Abstract. Stabilization of organic carbon in soils (SOC) 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 changed hydrological conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. We also detected fossil organic carbon (FOC) at several sites, constituting 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 analysis 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 versus 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 investigated tropical soils.

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中的作用

摘要。土壤中有机碳（SOC）的稳定取决于几种土壤特性，包括土壤的风化阶段和母体材料的矿物学。因此，由于土壤发育的差异，热带SOC稳定机制可能不同于温带土壤。为了更好地理解这些机制，我们调查了原始热带非洲山地森林下三种土壤深度沿从镁铁质到长铁质的地球化学梯度以及覆盖高原，斜坡和山谷位置的地形梯度的SOC动态。为此，我们进行了一系列的土壤C分级实验，并结合了土壤的地球化学成分分析和顺序提取了土壤成因氧化物。我们使用回归分析和降维相结合的方法研究了目标变量与预测变量之间的关系。在这里，我们表明，反应性次生矿物相与其他机制（例如通过粘土含量的聚集或碳稳定化）一起，有时甚至比其他机制更能推动SOC特性和稳定机制。SOC的关键矿物稳定机制与土壤地球化学密切相关，在研究区域之间存在差异。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，河流动力学和变化的水文条件对谷位置的SOC动态具有次要控制，导致那里的SOC储量比非谷位置的SOC高。我们还在多个地点检测到了化石有机碳（FOC），占贫C土壤中SOC总量的52.0±13.2％。有趣的是，这些土壤的SOC总量不超过没有FOC的站点。另外，FOC朝着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分解了FOC。回归分析表明，与土壤风化，母体材料地球化学和土壤肥力以及土壤深度有关的变量最多可解释SOC储量和Δ的75％的变化。^14C。此外，相同的变量解释了与微骨料相关的C相对富集相对于游离粉土和粘土相关的C分数的C相对丰度的44％的变化。然而，地球化学变量对于解释控制土壤深度时的SOC目标变量具有重要意义或保持着重要意义。我们得出的结论是，尽管气候持续了很长时间，但土壤母质的地球化学性质仍在热带土壤中留下了足迹，从而影响了SOC储量和与矿物有关的碳稳定机制。尽管确定的稳定机制和控制措施与其他气候区风化较弱的土壤相似，但它们的相对重要性在所调查的热带土壤中却明显不同。