Root decomposition in tropical grasses and legumes, as affected by soil texture and season

We aimed to determine how root decomposition is affected by soil texture and weather conditions, and to produce a data set for calibrating or validating litter decomposition models. Roots of tropical grasses and legumes were buried in the field in two acid Oxisols of widely differing textures during either the dry or the wet season. Results were analyzed, using four decay models: single exponential, single exponential with residue, double exponential, and a model in which the relative decomposition rate (RDR) decreases non-linearly as a function of the litter fraction left: 1/X.dX/dt = − k(XX0)1b. In the dry season, the RDR of all roots decreased steeply with a declining fraction of leftover root weight. This indicates a rapid loss of a small fraction of easily decomposable material, leaving behind a large fraction of recalcitrant material. In the wet season, RDR decreased more gradually. The basic pattern of decomposition was the same for both clay loam and sandy soil, although the absolute values for RDR and the steepness of its decrease differed. Given that the two soils differed widely in texture and consequent water retention characteristics, the differences in RDR may have resulted from differences in soil water regime. The very high C-to-P ratios in the roots (up to 1780) reflect low P availability in these strongly P-sorbing Oxisols. Because microbial C-to-P ratios in these soils ranged from 34 to 50, considerable P immobilization may be needed for decomposition to proceed. Thus, P is likely to be an important rate-controlling factor in litter decomposition in these soils.