NITROGEN MINERALIZATION FROM LEAVES AND LITTER OF TROPICAL PLANTS - RELATIONSHIP TO NITROGEN, LIGNIN AND SOLUBLE POLYPHENOL CONCENTRATIONS

Rates and patterns of nitrogen mineralization from decomposing plant materials are known to be affected by initial concentrations of N, lignin and soluble polyphenols, but published results show differences in which properties correlated best with N release. We incubated soil with fresh leaves and litter from 12 species commonly used in tropical agroforestry systems, including both legumes and non-legumes, to estimate how release of mineral N was affected by chemical composition. After 16 weeks, net accumulation of mineral N in incubations ranged from 81% of initial N in fresh Sesbania sesban to net depletion equivalent to 70% of initial N in Casuarina equisetifolia litter. Fresh legume leaves generally had net N accumulation, whereas legume litter and non-legume fresh leaves and litter had net depletion. At all sampling dates, the percent of leaf N accumulated was most highly correlated with initial N concentration. Phosphorus and the ratios lignin:N and (lignin + polyphenol):N also strongly correlated with N accumulation, although these indices themselves were correlated with N. We conclude that differences among previous studies in best chemical predictors of mineralization rates arose from relatively small ranges in chemical composition of materials used. Over our wide range of materials, initial soluble polyphenols were secondary to initial N, which explained most of the variation in N accumulation or depletion.