Agroforestry system effects on soil characteristics of tropical soils in the Sarapiqui Region of Costa Rica

Abstract

Agroforestry systems have been regarded as an effective alternative for land uses in the humid tropics because they present beneficial effects upon soil characteristics. The objectives of this study were to detail agroforestry systems and their impact on selected soil properties as compared to baseline data and pastures utilizing on-farm agroforestry plots established in the Sarapiqui Region of Costa Rica in 1990. Tree components of the agroforestry systems were Vochysia ferruginea and V. guateinalensis, Stryphitodeitdroit microstachyum and Hieroityma alchorneoides. The properties studied included soil physical (soil bulk density), chemical [pH, exchangeable bases, acidity and micronutrients, effective cation exchange capacity (ECEC), extractable P, soil organic C (SOC) and total Kjehdahl N (TKN)] and biological parameters (minerahzable C and N, microbial biomass C and N). Agroforestry plots, adjacent pasture areas and one undisturbed forest site were sampled (i) to 10 cm for bulk density, (ii) at three depth increments to 50 cm for chemical analyses and (iii) at two depth increments to 15 cm for the biological analyses. Baseline data were available for the chemical properties only. A decrease in exchangeable bases and pH was noted in the agroforestry plots. An increase in SOC was observed in the agroforestry systems whose tree components were heavy litter producers (Vochysia sp.). Extractable P was higher in the surface soil of the agroforestry plots. Most micronutrient levels increased over the time span considered. Total Kjehdahl N was not different in soils under pasture or agroforestry systems even using a N2-fixing species (S. microstachyum). Carbon to N ratios were not different from soil in pastures and agroforestry systems. Higher minerahzable C levels were observed in the pasture plots at the surface. No differences in minerahzable N in soils under pasture and agroforestry were observed. Soil microbial biomass C and N were not significantly different in pastures and agroforestry systems. Microbial biomass C and mineralizable C and N, and microbial biomass N and mineralizable N were poorly correlated with each other in both depths studied. Specific respiratory activity of the soils under both management systems was not significantly different, but was higher than soil from an undisturbed forest area.