Surface and subsurface N2O losses from dairy cropping systems

Dairy rotations rely on corn silage, which is estimated to have significant nitrous oxide (N2O) emissions. This study examined whether including legumes within rotations can reduce N2O emissions from the soil surface and dissolved in tile-drainage water. Emissions of N2O were measured from the soil surface and in tile drainage. Cropping systems were: corn–corn (CC), corn + cover crop-corn (C + cc), soybean–corn (SC) and alfalfa–alfalfa (AA) on a clay soil. Liquid dairy manure provided 2-year total N inversely related to legume cropping: 310 (CC), 280 (C + cc), 110 (SC), 50 kg N ha−1 (AA). Losses of N2O via tile drainage were 0.1–0.3% of total emissions. Ratios of N2O-N to NO3−-N in drainage were at least 63% lower than the IPCC default value (0.0075). Reductions of N2O emissions were only observed from established alfalfa in year 2. Compared to the SC treatment, which had the highest emissions in year 2, the AA treatment had 62% lower surface N2O and 88% lower dissolved N2O flux. Alfalfa had low yield in the first year, which led to high yield-scaled N2O emissions; thus, alfalfa may need to be grown 4 years to achieve a similar average yield scaled emission factor as CC. Silage corn had consistently high yield, averaging 317 kg ha−1 yr−1 for N yield, which was 36% higher than AA. As a result, CC had the lowest N2O emissions scaled by N-yield over the 2 years, averaging 2.6% of N-yield, which was 59% lower than AA on average.