Isotopologue fractionation during microbial reduction of N2O within soil mesocosms as a function of water-filled pore space

Isotopologue analyses of N2O within soil mesocosm experiments were used to evaluate the influence of N2O reduction on isotope fractionation. We investigated fractionation during N2O reduction at 60%, 80% and 100% water-filled pore space (WFPS) and found net isotope effects (NIE) for δ15N of 4.2–7.8‰, δ18O of 12.5–19.1‰, δ15Nα of 6.4–9.7‰ and δ15Nβ of 2.0–5.9‰. Consequently, N2O reduction has a marked affect on isotopologue values and the importance of this process in flux chamber studies should not be ignored. With the exception of SP (the difference between the δ15N of the central, α, and terminal, β, atoms) inverse relationships between the NIE, reaction rate and reaction rate constant and WFPS were observed. Isotopic discrimination in SP during N2O reduction was small and the average NIE for the treatments varied between 2.9‰ and 4.5‰. A strong correlation was evident between δ18O vs. δ15N and δ18O vs. δ15Nα during reduction with slopes of 2.6 and 1.9, respectively, which contrasts from a slope of <1 commonly observed for mixing between soil-derived and atmospheric N2O in flux chambers.