Disaggregating nitrous oxide emission factors for ruminant urine and dung deposited onto pastoral soils

The largest source of nitrous oxide (N2O) in New Zealand is excreta deposited by farmed, grazing ruminants. A country-specific excreta N2O emission factor (EF3PRP) of 1% is used to estimate these emissions. The aim of this study was to assess if disaggregation of EF3PRP by excreta type (urine vs. dung) is warranted. Cattle urine and dung was applied separately onto small plots of six different pastoral soils (well- and poorly-drained) across three climatic regions in New Zealand during two seasons (autumn and spring). Sheep dung was applied to other plots as a contrasting excreta type. N2O emissions were measured using static chambers. The average EF3PRP for cattle urine was 0.29% of the N applied, which was significantly greater than for cattle dung (0.04%) and sheep dung (0.01%). Drainage class did not always relate to soil water content and associated N2O emissions in the top soil, suggesting care is required when calculating national inventories based on soil drainage classes and their associated EF3PRP values. These results support the disaggregation of New Zealand's EF3PRP into urine and dung emission factors. However, further disaggregation of dung EF3PRP by animal type (cattle vs. sheep) is not justified, but further research is warranted.

► We determined separate N2O emission factors (EF3) for urine and dung deposited onto grazed pasture. ► EF3 values were as follows: cattle urine (0.29%), cattle dung (0.04%) and sheep dung (0.01%). ► Significant differences between dung EF3 and urine EF3 support disaggregation of these two sources. ► Our results do not support further disaggregation of dung EF3 into animal type (sheep vs. cattle). ► Intensively grazed sites showed a strong positive relationship between urine EF3 and initial soil moisture.