TRACING NITROGEN MOVEMENT IN CORN PRODUCTION SYSTEMS IN THE NORTH-CAROLINA PIEDMONT - A N-15 STUDY

Legume cover crops have been studied in the southeastern USA, but ther e have been no N-15 tracer studies comparing movement of legume N and fertilizer N for this region. Our study used N-15-enriched crimson clo ver (Trifolium incarnatum L.) (135 kg N ha(-1)) and either (NH4NO3)-N- 15 Or (NH4NO3)-N-15 (70 kg total N ha(-1)) to quantify N movement thro ugh soil inorganic and organic N pools and into corn (Zea mays L.). Cl over N mineralized rapidly, with 45% of the initially applied N detect able as inorganic N at 18 d following incorporation, Rapid nitrificati on of enriched NH4+ occurred, but, since the soil NO3- pool was larger in this treatment than in the enriched NO3- treatment (presumably due to chance), the relative enrichment of the soil NO3- pool was less th an with the application of enriched NO3-. At anthesis, 25% of the N in corn had been derived from the NO3- source, while only 11% had been d erived from the NH4+ source (P < 0.05). At physiological maturity in 1 990, the first growing season, 38 to 44% of each enriched source could be accounted for. By physiological maturity in 1991, 60% of the dover source but only 28 to 36% of the fertilizer sources could be accounte d for. This study demonstrates the substantial amounts of endogenous s oil N mineralization, inorganic N immobilization, and legume N persist ence in these cropping systems.