Chemical analysis of wheat straw reveals that internodes have larger cellulose but smaller hot-water-soluble fraction contents than either leaves or nodes. Lignin and hemicellulose contents are similar. The leaves and nodes of wheat straw lose weight more rapidly than the internodes during the early stages of aerobic decomposition. This is at least in part a consequence of the more rapid losses of the polysaccharides in leaves and the hot-water-soluble materials in nodes. Internodes also provide a poorer substrate than leaves and nodes for the anaerobes which produce acetic acid.
Summary When seedling development is slowed by the presence of straw in wet seed-beds both microbial products and compounds of plant origin contribute to phytotoxicity. Hot (100~ watersoluble extracts from fresh straw contained phytotoxic substances but these accounted for less of the phytotoxicity than the microbial products, primarily acetic acid, from anaerobic fermentation of the insoluble straw polysaccharides (cellulose and hemicelluloses). The water-soluble components however also included mineral salts required in the decomposition of these polysaccharides.
The losses of total weight and of the individual components of oat straw were followed under field conditions over one cropping season. Decomposition rates of cellulose and hemicellulose were closely similar, and together these polysaccharides accounted for most of the weight losses. Losses of lignin were small, and total weight loss can be calculated from the increase in percentage lignin in recovered samples. Straw retained the potential to produce phytotoxic concentrations of acetic acid only during the early stages of decomposition. Both the water soluble components and the readily available polysaccharides are probably substrates for the production of acetic acid. Early inrorporation of straw minimised the potential for toxin production during winter.
Inoculation of wheat straw, contained in glass columns and moistened by continuous recirculation of a solution containing mineral salts, with a cellulolytic fungus, Penicillium corylophilum, and a N2-fixing anaerobe, Clostridium butyricurn, increased the decomposition rate constant from 0.0096 d-l to 0.0139 d-l compared with non-inoculated straw. N 2 fixation during the utilization of the straw resulted in a gain of 1 1.5 mg N (g straw lost)-* over a period of 8 weeks at 25 "C.
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