Greenhouse gas emissions from upland crops are difficult to abate because they are dominated by nitrous oxide (N2O) production from soil processes. Using a review of peer-reviewed literature, we show that annual rotation of corn with soybean leads to a 50% reduction in 2-year emissions compared to continuous corn. Nevertheless, emissions from the corn-soybean rotation remain substantial and, despite optimum N fertilizer management in corn the corn phase of the rotation, approximately 40% of the emissions occur during the soybean phase the rotation. Using cropping systems models, we developed a strategy that combines cover crop management and earlier planting of longer growth soybean genetics to reduce emissions from soybean production by 33%. Earlier planting and cover crops reduce fallow periods that are critical sources of emissions. These practices, which complement N fertilizer management in maize, are widely accessible and represent an immediate opportunity to reduce emissions from soybean production. We recommend three key steps towards decarbonizing US crop production: 1) rotate corn with soybean; 2) optimize N fertilizer in corn; 3) reduce fallows in the soybean phase of the rotation.

This project demonstrated that rotation of corn with soybean is the most effective opportunity to reduce the carbon intensity of corn production. Rotating corn with soybean reduces the carbon intensity score of corn production by 50%. This project also explored opportunities to reduce the carbon intensity score of soybean production; earlier planting of longer maturity soybeans and cover crops can make a 33% reduction in the carbon intensity score of soybeans by increasing yield and reducing emissions of nitrous oxide and nitrate leaching. Specifically, we simulated the effects of moving soybean planting dates from county average to the 10th percentile of corn planting date or the earliest insurable soybean planting date if the 10th percentile of corn planting date preceded the earliest insurable soybean planting date. We also simulated planting of winter cereal rye cover crop after only the corn phase of the rotation and terminating the cover crop after soybean planting. Recently, insurance dates for soybean planting have been moved earlier based solely on the physiology of soybean growth. Our work emphasizes that the environmental benefits of earlier planting should also be considered when setting insurance dates for soybean planting.