During the last 40 years, nitrate (NO3−) has become one of the most common ground water contaminants. Stream riparian zones are considered important ecological ecotones that decrease the NO3− load of ground water discharging into streams. This study uses NO3−/Cl− ratios, natural abundances of15N and18O in NO3−, and an in situ15NO3− tracer experiment to evaluate NO3− reducing processes occurring in ground water within a narrow grassed buffer strip bordering a stream in an agricultural watershed. The NO3−/Cl− ratios (1.9‐0.0 [M/M]) indicate that both NO3− consuming processes and mixing of two ground water flow regimes with different NO3− loads contribute to the drop observed in ground water NO3− concentrations within the riparian zone. δ15N and δ18O of the ground water NO3− within the riparian zone (δ15N = 5.1 to 48.8 %c; δ18O = 1.1 to 17.8 %c) were enriched compared to the ground water below the adjacent cultivated field (δ15N = 3.8 to 10.1 %0; δ18O = 0.5 to 6.2 %c). A significant linear relationship (r2=0.97) between ground water δ15N and δ18O in NO3− was found, which is consistent with NO3− consumption by microbial denitrification. The estimated enrichment factors for15N are a factor of 1.5 higher than for18O. The in situ15NO3− tracer experiment conclusively confirmed that denitrification is occurring within the ground water of the riparian zone and demonstrates that denitrification rates can be directly measured in situ.

The stable isotopic composition (delta 15N and delta 18O) of nitrate was analyzed in two lysimeter field experiments in order to identify the conditions under which the dual isotope approach can be applied to identify the main source of nitrate in agricultural soils. The first field experiment involved six lysimeters beneath fields that had been fertilized for 10 yr with the same type of fertilizer (NH4NO3; delta 15N = +1.2@1000, delta 18O = +18.6@1000). The isotope ratios of NO3- in the leachate (delta 15N approximately 0@1000; delta 18O approximately +2@1000) could not be interpreted in a conventional way with either fertilizer or soil organic nitrogen as main sources. These results provided clear evidence for the microbial immobilization and subsequent mineralization and nitrification to NO3- (mineralization-immobilization turnover concept). This process masked the original oxygen isotope ratio of the fertilizer source during the summer when microbial activity was high. A second experiment involving the application of Ca(NO3)2 to three lysimeters during the winter confirmed that the dual isotope approach remains valid for the source identification of nitrate under conditions of low microbial activity. The study reveals the limitation of the dual isotope approach to characterize nitrate sources under biologically active conditions and the ability to quantify microbial processes when the main sources can be controlled.

Nitrogen elimination was investigated in two eutrophic Swiss lakes with different hypolimnetic oxygen conditions. Nitrogen burial was estimated from sediment-trap and sediment-core studies. Denitrification and NO, --ammonification rates were quantified with IsNO,-and acetylene incubation experiment! and whole-lake mass balances. The study confirmed earlier reports that the acetylene-block technique yields denitrification rates that are systematically too low. Denitrification rates obtained from isotope tracer experiments were compatible with nitrogen consumption rates observed in flux chamber experiments and whole-lake mass balances. The NO,--ammonification contributed <5% to the NO,-consumption rate in Lake Baldegg. Coupled nitrification-denitrification seemed to be insignificant at the deepest station of Lake Baldegg. The comparison of in situ denitrification rates measured at the deepest site (4.3 mmol m-* d-l) with the denitrification rate obtained from whole-lake mass balances (6.1 mmol rnd2 d-l) indicates that enhanced denitrification may be present in shallower sediments with a better supply of 0,. Mass transfer coefficients for NO,-were similar in both lakes (21.7 and 21.4 m yr-). The NO,-concentration seems to be a key parameter in determining denitrification rates.

As reported from marine systems, we found that also in 15 prealpine lakes N2O concentrations were strongly correlated with O2 concentrations. In oxic waters below the mixed surface layer, N2O concentrations usually increased with decreasing O2 concentrations. N2O is produced in oxic epilimnia, in oxic hypolimnia and at oxic-anoxic boundaries, either in the water or at the sediment-water interface. It is consumed, however, in completely anoxic layers. Anoxic water layers were therefore N2O undersaturated. All studied lakes were sources for atmospheric N2O, including those with anoxic, N2O undersaturated hypolimnia. However, compared to agriculture, lakes seem not to contribute signif cantly to atmospheric N2O emissions.

Nitrous oxide (N2O) production at the sediment surface of eutrophic Lake Baldegg was quantified with three independent methods: pore‐water samplers, benthic chambers, and mass balances of the aerated‐oxygenated hypolimnion. N2O production at the sediment surface was the most important source in this lake and led to an accumulation in the hypolimnion during summer stratification. Highest rates of N2O emission to the atmosphere (24 µmol m−2 d−1) were observed after the onset of winter overturn, when hypolimnetic water enriched in N2O came in contact with the atmosphere. During summer stratification N2O emissions to the atmosphere decreased to ∼4 µmol m−2 d−1. The winter fluxes are close to the highest reported N2O emissions from marine systems.