The impact of agriculture and other land uses on emissions of methane and nitrous and nitric oxides

Abstract

The growth in world population, and with it the growth of the demand for food and feed, has resulted in conversion of natural ecosystems to agricultural land, and a major intensification of the management systems practiced in agriculture. Increasing affluence in some developing countries has added to the pressures, through the accompanying trend towards a more meat-and milk-based diet. These changes have resulted in a large increase in ruminants and other livestock over the last few decades, and the trend is expected to continue. The direct methane emissions from ruminants have therefore increased, and so have those from animal manures generally. Taken together with CH 4 emissions from rice cultivation and landfills, the total from activities associated with land use is 25–45% of all anthropogenic CH 4 emissions. Reductions in emissions from rice, from manure management and from landfills are feasible, but only a fall in ruminant populations is likely to make a real difference in this particular source. The additional reactive nitrogen provided by the increased manure supply, coupled with the vastly expanded production of synthetic nitrogen fertilizers over the last 50 years, has increased the emissions of nitrous and nitric oxides, and agriculture is now the major anthropogenic source of this gas. Land use change can increase emissions for a period. Practices which improve overall N use efficiency in agriculture are capable of reducing emissions per unit of N used significantly, but the projected increases in fertilizer use in coming decades suggests that the overall trend will continue upwards for some time.