Decomposição da fi tomassa de plantas de cobertura e liberação de nitrogênio em função da quantidade de resíduos... Ciência Rural, v.44, n.5, mai, 2014. 801Decomposição da fi tomassa de plantas de cobertura e liberação de nitrogênio em função da quantidade de resíduos aportada ao solo sob sistema plantio direto Fitomass decomposition and nitrogen release of cover crops in function of the level of residue input to soil under no-tillage system ABSTRACT The no-tillage system success relies on higher addition of different types of crop residues to the soil, each one with specifi c decomposition kinetics which drives the net mineralization and immobilization of the nitrogen (N) in the soil. This manuscript had as objective to evaluate the decomposition rate and N released from winter cover crops residues under different above ground biomass input to the soil. The experiment was carried out in the experimental area of the Soil Department at Federal University of Santa Maria, in random complete block design, with three replications, during 2003/04 and 2004/05. The decomposition and the N released from black oat, oil radish and hairy vetch residues were investigated using mesh bags of decomposition containing fi tomass equivalent to 3, 6 and 9 Mg of dry matter ha -1 placed in plots with 80 m 2 with the same level of crop residues used in the mesh bags. They were collected at 7, 14, 28, 42, 63, 84, 113 and 141 days after cover crops management. The amount of residues input did not infl uence the decomposition rate. However, the N mineralization and immobilization processes were affected by the level of residues input and they were driven by C/N ratio. The larger decomposition residues rate and higher N release followed the order: vetch, radish oil and black oat. The C/N ratio drives the size of labil and recalcitrant compartiments of cover crops fi tomass.