Nitrous oxide and carbon dioxide emissions from monoculture and rotational cropping of corn, soybean and winter wheat

It is well established that nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) emissions from agricultural land are influenced by the type of crop grown, the form and amount of nitrogen (N) applied, and the soil and climatic conditions under which the crop is grown. Crop rotation adds another dimension that is often overlooked, however, as the crop residue being decomposed and supplying soluble carbon to soil biota is usually from a different crop than the crop that is currently growing. Hence, the objective of this study was to compare the influence of both the crop grown and the residues from the preceding crop on N 2 O and CO 2 emissions from soil. In particular, N 2 O and CO 2 emissions from monoculture cropping of corn, soybean and winter wheat were compared with 2 -yr and 3-yr crop rotations (corn-soybean or corn-soybean-winter wheat). Each phase of the rotation was measured each year. Averaged over three growing seasons (from April to October), annual N 2 O emissions were about 3.1 to 5.1 times greater in monoculture corn (2.62 kg N ha -1 ) compared with either monoculture soybean (0.84 kg N ha -1 ) or monoculture winter wheat (0.51 kg N ha -1 ). This was due in part to the higher inorganic N levels in the soil resulting from the higher N application rate with corn (170 kg N ha -1 ) than winter wheat (83 kg N ha -1 ) or soybean (no N applied). Further, the previous crop also influenced the extent of N 2 O emissions in the current crop year. When corn followed corn, the average N 2 O emissions (2.62 kg N ha -1 ) were about twice as high as when corn followed soybean (1.34 kg N ha -1 ) and about 60% greater than when corn followed winter wheat (1.64 kg N ha -1 ). Monoculture winter wheat had about 45% greater CO 2 emissions than monoculture corn or 51% greater emissions than monoculture soybean. In the corn phase, CO 2 emissions were greater when the previous crop was winter wheat (5.03 t C ha -1 ) than when it was soybean (4.20 t C ha -1 ) or corn (3.91 t C ha -1 ). Hence, N 2 O and CO 2 emissions from agricultural fields are influenced by both the current crop and the previous crop, and this should be accounted for in both estimates and forecasts of the emissions of these important greenhouse gases. Key words: Denitrification, soil respiration, rotation, crop residue