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Decomposition of leaf litter from tropical forage grasses and legumes

1993; Elsevier BV; Volume: 25; Issue: 10 Linguagem: Inglês

10.1016/0038-0717(93)90050-l

1879-3428

Richard J. Thomas, N.M. Asakawa,

Agroforestry and silvopastoral systems

In tropical pastures which are generally underutilized, nutrients are returned to the soil mainly via plant litter. The release and recycling of nutrients from litter is known to be a function of climate and litter composition. However, little is known about the decomposition of tropical forage species in pastures. We compared rates of above ground litter decomposition and changes in nutrient contents of six legume and four grass species in litter bags over the wet and dry season of a well-drained isohyperthermic savanna of Colombia. Decomposition constants and litter half-lives were estimated by fitting a single exponential model to the data. Litter production was measured monthly in pure stands using matched quadrats. Of the six legume species studied, rates of decomposition of organic matter (OM) were fastest in Stylosanthes capitata and Arachis pintoi and slowest in Desmodium ovalifolium. The remaining legumes, Centrosema acutifolium, Pueraria phaseoloides and Stylosanthes guianensis, decomposed at rates similar to the grasses Andropogon gayanus, Brachiaria decumbens, B. dictyoneura and B. humidicola. Rates of litter decomposition decreased and litter half-lives increased during the onset of the dry season. Rates of release of nutrients (N, P, K, Ca and Mg) generally followed a pattern similar to that of OM, although the absolute amounts of N, K and Ca released were greater in the legumes than in the grasses due to higher initial concentrations in legumes. Immobilization of N occurred only in litter with high C:N ratios (> 109) and high lignin:N ratios (> 25). Of the chemical variables studied the lignin:N and (lignin+polyphenol):N ratios were linearly correlated with the loss of OM from grass and legume litter (r = 0.66). Losses of N were best correlated with the % lignin + (C:N ratio) (r = 0.67) but lignin:N, (lignin + polyphenol):N and C-to-N ratios also gave similar correlations (r = 0.61−0.67). The linear relationship between the lignin-to-N ratio and the loss of OM was improved when rainfall was included in a multiple regression (r = 0.82). The use of polyphenols as an indicator of litter decomposition is discussed but not recommended using the current simple methodologies available. The rates of N released were estimated from the litter decay equations and the mean monthly rates of litter production from pure stands of each species and were related to the N requirements of a tropical pasture producing 0.85–1.7 kg DM m−2yr−1. The estimates indicated a slow and inadequate rate of N release from grasses and a wide variation in rate of release amongst the legumes and hence a variable, species-dependent, ability to supply a substantial proportion of a pasture's requirements from leaf litter decomposition.