The magnitude of nitrogen reactions in the soil varies according to climate conditions, soil type, soil tillage, method of nitrogen application, and plant residue management. This study tried to evaluate the effect of nitrogen application methods and of black oat (Avena strigosa) residues on N leaching and immobilization in a Brazilian Haplumbrept by two experiments carried out in a greenhouse. The treatments were combinations of three N fertilization methods (without urea, urea incorporated into the soil and urea applied over the soil surface) with three management practices of oat residues (without straw, straw incorporated into the soil and straw applied over the soil surface). As additional factors, the effect of soil pH (5.5 or 7.0) was also evaluated in the leaching experiment and the effect of the sowing date (0, 30, or 60 days after N and straw additions) in the experiment of N immobilization. An amount of 4.0 Mg ha-1 of oat straw (dry matter) was applied in both experiments, and N at rates of 200 or 100 kg ha-1, respectively, in the leaching and immobilization trials. Straw and N rates were calculated based on the surface area of the experimental units. Nitrogen leaching was measured weekly, for ten weeks, by percolation of distilled water through polyvinyl chloride columns. Nitrogen immobilization was estimated indirectly, by determining the dry matter and N accumulation in the maize plant shoots sown at the three dates in different experimental units. The amount of leached N varied from 25 to 70 % of the applied amount, was greater in the pH 5.5 than in the pH 7.0 treatments, and in treatments with incorporated urea than in those with urea applied over the soil surface. Nitrogen leaching was unaffected by oat residue application, regardless of the application method. Dry matter of the maize shoots (DMMS) and N accumulation were lowest for plants sown on the day of treatment addition, probably due to a higher immobilization of N during the initial period after the residue application. Addition of N increased the maize dry matter and shoot N uptake, but the urea application method had no influence on any of them. Residue deposition over the surface promoted higher DMMS and N uptake than residue incorporation, probably due to higher constant soil moisture and lower N immobilization.
Nitrogen reactions; fertilization methods; cover crops; soil pH