Tracking Deep Soil Nitrogen in Cover Crop Systems: A N-15 Isotope Study.

Large amounts of N remain in the soil after a corn or soybean crop. Cover crops that are planted by early September in the mid-Atlantic region can take up 100-200 kg of N/ha. We used a stable N isotope tracer (N-15) at two sites (with sandy loam and clay loam subsoils) to investigate the depth from which cover crops take up N, and whether the N is then cycled to the subsequent corn crop. We buried N-15 as KNO₃ at 60, 120, or 180 cm in late August, and planted cover crops above the N-15. We evaluated effects of cover crop species (cereal rye (Secale cereale); forage radish (Raphanus sativus); rye-radish mix; rye-radish-Crimson clover (Trifolium incarnatum) mix) and planting date (Sept 1; Oct 1) on cover crop N-15 uptake in December and April. We planted corn in May, and tested V5 stage biomass and grain for N-15. We found that Sept 1 planted cover crops took up N-15 that was buried at 60 cm, 120 cm, and small but statistically significant amounts from 180 cm. Oct 1 planted cover crops took up N-15 that was buried at 60 cm only. If the cover crops did not reach the N-15 from a burial depth in December, they still had not reached the N-15 from that burial depth in April. There was significantly more N-15 in corn grain following the radish-rye-clover cover crop than following no cover crop or the rye monoculture cover crop, providing evidence that some of the N-15 captured by the mixed cover crop is available for the following corn crop. In summary, early-planted rye and radish cover crops reached N from deep soil (up to 180 cm deep) by December, and a rye-radish-clover (but not a pure rye) cover crop recycled this N to the following corn crop.