Application of the N-15 technique to determine the contributions of nitrification and denitrification to the flux of nitrous oxide from aerated pig slurry

In order to elucidate and quantify nitrogen transformations occurring durin g aerobic treatment of pig slurry, two laboratory experiments were carried out with contrasting levels of aeration, high level (experiment 1) and low level (experiment 2) of aeration. During these experiments, after reaching steady-state conditions, a single pulse of NO3--N-15 tracer was added direc tly to the reactor. When nitrification conditions were prevailing (experime nt 1), no transformation of added NO3--N-15 occurred (98.7% N-15-recovery a s nitrates in the liquid slurry). Moreover, the unlabelled nitrous oxide(N2 O) measured (7.4% of the total nitrogen content of the raw slurry) strongly demonstrated that the source of N2O emitted was nitrification. During simu ltaneous nitrification and denitrification conditions (experiment 2), the a dded NO3--N-15 was found in gaseous nitrogen forms (N2O-N-15:27%; N-2-N-15: 18%) and in the liquid (TN-N-15:54%). From this result. it was evident that N2O was mainly emitted by denitrification process. Using the decrease of t he isotopic excess of the NO3--N-15 we calculated that 92% of NO2--N was di rectly denitrified (as gaseous forms) without any previous oxidation to nit rate. (C) 2001 Elsevier Science Ltd. All rights reserved.