In Southern Brazil, black oat (Avena strigosa, Schieb), is a common cover crop, preceding maize in no-tillage systems. However, where N supply is restricted or absent, this sequence can affect the corn yield due to the N immobilization process. The practice of fertilizing cover crops with N to control this effect is not well investigated yet. Main goal of this study was to evaluate the effect of N application on black oat regarding residue decomposition and residue N release, as well as corn yield. The experiment was carried out in 1998/99 and 1999/00 at the Federal University of Santa Maria in the experimental area of the Soil Science Department, Santa Maria county, state of Rio Grande do Sul, Brazil, on a typic Hapludalf with sandy loam A horizon. The experimental design was of completely randomized blocks with ten treatments and four replicates. Seven of the treatments were oat, which received N applications of 0, 40, 80, 120, 160, 200, and 240 kg ha-1, one with common vetch , and two fallow. All cover crops were followed by maize cultivated without N fertilization, except for one of the fallow treatments, after which maize received 160 kg ha-1N in split application. Lime, P, and K were applied at constant rates in all treatments. Residue decomposition of oat and vetch, and N release were determined using mesh bags for all treatments, except for fallow. In 1998, samples were collected after 0, 10, 20, 30, 50, 70, 90, and 110 days and after 0, 15, 30, 45, 65, 85, 105, and 125 days in 1999. Results indicated no significant difference in the residue decomposition rate in relation to any N rate, even when the C/N ratio of the oat residue was reduced from 50 (0 N) to 26 (240 N). Nitrogen released by the oat residues was directly proportional to the applied N dose. Although a positive effect of N fertilization of black oat was verified for the subsequent corn crop in nutrition and yield, none of the evaluated doses brought forth a comparable yield to the one obtained with fallow and subsequent maize with top dressing N. Consequently, the decision to apply all N on black oat would not be an efficient strategy to supply the corn with N, despite the positive effect of oat N fertilization on N availability in the subsequent crop.
conservation tillage; cover crop; available nitrogen; mesh bags; Avena strigosa
