The Effect of Including Legumes in Dairy Crop Rotations on Nitrous Oxide Emissions

This study examined whether substituting legumes for corn in dairy crop rotations could reduce N2O emissions. This was done by measuring the surface N2O emissions of four different two-year crop rotation treatments: corn-corn, corn (+cover crop)-corn, soybean-corn, and alfalfa-alfalfa over an 18 month period (May 1, 2014-Oct. 31, 2015) on sandy loam and clay soils. Emissions of dissolved N2O in subsurface tile drains in the clay soil were also measured for eight months (March 20, 2015-October 31, 2015) to determine the contribution of dissolved N2O to total N2O emissions as well as the effect of crop rotations on dissolved N2O emissions. In the sandy loam soil, alfalfa and soybeans had the lowest growing season N2O emissions (9.6 and 10.6 g ha-1 d-1, respectively), and alfalfa had the lowest annual emissions over two years at 3.0 and 3.6 kg N2O-N ha-1 yr-1. In the clay soil, the alfalfa had 3-4 times lower soil-surface N2O emissions than the other treatments in the second growing season only. Non-growing season emissions accounted for 56% of annual N2O emissions. Subsurface emissions accounted for 0.4% of N2O emissions over an eight month period and followed the same treatment effects as the clay soil surface emissions. This study suggests it is possible to reduce N2O emissions by substituting legumes for corn, especially when combined with the appropriate soil type; however, reductions can be negated by management practices that increase non-growing season emissions.