THE ORIGIN OF THE PENTOSE FRACTION OF SOIL POLYSACCHARIDE

Abstract

Summary Incubation of soil with monosaccharide for 224 days resulted in the evolution of about 80 per cent of the substrate carbon as CO 2 and the transformation of 3 per cent to soil sugars whether the substrate was 14 C‐glucose or xylose and whether the soil was pH 7.4 or pH 5.0. There was no detectable change in the total amounts of individual sugars in the soil during incubation. 14 C‐glucose and xylose gave the same distribution of radioactivity among the soil sugars : hexoses and 6‐deoxy‐hexoses were initially well labelled, with glucose having twice the specific activity of the other sugars. As the incubation progressed some activity appeared in the pentoses (the activity in xylose became very low within the first 14 days of the 14 C‐xylose incubation) and that in the hexoses slowly declined, with glucose no longer predominant. Nevertheless after 448 days the hexoses were still 3–4 times more radioactive than the pentoses. The activity in rhamnose did not decline with time so that eventually it became the most strongly labelled sugar. Incubation of soil with glucose and 14 C‐acetate showed very little transformation of the acetate to sugars indicating that glucose is not metabolized to C 2 compounds before it is transformed to other sugars. Ammo‐acids in soil incubated for 7 days with 14 C‐glucose had much lower levels of radioactivity than hexoses or 6‐deoxy‐hexoses. It is concluded that if soil pentose originates by microbial synthesis it must accumulate slowly by a long process of selective decomposition of a mixture of polysaccharides.