Biologicalnitrogen ixation and urea-N recovery in 'Coastcross-1' pasture treated with Azospirillum brasilense

The objective of this work was to quantify the inoculation effect of Azospirillum brasilense (Ab-V5 and Ab-V6 strains) on the forage yield, biological nitrogen fixation (BNF). and urea N-15 recovery of the forage grass 'Coastcross-1'. The experiment was carried out in a 2 (with or without inoculation) x 2 (without N fertilizer and with 100 kg ha(-1) N per year as urea) x 7 (cuts) factorial arrangement. The natural N-15 abundance method was used to determine BNF; to determine urea-N recovery, N-15-labeled urea was applied in microplots. Forage yield was higher in grasses subjected to inoculation. with 7.4 Mg he dry matter per year, for the treatment without N fertilizer, and 8.0 Mg ha(-1) dry matter per year for the treatment with 100 kg ha(-1) N per year, respectively, which shows an additive effect of inoculation and N fertilization. The contribution of BNF was 23.0 and 53.8 kg he per year for the unfertilized treatment, both in uninoculated and inoculated plants, respectively. Urea-N-15 recovery was 13.7 and 16.5 kg ha(-1) per year for uninoculated and inoculated treatments, respectively, corresponding to 13.7 and 16.5% of the urea-N applied. Inoculation with A. brasilense increases forage yield and the contribution of BNF to grass nutrition with N, as well as urea-N recovery by the forage grass.