Decomposition- and nitrogen-mineralization patterns of Leucaena leucocephala and Cassia siamea mulch under tropical semiarid conditions in Kenya

In agroforestry systems, loppings from trees and shrubs are commonly used, often as mulch, as sources of nutrients for interplanted crops. Therefore, it is important to understand the rates of mulch decomposition. This paper reports the results of a study on the decomposition and nitrogen (N)-mineralization patterns of the leaves, small twigs, and mulch (leaves plus twigs) of Leucaena leucocephala and Cassia siamea in a field experiment in an Alfisol in semiarid Kenya. Plant materials contained in 5mm nylon bags were placed below or on the soil surface in an alleycropping system involving the two tree species, with maize (Zea mays L.) as the intercrop. For plant materials of both species (except Leucaena leaves), there were two phases of decompositions: an initial rapid phase and a slower second one; Leucaena leaves had only a single phase. When placed below the soil surface, the decomposition rates of both Leucaena and Cassia mulch were similar (about 12% week(-1) in the first phase and 1% week(-1) in the second phase). When placed on the soil surface, Leucaena mulch decomposed 1.3 times more rapidly than Cassia mulch in the first phase. The patterns of N release from the mulch of both species were similar to those of mass loss. In general, mulch-decomposition and N-release rates of both species were related more to their C:N ratios than to polyphenol contents; while C:N ratio predicted the rate of first (rapid) phase, the rate of the second (slow) phase appeared to be regulated by lignin and polyphenol contents.