Improved isotopic model based on 15 N tracing and Rayleigh-type isotope fractionation for simulating differential sources of N 2 O emissions in a clay grassland soil.

Rationale Isotopic signatures of N2O can help distinguish between two sources (fertiliser N or endogenous soil N) of N2O emissions. The contribution of each source to N2O emissions after N-application is difficult to determine. Here, isotopologue signatures of emitted N2O are used in an improved isotopic model based on Rayleigh-type equations. Methods The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N2O, N-2 and CO2 over a 12-day incubation period. To determine the source of N2O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2O and those from the N-15-tracing technique. Results The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The N-15-enrichment analysis shows that initially similar to 70% of the emitted N2O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N-pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh-type model applied to N2O isotopocules data (delta N-15(bulk)-N2O values) shows poor agreement with the measurements for the original one-pool model for treatment 1c; the two-pool models gives better results when using a third-order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches. Conclusions The importance of N2O emissions from different N-pools in soil for the interpretation of N2O isotopocules data was demonstrated using a Rayleigh-type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N-pool sizes.