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The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with IF-labelled ryegrass for 1 year. The plants reduced the loss of ' 4 2 0 , from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little '42 label being associated with the shoot.Chemical fractionation of the root showed that the IF was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of WO, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.

The parameters of the extraction and hydrolysis of soil carbohydrates by methods involving 24N H,SO, and N H2S04 were studied for a sandy granitic loam. Sugars were measured by alkaline ferricyanide, orcinol, anthrone (hexoses), orcinol-ferric chloride (pentoses), cysteine-sulphuric acid (methyl pentoses), glucose oxidase (glucose), and also by analysis of the individual sugars by paper chromatography.After shaking the soil with z4N H2S04 at zoo C a further period of treatment with N H2S04 at IOOO C was required to obtain maximum hydrolysis of the soil carbohydrates. This period decreased from 17 h to 5 has the time with 24N H,S04 increased from z to 16 h. N H2S04 at 1 0 0 ' C alone was less efficient. The extraction of pentoses by 24N H,SO, reached a maximum within 8 h, and methyl pentoses within 4 h, both declining thereafter. The release of hexoses was continuing after 40 h.The best compromise involved extraction with 24N H,SO, for 16 h followed by hydrolysis with N H,S04 for 5 h. By this treatment yields of pentoses and methyl pentoses were respectively gg and gz per cent of the maximum obtainable.

Incubation of soil with "C-rye straw for 448 days resulted in the evolution of about 50 per cent of the carbon of the substrate as CO,. The two main sugars of the straw, glucose and xylose, were degraded to approximately the same extent (70 per cent). The same results were obtained whether the soil was derived from granitic or basic igneous parent material.There was very little transformation of the substrate to galactose, mannose, arabinose, rhamnose, or fucose, and a much slower rate of degradation than with soil incubated with 14C-glucose over a similar period.Hydrolysis of the soil samples by a preliminary treatment with 5 N H2S04, before treatment with 24 N H2S04, followed by heating with N H2SO,, did not release significantly greater amounts of sugar than treatment with 24 N H,SO, and N H2S04 alone. Separate analysis of the hydrolysates showed that 90 per cent of each of galactose, mannose, arabinose, xylose, rhamnose, or fucose had been extracted by 5 N H2S0, but only 50 per cent of the glucose.Fractionation of the straw-soil mixture after 224 days incubation showed that the specific activity of the glucose was higher in the humin fraction than in the fulvic acid, as would be expected if the remaining 14C were still in the form of unchanged plant material.This evidence that plant polysaccharide persists in soil could explain the presence of much of the xylose in the soil organic matter. IntroductionSOIL polysaccharide is mainly the product of two continuous processes :

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