Recovery of fertilizer nitrogen in crop residues and cover crops on an irrigated sandy soil

Nitrogen fertilizer inputs for intensive, irrigated crop production on sandy soils can contribute to elevated NO3 concentrations in groundwater. This study was conducted from 1995 to 1998 to determine the potential of a winter rye (Secale cereale L.) cover crop to recover fertilizer N from soil and crop residues and the availability of this N to corn (Zea mays L.). Nitrogen fertilizer treatments included no N and N-labeled (N-15-depleted) fertilizer (NLF) applied to sweet corn at 190 kg ha(-1) and to potato (Solanum tuberosum L.) at 224 kg ha-1. Cover crop treatments (fallow and winter rye) were established following harvest, plowed the following spring, and corn was grown with unlabeled fertilizer N at 112 kg ha(-1). Whole plant fertilizer N recovery averaged 54% for sweet corn and 34% for potato using the NLF (N-15 isotope) method, and was significantly lower than N recovery determined by the difference method. Total NLF recovery decreased between harvest and the following spring (from 66 to 43% following sweet corn and from 47 to 37% following potato), presumably due to mineralization and leaching of crop residue N. Winter rye NLF uptake averaged 2 kg ha(-1) and had no effect on total NLF recovery. Corn grain yields were significantly higher following potato than following sweet corn and following a winter rye cover crop compared with fallow in 2 of 3 yr. Corn NLF uptake averaged 3 kg ha(-1) indicating the yield benefit following potato or winter rye was due to a rotation effect rather than a direct N contribution. These results indicate that on irrigated sandy soils in this region most of the N fertilizer not removed in the harvested portion of crops will be lost by leaching during the growing season or by the following spring.