Consorciação de plantas de cobertura antecedendo o milho em plantio direto: I - Dinâmica do nitrogênio no solo

The dynamics of nitrogen in soils under no-tillage in Southern Brazil are poorly studied so far. A field experiment on a typic Hapludalf on the experimental area of the Soil Science Department, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul State, Brazil, was carried out in 1998/99 and 1999/00 to evaluate the effect of single crop and mixtures of black oat (BO) (Avena strigosa Schieb), common vetch (CV) (Vicia sativa L.) and oilseed radish (OR) (Raphanus sativus L. var. oleiferus Metzg.) on the soil nitrogen (N) dynamics. The experiment was set in a complete randomized block design with four replications. The treatments were: 100 % BO (80 kg ha-1 of seeds), 100 % CV (80 kg ha-1), 100 % OR (14 kg ha-1), 15 % BO + 85 % CV, 45 % BO + 55 % CV, and 30 % BO + 70 % OR. Additionally, two plots under winter fallow were also used for comparison. Other two control treatments were corn cultivated without N fertilizer in one plot and fertilized with 180 kg ha-1 of N-urea in another one. Mineral soil N was measured nine times, beginning ten days after cover crops were killed. Samples were taken from the 0-5, 5-15, 15-30, 30-60, and 60-90 cm soil layers. The sum of mineral N in the five soil depths was greater for single common vetch than for fallow and single BO treatment. At the end of the first month, common vetch had already approximately 30 kg ha-1 more N than the other treatments. The soil under oilseed radish had lower mineral N contents than single common vetch and similar values to those of the mixture between oat and common vetch. The results of this study indicated that the oat-vetch mixture reduced the amount of mineral soil N in relation to single vetch and this effect was proportional to the amount of oat in the crop mixture. It was also verified that the potential N loss by leaching was greater after single vetch than after oat and oilseed radish and greater than oat and vetch mixture.