Stabilization of soil organic carbon (SOC) against microbial decomposition depends on several soil properties, including the soil weathering stage and the mineralogy of parent material. As such, tropical SOC stabilization mechanisms likely differ from those in temperate soils due to contrasting soil development. To better understand these mechanisms, we investigated SOC dynamics at three soil depths under pristine tropical African mountain forest along a geochemical gradient from mafic to felsic and a topographic gradient covering plateau, slope and valley positions. To do so, we conducted a series of soil C fractionation experiments in combination with an analysis of the geochemical composition of soil and a sequential extraction of pedogenic oxides. Relationships between our target and predicting variables were investigated using a combination of regression analyses and dimension reduction. Here, we show that reactive secondary mineral phases drive SOC properties and stabilization mechanisms together with, and sometimes more strongly than, other mechanisms such as aggregation or C stabilization by clay content. Key mineral stabilization mechanisms for SOC were strongly related to soil geochemistry, differing across the study regions. These findings were independent of topography in the absence of detectable erosion processes. Instead, fluvial dynamics and changes in soil moisture conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. At several sites, we also detected fossil organic carbon (FOC), which is characterized by high $\mathrm{C}/\mathrm{N}$ ratios and depletion of N. FOC constitutes up to 52.0 ± 13.2 % of total SOC stock in the C-depleted subsoil. Interestingly, total SOC stocks for these soils did not exceed those of sites without FOC. Additionally, FOC decreased strongly towards more shallow soil depths, indicating decomposability of FOC by microbial communities under more fertile conditions. Regression models, considering depth intervals of 0–10, 30–40 and 60–70 cm, showed that variables affiliated with soil weathering, parent material geochemistry and soil fertility, together with soil depth, explained up to 75 % of the variability of SOC stocks and Δ¹⁴C. Furthermore, the same variables explain 44 % of the variability in the relative abundance of C associated with microaggregates vs. free-silt- and-clay-associated C fractions. However, geochemical variables gained or retained importance for explaining SOC target variables when controlling for soil depth. We conclude that despite long-lasting weathering, geochemical properties of soil parent material leave a footprint in tropical soils that affects SOC stocks and mineral-related C stabilization mechanisms. While identified stabilization mechanisms and controls are similar to less weathered soils in other climate zones, their relative importance is markedly different in the tropical soils investigated.

中文翻译：

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地球化学在热带雨林土壤中有机碳稳定对抗微生物分解中的作用

土壤有机碳 (SOC) 对微生物分解的稳定性取决于几种土壤特性，包括土壤风化阶段和母体材料的矿物学。因此，由于土壤发育的不同，热带 SOC 稳定机制可能不同于温带土壤中的 SOC 稳定机制。为了更好地理解这些机制，我们沿着从基性岩到长英质岩的地球化学梯度和覆盖高原、斜坡和山谷位置的地形梯度，研究了原始热带非洲山林下三个土壤深度的 SOC 动态。为此，我们进行了一系列土壤 C 分馏实验，并结合对土壤地球化学成分的分析和土壤氧化物的连续提取。使用回归分析和降维的组合来研究我们的目标变量和预测变量之间的关系。在这里，我们展示了反应性次生矿物相驱动 SOC 特性和稳定机制，以及有时比其他机制（如粘土含量引起的聚集或 C 稳定）更强烈。SOC 的关键矿物稳定机制与土壤地球化学密切相关，在研究区域之间有所不同。在没有可检测的侵蚀过程的情况下，这些发现与地形无关。相反，河流动力学和土壤水分条件的变化对山谷位置的 SOC 动力学有二级控制，导致那里的 SOC 储量高于非山谷位置。在几个地点，我们还检测到化石有机碳 (FOC)，$\mathrm{C}/\mathrm{N}$N. FOC 的比率和消耗 占碳消耗底土中总 SOC 储量的52.0  ± 13.2 %。有趣的是，这些土壤的总 SOC 储量并未超过没有 FOC 的地点。此外，FOC 向更浅的土壤深度强烈下降，表明微生物群落在更肥沃的条件下可分解 FOC。回归模型，考虑 0-10、30-40 和 60-70厘米的深度间隔 ，表明与土壤风化、母质地球化学和土壤肥力相关的变量以及土壤深度解释了高达 75% 的 SOC 变异性股票和 Δ ¹⁴ C. 此外，相同的变量解释了与微团聚体相关的 C 相对丰度与游离淤泥和粘土相关的 C 部分相关的 44% 的变异性。然而，在控制土壤深度时，地球化学变量在解释 SOC 目标变量方面获得或保持了重要性。我们得出结论，尽管长期风化，土壤母质的地球化学特性会在热带土壤中留下足迹，影响 SOC 储量和与矿物相关的 C 稳定机制。虽然确定的稳定机制和控制与其他气候区风化程度较低的土壤相似，但它们的相对重要性在所调查的热带土壤中明显不同。