Decomposition and nutrient release from pruning residues of two indigenous agroforestry species during the wet and dry seasons

The decomposition of leaves from Cordia africana Lam. and Albizia gummifera G. F.Gmel was investigated during the wet and dry seasons at Wondo Genet (Ethiopia). Litterbags of leaves were buried in soils under farmland and shaded-coffee agroforestry systems. Residual matter was recovered after 4, 8, 12, and 16 weeks and analysed for nitrogen (N), phosphorus (P), potassium (K), cellulose, lignin, soluble polyphenol and condensed tannin content. Mass-loss and release of N, polyphenols and condensed tannins were greater from Albizia leaves than from Cordia leaves, suggesting that a high polyphenol content does not necessarily retard decomposition. The rates of mass loss and release of the majority of leaf constituents were considerably faster during the wet season than during the dry season. Lignin decomposition, however, proceeded more rapidly during the dry season, and no significant seasonal differences were observed for polyphenol decomposition. The decomposition kinetics of most leaf components during the wet season were best described by a single-exponential model, but a quadratic model provided the best fit during the dry season. Initial leaf chemistry and season were important decomposition factors, while land-use effects were negligible. However, land-use effects showed distinct seasonal differences, with leaf litter decomposing more rapidly in soil under shaded-coffee than under farmland management, especially during the wet season. This study also demonstrated that polyphenol content does not show the predictive effects it has been attributed to have and that other constituents, such as condensed tannins, would be better suited for this purpose.