Role of N2-fixation in the sustainability of the ponded grass pasture system

Abstract

Ponded pastures in northern Australia produce green fodder in the seasonally dry winter period and may be employed to reduce grazing pressure on dryland pastures. The soils under ponded pasture, currently 26,000 ha in Queensland, are inherently infertile. This study was conducted to determine if a non-symbiotic association between bacteria and grass roots was responsible for the supply of N to ponded grasses. Intact soil-plant cores were obtained from a new ponded pasture of Aleman (Echinochloa polystachya) and an 8-year-old pasture of Hymenachne (Hymenachne amplexicaulis). Nitrogenase (N2-ase) activity was measured using the acetylene reduction assay and bacteria were selectively isolated to N-free malate medium from root segments of the most active plants. N2-ase activity of the intact soil-plant cores ranged from 76 to 380 g N ha−1 d−1 for the Aleman pasture and from 5 to 179 g N ha−1 d−1 for the Hymenachne pasture. Assays on excised roots showed the greatest activity on adventitious roots formed on the submerged nodes of Hymenachne stems. No major differences in colony morphology were detected in N2-fixing bacteria isolated from the roots of the two grasses. An association appeared to exist between bacteria and the roots of both grasses with most of the N for the young Aleman pasture being fixed N, whereas the fixed N supply for the older Hymenachne pasture was supplemented by the mineralization of organic N.