This Issue describes how agriculture provides important ecosystem services in the forms of food and fiber, but can also convey many disservices to agroecosystems themselves and to the ecosystems affected by agricultural practices. In particular, agricultural activities contribute substantial amounts of greenhouse gases, including more methane and nitrous oxide than any other human activity. For example, Duxbury (1994) estimated that agriculture contributes 25%, 65% and 90% of all anthropogenic emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), respectively.