate origin is the first and crucial step for water resource preservation. Here, for the first time, we combined at the catchment scale stable isotopic tracers (δ15N and δ18O of nitrate and δ11B) and fecal indicators to trace nitrate sources and pathways to the stream. We tested this approach on two rivers in an agricultural region of SW France. Boron isotopic ratios evidenced inflow from anthropogenic waters, microbiological markers revealed organic contaminations from both human and animal wastes. Nitrate δ15N and δ18O traced inputs from the surface leaching during high flow events and from the subsurface drainage in base flow regime. They also showed that denitrification occurred within the soils before reaching the rivers. Furthermore, this study highlighted the determinant role of the soil compartment in nitrate formation and recycling with important spatial heterogeneity and temporal variability.

Nitrate content of surface waters results from complex mixing of multiple sources, whose signatures can be modified through N reactions occurring within the different compartments of the whole catchment. Despite this complexity, the determination of nitrate origin is the first and crucial step for water resource preservation. Here, for the first time, we combined at the catchment scale stable isotopic tracers (δ 15 N and δ 18 O of nitrate and δ 11 B) and fecal indicators to trace nitrate sources and pathways to the stream. We tested this approach on two rivers in an agricultural region of SW France. Boron isotopic ratios evidenced inflow from anthropogenic waters, microbiological markers revealed organic contaminations from both human and animal wastes. Nitrate δ 15N and δ 18 O traced inputs from the surface leaching during high flow events and from the subsurface drainage in base flow regime. They also showed that denitrification occurred within the soils before reaching the rivers. Furthermore, this study highlighted the determinant role of the soil compartment in nitrate formation and recycling with important spatial heterogeneity and temporal variability.

Wood samples (cm-pieces and powders) were incubated in distilled water (DW) and river water (RW) to assess the effects of early diagenesis on carbon and nitrogen dynamics (δ13C and δ15N). Bacterial activity (BiologECO) was twice higher in DW than RW, although mass loss of wood pieces in RW was 3× greater than in DW (30% vs 10%). This difference was attributed to white-rot fungi that colonized the wood pieces in RW, whereas soft-rot fungi developed in DW. These results confirm that fungi are the main agents of degradation in wood, and that white-rot fungi are much more efficient than soft-rot fungi for wood degradation. Despite obvious degradation in both type of water, wood δ13C was not significantly modified. In contrast, N dynamics showed complex and opposite patterns in both types of water, highlighting the strong impact of early diagenesis on the 15N of organic matter (unlike C). But, most of the 15N changes could be attributed to fungi that relocate N between wood and its micro environment in response to N-availability. Altogether, this suggests that early diagenesis may average an environmental signal by integrating the 15N values of individual signals (woods, fungi, water) and microbial processes.

Earthworms are known to have a major impact on organic matter dynamics in soils. The precise dynamics of carbon incorporation and/or decomposition in soil under the influence of earthworms still need to be investigated. In a mesocosm experiment, the fate of Ryegrass root and shoot litter was monitored in the soil, in the presence and absence of anecic earthworms Lumbricus terrestris L. Residues were13C labelled and deposited onto the soil surface. Incorporation of 13C in surface casts and in the 0–20 and 40–60cm soil layers was monitored 1, 2, 4, 8, 24 and 54weeks after adding labelled litter. Organic carbon content and δ13C values were obtained for all samples, allowing the determination of the percentage of carbon derived from labelled litter (Clab). Roots and shoots were incorporated in the 0–20cm soil layer during the year of experiment, Clab reaching 11.4% of the soil organic carbon after 54weeks. On the contrary, no significant contribution from labelled residues was observed in the 40–60cm layer. Roots decomposed at a slower rate compared to shoots. Litter incorporation was observed in casts from the very first weeks of experiment (Clab from 34.8 to 51.4% after 2weeks). In the soil, a significant effect of earthworms on the Clab was detected after 24weeks. Earthworms accelerated root and shoot decomposition in the soil. They also enhanced, in the presence of shoot residues, the decomposition of the organic matter originally present in the soil. However, after one year, earthworms smoothed the difference between residue types in casts and to a lesser extent in soil, revealing their capacity to enhance the decomposition of both roots and shoots.