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

Recent studies have suggested that the direction and magnitude of changes in soil carbon (C) pools following land-use change in the tropics depend upon initial site conditions, vegetation productivity, and management. Despite observations that soil C pools both increase and decrease following deforestation, global assessments of carbon dioxide fluxes due deforestation usually assume a single rate of loss. The goal of my dissertation was to understand how the response of the soil C pool to land-use change varies geographically for a 140,000-ha region in Costa Rica, and how to extrapolate site-specific changes in soil C pools to estimate regional CO₂ fluxes. I collected an extensive data set for 110 managed and forested sites in northeastern Costa Rica that included: soil C, indices of vegetation productivity, soil texture, mineralogy, elevation, topographic relief, and landcover history. Managed sites were paired with reference forest sites on similar soils and topography to estimate pre-conversion conditions.; In this region, the direction and magnitude of the changes in soil C pools following conversion of mature forests to pasture varied as a function of non-crystalline clays in the low-elevation soils and %slope in the high-elevation soils. The conversion of old pastures to intensively managed cash crops reduced soil C storage to a greater extent than the conversion of forest to pasture. Old pastures that had regenerated to secondary forests or tree plantations did not show increased soil C storage. As a whole, soils in this region have been a small source of carbon dioxide to the atmosphere over the past 50 years as managed lands have replaced native forests.