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 delta N-15 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 N2O-N 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 parts per thousand in the morning to a minimum value of -0.1 parts per thousand in the afternoon. Based on SP values reported for bacterial denitrification (-5 to 0 parts per thousand; Toyoda et al., 2005: Sutka et al., 2006), hydroxylamine oxidation (nitrification) and fungal denitrification (33-37 parts per thousand; Sutka et al., 2006) we found that production attributable to bacterial denitrification increased from between 52.9 and 60.9% in the morning to between 87.5 and 100% in the afternoon. Further, we observed diurnal variation in flux and SP that is consistent with increased production from bacterial denitrification associated with temperature-driven increases in respiration. (C) 2009 Elsevier Ltd. All rights reserved.