Cover crops are often included in BMPs for reducing soil erosion, building soil organic matter, and reducing nitrate leaching. Because of additional management requirement and uncertainties about their impact on nitrogen availability and cash crop yields, cover crops have not been widely adopted in conventional fields. This study aims to reduce these uncertainties for corn growers in Minnesota by measuring the impact of select cover crops on corn yield, nitrogen demand, and nitrogen uptake in a soybean-corn rotation in southern Minnesota. Additionally, plant available nitrogen was measured through the corn growing season to determine if yield reductions were caused by asynchrony in nitrogen mineralization and corn nitrogen demand.
In 2015 and 2016, five cover crop treatments—medium red clover (Trifolium pratense) seeded 12 lb/ac, field pennycress (Thalspi avense) 9 lb/ac, cereal rye (Secale cereal) 150 lb/ac, cereal rye plus radish (Raphanus sativus) 75 lb/ac and 5 lb/ac, and control—were established following soybeans at three Minnesota locations. In spring 2016 and 2017, cover crops were terminated, corn was planted, and 6 nitrogen rates were applied, ranging from 0 to 250 lb/ac. Corn yield was measured and plateau nitrogen rates determined for each cover crop treatment. Additionally, plant available nitrogen in soil was tracked through the growing season using two methods: KCl extractable nitrogen in 1 ft cores; and plant available nitrogen supplied over time on Plant Root Simulator (PRS) resin probes.
Cover crops varied in the amount of biomass produced and nitrogen taken up, which impacted the residual soil nitrate-N present prior to corn growth. Cover crops had a significant impact on cumulative nitrate-N adsorbed by PRS probes only in 2016. Cumulative nitrate-N release was greatest in medium red clover and control plots, and lowest in cereal rye and cereal rye plus radish plots; pennycress plots varied somewhat by location. These rankings remained fairly consistent throughout the growing season, suggesting that N accumulated in cover crop biomass was lost or remained immobilized. Plateau values for corn grain response to N varied by year and location. Plateau N rates were consistently lower for control and medium red clover than for cereal rye and cereal rye plus radish. In 2016, certain treatments (varying by location) failed to reach a plateau at the maximum applied rate of 250 lb/ac. These results suggest that although non-legume cover crops can reduce the amount of leaching-susceptible soil nitrogen, farmers should not expect to recover this nitrogen during the growing season. Additionally, no nitrogen benefit (compared to control) was shown after medium red clover (a legume), likely due to poor growth especially in 2017.