NITROGENASE ACTIVITY (C2H2 REDUCTION) IN STRAW-AMENDED WHEAT BELT SOILS IN RESPONSE TO DIAZOTROPH INOCULATION

A series of laboratory-based experiments involving different soils and different diazotrophs, was undertaken to study the effect of inoculation on ARA (C2H2 reduction activity) associated with straw breakdown. Twenty soils from the New South Wales wheat belt representing a range of pH values, clay contents and cropping histories were amended with sterile (gamma-irradiated) straw and inoculated with Azospirillum brasilense Sp7 and Cellulomonas sp.CS1-17. In the absence of inoculation all soils showed nitrogenase activity (ARA) following straw incorporation, indicating the widespread distribution of diazotrophs in these soils and the presence of cellulolytic organisms capable of supplying the indigenous diazotrophs with energy from the breakdown of straw. Without inoculation, there was more than a 10,000-fold range in the number of diazotrophs present in the 20 soils after the first 30 days of incubation. Increasing the moisture content from 100% Field Capacity (FC) to 1.5 x FC resulted in significant increases in ARA in most soils. There was no significant effect of co-inoculation with A. brasilense and Cellulomonas on ARA, although most of the inoculated treatments showed a further increase in numbers of diazotrophs after 30 days. ARA was correlated significantly with soil pH, and with the clay content of the soil. There was no significant correlation between ARA and soil organic matter or available N in the soils at the time of collection. Eighteen strains of diazotrophs, representing eight genera, were inoculated into a red podsolic soil amended with sterile wheat straw. Sixteen of the strains showed a significant increase in ARA compared with the uninoculated controls; co-inoculation with Cellulomonas sp.CS1-17 had no effect. Inoculation of five strains of diazotroph into six straw-amended soils, including the red podsolic soil, gave some early increases in ARA but only one soil showed a significant benefit from inoculation during the first 3 weeks of the experiment. We conclude that most wheat belt soils have the potential for nitrogenase activity when provided with a substrate but responses to inoculation with N2-fixing bacteria will depend on various biological and environmental factors.