Gaseous nitrogen losses from a grassland area used for overwintering cattle

The grassland area where cattle overwinter was identified as an important point source of N2O due to soil compaction and excrement deposition. The rates of N2O emission were mostly directly related to the intensity of animal impact. However, laboratory measurements had shown a much greater potential for N2O production from soil in a severely impacted site than indicated by field measurements, possibly due to factors affecting the mole fraction of N2O. The 15N gas-flux method was used to measure the emissions of N2O and N2 at three sites along a gradient of animal impact. Over the experimental period (72 h), the loss of NO3−-N as N gases was 60%, 12% and 3%, and the mole fraction of N2O was 0.04, 0.15 and 0.75 for the severe, moderate and control treatments, respectively. We hypothesize that soil pH, which is enhanced in impacted sites (from 5.7 up to 7.8), may control mole fraction of N2O in such a way that under alkaline conditions most of the nitrogen escapes as N2.