Decomposition of plant material in Australian soils. II. Residual organic 14C and 15N from legume plant parts decomposing under field and laboratory conditions

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Abstract
14C- and 12N-labelled Medicago littoralis and Medicago truncatula plant parts, ground or unground, were added at a rate equivalent to 50 kg nitrogen ha-l to a calcareous soil in the field and allowed to decompose for two years. Both plant types behaved similarly but the various plant parts decomposed to different extents. After 4 weeks' and 2 years' decomposition respectively, the residual organic 14C in soil from leaves of both Medicago species accounted for about 62% and 20% of input, from stems 70% and 24% and from roots 80% and 32%. Average residual organic 15N accounted for 64% and 40% of leaf 15N, 87% and 56% of stem and 81% and 50% of root 15N. Grinding had no effect on the residual 14C and 15N of plant parts. After 2 years' decomposition the proportion of residual and 15N present as labelled biomass was greatest for leaf residues. Results from laboratory studies of 20 weeks' decomposition of ground and unground Medicago littoralis plant parts under continuously moist and intermittently dry and rewetting conditions were consistent with field results. Grinding significantly promoted pod decomposition under most incubation conditions. Drying and rewetting promoted decomposition of the plant parts. Pods were affected more than other parts. The longer the time moist following drying, the greater the decomposition. The more frequent the drying and wetting cycles, the greater the decomposition.