Estimation of biological nitrogen fixation associated with Brachiaria and Paspalum grasses using15N labelled organic matter and fertilizer

Abstract

Six pasture grasses, Paspalum notatum cv batatais, P. notatum cv pensacola, Brachiaria radicans, B. ruziziensis, B. decumbens and B. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of15N labelled organic matter or frequent small (2 kg N. ha-1) additions of15N enriched (NH4)2SO4. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in15N enrichment in plant-available nitrogen (NO3-+NH4+) with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different15N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the15N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in15N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same15N enrichment in soil derived N. Data from the final harvests of this treatment indicated that B. humidicola and B. decumbens obtained 30 and 40% respectively of their nitrogen from N2 fixation amounting to an input of 30 and 45 kg N.ha-1 year-1 respectively. © 1986 Martinus Nijhoff Publishers.