Absorption of soil native nitrogenby corn under no-tillage in succession to cover crops

In untilled soils with green manure cultivation in during the dry season and the management of the nitrogen fertilization usually change the dynamics and the recovery of the nitrogen in the soil-plant system. The objective of this study was to evaluate the amount of native soil nitrogen absorbed by corn under no-tillage and submitted to different N rates in succession to sunnhemp (Crotalaria juncea), millet (Pennisetum americanum) and fallow in a Red Latosol in the Cerrado. The experiment was carried out on an Experimental Farm of the Faculty of Engineering, Sao Paulo State University (UNESP-Ilha Solteira), which is situated in Selviria, state of Mato Grosso do Sul, Brazil, during the 2001/02 and 2002/03 growing seasons. The experiment was set up in a randomized complete block design, with 15 treatments and four replications in a 5 x 3 factorial scheme, which consisted of the combination of five urea-N rates (0, 30,80, 130 and 180 kg ha(-1) of N) and three preceding cover crops (sunnhemp, millet and fallow ground). The amount of soil native Nabsorbed by corn was calculated by the difference between the total N accumulated in the corn plant at physiologic maturation stage and the amount of N coming from urea, millet or sunnhemp as determined by the N-15 isotopic method. The soil supplied greater N amounts to corn than the inorganic fertilizer and green manures combined. The N rates and cover crop systems significantly influenced the uptake of soil native nitrogen by corn plants.