Nitrogen cycle in pure grass and grass/legume pastures. Evaluation of pasture sustainability

To evaluate the impact on pasture sustainability of the introduction of a forage legume (Desmodium ovalifolium) into a Brachiaria humidicola pasture, studies of the contribution of legume N2 fixation, litter recycling, the plant material on offer and the animal live weight gain were made on grazed pure grass and mixed pastures at three different grazing pressures at the CEPLAC field station at Itabela in the Atlantic forest region of souther Bahia. The contribution of biological nitrogen fixation (BNF) to D. ovalifolium was estimated to be approximately 50% of plant N in a satellite experiment using the 15N isotope dilution technique. Evaluation of the plant material on offer showed that the proportion of legume in the mixed swards ranged between 39% (at the lowest stocking rate of two animals/ha) and 16 and 2% (three and four animals/ha, respectively). The total contribution of BNF was calculated at 74, 28 and 3 kg N·ha·a-1 for the three stocking rates, respectively. Litter deposition varied little between the different treatments, but the N content of the litter was considerably higher in the mixed sward and was considerably lower at the highest stocking rate in both pastures. Analysis of the 13C isotopic abundance of the soil organic matter showed that 5 years after establishment of the pure B. humidicola pasture, approximately 27% of the soil carbon was derived from grass at a depth of 0-5 cm; a slightly lower proportion (20%) was derived from grass at 5-15 cm. Analysis of the 13C abundance of cattle faeces indicated that the proportion of legume consumed by cattle in the mixed sward was between 10 and 37%. The data indicate that selection of an appropriate grazing pressure can greatly influence recycling. Therefore, presumably pasture sustainability and the introduction of a legume in the pasture are beneficial to both animal production and pasture sustainability. (author). 24 refs, 3 figs, 2 tabs