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.