Recycling 15N Fertilizer Using Cover Crops in a Corn System.

Cover crops can serve as temporary storage vessels for nutrients by capturing residual soil nutrients following cash crop harvest and relinquishing nutrients to subsequent cash crops upon decomposition. Quantifying nutrients recycled by cover crops can be challenging due to the many loss and transformation pathways nutrients endure; however, fertilizer enriched with a stable isotope, such as ¹⁵N, can provide an accurate and safe method to trace the cycling of residual fertilizer in a cropping system. The objective of this study was to assess the uptake of fertilizer N by corn that was applied to preceding cover crops. Cereal rye (Secale cereale), tillage radish (Raphanus sativus L.), and cereal rye-tillage radish mixture were planted in September 2014 and 2015 and terminated in March 2015 and February 2016, respectively. Urea fertilizer enriched with ¹⁵N was applied at four N rates (0, 34, 67, and 101 kg N ha^-1) to cover crops and fallow plots two weeks after cover crop planting. Corn (Zea mays L.) was no-till drilled into cover crop residue four weeks after cover crop termination. At the V6 growth stage, corn was sampled and analyzed for biomass accumulation, total N uptake, and ¹⁵N uptake. In 2015, corn following tillage radish produced nearly 4 times more biomass than corn proceeding cereal rye or the cereal rye-tillage radish mixture. Biomass accumulation in 2016 was 1.5 and 3 fold greater for corn planted into tillage radish residue than corn planted into cereal rye-tillage radish mixture and fallow soil, respectively. Corn following tillage radish that received the highest N rate recovered the most ¹⁵N both years with uptakes of 1.7 and 2.8 kg ¹⁵N ha^-1 in 2015 and 2016, respectively. Results indicate that tillage radish could supply a greater amount of available N to subsequent corn early in the growing season than cereal rye.