N2O source partitioning in soils using (15)N site preference values corrected for the N2O reduction effect.

Abstract

The aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP-based N2O source partitioning. The N2O SP values (n = 431) were derived from six soil incubation studies in N2-free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N2 were compensated for using three different approaches: a closed-system model, an open-system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end-member isotopic mass balance. The average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed-system model, resulting in -2.6 ‰ (±9.5), while the open-system model and the dynamic apparent NIE model gave average SP0 values of 2.9 ‰ (±6.3) and 1.7 ‰ (±6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (±21.0) according to the closed-system model, while the open-system model and the dynamic apparent NIE function resulted in values of 31.0% (±14.0) and 28.3% (±14.0), respectively. Using a closed-system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account.