Abstract. Soil organic carbon (SOC) constitutes the largest
terrestrial C stock, particularly in the Andosols of volcanic areas.
Quantitative information on distribution of SOC stocks is needed to
construct a baseline for studying temporal changes in SOC. The spatial
variation of soil short-range-order minerals such as allophane usually
explains the variability of topsoil SOC contents, but SOC data for deeper
soil layers are needed. We found that within a 1 km2 Costa Rican
basin covered by coffee agroforestry, SOC stocks in the upper 200 cm of soil
were highly variable (24 to 72 kg C m−2). Topsoil SOC stocks were not
correlated with SOC stocks present in deeper layers. Diffuse-reflectance
mid-infrared (MIR) spectroscopy made possible the analysis of a large number
of samples (69 soil profiles, i.e. 598 soil samples) for ammonium-oxalate and
sodium-pyrophosphate-extractable forms of Al, Fe, and Si, as well as SOC
content and bulk density. Using the MIR spectra, we identified two different
soil materials, which were identified as allophanic and halloysitic soil material. Allophanic soil occurred
on top of the halloysitic soil. The thickness of the allophanic soil material, rich in SRO
minerals and related to a young andic A horizon, explained the variability of
SOC. This study illustrates that knowledge of topography and pedogenesis is
needed to understand and extrapolate the distribution of SOC stocks at
landscape scales.
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