Pii: S0043-1354(00)00561-3

In order to elucidate and quantify nitrogen transformations occurring during 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 steadystate conditions, a single pulse of NO3 -N tracer was added directly to the reactor. When nitrification conditions were prevailing (experiment 1), no transformation of added NO3 -N occurred (98.7% Nrecovery as nitrates in the liquid slurry). Moreover, the unlabelled nitrous oxide (N2O) measured (7.4% of the total nitrogen content of the raw slurry) strongly demonstrated that the source of N2O emitted was nitrification. During simultaneous nitrification and denitrification conditions (experiment 2), the added NO3 -N was found in gaseous nitrogen forms (N2ON : 27%; N2N : 18%) and in the liquid (TN-N : 54%). From this result, it was evident that N2O was mainly emitted by denitrification process. Using the decrease of the isotopic excess of the NO3 -N we calculated that 92% of NO2 -N was directly denitrified (as gaseous forms) without any previous oxidation to nitrate. # 2001 Elsevier Science Ltd. All rights reserved Key words}nitrous oxide, nitrification, denitrification, N, pig slurry