Abstract: Plants affect the spatial distribution of soil microorganisms, but the influence of the local abiotic context is poorly documented. We investigated the effect of a single plant species, the cushion plant Silene acaulis, on habitat conditions, and microbial community. We collected soil from inside (In) and outside (Out) of the cushions on calcareous and siliceous cliffs in the French Alps along an elevation gradient (2,000-3,000 masl). The composition of the microbial communities was assessed by Capillary-Electrophoresis Single Strand Conformation Polymorphism (CE-SSCP). Univariate and multivariate analyses were conducted to characterize the response of the microbial beta-diversity to soil parameters (total C, total N, soil water content, [Formula: see text], and pH). Cushions affected the microbial communities, modifying soil properties. The fungal and bacterial communities did not respond to the same abiotic factors. Outside the cushions, the bacterial communities were strongly influenced by bedrock. Inside the cushions, the bacterial communities from both types of bedrock were highly similar, due to the smaller pH differences than in open areas. By contrast, the fungal communities were equally variable inside and outside of the cushions. Outside the cushions, the fungal communities responded weakly to soil pH. Inside the cushions, the fungal communities varied strongly with bedrock and elevation as well as increases in soil nutrients and water content. Furthermore, the dissimilarities in the microbial communities between the In and Out habitats increased with increasing habitat modification and environmental stress. Our results indicate that cushions act as a selective force that counteracts the influence of the bedrock and the resource limitations on the bacterial and fungal communities by buffering soil pH and enhancing soil nutrients. Cushion plants structure microbial communities, and this effect increases in stressful, acidic and nutrient-limited environments.
Abstract:
Aims
Climate-induced changes in snow cover are likely to affect cold arctic and alpine ecosystems functioning and major processes such as wintertime plant litter decomposition. However, it remains poorly studied in subalpine systems where the snowpack may be irregular. In this paper we explored the dynamic of the winter plant litter decomposition process, its magnitude and its relationship with the snowpack properties.
Methods
In subalpine grasslands of the Central French Alps, we performed a litter bag experiment monitoring over a whole winter the litter decomposition from the exploitative Dactylis glomerata and the conservative Festuca paniculata, under two contiguous experimental sites with snowpacks differing in depth and physical properties.
Results
Litter decomposition rates were stable during winter and 3-fold higher under deeper and permanent snowpack with higher thermal resistance. Litter quality appeared only significant under thinner snowpack with higher decomposition rates for the exploitative species. A snowpack with higher thermal resistance created an insulating layer promoting the decomposition process.
Conclusion
These results suggest that the temporal (permanence vs. intermittency) and physical (depth and thermal resistance) characteristics of the snowpack should be considered when studying the response of winter ecosystems functioning to global changes.
Abstract: Mycorrhizal fungi or endphytes colonize plant roots and their occurrence and composition depend on biotic and abiotic characteristics of the ecosystem. We investigated the composition of these microbial communities associated with Festuca paniculata, a slow growing species, which dramatically impacts functional plant diversity and the recycling of organic matter in subalpine grasslands. F. paniculata individuals from both mown and unmown grasslands were randomly collected and the microscopic observation of the plant roots revealed a difference in fungal colonization according to management. The ITS regions of root-associated fungi were amplified, cloned and sequenced. Bioinformatic analysis revealed a total of 43 and 35 phylotypes in mown and unmown grasslands respectively, highlighting a remarkable difference in the composition between both fungal communities. The phylotypes were assigned to 9 classes in which two classes Eurotiomycetes and Lecanoromycetes were specific to mown grasslands, while Tremellomycetes were specific to unmown grasslands and only five phylotypes were common to both locations. The comparative analysis of fungal lifestyles indicated the dominance of saprobes and a large proportion of endophytes compared to the mycorrhizal fungi (7/1 and 11/2 phylotypes in mown and unmown grasslands, respectively). Endophyte richness was greater in the unmown gassland than in the mown grassland and their relative proportion was twice higher. Our results suggest that endophytes may offer potential resources to F. paniculata and play an important role in the regulation of plant diversity.
Abstract: P>1. Nitrogen (N) cycling is a key process determining ecosystem functioning in subalpine grasslands where traditional mowing and manuring are being abandoned. However, the roles of the plant and microbial communities in mediating changes in N availability are still poorly understood. 2. We inoculated 15 subalpine grassland fields with dual-labelled ammonium nitrate (15NH(4)+, 15NO(3)-) during July 2005 and used pool dilutions over 1 month to calculate inorganic N fluxes into the microbial pool and uptake in plant communities by grasses, forbs and legumes. The effects of current land abandonment were assessed by comparing manured and mown terraces (ancient croplands) with other terraces where these practices have ceased, and mown versus unmown unterraced meadows. 3. Rapid cycling of inorganic N and high soil N availability in forb-dominated manured and mown terraces resulted from fast plant N uptake and low microbial C:N ratio. In grass-dominated unmown terraces, N cycling was slower and N retention was greater; microbial N uptake remained similar to that in the other terraces, although a higher C:N ratio suggested a shift towards fungal dominance. 4. In unterraced meadows, pH was low due to reduced mixing of soil with the underlying calcareous rock. Soil [NH4+] was high and [NO3-] low, but current management had no effect on N pool size, although plant N uptake was greater in the mown than unmown fields. This may be partially explained by high N retention by dominant Festuca paniculata tussocks. The microbial N pool and N uptake were both low and the microbial C:N ratio was high, suggesting that fungi slowed N cycling and reduced the influence of mowing on N turnover. 5. Synthesis. In these marginal long-term grasslands, with low productivity and high biodiversity value, changes in ecosystem function associated with reduced management intensity were mediated through slower N cycling. This response was expressed as more gradual nutrient uptake but greater retention by unmown plant communities, slower microbial uptake and smaller soil N pools. In contrast to more productive ecosystems, such as north-western European grasslands, reduced management is detrimental to both biodiversity and the maintenance of soil-related ecosystem services. These costs will need to be balanced against potential benefits, such as carbon storage.
Notes: Matthew Robson, T. Baptist, Florence Clement, Jean-Christophe Lavorel, Sandra
Abstract: The snowpack is a complex photochemical reactor that emits a wide variety of reactive molecules to the atmosphere. In particular, the photolysis of nitrate ions, NO3-, produces NO, NO2, and HONO, which affects the oxidative capacity of the atmosphere. We report measurements in the European High Arctic where we observed for the first time emissions of NO, NO2, and HONO by the seasonal snowpack in winter, in the complete or near-complete absence of sunlight and in the absence of melting. We also detected unusually high concentrations of nitrite ions, NO2-, in the snow. These results suggest that microbial activity in the snowpack is responsible for the observed emissions. Isotopic analysis of NO2- and NO3- in tie snow confirm that these ions, at least in part do not have an atmospheric origin and are most likely produced by the microbial oxidation of NH4+ coming from clay minerals into NO2- and NO3-. These metabolic pathways also produce NO. Subsequent dark abiotic reactions lead to NO2 and HONO production. The snow cover is therefore not only an active photochemical reactor but also a biogeochemical reactor active in the cycling of nitrogen and it can affect atmospheric composition all year round.
Notes: Amoroso, A. Domine, F. Esposito, G. Morin, S. Savarino, J. Nardino, M. Montagnoli, M. Bonneville, J. -M. Clement, J. -C. Ianniello, A. Beine, H. J.
Abstract: Trait differentiation among species occurs at different spatial scales within a region. How does the partitioning of functional diversity help to identify different community assembly mechanisms? Northeastern Spain. Functional diversity can be partitioned into within-community (alpha) and among-communities (beta) components, in analogy to Whittaker's classical alpha and beta species diversity concept. In light of ecological null models, we test and discuss two algorithms as a framework to measure alpha and beta functional diversity (the Rao quadratic entropy index and the variance of trait values). Species and trait (specific leaf area) data from pastures under different climatic conditions in NE Spain are used as a case study. The proposed indices show different mathematical properties but similarly account for the spatial components of functional diversity. For all vegetation types along the climatic gradient, the observed alpha functional diversity was lower than expected at random, an observation consistent with the hypothesis of trait convergence resulting from habitat filtering. On the other hand, our data exhibited a remarkably higher functional diversity within communities compared to among communities (alpha >beta). In contrast to the high species turnover, there was a limited functional diversity turnover among communities, and a large part of the trait divergence occurred among coexisting species. Partitioning functional diversity within and among communities revealed that both trait convergence and divergence occur in the formation of assemblages from the local species pool. A considerable trait convergence exists at the regional scale in spite of changes in species composition, suggesting the existence of ecological redundancy among communities.
Notes: de Bello, Francesco Thuiller, Wilfried Leps, Jan Choler, Philippe Clement, Jean-Christophe Macek, Petr Sebastia, Maria-Teresa Lavorel, Sandra
Abstract: In alpine ecosystems, tannin-rich-litter decomposition occurs mainly under snow. With global change, variations in snowfall might affect soil temperature and microbial diversity with biogeochemical consequences on ecosystem processes. However, the relationships linking soil temperature and tannin degradation with soil microorganisms and nutrients fluxes remain poorly understood. Here, we combined biogeochemical and molecular profiling approaches to monitor tannin degradation, nutrient cycling and microbial communities (Bacteria, Crenarcheotes, Fungi) in undisturbed wintertime soil cores exposed to low temperature (0 degrees C/-6 degrees C), amended or not with tannins, extracted from Dryas octopetala. No toxic effect of tannins on microbial populations was found, indicating that they withstand phenolics from alpine vegetation litter. Additionally at -6 degrees C, higher carbon mineralization, higher protocatechuic acid concentration (intermediary metabolite of tannin catabolism), and changes in fungal phylogenetic composition showed that freezing temperatures may select fungi able to degrade D. octopetala's tannins. In contrast, negative net nitrogen mineralization rates were observed at -6 degrees C possibly due to a more efficient N immobilization by tannins than N production by microbial activities, and suggesting a decoupling between C and N mineralization. Our results confirmed tannins and soil temperatures as relevant controls of microbial catabolism which are crucial for alpine ecosystems functioning and carbon storage.
Abstract: Nitrogen (N) availability in grasslands varies with agricultural land use. Traditional management regimes of mowing for hay and manuring in subalpine meadows maintain plant communities with exploitative functional strategies suited to fertile soils with fast turnover of nutrients. We investigated whether the neglect of traditional practices has led to a reduction in N availability in two parallel ecosystems (terraced and unterraced fields). Soil nitrate and ammonium contents were assessed using soil cores and ion exchange resins over a 1-year period, and assays of microbial nitrifying and denitrifying enzyme activities, made early in the growing season. A large difference in pH between the two ecosystems, caused by historical ploughing, facilitated greater N availability in terraced than unterraced fields. Abandonment of manuring and mowing caused a reduction in N availability and N transformation processes, which correlated with a shift in the plant community towards more-conservative functional strategies and greater dominance by grasses. We propose that positive feedback between the grassland management regime and dominant plant functional strategy maintained high N availability in these managed subalpine grasslands. When traditional practices of mowing and manuring are neglected, direct management effects combined with the spread of grass species with conservative strategies force down N availability in the soil, reduce microbial activity, change the pH, and lead to a long-term loss of characteristic herbaceous subalpine-meadow species. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: Many studies have shown that intensive agricultural practices significantly increase the nitrogen concentration of stream surface waters, but it remains difficult to identify, quantify, and differentiate between terrestrial and in-stream sources or sinks of nitrogen, and rates of transformation. In this study we used the delta N-15-NO3 signature in a watershed dominated by agriculture as an integrating marker to trace (1) the effects of the land cover and agricultural practices on stream-water N concentration in the upstream area of the hydrographic network, (2) influence of the in-stream processes on the NO3-N loads at the reach scale (100 m and 1000 m long), and (3) changes in delta N-15-NO3 signature with increasing stream order (from first to third order). This study suggests that land cover and fertilization practices were the major determinants of delta N-15-NO3 signature in first-order streams. NO3-N loads and delta N-15-NO3 signature increased with fertilization intensity. Small changes in delta N-15-NO3 signature and minor inputs of groundwater were observed along both types of reaches, suggesting the NO3-N load was slightly influenced by in-stream processes. The variability of NO3-N concentrations and delta N-15 signature decreased with increasing stream order, and the delta N-15 signature was positively correlated with watershed areas devoted to crops, supporting a dominant effect of agriculture compared to the effect of in-stream N processing. Consequently, land cover and fertilization practices are integrated in the natural isotopic signal at the third-order stream scale. The GIS analysis of the land cover coupled with natural-abundance isotope signature (delta N-15) represents a potential tool to evaluate the effects of agricultural practices in rural catchments and the consequences of future changes in management policies at the regional scale.
Abstract: Plant uptake and denitrification are considered to be the most important processes responsible for N retention and mitigation in riparian buffers. In many riparian buffers, however, nutrients taken up by plants remain in the system only temporarily and may be gradually released by mineralization later. Still, plants increase the residence time of nutrients considerably by reducing their mobility. We investigated the importance of plant N uptake and N immobilization in litter for N retention in riparian buffers. Nitrogen uptake in vegetation and N dynamics in litter were measured over a two-year period in a range of forested and herbaceous riparian buffers along a climatic gradient in Europe, receiving different loadings of N-enriched groundwater. Plant production, nitrogen uptake, and N retention were significantly higher in the forested buffer sites compared to the herbaceous buffer sites. However, in herbaceous buffers, periodic harvesting of herbaceous biomass contributed considerably to the N retention. No relationship between lateral N loading and plant productivity or N uptake was observed; this indicated that plant growth was not N-limited. In the winter period, decaying leaf litter had a small but significant role in N retention in a majority of the riparian ecosystems studied. Moreover, no responses to the climatic gradient were found. Generally, we can state that annual N retention in the vegetation and litter compartment is substantial, making up 13-99% of the total N mitigation. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: In exploring the dynamics of iron and nitrogen cycling in sediments from riparian forests we have observed a redox reaction that has not been previously described. During incubations of soil slurries under strictly anaerobic conditions, we repeatedly measured an unexpected production of both nitrite (NO2-) and ferrous iron [Fe(II)]. Using this indirect evidence we hypothesize that, under anaerobic conditions, there is a biological process that uses ferric iron [Fe(III)] as an electron acceptor while oxidizing ammonium (NH4+) to NO2- for energy production. This NH4+ oxidation under iron reducing anaerobic conditions is thermodynamically feasible and is potentially a critical component of the N cycle in saturated sediments. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Riparian zones have long been considered as nitrate sinks in landscapes. Yet, riparian zones are also known to be very productive ecosystems with a high rate of nitrogen cycling. A key factor regulating processes in the N cycle in these zones is groundwater table fluctuation, which controls aerobic/anaerobic conditions in the soil. Nitrification and denitrification, key processes regulating plant productivity and nitrogen buffering capacities are strictly aerobic and anaerobic processes, respectively. In this study we compared the effects of these factors on the nitrogen cycling in riparian zones under different climatic conditions and N loading at the European scale. No significant differences in nitrification and denitrification rates were found either between climatic regions or between vegetation types. On the other hand, water table elevation turned out to be the prime determinant of the N dynamics and its end product. Three consistent water table thresholds were identified. In sites where the water table level is within -10 cm of the soil surface, ammonification is the main process and ammonium accumulates in the topsoils. Average water tables between <LF>-10 and -30 cm favour denitrification and therefore reduce the nitrogen availability in soils. In drier sites, that is, water table level below -30 cm, nitrate accumulates as a result of high net nitrification. At these latter sites, denitrification only occurs in. ne textured soils probably triggered by rainfall events. Such a threshold could be used to provide a proxy to translate the consequences of stream flow regime change to nitrogen cycling in riparian zones and consequently, to potential changes in nitrogen mitigation.
Abstract: 1. Nitrogen (N) loss from agricultural fields and urban areas to stream and groundwaters is a world-wide environmental problem. Excessive nitrogen loading is partly responsible for eutrophication of fresh water and estuarine ecosystems, while elevated nitrate in drinking water has consequences for human health. Under certain conditions, riparian zones improve water quality by removing groundwater nitrate before it enters adjacent stream ecosystems. Nitrate decline along riparian flow paths is most often attributed to denitrification activity and vegetation uptake, but spatio-temporal distributions and rates are notoriously difficult to establish. 2. We used natural delta(15)N techniques in two riparian wetlands with differing vegetation to distinguish between the two processes responsible for reducing nitrate fluxes. We collected groundwater and above-ground vegetation samples along riparian transects where hydrology and groundwater chemistry had been investigated previously. 3. By measuring the natural abundance distribution of nitrogen isotopes in both the groundwater nitrate and riparian plant tissues along the transects, we attempted to determine to what extent the groundwater nitrate decline (from c. 15 to < 1 mg N L-1) observed in these two riparian sites with contrasting vegetation resulted from denitrification and/or plant uptake. 4. Denitrifying bacteria preferentially use the lighter isotope and hence tend to increase delta(15)N-NO3. In most groundwater samples we observed a significant increase of delta(15)N-NO3 (from +5 to +28parts per thousand) as nitrate concentrations declined, which demonstrated that denitrification was predominantly responsible for nitrate retention even when the water table was low. 5. This isotopic approach provided evidence of seasonal variation in the occurrence of denitrification, and helped to delimit the area where denitrification was most active. delta(15)N of overlying vegetation along the riparian transects was higher (from +1.7 to +14.2parts per thousand) than the typical range for terrestrial plants, and was related to the isotopic composition of nitrate in underlying groundwater when the water table was high. Thus, in this case, both plant uptake and denitrification contributed to the observed nitrate decline. However, given that the roots were limited to the upper 50 cm of soil, direct uptake of groundwater nitrate by riparian vegetation was only important when the water table was high. 6. Measurement of delta(15)N in plants may be a simple and powerful means of identifying buffer zones where denitrification is actively processing allochtonous nitrate in riparian ecosystems and their surrounding catchments. 7. Synthesis and applications. The isotopic approach described in this paper is a useful diagnostic tool for easily identifying actual denitrification locations where groundwater nitrate removal is taking place. It should allow investigation at a landscape scale of the spatio-temporal patterns of biogeochemical hot spots where denitrification rates are disproportionately high relative to the surrounding area. This could provide a sound basis for landscape management and restoration in the context of diffuse nitrogen pollution control.
Abstract: Three main reservoirs were identified that contribute to the shallow subsurface flow regime of a valley drained by a fourth-order stream in Brittany (western France). (i) An upland flow that supplied a wetland area, mainly during the high-water period. It has high N-NO3- and average Cl- concentrations. (ii) A deep confined aquifer characterized by low nitrate and low chloride concentrations that supplied the floodplain via flow upwelling. (iii) An unconfined aquifer under the riparian zone with high Cl- and low N-NO3- concentrations where biological processes removed groundwater nitrate. This aquifer collected the upland flow and supplied a relict channel that controlled drainage from the whole riparian zone. Patterns of N-NO3- and Cl- concentrations along riparian transects, together with calculated high nitrate removal, indicate that removal occurred mainly at the hillslope-riparian zone interface (i.e. first few metres of wetland), whereas dilution occurred in lower parts of the transects, especially during low-water periods and at the beginning of recharge periods. Stream flow was modelled as a mixture of water from the three reservoirs. An estimation of these contributions revealed that the deep aquifer contribution to stream flow averaged 37% throughout the study period, while the contribution of the unconfined reservoir below the riparian zone and hillslope flow was more variable (from ca 6 to 85%) relative to rainfall events and the level of the riparian water table. At the entire riparian zone scale, NO3- removal (probably from denitrification) appeared most effective in winter, despite higher estimated upland NO3- fluxes entering the riparian zone during this period. Copyright (C) 2003 John Wiley Sons, Ltd.
Abstract: We evaluated nitrogen (N) removal efficiency by riparian buffers at 14 sites scattered throughout seven European countries subject to a wide range of climatic conditions. The sites also had a wide range of nitrate inputs, soil characteristics, and vegetation types. Dissolved forms of N in groundwater and associated hydrological parameters were measured at all sites; these data were used to calculate nitrate removal by the riparian buffers. Nitrate removal rates (expressed as the difference between the input and output nitrate concentration in relation to the width of the riparian zone) were mainly positive, ranging from 5% m(-1) to 30% m(-1), except for a few sites where the values were close to zero. Average N removal rates were similar for herbaceous (4.43% m(-1)) and forested (4.21% m(-1)) sites. Nitrogen removal efficiency was not affected by climatic variation between sites, and no significant seasonal pattern was detected. When nitrate inputs were low, a very large range of nitrate removal efficiencies was found both in the forested and in the nonforested sites. However, sites receiving nitrate inputs above 5 mg N L-1 showed an exponential negative decay of nitrate removal efficiency (nitrate removal efficiency = 33.6 e(-0.11) (NO3,) (input), r(2) = 0.33, P < 0.001). Hydraulic gradient was also negatively related to nitrate removal (r = -0.27, P < 0.05) at these sites. On the basis of this intersite comparison, we conclude that the removal of nitrate by biological mechanisms (for example, denitrification, plant uptake) in the riparian areas is related more closely to nitrate load and hydraulic gradient than to climatic parameters.
Abstract: We investigated the seasonal patterns of denitrification rates and potentials in soil profiles along the topohydrosequence formed at the upland-wetland interface in three riparian wetlands with different vegetation cover (i.e., forest, understory vegetation, and grass). Denitrification was measured using the acetylene inhibition method on soil cores and slurries, which provided a means of comparing the relative activity of this process in different locations. We evaluated, on a seasonal basis, the respective importance of the vegetative cover and the hydromorphic gradient as factors limiting denitrification. Regardless of the season, vegetation type, or lateral position along each topohydrosequence in the riparian wetlands, strong significant gradients of both in situ and potential denitrification rates were measured within a soil profile. Results confirm that the upper organic soil horizon is the most active, when in contact with the ground water. In deeper soil horizons, denitrification activity was low (from 0.004 to 0.5 mg N kg(-1) dry soil d(-1)), but contributed significantly to the reduction of ground water NO3- load along the riparian ground water flowpath (front 9.32 to 0.98 mg NO3-N L-1). Along the soil topohydrosequence, the denitrifying community of the upper soil horizons did not vary significantly on a seasonal basis despite the large seasonal ground water fluctuations. Along each topohydrosequence, the denitrification-limiting factor gradually shifted front anaerobiosis to NO3- supply. In situ denitrification rates in the forested, understory vegetation and grass sites were not significantly different. This result emphasizes the importance of the topography of the valley rather than the vegetation cover in controlling denitrification activity in riparian wetlands.
Abstract: Soil saturation is known to be of crucial importance, to denitrification and other nitrogen cycling processes within the riparian zone. Since denitrification potential generally increases towards the soil surface, water table elevation can control the degree to which nitrate reduction is optimised. Given their topographic location and sedimentary structure, most floodplains are characterised by high water tables. However, detailed field data on water table levels, hydraulic gradients and flow patterns within the riparian zone are generally lacking. This paper presents data collected as part of a pan-European study of nitrate buffer zones, the Nitrogen Control by Landscape Structures in Agricultural Environments project (NICOLAS). An identical experimental design was employed at each site, allowing riparian zone hydrology and nitrogen cycling processes to be explored across a wide range of temperate climates; only the hydrological data are discussed here. A grid of dipwells at 10-metre spacing was installed at each site and manual measurements made at least once a month for a minimum of one year. In addition, at least one dipwell in each grid was monitored continuously using a data logger. All the riparian zones studied displayed a clear annual cycle of water table elevation, although other factors seemed equally important in influencing the range of variation. Where the riparian zone was flat, the water level in the adjoining river or lake proved more significant in controlling water table levels within the riparian zone than was originally anticipated. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Understanding the environmental consequences of changing water regimes is a daunting challenge for both resource managers and ecologists. Balancing human demands for fresh water with the needs of the environment for water in appropriate amounts and at the appropriate times are shaping the ways by which this natural resource will be used in the future. Based on past decisions that have rendered many freshwater resources unsuitable for use, we argue that river systems have a fundamental need for appropriate amounts and timing of water to maintain their biophysical integrity. Biophysical Integrity is fundamental for the formulation of future sustainable management strategies. This article addresses three basic ecological principles driving the biogeochemical cycle of nitrogen in river systems. These are (1) how the mode of nitrogen delivery affects river ecosystem functioning, (2) how increasing contact between water and soil or sediment increases nitrogen retention and processing, and (3) the role of floods and droughts as important natural events that strongly influence pathways of nitrogen cycling in fluvial systems. New challenges related to the cumulative impact of water regime change, the scale of appraisal of these impacts, and the determination of the impacts due to natural and human changes are discussed. It is suggested that cost of long-term and long-distance cumulative impacts of hydrological changes should be evaluated against short-term economic benefits to determine the real environmental costs.
Abstract: Water levels, and nitrate and chloride concentrations have been monitored in ground waters (1 - 8 m depth) for over two years in a wetland in Brittany, France. Piezometric measurements allow to distinguish 3 different watershed and quite complex water-flow along the wetland. Hydrochemical measurements give constraints to the origin of the flow and both methods give a complete hydrological scheme. The contribution of a deep watershed, and the influence of evapo-transpired fluxes from a forest, along the wetland are evidenced. Nitrogen isotopic determinations indicate the nature of the nitrate reduction process. The combined hydrological and hydrochemical data allow to determine fluxes of nitrate of 8-10 kg/hr for the whole zone and denitrification rates.
