Isotopologue signatures of N₂O produced by denitrification in soils

Abstract

We investigated oxygen and site‐specific nitrogen isotope effects of N 2 O produced in the NO 3 − ‐to‐N 2 O step of denitrification. Arable sand and silt loam soils with varying NO 3 − availability were incubated under N 2 /C 2 H 2 atmosphere in order to establish anaerobic conditions and to block N 2 O reduction. Dual isotope and isotopomer ratios of emitted N 2 O were determined by analysis of δ 18 O, average δ 15 N ( δ 15 N bulk ) and 15 N site preference (SP is equal to difference in δ 15 N between the central and peripheral N positions of the asymmetric N 2 O molecule). The average N enrichment factor of the NO 3 − ‐to‐N 2 O step ranged from −47.9 to −53.6‰, which is between the reported ranges of enrichment factors of nitrification and denitrification. SP varied with time, and mean values were between 3.1 and 8.9‰, which is higher compared to SP reported from pure cultures of denitrifiers but lower compared to nitrifiers and fungal denitrifiers. This shows that SP of N 2 O produced in soils might be seen as a semiquantitative indicator for the different pathways of N 2 O production but not specific enough to quantify the relative contribution of denitrification to the total N 2 O flux in systems where several source processes are important. δ 18 O of N 2 O from all treatments was less variable compared to δ 18 O of soil NO 3 − , indicating that there was a relatively large O exchange with water during N 2 O formation.