Soil phosphorus dynamics in a humid tropical silvopastoral system

In developing countries of the humid tropics, timber exploitation and agricultural expansion frequently result in deforestation. Extensive land management, coupled with inherently low soil fertility invariably produce declines in agricultural/livestock productivity which eventually lead to land abandonment and further deforestation. Phosphorus is often the major nutrient limiting plant growth in soils of the tropics due to their high P retention capacities. Agroforestry systems, which combine fast growing trees with annual or perennial crops and/or pasture, have been considered as viable alternatives to current land use practices. Several hypotheses suggest that by combining trees with crops or pasture, especially leguminous species which fix atmospheric nitrogen, farmers can improve soil chemical, physical and biological properties through enhanced nutrient cycling, improved soil structure and increased soil organic matter. However, few of these hypotheses have been tested rigorously under field or on-farm conditions.;In this experiment, I established Erythrina berteroana (an arboreous legume) in native grass pastures in the Atlantic coastal plain of Costa Rica to determine the effects of tree pruning and cattle grazing on soil P availability. I measured soil P fluxes as well as changes in pasture biomass over an 18-month period using a 2 x 2 factorial experimental design (trees and grazing were the two factors). To detect small-scale spatial and temporal changes in labile soil P, I modified a technique using anion exchange membranes (AEM) in situ. In a separate field experiment, I determined decomposition rates and P release characteristics of the principal silvopastoral system components (Erythrina leaves, pasture grass clippings and cattle dung).;AEM-P correlated significantly with soil solution P and behaved like a dynamic exchanger in this high P retaining soil. Overall, Erythrina leaves decomposed faster than both pasture grass and cattle dung. All residues decomposed as density-dependent exponential decay (3-parameter) functions; however, P release characteristics for Erythrina and pasture residues followed negative exponential functions and released 4-5 times less P than dung. Labile soil P pulsed in relation to the acceleration-deceleration of the rate of residue P release.;Phosphorus fluxes after tree pruning and grazing were highly dynamic for all treatments. Tree pruning increased labile soil P over time when coupled with grazing. Pasture biomass production was greatest in the grazed tree treatment. Pasture biomass P production and concentration was greatest in the non-grazed treatment. Trees and grazing together tended to increase nutrient (P) turnover which stimulated biomass production. In contrast, trees without grazing promoted nutrient (P) accumulation in pasture biomass.