Nitrogen immobilization of urea and ammonium sulphate applied to maize before planting or top dressing in a no-till system

Ammonium sulfate (AS) and area (U), labeled with N-15, were applied to no-till maize, 33 days before and 10 days after sowing, at a single rate of 80 kg ha(-1) of N incorporated 57 cm deep along furrows spaced 0.8 m. Corn was sown after black oat (Avena strigosa Schieb.).The amount of immobilized N and of N-fertilizer recovered by corn plants was evaluated at the following plants stages: 4-5 leaves, 11-12 leaves, flowering, and at harvest. Treatments were applied in a randomized block design with three replications. The analysis of variance was performed based on one factorial scheme (2 x 6) with two sources in preplanting (AS and U) on six sampling dates, and on a second (2 x 3) with two sources in top-dressing on three sampling dates. This field experiment was carried out on a Typic Acrustox in Uberlandia, Minas Gerais state, Brazil. Pre-planting applications resulted in a maximal AS-N immobilization 22 days after fertilizer application (9.1 kg ha(-1) or 11.4 % of applied N), whereas the maximum immobilization of U-N occurred 11 days after fertilizer application (46.5 kg ha(-1) or 58.1 % of applied N). Until harvest, the plants (aerial part, grains and roots) had accumulated 66.0 and 47.9 of AS-N and U-N, respectively (use efficiency of 82.5 and 59.9 % of applied N). N top-dressing resulted in 12.5 % less N immobilized applied as both sources in all growth stages, evidencing that the soil biomass did not compete with the plants for N-fertilizer, in agreement with results of the growing season 1999/2000. In both growing seasons (1999/2000 and 2000/2001) corn plants assimilated an average 8.9 and 15.4 kg ha(-1) of AS-N for each kg of immobilized N-fertilizer from pre-planting and top-dressing, respectively, in the stage of 11-12 leaves and at flowering. For U-N these values were 4.5 and 5.2 kg ha(-1), respectively, presenting a lower ratio of immobilized AS-N in top-dressing. Highest corn yields were obtained in the AS treatments (mean grain yield of 8.543 kg ha(-1)) independent of the application time. Urea treated plants obtained average grain yields of 7.767 kg ha(-1) for both application periods. These results show that for pre-planting fertilizers the immobilization-mineralization N turnover was faster in the AS than in the U-N treatment. Consequently, N assimilation by corn plants was higher in the AS treatments. Only U-N was significantly immobilized more also in the second growing season when top-dressed, thus limiting plant N uptake.