Stable isotope discrimination during soil denitrification: Production and consumption of nitrous oxide

Measuring the stable isotope composition of nitrous oxide (N 2 O) evolved from soil could improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N 2 O formation in soil. However, interpretation of the isotopic data in N 2 O is complicated by the lack of knowledge of fractionation parameters by different microbial processes responsible for N 2 O production and consumption. Here we report isotopic enrichment for both nitrogen and oxygen isotopes in two stages of denitrification, N 2 O production and N 2 O reduction. We found that during both N 2 O production and reduction, enrichments were higher for oxygen than nitrogen. For both elements, enrichments were larger for N 2 O production stage than for N 2 O reduction. During gross N 2 O production, the ratio of δ 18 O‐to‐ δ 15 N differed between soils, ranging from 1.6 to 2.7. By contrast, during N 2 O reduction, we observed a constant ratio of δ 18 O‐to‐ δ 15 N with a value near 2.5. If general, this ratio could be used to estimate the proportion of N 2 O being reduced in the soil before escaping into the atmosphere. Because N 2 O‐reductase enriches N 2 O in both isotopes, the global reduction of N 2 O consumption by soil may contribute to the globally observed isotopic depletion of atmospheric N 2 O.