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, p olyphenol, and lignin concentration of the leaves. Cumulative N minera lized was not correlated to initial N, soluble polyphenol and insolubl e tannin concentrations, but was correlated to lignin and neutral dete rgent 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+polyphen ol):N (r=0.75 at P<0.05) were negatively corre lated with N release. T otal polyphenol content was not a useful predictor of N release, but t he reactivity of the polyphenols as measured by their protein-binding capacity can be a useful predictor. In addition to measur ing the conc entration of polyphenols, their reactivity with proteins must be measu red, in order to assess their role in regulating N release. The (ligni n+polyphenol):N ratio could be used to screen leguminous tree leaves f or 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.