Quantification of the contribution of biological nitrogen fixation to tropical green manure crops and the residual benefit to a subsequent maize cropusing N-15-isotope techniques

In this study the contribution of biological N-2 fixation (BNF) to legumino us green manures was quantified in the field at different sites with differ ent N-15 methodologies. In the first experiment, conducted on a Terra Roxa soil in Cuba, the BNF contribution to three legumes (Crotalaria juncea, Muc una aterrima and Canavalia ensiformis) was quantified by applying N-15-labe lled ammonium sulphate to the soil. The second experiment was planted in a very low fertility sandy soil near Rio de Janeiro, and the N-15 natural abu ndance technique was applied to quantify BNF in C. juncea, M. niveum and so ybean. In both studies the advantages of using several non-N-2-fixing refer ence plants was apparent and despite the much greater accumulation of the C . juncea in the experiment performed on the fertile soil of Cuba, the above ground contributions of BNF at both sites were similar (40-80 kg N ha(-1)) and greater than for the other legumes. In a further experiment the possib le contribution of root-derived N to the soil/plant system of two of the le gumes was quantified using a N-15-leaf-labelling technique performed in pot s. The results of this study suggested that total below-ground N could cons titute as much as 39 to 49% of the total N accumulated by the legume crops. (C) 2001 Elsevier Science B.V. All rights reserved.