Changes in the monosaccharide composition during the decomposition of rice and barley straws under field conditions were examined. The proportion of mannose, galactose, fucose, rhamnose, and ribose increased consistently with time, whereas the proportion of cellulosic glucose became smaller consistently. Changes in the proportion of hemicellulosic glucose, arabinose, and xylose followed a somewhat zigzag course, though a consistent increase in the proportion of hemicellulosic glucose was observed after the flush of decomposition.The molar ratio of xylose to mannose decreased sharply at the flush of decomposition, then gradually afterwards, and reflected well the decomposition of the straw saccharides. The ratio of hemicellulosic glucose to cellulosic glucose became consistently larger with time after the flush of decomposition. Samples which contained less straw saccharides with the progression of the decomposition appeared to have larger molar ratios of hexoses to pentoses, deoxyhexoses to pentoses, and hemicellulosic saccharides to cellulosic saccharides.Even after 3 years of incubation, the monosaccharide composition of the straw, though more similar, was still quite different from that of soil, reflecting the persistence of the remaining straw saccharides.

The monosaccharide composition of Ando soils, which originate from volcanic ash and have high organic matter content (8-21% carbon), was quite different from that of non-volcanic ash soils (1.2-1.9% carbon), being richer in mannose, fucose and ribose, whereas there was less glucose in cellulose-like form. arabinose, xylose and rhamnose. The Ando soils were also characterized by a lower percentage of organic carbon in the form of saccharide (4.4-7.4%) in comparison with non-volcanic ash soils (10.5%), though the former soils contain a greater amount of saccharides.The monosaccharide composition of Ando soils was unrelated to the vegetation, land usage, or climatic conditions, and is presumed to be a soil characteristic resulting from the preferential accumulation of microbial polysaccharides.

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The decomposition of rice and barley straw saccharides under field conditions was well represented by the first-order kinetics model of Yl = CIe-klr + C2e-kit, where Yr is the remaining amount at time t, kl and k2 are the decomposition rate constants (time-') for the labile fraction (CI) and the non-labile fraction (C2), respectively. About 82% of the total saccharide content of the rice straw was in the labile fraction with a rate constant 0.64-0.8 1, the values for the barley straw being 70-92% and 0.50-0.6 I , respectively. The non-labile fraction appeared to persist for a long time, with a half-value period of 9-59 months. Barley straw and its saccharides decomposed at a slower rate than rice straw. Much more favourable initial decomposition was observed in a paddy field than in an upland field.Mannose, fucose, rhamnose and ribose were synthesized in parallel with the flush of decomposition of straw saccharides.

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