Nitrous oxide emissions from cattle urine deposited onto soil supporting a winter forage kale crop

Wintering cows on forage crops leads to urine being excreted onto wet, compacted soils, which can lead to significant gaseous emissions of nitrous oxide (N2O). A field trial was conducted to determine the N2O emission factor (EF3) for dairy cows wintered on a kale forage crop in Otago, New Zealand. Urine was collected from non-lactating dairy cows on a forage kale diet and applied at 550 kg N ha-1 to compacted and non-compacted soil in a kale field. The effect of compaction increased soil bulk density and decreased total porosity and macroporosity, thereby increasing anaerobic conditions compared to non-compacted soil. This increased cumulative N2O emissions from urine and non-urine treated soil more than two-fold. Cumulative N2O losses over 4 months were 3.8 and 1.4 kg N2O-N ha-1 from urine applied to a compacted and non-compacted poorly drained soil. The corresponding EF3 value for urine applied to compacted soil was approximately 2.5 times greater than for urine applied to non-compacted soil, at 0.39% and 0.16%, respectively (P < 0.05). Combining our results with previous studies where brassica-fed livestock urine was applied to soils supporting a forage brassica crop suggested urine EF3 derived from grazing of brassica crops was significantly related to soil water content (P < 0.05).