Litter decomposition by four functional tree types for use in silvopastoral systems

Litter fall and decomposition from trees in a silvopastoral system is considered to be an important factor contributing to soil quality. The hypotheses of this study are that different functional types of trees and seasonal differences in the growth of the trees affects litter quality and quantity and the rate of decomposition. Litter was collected in traps from four tree species representing four functional types, i.e. black alder (deciduous N-fixer), Tasmanian blackwood (evergreen N-fixer), red oak (deciduous non-N-fixer) and Eucalyptus (evergreen non-N-fixer). The rate of litter decomposition was measured using litter bags. A high rate of decay was linked to high amounts of microbial activity associated with the litter and this was affected by the differences in N and lignin content of the leaves. The leaf litter which exhibited the most rate of rapid decay and cycling of nutrients was that from the deciduous N-fixing tree species, which contained low amounts of lignin and C and high amounts of N. Tasmanian blackwood also produced N-rich litter, but the decay rate was retarded by higher contents of lignin. Non-N-fixing tree species produced leaf litter which decayed slowly due to both high contents of lignin and low amounts of N, regardless of whether the species were deciduous or evergreen. The initial lignin content and the lignin-to-N ratio in leaf litter emerge as likely controls of the rate of decay and therefore as predictors of the relative abilities of the different tree species to contribute nutrients to the pasture systems. These litter quality characteristics appear to be independent of the functional type categories. Seasonal effects on leaf litter quantity and quality were not as important as the over-riding chemical differences between tree species. A mixture of litter types could be used to manipulate the timing of nutrient release to benefit pasture growth.