Decomposition of Arboreal Legume Fractions in a Silvopastoral System

Deposition and decomposition of arboreal vegetation represent crucial but poorly understood soil nutrient and organic matter cycling components of warm climate silvopastoral systems. Biomass loss and nutrient release from decomposition bags with leaves (Exp. 1) or branches (Exp. 2) of the arboreal legumes gliricidia [Gliricidia sepium (Jacq.) Kunth] and sabia (Mimosa caesalpiniifolia Benth.) in a signal grass [Brachiaria decumbens (Stapf) R.D. Webster] silvopastoral system were measured in 2011 and 2012. In Exp. 2, branches were divided into 0- to 3-, >3- to 9-, and >9- to 20-cm circumference classes. The incubated nylon bags were collected 0, 4, 8, 16, 32, 64, 128, and 256 d after placement on the soil surface. Gliricidia leaves and branches, in general, had higher N and lower acid detergent lignin (ADL) concentrations and ADL/N ratios than those of sabia. Gliricidia leaf biomass loss rates were greater (P < 0.001; k = 0.0038 g g(-1) d(-1)) than those of sabia (k = 0.0012 g g(-1) d(-1)), as were the branch biomass loss rates (gliricidia, k = 0.0018 g g(-1) d(-1); sabia, k = 0.0005 g g(-1) d(-1)). Leaf N release from the fixed amounts incorporated as annual litterfall corresponded to 47 and 14 kg N ha(-1) from gliricidia and sabia, respectively. The differences in concentrations and decomposition rates of the various fractions indicate that the two legume species differ in their nutrient recycling capacities.