Document: JEN-3-34-60

Geographic variation in soil organic carbon dynamics following land-use change in Costa Rica.

POWERS, J.S.*

Duke University, Durham, NC 27708 ¹

Abstract:
Recent studies have suggested that the direction, magnitude, and rate of change in soil organic carbon (SOC) pools following deforestation in the tropics are dependent upon initial soil conditions and site factors. However, most global or continental-scale assessments of the flux of CO₂ due to SOC oxidation with land-use change assume an average rate of SOC decay. To assess the role of geographic variation in mediating SOC dynamics following forest conversion, I am quantifying the relationship between changes in SOC pools and site factors such as soil order, texture, clay mineralogy, elevation, vegetation productivity indices, and land-use history for a 140,000-ha landscape in northeastern Costa Rica. Composited soil samples were collected from 110 geo-referenced sites: 40 forests, 38 pastures, 27 agricultural fields, and 5 tree plantations. Managed sites were paired with forests to estimate pre-disturbance conditions. The study design allows for analysis of abiotic and biotic controls on SOC stocks across a number of spatial scales and land-use transitions. Within an 800-ha primary rainforest (the La Selva Biological Station), SOC stocks were correlated with leaf litter standing crops and ¹³C decreased with increasing slope, an indication of the role of geomorphic processes in redistributing carbon. Within all of the forested sites in the region, SOC concentrations increased with elevation and decreased with slope. In general, pastures in the higher elevations (above 350 m) tended to gain SOC compared to forests, while pastures in the lowlands had lower SOC stocks than corresponding forests. Given that most of the lowlands in this region have been converted to pastures within the last 40 years, it is likely that land-use change has resulted in substantial fluxes of CO₂ to the atmosphere.

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This abstract is being presented at: 3:30 PM in session:
CLIMATE CHANGE