Isotopologue data reveal bacterial denitrification as the primary source of N2O during a high flux event following cultivation of a native temperate grassland

The source of N2O in terrestrial ecosystems has long been debated. Both nitrification and denitrification produce N2O but their relative importance remains uncertain. Here we apply site preference, SP (the difference in d15N between the central and outer N atom in N2O), to estimate the relative importance of bacterial denitrification (including nitrifier denitrification) to total N2O production from soil. We measured SP over a diurnal cycle following the third year of tillage of a previously uncultivated grassland soil at the Kellogg Biological Station (KBS) in southwestern Michigan. Fluxes of N2O in our study ranged between 7.8 and 12.1 g N2OeN ha 1 d 1 and were approximately 3 and 10 times greater than fluxes observed in managed agricultural and successional fields, respectively, at KBS. Consequently, our study captured a period of high flux resulting from the cultivation of a historically never-tilled soil. Concentration weighted SP values decreased from 12.9& in the morning to a minimum value of 0.1& in the