The use of mineral fertilizers and legumes is the most important strategy used to supply nitrogen for cash crops. Since there is a risk of environmental pollution by nitrogen fertilization, it is very important to improve the approaches of nitrogen use recommendation to achieve sustainable agricultural practices. The organic matter content has been used as a parameter to estimate N availability, but in some cases it has been inefficient. In order to develop an equation to estimate N availability in conservation tillage systems, a long-term experiment on a Paleudult of Rio Grande do Sul was used. The experimental design applied was a split-split plot with three replications. The main plots received tillage systems consisting of conventional, minimum and no-tillage. Subplots received winter cover crops in corn production systems: black oat/corn, common vetch/corn and mixture of black oat + common vetch/corn + cowpea. Subplots were split in four treatments: bare soil without cover crop previously to corn, cover crops with 0, 90 and 180 kg ha-1 of N. Based on the results, an equation was developed that considers the effect of soil tillage on N mineralization and the cover crops contribution on N availability, as a consequence of the N content in above ground biomass and C/N ratio. With this equation, it was possible to estimate the N requirement to be supplied by mineral fertilization aiming to achieve a target yield. Although validated with a limited data, the equation showed a good agreement with the experimental data.
conservation tillage; legume cover crop; nitrogen mineralization