Diverse rotations impact microbial processes, seasonality and overall nitrous oxide emissions from soils

Many studies haveexamined soil‐borne nitrous oxide (N 2 O) emissions from crops, but little effort has gone into determining the N 2 O emissions from each phase of a crop rotation. A 4‐yr study on a long‐term field experiment compared growing season N 2 O emissions from continuous corn (CC; Zea mays L.) and a 4‐yr crop rotation involving corn (RC), oat (Avena Sativa L .) underseeded to alfalfa ( Medicago sativa  L.) (RO), and 2 yr of alfalfa (RA1, RA2). Molecular microbial biomass (DNA yield), as well as N‐cycling functioning genes (mineralization, nitrification, and denitrification), were also evaluated. Although 4‐yr cumulative N 2 O emissions from RC (9.25 kg N ha –1 ) were significantly greater than from CC (7.94 kg N ha –1 ), cumulative emissions from the entire rotation were 54% lower (3.69 kg N ha –1 ) than CC because of low emissions from RO (3.1 kg N ha –1 ), RA1, and RA2 (1.11–1.27 kg N ha –1 ). Years that had substantial early‐season precipitation combined with high soil inorganic N from alfalfa plow‐down contributed to elevated N 2 O emissions from RC. Improved soil conditions and fertility under rotation increased RC grain yields by 35% (9.45 Mg ha –1 ) compared with CC (7.01 Mg ha –1 ). Microbial biomass was 73% greater in RC compared with CC. Nitrogen mineralization genes were 19% greater in RC but they were not correlated to N 2 O emissions, whereas bacterial nitrifiers were positively correlated. Denitrification was likely responsible for N 2 O emissions under CC, while nitrifier‐denitrification appeared to be the primary pathway under RC. The N 2 O emissions and microbial processes from all phases of a rotation should be considered for environmental modeling and policy decisions.