Green waste compost reduces nitrous oxide emissions from feedlot manure applied to soil

Australia produces in excess of 1 million tonnes of feedlot manure (FLM) annually. Application of FLM to grain cropping and grazing soils could provide a valuable nutrient resource. However, because of high nutrient concentration, especially of N (>2%), FLM has the potential for environmental pollution, for example, N pollution to the water bodies and N2O emission to the atmosphere. Therefore, controlling N supply from FLM is essential for the judicious utilisation of FLM in the field as well as reducing N2O emission to the atmosphere. We utilised the low N concentration green waste compost (GWC, about 3 million tonnes produced annually) as a potential management tool to assess its effectiveness in regulating N release from FLM and controlling the rates of N2O emission from field application when both FLM and GWC were applied together to sorghum (Sorghum bicolor Moench) grown on a Vertisol. We measured N2O emission rates during the sorghum crop and clean fallowing over one-year period in the field. Annual soil N2O emissions were 5.0 kg N2O ha−1 from urea applied at 150 kg N ha−1, 5.1 and 5.5 kg N2O ha−1 from FLM applied at 10 and 20 t ha−1 respectively, 2.2 kg N2O ha−1 from GWC applied at 10 t ha−1, 4.3 kg N2O ha−1 from FLM and GWC applied together at 10 t ha−1 each, and 3.3 kg N2O ha−1 from the unamended soil. Thus, we found that GWC application reduced N2O emissions below those from an unamended soil while annual emission rate from FLM approached that from fertiliser N application (∼0.7% N2O emission factor). A mixture of FLM + GWC applied at 10 t ha−1 each reduced N2O emission factor by 64% (the emission factor was 0.22%), most likely by reducing the amount of mineral N in the soil because soil NH4-N and NO3-N and the rate of N2O emission were significantly correlated in this soil. Since the global warming potential of N2O is 298 times that of CO2, even a small reduction in N2O emission from GWC application has a significant and positive impact on reducing global warming.