The effect of nitrogen concentration in synthetic cattle urine on nitrous oxide emissions

This study determined a relationship between N concentration in synthetic cattle urine and the nitrous oxide (N2O) emission factor (EF3; N2O-N emitted as % of urine N applied) under field conditions. The results will improve the assessment of the efficacy of N2O mitigation options that affect urinary N concentration and deposition rates. Field studies on two free draining soils and one poorly draining soil were conducted using synthetic urine with N concentrations of 0, 2, 4, 6, 8, 10 and 12 g N L−1 (equivalent to application rates of 0 to 1200 kg N ha−1). The study on the poorly draining soil also included a urine N concentration of 14 g N L−1 (1400 kg N ha−1). N2O emissions were measured for up to 18 weeks after urine application using a static chamber methodology. The EF3 values ranged from 0.03 to 0.34% of urine N applied on the free draining soils and from 0.5 to 0.9% on the poorly draining soil. There was a statistically significant (p < 0.05) trend of increasing EF3 with increasing N application rate on the free-draining soils, with EF3 increasing 3- to 4-fold between the lowest and highest N rate. No such trend was found on the poorly draining soil, indicating that for this soil the N2O emission factor was independent of the N application rate. These results suggest that the urine N concentration only affected the N2O emission factor when values were very low. We therefore conclude that mitigation strategies which reduce the urine N concentration in individual urination events, but not the overall total amount of urine N excreted over the whole farm system, may have limited impact on reducing total direct N2O emissions.