Isotopic Evaluation of Deep Soil Nitrogen Uptake By Cover Crop Systems.

Transect sampling on 14 farms in Maryland and Pennsylvania found that 100 to 500 kg/ha of mineral nitrogen (N) remain in the upper 2 m of soil after corn and soybean senescence. This N represents an untapped potential resource for farmers, and capturing the deep N (defined here as that located at 50 to 210 cm) could significantly reduce N loading into bodies of water, especially eutrophication-sensitive estuaries like the Chesapeake Bay. Deep soil N has been largely unstudied and is not effectively addressed currently in typical cover cropping practices. We hypothesize that deep-rooted, early planted cover crops can capture deep-soil N and recycle it for subsequent crops rather than allow its loss to eutrophication-sensitive waters. In order to assess the depth from which cover crops take up N, we buried 15-N labeled potassium nitrate (15N, 99%) as a tracer in late August at 100 and 200 cm depths in a sandy (Downer) and a clayey (Christiana) soil. We planted forage radish (Raphanus sativus) and rye (Secale cereale) on September 1 or October 1 and assessed 15-N content on Nov 14, 2014. There was a significant interaction between N-15 burial depth and cover crop planting date at both the clayey site and sandy site. For the 100 cm burial depth, the cover crop 15-N content was higher for the September 1 planted cover crops than October 1 planted cover crops. The early planted cover crops over the 100 cm buried tracer had much greater 15-N enrichment on the site with sandy soils than on the site with clayey soils. No 15-N enrichment was detected in the October 1 planted cover crops or from the 200 cm placed tracer. There were no differences in 15-N enrichment among cover crop species. Concentration of total N in the tissue was much higher and C/N ratio much lower for the younger, later planted cover crops, especially for forage radish.