NITROGENASE ACTIVITY (C2H2 REDUCTION) BY FREE-LIVING BACTERIA IN SOILIN A LONG-TERM TILLAGE AND STUBBLE MANAGEMENT EXPERIMENT ON A VERTISOL

The effects on nitrogenase activity (C2H2 reduction) of cereal stubble management, tillage and N fertilizer practices were studied in a long -term experiment on a vertisol at the Hermitage Research Station in so uthern Queensland, Australia. The experiment compared 8 treatments com prising 3 factors (1) tillage (zero, mechanical cultivation), (2) stub ble (burnt, retained) and (3) N fertilizer applied at sowing (0, 69 kg N ha-1 yr-1 as urea) in four randomised blocks. Measurements of nitro genase activity, using in situ C2H2 reduction assays, began on 1 March 1991 following stubble treatment and continued for 4.5 weeks. Followi ng the application of water to wet the soil to field capacity, nitroge nase activity was observed in all treatments. Nitrogenase activity was greater in the stubble-retained treatments than in the stubble-burnt treatments although this was significant at only 5 of the 8 sampling t imes. The results suggested that the application of N fertilizer depre ssed nitrogenase activity and that cultivation encouraged activity com pared with the zero-tillage treatments, but these effects were only si gnificant at one each of the 8 sampling times. By using an appropriate mix of management practices it may be possible to promote N2 fixation by free-living bacteria using cereal stubble for energy.