Improved isotopic model based on 15N tracing and Rayleigh-type isotope fractionation for simulating differential sources of N2O 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 effect 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, N2 and CO2 over a 12-day incubation period. To determine the source of N2O emissions those were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2O and the 15N-tracing technique.
RESULTS: The spatial dispersal of N and C significantly affected the quantity, but not timing of gas fluxes. Cumulative emissions are larger for 3c than 1c. The 15N-enrichment analysis shows that initially ~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 1c. The Rayleigh type model applied to N2O isotopocules data (δ15Nbulk-N2O) shows poor agreement with the measurements for the original 1-pool model for 1c; the 2-pool models gives better results when using a third order polynomial equation. In contrast, in 3c little difference is observed between both modelling approaches.
CONCLUSIONS: The importance of N2O emissions from different N-pools in soil for the interpretation of N2O isotopocule data was demonstrated using a Rayleigh type model. Earlier statements concerning exponential increase of native soil nitrate (NO3-) pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N-pool sizes.