Advancing the planting date and implementing cover crops to reduce N2O emissions from US soybean crops

Strategies to reduce the emissions of N2O, a potent greenhouse gas mostly derived from agriculture, are generally based on nitrogen fertilizer management. However, a fraction of the agriculture-derived N2O comes from soybean, a crop that rarely receives nitrogen fertilizer. Our goal was to analyze the ability of two easily-implemented strategies for current farmers to reduce N2O emissions from soybean in US corn-soybean rotation crop systems: planting of cover crops between the corn and soybean growth and advancing the soybean planting date. This second strategy relies on the fact that the optimum agronomic seeding date of soybean is the same as corn, but it is generally seeded around 2 weeks later due to the limited availability of planter machinery, which leads farmers to first plant the crop with greater risk (i.e., corn).

We used 2 field experiments in central Iowa. The first, measured N2O emissions during 3 growing seasons in a corn-soybean rotation (including both phases of the rotation each year) with and without a winter cereal rye cover crop. The second experiment, measured N2O emissions at least once per week for 2 consecutive years in a corn-soybean rotation using 2 different seeding dates for both crops. Then, we used APSIM process-based simulation model to evaluate the effects of both strategies on grain yield and N2O emissions for a set of historical weather years (1991-2020).

Annual N2O emissions in the soybean phase of the rotation using the actual planting date were 2.01 kg N ha-1 yr-1 averaged across the 30 years. Advancing the soybean planting date equal to the corn planting date made a small reduction in annual emissions (6% reduction). But including winter cover crops in the rotation reduced annual N2O emissions in the soybean phase by 27%. Neither advancing the soybean planting date nor planting cover crops affected the corn yield.