Recovery of 15N By Tillage Radish and Rye Cover Crop in Arkansas.

Cover crops have been extensively studied for their role as catch crops to recover and recycle excess nutrients.  Cereal rye (Secale cereal) is commonly used as a cover crop due to high biomass production.  Tillage radish (Raphanus sativus L.) has recently been introduced as a cover crop effective at alleviating soil compaction and scavenging nutrients with its fast-growing enlarged taproot.  Research on these cover crops for nutrient cycling in Arkansas production systems is very limited.  The objective of this study was to evaluate the recovery of fertilizer N by rye and tillage radish cover crops in Arkansas.  Tillage radish, rye, and a tillage radish-rye blend was planted in September 2014.  Uptake and release of fertilizer N by these single cover crops and the tillage radish-rye blend was assessed by applying ¹⁵N-enriched fertilizer to cover crops.  Urea labeled with 2.5 atom % ¹⁵N was applied 2 weeks after cover crop planting at four rates (0, 34, 67, and 101 kg N ha^-1).  Cover crop biomass samples were collected prior to termination in March 2015 and analyzed for N uptake and fertilizer N recovery efficiency.  Total N in rye shoots generally increased as N rate increased; however, total N in radish shoots was lowest under the highest N rate (101 kg N ha^-1).  Tuber total N was generally lower in radishes mixed with rye than in radishes alone, indicating that competition between the species could play a role in N recovery.  Average C/N ratios were higher in rye than tillage radish shoots, which could imply a greater immobilization of N by rye and rapid mineralization of organic-N from tillage radish.  The results obtained from this study indicate that tillage radish may be more effective at scavenging nutrients from the soil, especially at moderate N rates (34 or 64 kg N ha^-1).