Quantification of the contribution of N2 fixation to tropical forage legumes and transfer to associated grass

Pastures, mainly dedicated to beef cattle production, occupy approximately 300 million ha of the savanna regions of South America. The productivity of the pasture grasses is principally limited by the availability of P and N, and in the latter case it is rarely economically viable to apply N fertilizer. The introduction of pasture legumes into these pastures is seen as the best strategy to improve N nutrition of the grasses despite the difficulties to be overcome with persistence of the legume in the sward. The benefit to productivity comes not only from the improved quality of the animal diet when the legume is consumed, but also the N transferred to the grass in the litter and decaying roots of the legume, and there may be some direct transfer of fixed N from legume to grass. In our study the 15N isotope dilution (ID) and total N difference techniques were employed to quantify the contribution of biological N2 fixation (BNF) to seven different tropical forage legumes and possible transfer of fixed N to the grass Brachiaria brizantha with three of them (a Centrosema hybrid, Galactia striata and Desmodium ovalifolium) in mixed swards. Amongst the legumes, Stylosanthes guianensis was found to obtain the largest contribution from BNF (between 75–97 kg N ha−1 in 97 days of growth), and kudzu (Pueraria phaseoloides) and D. ovalifolium were found to have a more limited capacity to obtain N from this source. In the mixed swards the 15N data suggested that over 30% of the N accumulated by the grass could be derived from N fixed by the associated legume, but these estimates should be treated with caution as the results suggested that there may be serious errors associated with the use of the 15N ID technique in the field to estimate this transfer of fixed N from legume to associated grass.