Laboratory incubation studies to investigate the control of isotopologue signatures of soil- emitted N2O

Isotopologue signatures of N2O such as δ18O, average δ15N (δ15Nbulk) and 15N site preference (SP = difference in δ15N between the central and peripheral N positions of the asymmetric N2O molecule) can be used to constrain the atmospheric N2O budget and to characterize N2O turnover processes. However, the use of this approach to study N2O dynamics in soils requires knowledge of isotopologue fractionation factors (ɛ) for the various partial processes involved, e.g. N2O production by nitrification or denitrification, and N2O reduction by denitrification. The aim of our study is to investigate the control of isotopologue signatures of soil-emitted N2O. Two soils were incubated in the laboratory under varying conditions in order to manipulate the partial processes of N2O turnover. ɛ of δ18O, δ15Nbulk and SP was determined in experiments, where only one of the partial processes was governing the isotopic signature of N2O in the incubation system of the respective treatment. ɛ of nitrification was determined by favoring N2O fluxes originating from NH4+ oxidation using high NH4+ levels and low soil moisture (30 to 50 % water-filled pore space). ɛ of N2O production by denitrification was obtained by anaerobic incubation of NO3- amended soils when N2O reduction was inhibited by 10 kPa acetylene. ɛ of N2O reduction was derived by (i) comparing treatments with and without inhibition of N2O reduction or (ii) by monitoring the time course of isotopic signatures of N2O applied to the headspace of NO3-- depleted anaerobic soil. To investigate the combined effect of various simultaneous processes, isotopologue signatures of N2O emitted from soils incubated under varying N-fertilizer level and moisture were measured while gross rates of the partial processes were determined independently. Results of fractionation factors and isotopologue signatures will be presented and discussed in view of previous studies and theoretical considerations.