A review of emissions of methane, ammonia, and nitrous oxide from animal excreta deposition and farm effluent application in grazed pastures

The agricultural sector in New Zealand is the major contributor to ammonia (NH 3 ), nitrous oxide (N 2 O), and methane (CH 4 ) emissions to the atmosphere. These gases cause environmental degradation through their effects on soil acidification, eutrophication, and stratospheric ozone depletion. With its strong agricultural base and relatively low level of heavy industrial activity, New Zealand is unique in having a greenhouse-gas-emissions inventory dominated by the agricultural trace gases, CH 4 and N 2 O, instead of carbon dioxide which dominates in most other countries. About 96% of this anthropogenic CH 4 is emitted by ruminant animals as a byproduct during the process of enteric fermentation. Methane is also produced by anaerobic fermentation of animal manure and many other organic substrates. In pastoral soils, NH 3 and N 2 O gases are generated from N originating from dung, urine, biologically fixed N 2 , and fertiliser. The amount of these gaseous emissions depends on complex interactions between soil properties, climatic factors, and agricultural practices. In this review paper, the animal-excretal inputs and farm-effluent applications to New Zealand pastures are quantified. Data from overseas and New Zealand studies on CH 4 , NH 3 , and N 2 O emissions from excretal deposition and animal effluents, and the factors affecting these emissions, are synthesised with an aim to improve the New Zealand estimates of emissions from these sources. The practical implications of these emissions are described in relation to environmental impacts and management strategies for reducing these emissions.