Nitrous oxide, carbon dioxide and methane emissions from irrigated cropping systems as influenced by legumes, manure and fertilizer

Irrigated land in southern Alberta is intensively managed, producing high yields but also requiring higher inputs, notably of nitrogen (N), than adjacent rainfed lands. The higher N inputs, combined with enhanced soil moisture, might stimulate nitrous oxide (N2O) emissions, but the influence of management on these emissions has not been widely studied. Our objective was to assess soil N2O emissions, along with those of carbon dioxide (CO2) and of methane (CH4), from irrigated cropping systems as influenced by source of N. We used a chamber technique to measure year-round emissions for 3 yr in long-term irrigated crop rotations receiving N as legume crop residues, non-legume crop residues, livestock manure or ammonium nitrate fertilizer. Unlike CO2 fluxes, which peaked during the growing season, those of N2O showed no consistent seasonal trends; emissions occurred sporadically in bursts throughout the year. Depending on management practices, 0.4 to 4.0 kg N2O-N ha(-1) yr(-1) was emitted to the atmosphere. The amount of N2O emitted from the alfalfa system, averaged over all manure and fertilizer N amendments, was more than twofold that emitted from the corn system. The proportions of fertilizer-N released as N2O were 0.95% for the alfalfa system and 1.30% for the corn system. After livestock manure or legume residues were incorporated, soil CO2 and N2O emissions appeared to be intertwined, but during the early spring N2O emissions were decoupled from CO2. Furthermore, N2O emissions were highly variable in space; at three of 54 chambers, N2O fluxes were consistently 12 to 55 times greater than those for other chambers in the same treatment. Such complexity conceals the underlying processes of net N2O production and transport to the soil surface.