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.

中文翻译：

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短程矿物是解释深层土壤有机碳储量分布的有力因素：哥斯达黎加安多索尔州一个咖啡农林业种植园的案例

土壤有机碳（SOC）是最大的陆地碳储量，尤其是在火山区的安多索尔。需要SOC数量分布的定量信息来构建研究SOC时变的基线。土壤短程矿物（如Allophane）的空间变化通常可以解释表土SOC含量的变化，但需要更深土壤层的SOC数据。我们发现，在哥斯达黎加一个咖啡农林业覆盖的1  km ²盆地内，土壤200厘米上部的SOC储量 变化很大（24至72  kg C m ^-2）。表土土壤有机碳储量与深层土壤有机碳储量无关。漫反射中红外（MIR）光谱法使得分析大量草酸铵和焦磷酸钠可提取形式的Al，Fe和Si的样品（69个土壤剖面，即598个土壤样品）成为可能。以及SOC含量和堆积密度。使用miR光谱，我们确定了两个不同的土壤材料，其被确定为allophanic和halloysitic土料。Allophanic土壤发生在顶部halloysitic土壤。异色胶的厚度土壤材料中富含SRO矿物，与年轻的Andic A层有关，这解释了SOC的变异性。这项研究表明，需要了解地形和成岩作用，才能了解和推断景观尺度上SOC的分布。