We document an enrichment of both the Δ18OAtm.O2 and Δ15NAtm.N2 values of soil-derived N2O collected from landfill cover soils relative to tropospheric N2O. The isotopic values of N2O vary from -5.1? to +19.4? and from +19.0? to +33.5? for Δ15NAtm.N2 and Δ18OAtm.O2, respectively. A tight linear correlation for Δ18OAtm.O2 versus Δ15NAtm.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 15N and 18O relative to tropospheric N2O.