Abstract. Earlier studies have demonstrated that soil texture and geochemistry strongly affect soil organic carbon (SOC) content. However, those findings primarily rely on data from temperate regions with soil mineralogy, weathering status and climatic conditions that generally differ from tropical and sub-tropical regions. We investigated soil properties and climate variables influencing SOC concentrations across sub-Saharan Africa. A total of 1,601 samples were analyzed, collected from two depths (0–20 cm and 20–50 cm) at 45 sentinel sites from 17 countries as part of the Africa Soil Information Service (AfSIS) project. The dataset spans climatic conditions from arid to humid and includes soils with a wide range of pH_H20 values, weathering status, soil texture, exchangeable cations, extractable metals and a variety of important land cover types. The most important SOC predictors were identified by linear mixed effects models, regression trees and random forest models. Our results indicate that SOC is primarily controlled by aridity index (PET/MAP), exchangeable calcium (Ca_ex) and oxalate-extractable aluminum (Al_ox); this was found across both depth intervals. Oxalate-extractable iron (Fe_ox) emerged as the most important predictor for both depth intervals in the regression tree and random forest analyses. However, its influence on SOC concentrations was strong only below Fe_ox concentrations of 0.25 wt %. This suggests that Fe_ox can act as a pedogenic threshold – even on a continental scale. Across model-ling approaches, clay and fine silt content (Our findings indicate that the key controlling factors of SOC across sub-Saharan Africa are similar to what has been reported for temperate regions – except for soil texture and vegetation cover. However, the strength and importance of the controlling factors vary across the environmental gradient we studied.

中文翻译：

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整个撒哈拉以南非洲大陆对土壤有机碳的规模控制

摘要。较早的研究表明，土壤质地和地球化学强烈影响土壤有机碳（SOC）含量。但是，这些发现主要依赖于温带地区的土壤矿物学，风化状况和气候条件的数据，这些数据通常不同于热带和亚热带地区。我们调查了影响撒哈拉以南非洲地区SOC浓度的土壤特性和气候变量。作为非洲土壤信息服务（AfSIS）项目的一部分，共分析了1,601个样品，从17个国家的45个哨兵点的两个深度（0–20 cm和20–50 cm）中采集。该数据集涵盖从干旱到潮湿的气候条件，并包括pH _H20范围很广的土壤值，风化状况，土壤质地，可交换的阳离子，可提取的金属和多种重要的土地覆盖类型。通过线性混合效应模型，回归树和随机森林模型确定了最重要的SOC预测因子。我们的结果表明，SOC主要受干旱指数（PET / MAP），可交换钙（Ca _ex）和草酸盐可提取铝（Al _ox）的控制。在两个深度间隔中都发现了这一点。草酸可萃取铁（Fe _ox）成为回归树深度间隔和随机森林分析中最重要的预测指标。但是，其对SOC浓度的影响仅在0.25wt％的Fe _ox浓度以下才强烈。这表明Fe _ox甚至在整个大陆范围内，都可以作为成岩作用的门槛。在各种模型方法中，粘土和细粉含量（我们的研究结果表明，整个撒哈拉以南非洲地区SOC的关键控制因素与温带地区的报道相似，除了土壤质地和植被覆盖。但是，强度在整个研究的环境梯度中，控制因素的重要性各不相同。