Short-range-order minerals as powerful factors explaining deep soil organic carbon stock distribution the case of a coffee agroforestry plantation on Andosols in Costa Rica

摘要

Abstract. Soil organic carbon (SOC) constitutes the largestterrestrial C stock, particularly in the Andosols of volcanic areas.Quantitative information on distribution of SOC stocks is needed toconstruct a baseline for studying temporal changes in SOC. The spatialvariation of soil short-range-order minerals such as allophane usuallyexplains the variability of topsoil SOC contents, but SOC data for deepersoil layers are needed. We found that within a 1 km~(2) Costa Ricanbasin covered by coffee agroforestry, SOC stocks in the upper 200 cm of soilwere highly variable (24 to 72 kg?C?m~(?2) ). Topsoil SOC stocks were notcorrelated with SOC stocks present in deeper layers. Diffuse-reflectancemid-infrared (MIR) spectroscopy made possible the analysis of a large numberof samples (69 soil profiles, i.e.?598 soil samples) for ammonium-oxalate andsodium-pyrophosphate-extractable forms of Al, Fe, and Si, as well as SOCcontent and bulk density. Using the MIR spectra, we identified two differentsoil materials, which were identified as allophanic and halloysitic soil material. Allophanic soil occurredon top of the halloysitic soil. The thickness of the allophanic soil material, rich in SROminerals and related to a young andic A horizon, explained the variability ofSOC. This study illustrates that knowledge of topography and pedogenesis isneeded to understand and extrapolate the distribution of SOC stocks atlandscape scales.