ROLE OF N-2-FIXATION IN THE SUSTAINABILITY OF THE PONDED GRASS PASTURE SYSTEM

Ponded pastures in northern Australia produce green fodder in the seas onally dry winter period and may be employed to reduce grazing pressur e on dryland pastures. The soils under ponded pasture, currently 26,00 0 ha in Queensland, are inherently infertile. This study was conducted to determine if a non-symbiotic association between bacteria and gras s roots was responsible for the supply of N to ponded grasses. Intact soil-plant cores were obtained from a new ponded pasture of Aleman (Ec hinochloa polystachya) and an 8-year-old pasture of Hymenachne (Hymena chne amplexicaulis). Nitrogenase (N-2-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. N-2-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 gr eatest activity on adventitious roots formed on the submerged nodes of Hymenachne stems. No major differences in colony morphology were dete cted in N-2-fixing bacteria isolated from the roots of the two grasses . An association appeared to exist between bacteria and the roots of b oth grasses with most of the N for the young Aleman pasture being fixe d N, whereas the fixed N supply for the older Hymenachne pasture was s upplemented by the mineralization of organic N.