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

Many studies haveexamined soil‐borne nitrous oxide (N2O) emissions from crops, but little effort has gone into determining the N2O emissions from each phase of a crop rotation. A 4‐yr study on a long‐term field experiment compared growing season N2O 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 N2O 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 N2O 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 N2O emissions, whereas bacterial nitrifiers were positively correlated. Denitrification was likely responsible for N2O emissions under CC, while nitrifier‐denitrification appeared to be the primary pathway under RC. The N2O emissions and microbial processes from all phases of a rotation should be considered for environmental modeling and policy decisions. Core Ideas Nitrifier denitrification was likely the primary N2O pathway under rotation corn (RC) treatments. N2O emissions under continuous corn (CC) appeared to be controlled primarily by denitrification. N2O emissions were stimulated by early‐season rainfall in RC and mid‐season rainfall in CC. The average N2O emissions over the 4‐yr rotation were 54% lower than CC. N2O emissions from RC were 16.5% greater than from CC.