Experimental determinations of isotopic fractionation factors associated with N2O production and reduction during denitrification in soils

Quantifying denitrification in arable soils is crucial in predicting nitrogen fertiliser losses and N2O emissions. Stable isotopologue analyses of emitted N2O (delta N-15, delta O-18 and SP = N-15 site preference within the linear N2O molecule) may help to distinguish production pathways and to quantify N2O reduction to N-2. However, such interpretations are often ambiguous due to insufficient knowledge on isotopic fractionation mechanisms. Here we present a complex experimental approach to determine the net fractionation factors (eta) associated with denitrification. This determination is based on three laboratory experiments differing in their experimental set-up and soil properties. Static and dynamic incubation techniques were compared. All available methods for independent determination of N2O reduction contribution were used, namely, N-2-free atmosphere incubation, acetylene inhibition technique and N-15 gas-flux method. For N2O production: (i) the determined difference in delta O-18 between soil water and produced N2O vary from +18 parts per thousand to +42 parts per thousand and show very strict negative correlation with soil water saturation; (ii) the determined eta N-15 of N2O production vary from -55 parts per thousand to -38 parts per thousand and the fractionation decreases with decreasing substrate availability; (iii) the determined SP of produced N2O vary from -3 parts per thousand to +9 parts per thousand. For N2O reduction: (i) the determined eta O-18 and eta N-15 of N2O reduction vary in very wide ranges from -18 parts per thousand to +4 parts per thousand and from -11 parts per thousand to +12 parts per thousand, respectively, and depend largely on the differences in experimental setups; whereas (ii) the determined eta SP of N2O reduction shows a very consistent value with all previous studies and varies in a rather narrow range from -2 parts per thousand to -8 parts per thousand. It can be concluded that eta values of N2O production determined during laboratory incubations yield only roughly estimates for respective values expectable under field study conditions. eta O-18 and eta N-15 associated with N2O reduction may vary largely, probably depending on spatial and temporal coincidence of N2O production and reduction, and are hence not yet predictable for natural conditions. However, the eta SP of N2O reduction appeared to be relatively robust and a most probable value of about -5 parts per thousand can be used to constrain N2O reduction based on SP of soil emitted N2O. (C) 2014 Elsevier Ltd. All rights reserved.