BIOLOGICAL N-2-FIXATION BY 3 TROPICAL FORAGE LEGUMES AND ITS TRANSFERTO BRACHIARIA-HUMIDICOLA IN MIXED SWARDS

A study was conducted (a) to determine the biological N-2-fixation (BN F) by three tropical pasture legumes (Centrosema pubescens, Stylosanth es hamata and Pueraria phaseoloides) grown in monoculture and associat ed with the grass Brachiaria humidicola; and (b) to assess the potenti al transfer of fixed N from the legume to the companion grass. To calc ulate the proportion of N in the legume derived from air, the N-15 iso tope dilution technique was used. The fertilizer (ammonium sulfate 10% enriched with N-15) was applied in seven split doses during the growi ng season. Aboveground biomass of the grass and the legumes in monocro p and in mixture were determined twice a year, after 3 months of regro wth, and samples were analyzed for total N and N-15. Over the two eval uation periods, the association B. humidicola/C. pubescens was the mos t stable, with a proportion of 20-30% of the legume in the mixture. As the sole crop, B. humidicola had the highest production of shoot biom ass (972 g m(-2)) among all treatments during the first regrowth perio d (middle of the rainy season). Total shoot biomass in the association s ranged from 574 (B. humidicola/C. pubescens) to 807 g m(-2) (B. humi dicola/S. hamata). Dry matter production for the second evaluation (en d of the rainy season) showed the same trend, but lower yields were ob tained due to soil drying conditions. The N content in the shoot bioma ss was always higher in the legumes than in grass. However, B. humidic ola in monoculture showed the highest total N accumulation (18.1 g m(- 2)) due to its higher growth rate. Legumes in monocrop had a very sign ificant proportion of N derived from BNF (47-69%), with C. pubescens a s the better N-2-fixing legume (51-69%). A higher contribution from BN F also was observed in the legumes when mixed with the grass (57-76%). No clear transfer of N from the legume to the companion grass was obs erved, since the N-15 in B. humidicola was always higher in the mixed swards. This grass appeared to have a different pattern of N uptake, o btaining a very significant amount of unlabeled N from outside the mar ked plots. (C) 1997 Elsevier Science Ltd.