Nitrogen fixation by three tropical forage legumes in an acid-soil savanna of Colombia

The proportion of nitrogen derived from fixation was measured by 15N isotope dilution in three tropical grass-legume pastures on two Oxisols differing in texture over a 3 year period using three non-fixing controls. The use of the companion pasture grass, Brachiaria dictyoneura, was satisfactory as a non-fixing control when compared with a non-fixing Panicum maximum ecotype KK16 and native savanna grasses. Amounts of nitrogen fixed ranged from 0.3 to 40 kg N ha−1 12 weeks−1 during the wet season and were greatest with Stylosanthes capitata followed by Centrosema acutifolium and Arachis pintoi mainly as a result of greater legume biomass in the former compared with the latter two species. There were little or no differences amongst the three legumes in kg N fixed t legume DM−1. Amounts of nitrogen fixed per unit area decreased over the 3 year period mainly as a result of a loss of legume biomass. The % N derived from fixation (%Ndfa) was generally greater than 80% in all legumes on both soil types and there were little or no differences in %Ndfa with two levels of fertilization given at establishment. The %Ndfa did not change over time. In a separate experiment with A. pintoi, %Ndfa did decrease with increasing legume proportion in the sward. The results indicate that in tropical pastures sown on low fertility acid soils the amounts of nitrogen fixed by forage legumes are dependent on legume growth and persistence. Soil type, level of fertilization of age had little effect on the N2-fixation process as measured by %Ndfa. Based on these results approximate amounts of nitrogen fixed may be estimated by simple measurements of legume biomass-N × 0.8.