Field measurements of nitrogenase activity in soils amended with wheat straw

Abstract

Nitrogenase activity and decomposition of straw were examined in situ in two areas (Gunnedah and Cowra) representative of large areas of the New South Wales wheat belt. Measurements of nitrogenase activity were made by adapting the acetylene reduction assay for use in the field. Evolution of CO2 was monitored as an indirect measure of decomposition of straw. The addition of straw to soil stimulated nitrogenase activity which was related to the amount of straw added and the rate of straw decomposition. There were significant levels of activity provided the soil was moist and warm. Nitrogenase activity increased with mean daily soil temperature (up to at least 30¦C) and decreased as the soil dried from field capacity. It is concluded that nitrogen fixation by free-living nitrogen-fixing microorganisms in soils amended with straw may contribute to the nitrogen status of the soil and thus reduce the need for nitrogen fertilizers.