Emissions of N 2 O from Alfalfa and Soybean Crops in Eastern Canada

There is considerable uncertainty relative to the emissions of N 2 O from legume crops. A study was initiated to quantify N 2 O 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 N 2 O 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 N 2 O emissions ( F N2O ) were measured 47 times during the 2001 and 2002 growing seasons. Soil water, NH 4 –N, NO 3 –N, and N 2 O contents, and soil temperature were also determined to explain the variation in gas fluxes. Emissions of N 2 O were small under the grass where very low soil mineral N content probably limited denitrification and N 2 O production. Soil mineral N contents under legumes were up to 10 times greater than under timothy. However, soil mineral N contents and F N2O were not closely related, thus suggesting that the soil mineral N pool alone was a poor indicator of the intensity of N 2 O production processes. Higher F N2O 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 N 2 O emissions associated with soybean and alfalfa production in eastern Canada.