Tillage effects on soil nitrogen and plant biomass in a corn-alfalfa rotation

Alfalfa (Medicago sativa L.) stands have been reported to either decrease or increase nitrate (NO3) leaching, depending on tillage management. The fate of alfalfa-generated N and its interaction with N fertilization in a subsequent corn (Zea mays L.) crop remains uncertain. Alfalfa contributions to corn yields and soil mineral N pools were studied under conventional tillage (CT) and no-tillage (NT) systems over a 3-yr corn-alfalfa-corn sequence. The field experiment compared CT vs. NT managements in fertilized and nonfertilized Kalamazoo loam soils (Typic Hapludalf), Four of the nonfertilized plots were equipped with undisturbed monolith lysimeters to monitor NO3 leaching. Living alfalfa stands lost 6 kg N ha(-1) to deep leaching over a l-yr period, which is much less than the 20 kg NO3-N ha(-1) yr(-1) lost by nonfertilized corn following corn. Within the first 6 mo following the spray-killing of the stand, alfalfa plant tissue decomposition contributed to soil mineral N pools up to 115 kg N ha(-1). During a dry year, all corn N requirements were met by plant tissue decomposition of alfalfa and soil organic matter mineralization, while applied N fertilizer accumulated in the soil profile and was highly susceptible to spring leaching. Similar quantities of N were lost to deep leaching from CT and NT systems during corn production, as higher drainage rates in NT systems were compensated by lower NO3-N concentration of the leachates.