NITROGENASE ACTIVITY (C2H2 REDUCTION) BY FREE-LIVING BACTERIA IN SOIL IN 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 southern Queensland, Australia. The experiment compared 8 treatments comprising 3 factors (1) tillage (zero, mechanical cultivation), (2) stubble (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 nitrogenase activity, using in situ C2H2 reduction assays, began on 1 March 1991 following stubble treatment and continued for 4.5 weeks. Following the application of water to wet the soil to field capacity, nitrogenase 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 times. The results suggested that the application of N fertilizer depressed nitrogenase activity and that cultivation encouraged activity compared with the zero-tillage treatments, but these effects were only significant 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.