Nitrogenase activity (C₂H₂ reduction) in soils following wheat straw retention: effects of straw management

Abstract

The effects of stubble management practices on straw decomposition (C02 production) and nitrogenase activity C2H2 reduction) were examined in situ on a black earth (pH 7.4, clay content 51%)) near Gunnedah in the wheat-belt of New South Walcs. Straw treatments were: (1) mulching (bladeploughed) or surface cultivation (scarification), (2) burning and cultivation, (3) no-tillage, and (4) incorporation (disc-ploughed). In 1979, the straw was mulched on the surface in treatment 1. When moisture was applied, preliminary measurements (1 980) showed that nitrogenase activity was highest in the incorporated treatment with less in the surface mulched and no-tillage treatments respectively. There was only a small amount of activity in the burnt treatment due to some straw remaining. In a longer-term study in 1985 and 1986 straw in treatment 1 was lightly mixed near the soil surface by scarification. Following moisture application, nitrogenase activity was significantly higher in the scarified treatment than in the incorporated treatment, indicating that depth of mixing of straw with soil was important. Nitrogenase activity in the no-tillage treatment was similar to that in the incorporated treatment, and there was substantially less activity in the burnt treatment. Production of CO2 was similar in the straw-retained treatments, but significantly lower in the burnt treatment. In a series of short-term assays throughout 1985, microbial activity from January to May 1985 decreased with falling soil temperature. With the increase in temperature from July to November 1985, there was no corresponding increase in activity. Despite changes in microbial activity throughout the year, there was little change in the numbers of N2-fixing bacteria in the 14-month period from February 1985 to March 1986, indicating stability in the potential for N2 fixation. Although surface-cultivation (scarification) of straw is apparently the most favourable for free-living N2 fixation, other factors such as erosion and disease control need to be considered in deciding which straw management practice is to be adopted.