THE EFFECT OF OXIDATION BY PERIODATE ON SOIL CARBOHYDRATE DERIVED FROM PLANTS AND MICROORGANISMS

It has previously been shown that treatment of soil with periodate and tetraborate releases much of the carbohydrate and destroys an equivalent proportion of the soil aggregates. The residual carbohydrate is proportionately richer in glucose, arabinose and xylose, sugars characteristic of plant remains, than the whole soil. The effect of sodium periodate (0.02 M, 6-168 h) and sodium tetraborate (0.1 M, 6 h) treatment of soil on carbohydrates of different origin was examined using 14C-labelled soil in which the label was present in microbial products arising from 7 and 28 day incubations of 14C-glucose in soil, or in both plant and microbial materials resulting from 12 week incubations of 14C-labelled barley leaf and 1 year incubations of 14C-labelled ryegrass in soil. Arabinose and xylose were the sugars most resistant to periodate in the glucose incubated soil; in the ryegrass incubation arabinose, xylose and glucose were more persistent than galactose, mannose and rhamnnose. In the barley leaf incubation arabinose was more persistent than galactose and rhamnose. Thus periodate oxidation did not distinguish between sugars of different origin in soil and it was concluded that in the case of arabinose and xylose the persistence related to differences in chemical structures rather than to physical factors such as particle size of the plant fragments. The composition of the more stable residue can therefore not be used as an indication of polysaccharide origin in any comparison of the relative effects of plant and microbially derived material as aggregating agents.