The goal of this study was to examine spatial variation of soil organic C and its stable isotopic composition (δ13C) in 1-ha plots of mature rain forest and a cattle pasture dominated by C4 grasses in Costa Rica. Soil samples were taken from 80 mapped locations per plot and analyzed for organic C and δ13C. The range of values for soil C concentrations was similar between forest and pasture, although the mean values were higher in the forest. δ13C was narrowly constrained in the forest (from -27.96 to -26.09‰) but varied from -15.09 to -28.59‰ in the pasture. Variograms revealed spatial autocorrelation of soil C and δ13C in the pasture and organic C concentration in the forest soil. The large range and spatial variability of δ13C in the pasture site may be due to varying contributions of C3 and C4 vegetation to the soil C pool, which may limit the usefulness of the natural abundance technique as a precise tracer of soil C dynamics in this pasture.
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Acknowledgements I thank Mr. Machado-Lopez for letting me conduct my study in his field. Peter Tiffin, Ed Veldkamp, and two anonymous reviewers gave helpful comments on previous drafts. This study was made possible through funding from the Duke University Latin American Studies Program and Center for International Studies, a NASA G.S.R.P. Fellowship, an NSF Dissertation Improvement Grant (No. 9972569), a Lawrence J. Giles Award for Phytotron Research (Duke University) and a Catherine Keever Award (Duke University).
- Soil carbon
- Spatial variation
- Stable isotopes