The biogeochemical controls of the δ 15N and δ 18O of N2O produced in landfill cover soils

We document an enrichment of both the delta(18)O(Atm.O2) and delta(15)N(Atm.N2) values of soil-derived N2O collected from landfill cover soils relative to tropospheric N2O. The isotopic values of N2O vary from -5.1 parts per thousand to +19.4 parts per thousand and from +19.0 parts per thousand to +33.5 parts per thousand for delta(15)N(Atm.N2) and delta(18)O(Atm.O2), respectively. A tight linear correlation for delta(18)O(Atm.O2) versus delta(15)N(Atm.N2) is apparent, reflecting coupled microbial processes that produce N2O that may be isotopically enriched or depleted in relation to tropospheric N2O, Several explanations are provided to explain this correlation, including evidence for NH3 limitation during nitrification, which would be expected to diminish isotopic fractionation and consequently result in more enriched isotopic values of N2O. Desiccation effects on nitrification were also observed, which contribute to NH3 limitation and thus could influence the isotopic signature of N2O. Our results indicate that the N2O isotopic composition from soils may vary greatly depending on the season and soil moisture conditions and may at times be enriched in N-15 and O-18 relative to tropospheric N2O.