There is considerable uncertainty relative to the emissions of N2O from legume crops. A study was initiated to quantify N2O fluxes from soils cropped to alfalfa (Medicago sativa L.) and soybean (Glycine max L.), and to improve our understanding of soil and climatic factors controlling N2O emissions from these crops. Measurements were made on three soils cropped to alfalfa, soybean, or timothy (Phleum pratense L.), a perennial grass used as a control. In situ soil-surface N2O emissions (FN2O) were measured 47 times during the 2001 and 2002 growing seasons. Soil water, NH4-N, NO3-N, and N2O contents, and soil temperature were also determined to explain the variation in gas fluxes. Emissions of N2O were small under the grass where very low soil mineral N content probably limited denitrification and N2O production. Soil mineral N contents under legumes were up to 10 times greater than under timothy. However, soil mineral N contents and FN2O were not closely related, thus suggesting that the soil mineral N pool alone was a poor indicator of the intensity of N2O production processes. Higher FN2O were measured under legume than under timothy in only 6 out of 10 field comparisons (site-years). Moreover, the emissions associated with alfalfa (0.67-1.45 kg N ha-1) and soybean (0.46-3.08 kg N ha-1) production were smaller than those predicted using the emission coefficient proposed for the national inventory of greenhouse gases (alfalfa = 1.60-5.21 kg N ha-1; soybean = 2.76-4.97 kg N ha-1). We conclude that the use of the current emission coefficient may overestimate the N2O emissions associated with soybean and alfalfa production in eastern Canada.
