Gaseous Emissions of Nitrogen from Grazed Pastures: Processes, Measurements and Modelling, Environmental Implications, and Mitigation

"One feature that has become increasingly clear is that large gaseous and leaching losses are liable to occur from the livestock excreta returned to the soil by grazing animals. Urine patches in particular contain extremely high but localized concentrations of plant-available N. These concentrations greatly exceed the uptake capacity of the grass, and urine patches are therefore especially susceptible to ammonia volatilization, denitrification and leaching" (Whitehead, 1995). Grazing-managed pasture is a major system of livestock production in many countries. Grazed pastures receive large inputs of nitrogen (N), derived from biological fixation of atmospheric N, through the addition of manures and fertilizers, and the deposition of animal excreta. However, only a small proportion of the N (<15%) consumed by grazing animals is converted to milk or live weight gain, the reminder is excreted. Loss of N occurs mainly through ammonia (NH₃) volatilization, release of gaseous N such as nitric oxide (NO) and nitrous oxide (N₂O) through biological denitrification, and nitrate (NO₃^-) leaching, which has both economical and environmental implications. Nitrogen is an important plant nutrient and its loss affects both the quality and quantity of feed, thereby leading to poor animal production. Recently there have been increasing concerns about the environmental impacts of N loss through leaching (i.e., methaemoglobinaemia) and gaseous emission (i.e., greenhouse gas). In this chapter, the various sources of N input to grazed pasture are discussed in relation to the dynamics of N, the measurement and modelling gaseous emissions of N, and the implications of gaseous emission in relation to economic loss and environmental degradation. The dynamics of N transformations in soil-plant system with particular emphasis on the biochemistry of gaseous emission, and the measurement techniques and the use of process-based models to predict gaseous emissions are discussed. The practical implications of gaseous emission are discussed in relation to acid rain and climate change (i.e., the Kyoto Protocol). Grazing and farm management practices to mitigate gaseous emissions are highlighted.