Recent studies suggest that the direction and magnitude of changes in soil organic carbon (soil C) pools following forest-to-pasture conversion in the tropics are dependent upon initial soil conditions and local factors (e.g. pre-conversion soil C content, soil texture, vegetation productivity, and management practices). The goal of this study was to understand how landscape-scale variation in soil-forming factors influenced the response of soil C pools to forest clearing and pasture establishment in northeastern Costa Rica. We measured soil C and its stable isotopic composition in 24 paired pasture and reference forest sites distributed over large gradients of edaphic characteristics and slope throughout a 1400 km2 region. We used the large difference in stable C isotopic signatures of C3 vegetation (rain forest) versus C4 vegetation (pasture grasses) as a tracer of soil C dynamics. Soil C pools to 30 cm depth ranged from 26% lower to 23% higher in pastures compared to paired forests. The presence of non-crystalline clays and percent slope explained between 27 and 37% of the variation in the direction and magnitude of the changes in soil C storage following pasture establishment. Stable carbon isotopes (δ13C) in the top soil (0-10 cm) showed a rapid incorporation of pasture-derived C following pasture establishment, but the vegetation in these pastures never became pure C4 communities. The amount of forest-derived soil C in pasture topsoils (0-10 cm) was negatively correlated to both pasture age and the concentrations of non-crystalline iron oxides. Together these results imply that site factors such as soil mineralogy are an important control over soil C storage and turnover in this region.
Bibliographical noteFunding Information:
We would like to thank Peter Tiffin, Bill Schlesinger, Dan Richter, Elissa Levine, and three anonymous reviewers for helpful comments on earlier versions of this manuscript. We thank Jim Clark for statistical advice and help with the bootstrap analyses. Lars Szwec operated the mass spectrometer. This study was made possible through funding from the following sources to J.S.P.: a NASA Graduate Student Researcher Program Fellowship, an NSF Dissertation Improvement Grant (No. 9972569), an Andrew W. Mellon Foundation Fellowship for Ecosystem Studies awarded through the Organization for Tropical Studies, a National Security Education Program Fellowship, a Center for International Studies Travel Grant from Duke University, and a Tinker Field Research Grant from the Duke University Latin American Studies Program.
- Costa Rica
- Land-use change
- Soil carbon
- Stable carbon isotopes