Biological fixation of nitrogen in three tropical feed crops leguminous and its transfer to Brachiaria humidicola in association

In Cojedes State drained savannas an experiment was carried out with the purpose of: a) to determine the biological fixation of nitrogen (BFN) in three tropical feed crops leguminous (Centrosema pubescens Cp, Stylosanthes hamata Sh and Pueraria phaseoloides, Pp) cultivated in monoculture and associated with the gramineous Brachiaria humidicola (Bh); b) to evaluate the transfer potential of N fixed to the air to the companion gramineous. To calculate the proportion of N biologically fixed, the technique of isotopic dilution was used with N15. The fertilizer (enriched ammonium sulphate to 10% with N15) was added during the rainy season in two regrowth periods. In each case, the aerial biomass was determined after 90 days of growth, being analyzed the total content of N and N15 in the foliage. In both periods of evaluation, the association Bh / Cp was stabler, with a proportion of the leguminous in the mixture 20-30%. As monoculture, Bh had the biggest production of aerial biomass (972 gm-2) among all the treatments for the first period of evaluation (middle rainy season). The total production of dry matter (DM) in association, was modified between 574 (Bh/Cp) and 807 gm-2 (Bh/Sh). The production DM for the second period of evaluation (end rainy season) followed the same tendency, being observed, however, a general decrease in the yields due to the beginning of drought. The content of N in the leguminous was always higher than in Bh. Nevertheless, Bh in association reached an accumulation bigger than N (14 gm-2) due to its higher rate of growth. The leguminous alone had a significant proportion of N (47-69%) derivated of the BFN. Cp was the one that showed higher values of BFN (51-69%). Likewise, one observes a high proportion (57-76%) of element starting from the BFN when the leguminous were cultivated in association. In this sense one doesn't observe a clear transfer of N from the leguminous to the gramineous, since the contents of N15 in Bh they were always higher than in the leguminous in association. This could be attributed to that the gramineous possesses a different pattern of N absorption, obtaining a great proportion of this element outside of the allotments fertilized with N15