In order to evaluate the amount of immobilized N and the amount of N-fertilizer recovered by corn plants at the stages:5-6 leaves, 11-12 leaves, flowering, and physiologic maturation, ammonium sulfate (AS) and urea (U), labeled with 15N, were applied to maize in a no-till system 43 days before planting and 31 days after planting at a single rate of 80 kg ha-1 of N incorporated at 5-7 cm depth and in a spacing of 0.8 m. Corn was sown after black oat (Avena strigosa Schieb.). The treatments were applied in split-plots set up in the completely randomized design with three replications. The original plots with two N sources were halved for the application time factor (pre-planting and top-dressing application). The field experiment was carried out on a Typic Acrustox on the farm Floresta do Lobo, Uberlandia, state of Minas Gerais. Results show that in pre-planting application treatments the maximum N-AS immobilization occurred 19 days after the fertilizer application (13.3 kg ha-1 or 16.6 % of N applied) whereas the maximum immobilization of N-U occurred 40 days after fertilizer application (13.7 kg ha-1 or 17.1 % of N applied). The highest amount of 15N-fertilizer taken up by corn plants was observed between the stages 5-6 leaves and 11-12 leaves, at ratios of 44.1 % of N-AS and 23.4 % of N-U. The immobilized N in the treatment with top-dressed AS was lower than 3.5 % of the applied N whereas in the treatment with top-dressed U it was 9.9 kg ha-1 and 7.9 kg ha-1 at the stages 11-12 leaves and flowering, respectively. The recovery of N-fertilizer measured at the stage of physiologic maturation in the treatments with AS and U was 61.8 % and 42.0 % respectively. The 15N-fertilizer amounts recovered by corn plants per kg of 15N immobilized in the treatments with AS were 8.0 kg ha-1 and 16.7 kg ha-1 respectively for pre-planting and top-dressing application. Independent of the application time, the ratio N-fertilizer recovery/immobilized N-fertilizer in the U treatments was 3.1 kg ha-1. The highest corn yields were obtained in the AS treatments (grain mean of 7,824 kg ha-1), independent of the application time. The average crop yield in the treatments with U in both application periods was 6,977 kg ha-1. These results show that when the fertilizers were applied at pre-planting the immobilization-mineralization turnover was faster in the AS than in the U treatment. Consequently, the N-assimilation by corn plants was higher in the AS treatments. For top-dressing application, only U was significantly immobilized.
plant N-recovery; phenology of corn plants; 15N isotope; rate of plant N- fertilizer
