Mineralization of nitrogen from decomposing leaves of multipurpose trees as affected by their chemical composition

Nitrogen release patterns from seven leguminous trees were determined from 8-week laboratory incubations. The quantities of extractable NH4+-N and NO3– released to the soil to which the leaves had been applied was determined at weekly intervals and was related to the initial N, polyphenol, and lignin concentration of the leaves. Cumulative N mineralized was not correlated to initial N, soluble polyphenol and insoluble tannin concentrations, but was correlated to lignin and neutral detergent fibre N (NDF-N) concentrations. The ratios of NDF-N : N (r=0.68 at P<0.05), soluble polyphenol : N (r=0.70 at P<0.05) and (lignin+polyphenol) : N (r=0.75 at P<0.05) were negatively correlated with N release. Total polyphenol content was not a useful predictor of N release, but the reactivity of the polyphenols as measured by their protein-binding capacity can be a useful predictor. In addition to measuring the concentration of polyphenols, their reactivity with proteins must be measured, in order to assess their role in regulating N release. The (lignin+polyphenol) : N ratio could be used to screen leguminous tree leaves for their potential to release N in short-term experiments. This would allow for rapid screening of a large number of trees without detailed and expensive field experimentation.