Nitrous oxide emission from urine-treated soil as influenced by urine composition and soil physical conditions

Résumé

Urine patches from cattle and sheep on pastures represent considerable, highly localized N applications. Subsequent nitrification and denitrification of the nitrogenous compounds may result in high nitrous oxide (N2O) emissions. Not much is known about the extent of these emissions, or about possible mitigation options. The aims of this study were to experimentally quantify the effects of urine composition, dung addition, compaction and soil moisture on N2O emissions from urine patches. For an incubation study at 16 °C, soil was collected from a typic Endoaquoll, and N2O production was monitored during a 103-day period. Emissions for the whole period averaged 0.3 and 0.9% of the applied urine-N for dry and moist soil, respectively. When compacted or when dung was added, emissions from moist soils increased to 4.9 and 7.9%, respectively. Both addition of dung and soil compaction resulted in a delay of the peak N2O emission of approximately 10-15 days. No significant effect of amount of urine-N on emission percentages was detected. Changing the volume of urine with equal amounts of urine-N resulted in highly significant effects, peaking with an emission of 2.3% at a water-filled pore space (WFPS) of 78%. When the soil was water-saturated, N2O production was delayed until evaporation had decreased moisture contents. We concluded that denitrification was the main N2O forming process in the incubation study. Emission factors for urine reported in the literature do not generally include the potentially considerable effects of compaction or combination with dung. We conclude that realistic emission factors should take into account such an effect, together with estimates for the occurrence of camping areas in pastures. From our results, the best mitigation strategies appear to be increasing the volume of urine through feed additives, and avoiding compaction and promoting more homogeneous application of N through a lower cattle stocking rate. Also, research efforts may be targeted at management practices to avoid camping areas in pastures.