Isotopic investigation of denitrification in a riparian ecosystem in western France.
Abstract
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. We used natural δ15N 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. 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. Denitrifying bacteria preferentially use the lighter isotope and hence tend to increase δ15N-NO3. In most groundwater samples we observed a significant increase of δ15N-NO3 (from +5 to +28 per mil) as nitrate concentrations declined, which demonstrated that denitrification was predominantly responsible for nitrate retention even when the water table was low. This isotopic approach provided evidence of seasonal variation in the occurrence of denitrification, and helped to delimit the area where denitrification was most active. δ15N of overlying vegetation along the riparian transects was higher (from +1.7 to +14.2 per mil) 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. Measurement of δ15N 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. 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.