Tracing nitrogen movement in corn production systems in the North Carolina Piedmont: A nitrogen-15 study

Legume cover crops have been studied in the southeastern USA, but there 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 clover (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 through soil inorganic and organic N pools and into corn (Zea mays L.). Clover N mineralized rapidly, with 45% of the initially applied N detectable as inorganic N at 18 d following incorporation, Rapid nitrification 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 than 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 derived from the NH4+ source (P < 0.05). At physiological maturity in 1990, 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 accounted for. This study demonstrates the substantial amounts of endogenous soil N mineralization, inorganic N immobilization, and legume N persistence in these cropping systems.