How Much Nitrogen Is Left in the Soil Profile after Summer Annual Crops? a Deep N Survey on Mid-Atlantic Farms.

Summer annual crops are either fertilized with large amounts of N (e.g., corn) or they fix large amounts of N (e.g., soybean). In addition, organic matter is releasing N by mineralization during most of the year. We hypothesized that summer crops do not access and take up all of the mineral N in the soil profile, and that large amounts of mineral N remain in the soil, particularly in deeper layers. We investigated the amount of mineral N remaining in the soil in September (after crop N uptake had essentially ceased) for 17 fields with residual soils and 12 fields with Coastal Plain soils by taking 210 cm deep soil cores. We found an average of 286 kg/ha mineral N (NO₃ + NH₄). The ratio of NO₃-N/NH₄-N was 0.95 in the fields with residual soils and 0.74 in the fields with Coastal Plain soils. The ratio of NO₃/NH₄ in the 90-150 cm soil depth was significantly higher (p = 0.0241) for residual soils (1.25) than for Coastal Plain soils (0.69). We took deep soil cores in side-by-side corn-soybean fields in September, and found significantly higher levels of NO₃ following soybean than following corn, but similar levels of NH₄ in corn versus soybean. Across all 29 farms, 76% of the mineral N (in the 0-210 cm soil profile) remaining in September was in deeper soil layers (30-210 cm deep). The deeper the mineral N is in the profile, the greater the risk that it will leach out of the soil and into groundwater over the winter. This pool of deep soil N could serve as a valuable resource for farmers if cover crops could capture and bring it to the surface where it could be recycled to subsequent crops and potentially allow farmers to decrease fertilizer N inputs.