Application of the 15N 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 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 steady-state conditions, a single pulse of NO3−-15N tracer was added directly to the reactor. When nitrification conditions were prevailing (experiment 1), no transformation of added NO3−-15N occurred (98.7% 15N-recovery 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−-15N was found in gaseous nitrogen forms (N2O-15N : 27%; N2-15N : 18%) and in the liquid (TN-15N : 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−-15N we calculated that 92% of NO2−-N was directly denitrified (as gaseous forms) without any previous oxidation to nitrate.