THE DECOMPOSITION OF STRAW IN SOIL - USE OF MULTIPLE INTERNAL REFLECTANCE INFRARED-SPECTROSCOPY

Changes in the surface layers of wheat and barley straw internodes incubated in soil at 15-degrees-C for up to 448 d were analysed using multiple internal reflectance (MIR) infrared spectroscopy. Results were compared with chemical analyses of total C and N and polysaccharides in straw and soil-straw mixtures. Detectable spectroscopic changes occurred only at the inner surface of the straw. There was progressive enhancement of lignin absorption bands relative to those of polysaccharide and enhanced acetyl absorption. There was a greater association of clay mineral matter with the inner surfaces of the wheat straw than with those of the barley. Using the absorbance ratio of carbohydrate to lignin and assuming a negligible change in lignin content, it was estimated that for wheat straw about 75% of the carbohydrate in the surface layers was lost within the first 14 d of incubation. The barley straw showed no loss until 28 d, and 75% loss only after 112 d. In the early phases, the pattern of decomposition of straw carbohydrate in soil as revealed by MIR resembled that in sheep's rumen, but the rate of decomposition was much slower. Changes in the carbohydrate content of the soil-straw mixtures measured chemically paralleled those observed spectroscopically, but only gross chemical changes were observable. MIR infrared spectroscopy was shown to provide a sensitive measure of the early stages of plant tissue decomposition.