Studies on the rate of decomposition of plant residues in soil by following the changes in sugar components

SUMMARY Uniformly 14 C labelled straw and ryegrass leaf were incubated in soil in the proportions 0.5 and 1% w/w, respectively at 12°C for various periods up to 5 years. Samples were hydrolysed with 2.5 m H 2 SO 4 under reflux for 20 min to release non‐cellulosic sugars and subsequently with 12 M/0.5 m H 2 SO 4 to release glucose from cellulose‐like polymers. Total radioactivity was determined in whole soil and hydrolysates and in individual sugars. Fitting the data to simple exponential equations, total and 2.5 m H 2 SO 4 hydrolysate radioactivity were best expressed by three part exponential functions, whereas radioactivity in 12M/0.5M H 2 SO 4 hydrolysates and sugars were best expressed by two part functions, except for arabinose from the straw, for which a three‐part function was better. A comparison of the fit was made with that using the single‐term exponential expressions devised by Janssen, 1984 and Weibull, 1951 and with a modification of the two part exponential expression by the substitution of one term by the Weibull expression. Apart from the Weibull expression on its own, there was no significant difference between the expressions. It is suggested that, even for well defined carbohydrate components of the substrate, the decomposability changes with time because of their increasing inaccessibility to microorganisms, rather than that several components are present each with a different constant rate of decomposition.