LAUSR

Abstract Plant use efficiency of applied N is estimated to be around 50 % for most crops. In coastal plain sandy soils particularly, leaching along with volatilization in warmer climates may be a predominant pathway for N loss to the environment. A replicated field study to determine both crop N requirement and recovery efficiency of N (REN) in tomatoes (var. BHN 602) grown on sandy soils under a plastic-mulched bed system was conducted in north Florida. Isotope labeled-ammonium nitrate (15NH415NO3) was applied in spring at four different N rates (0, 168, 224, 280 kg N ha−1). A subsequent study in the fall was conducted in order to determine the recovery of residual N fertilizer in tomatoes. In spring, no appreciable response to applied fertilizer N rate above 168 kg N ha−1 (lowest rate) were observed, indicating recommended N rates may be more than what is required. On average, approximately, 62 % of N accumulation in the plant came from fertilizer, whereas 38 % came from soil N. In the fall, recovery of residual 15N fertilizer at harvest ranged from 1.9 to 5.4 kg N ha−1. At the end of both studies, a 15N balance was calculated to estimate total crop and soil recovery of N fertilizer, where approximately 15.4 % of applied 15N fertilizer was recovered. Unrecovered N in this study shows that optimizing N fertilizer management in warmer climates is critical in order to avoid unnecessary excess application of fertilizer and minimize loss of N to the environment.