Straw decomposition and nitrogenase activity (C2H2 reduction) by free-living microorganisms from soi: effects of pH and clay content

The effects of pH and clay content on decomposition of straw (CO2 production) and nitrogenase activity (C2H2 reduction) were measured in laboratory experiments using microorganisms extracted from soils collected from two areas (Gunnedah and Cowra) of the wheat belt of New South Wales, Australia. Assay systems containing sand and various substrates or finely-ground straw were inoculated with soil:water (1:5) extracts prepared from these soils. Straw decomposed at all the levels of pH tested (initial pH 5.5-9.6) although decomposition was inhibited partially in the presence of added clay (5% montmorillonite). Nitrogenase activity by free-living bacteria, extracted from both soils, was best when the initial pH was 7-7.5, regardless of the pH of the original soil (Gunnedah, pH 7.4; Cowra pH, 5.6). At pH-values outside the 7-7.5 range, nitrogenase activity decreased. The effect of clay on nitrogenase activity by bacteria extracted from each of the soils was complex. Montmorillonite enhanced nitrogenase activity of bacteria from both soil extracts when (1% w/w) glucose, xylan, fructose, cellobiose or K-malate were provided as energy sources. For the Cowra soil extract, the pH range for nitrogenase activity increased from pH 5.5-9, in systems without clay, to pH 3.6-10 in the presence of 5% montmorillonite. Organisms from the high clay-containing Gunnedah soil used straw more efficiently to support nitrogenase activity with 5% montmorillonite than without. However, with extracts from the low clay-containing Cowra soil, the effect was reversed. The results show that, despite uniform conditions of moisture and temperature, soils with similar histories of straw retention may differ in their potential for straw-associated N2 fixation because of limitations imposed by soil characteristics such as pH and clay content.