Black oats and common vetch are the main winter cover crops in southern Brazil. Grass-legume bicultures grown as winter cover crops are rarely used by farmers, although they can provide a more balanced biomass C/N ratio than single crops, besides erosion control and N supply to corn. In order to evaluate this hypothesis, this work was carried out on a Hapludalf with a sandy-loam texture, located at the Federal University of Santa Maria (RS). The treatments comprised different seeding density rates of common vetch (V) and black oats (O): T1: 100% V (80 kg ha-1); T2: 90% V (72 kg ha-1) + 10% O (8 kg ha-1); T3: 75% V (60 kg ha-1) + 25% O (20 kg ha-1); T4: 50% V (40 kg ha-1) + 50% O (40 kg ha-1); T5: 25% V (20 kg ha-1) + 75 % O (60 kg ha-1) and T6: 100% O (80 kg ha-1). Two more treatments including winter fallow were also used: T7 was cultivated with corn with 75 kg ha-1 of N-urea and T8 was cultivated with corn without N fertilizer. In succession to vetch as a single crop, with a C/N ratio of 13.5, corn grain yield was 5.44 Mg ha-1, which did not differ from the fallow treatment with N-urea application. However, vetch residue was rapidly decomposed, so that only 19.5% of the original biomass remained on the soil surface 6 months after its management. By including oat in the mixture with vetch, gradual increase in biomass C/N ratio was observed, reducing the N supply to corn and increasing residue persistence. The mixture which best comprised biomass production, soil cover by cover crops residues and N supply to corn was that consisting of 10% of oat plus 90% of vetch (C/N ratio of 18.6).
cover crops; nitrogen; no-tillage; Vicia sativa L.; Avena strigosa Schieb.; Zea mays L.