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

In this study the contribution of biological N-2 fixation (BNF) to leguminous green manures was quantified in the field at different sites with different N-15 methodologies. In the first experiment, conducted on a Terra Roxa soil in Cuba, the BNF contribution to three legumes (Crotalaria juncea, Mucuna aterrima and Canavalia ensiformis) was quantified by applying N-15-labelled 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 abundance technique was applied to quantify BNF in C. juncea, M. niveum and soybean. In both studies the advantages of using several non-N-2-fixing reference 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 possible contribution of root-derived N to the soil/plant system of two of the legumes was quantified using a N-15-leaf-labelling technique performed in pots. The results of this study suggested that total below-ground N could constitute as much as 39 to 49% of the total N accumulated by the legume crops. (C) 2001 Elsevier Science B.V. All rights reserved.