Using Cover Crops to Capture and Recycle Deep Soil N: On-Farm Experiments.

In the mid-Atlantic USA, summer annual crops are typically growing from May-August, leaving eight months when no crop is taking up soil N. In a prior survey of 29 mid-Atlantic farms, we found on average 284 kg/ha of mineral N in the soil (0-210 cm deep) in September (after the annual crop ceases N uptake), 76% occurring deeper than 30 cm. We hypothesized that deep-rooted cover crops planted by early September could capture large amounts of N, and release this N on the soil surface following cover crop termination. We performed experiments on 25 farms across Maryland and Pennsylvania evaluating the following cover crop treatments: 1) forage radish, 2) winter cereal (e.g., cereal rye, triticale, oats), 3) cover crop mix including radish, a winter cereal, and a legume (usually Crimson clover), and 4) no-cover crop control. We measured soil N (0-210 cm deep) and cover crop N in November and April. Results were highly variable because of differences in cover crop planting dates, levels of soil fertility, and growing degrees available in fall. Cover crop N uptake ranged from 4 to 343 kg N/ha. On most farms all cover crops reduced profile nitrate in fall, and radish sometimes (but not always) significantly increased topsoil nitrate in spring. Following cover crop termination, we planted corn on the plots using no starter N fertilizer. At the V5 stage, corn following radish had significantly higher biomass than corn following no-cover crop or mixed species, which had significantly higher biomass than corn following winter cereal. Corn following radish, mixed species, and no-cover crop had higher grain yields than corn following winter cereal receiving the same amount of N fertilizer. Planting a deep-rooted cover crop immediately following annual crop senescence can help reduce future fertilizer needs and prevent N from leaching into bodies of water.