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 u sed, often as mulch, as sources of nutrients for interplanted crops. T herefore, it is important to understand the rates of mulch decompositi on. 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 i n a field experiment in an Alfisol in semiarid Kenya. Plant materials contained in 5mm nylon bags were placed below or on the soil surface i n an alleycropping system involving the two tree species, with maize ( Zea mays L.) as the intercrop. For plant materials of both species (ex cept Leucaena leaves), there were two phases of decompositions: an ini tial rapid phase and a slower second one; Leucaena leaves had only a s ingle phase. When placed below the soil surface, the decomposition rat es 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 o n the soil surface, Leucaena mulch decomposed 1.3 times more rapidly t han Cassia mulch in the first phase. The patterns of N release from th e 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 (sl ow) phase appeared to be regulated by lignin and polyphenol contents.