The role of clay minerals and metal oxides for organic matter stabilization in highly weathered tropical soils

在高度风化的酸性热带土壤中，土地利用从自然生态系统到农业生态系统的变化对土壤有机质（SOM）储存和稳定的影响尚不清楚，尽管这些土壤在很大程度上受到这种扰动的影响。由于热带土壤的功能高度依赖于土壤有机质，因此对土壤有机质损失及其机制的不确定性具有巨大的影响。因此，迫切需要确定在这种高度风化的热带土壤中SOM稳定的最重要性质和机制。这对于减轻土地利用变化造成的SOM损失至关重要。金属氧化物和粘土矿物是这些土壤中SOM最重要的储存和稳定剂。然而，对于SOM的储存和稳定是否存在金属氧化物与无氧化物粘土的最佳比例，以及作为关键机制的矿物-有机结合（MOA）和团聚体的形成是否存在相同的最佳比例，目前也没有相关信息。所要求的项目的总体目标是解开金属氧化物和粘土矿物之间复杂的相互作用，在潮湿的热带非洲高度风化的酸性土壤中储存和稳定SOM。我们将利用东乌桑巴拉山脉（坦桑尼亚东北部）金属氧化物与粘土比例的独特梯度来研究两个关键稳定机制的相对重要性——MOA的形成和聚集体中遮挡对SOM的物理保护。与天然森林相比，农田土壤含有特别稳定的SOM，因为它抵抗干扰并增强微生物分解。因此，金属氧化物到粘土的梯度将与从天然林到农田的土地利用梯度相结合，从而得出稳定SOM的结论。初步的实地调查显示，在地形、气候和母质方面情况非常相似。土壤以高金属氧化物含量（赤铁矿、三水石）和高岭石为主要粘土矿物的Acrisols为主。森林和农田表层土的样品将沿着金属氧化物和粘土的梯度采集。样品将进入组合密度和骨料尺寸分馏法，以区分moa和骨料提供的SOM存储。基本的土壤表征结合了表征土壤矿物质及其与SOM相互作用的先进方法（XRD， XRF， XPS， FTIR， SEM， NanoSIMS等成像技术）将应用于这些组分。此外，将使用13C和15N标记化合物的培养实验和14C分析来确定金属氧化物和粘土对SOM稳定的影响及其机制。这种改进的定量过程知识将有助于更好地预测土地利用变化对这些脆弱热带土壤中SOM的影响，从而为防止SOM损失的可持续土地利用策略奠定基础。

Effects of land-use change from natural to agricultural ecosystems on storage and stabilization of soil organic matter (SOM) are not well understood in highly weathered acidic tropical soils, although these soils are largely affected by such perturbations. The uncertainty about losses of SOM and the responsible mechanisms have a huge impact because functioning of these tropical soils highly depends on SOM. Therefore, a strong need exists to determine the most important properties and mechanisms responsible for stabilization of SOM in such highly weathered tropical soils. This is crucial for the mitigation of SOM losses upon land-use change. Metal oxides and clay minerals are the most important agents for storage and stabilization of SOM in these soils. However, neither is information available if an optimum ratio of metal oxides to oxide-free clay exists for SOM storage and stabilization, nor if the formation of mineral-organic associations (MOA) and aggregates as key mechanisms have the same optimum. The overall objective of the requested project is to disentangle the complex interactions between metal oxides and clay minerals on storage and stabilization of SOM in highly weathered acidic soils of humid tropical Africa. We will use a unique gradient in the ratio of metal oxides to clay in the East Usambara Mountains (NE Tanzania) to study the relative importance of the two key stabilization mechanisms - the formation of MOA and physical protection of SOM by occlusion in aggregates. Cropland soils contain particularly stable SOM compared to natural forests, as it resists disturbance and enhanced microbial decomposition. Therefore, the gradient in metal oxides to clay will be combined with a gradient in land-use from natural forest to cropland allowing conclusions about stable SOM. A preliminary field survey revealed highly comparable conditions in terms of relief, climate and parent material. Soils are mostly Acrisols characterized by high metal oxide contents (hematite, gibbsite) and kaolinite as the dominant clay mineral. Samples of forest and cropland topsoils will be taken along the gradient in metal oxides and clay. Samples will enter a combined density and aggregate size fractionation to differentiate between SOM storage provided by MOAs and aggregates. A basic soil characterization combined with advanced approaches to characterize soil minerals and their interactions with SOM (XRD, XRF, XPS, FTIR; imaging techniques as SEM, NanoSIMS) will be applied to these fractions. In addition, incubation experiments including the use of 13C and 15N labeled compounds and 14C analysis will be used to determine the impact of metal oxides and clay on SOM stabilization and the responsible mechanisms. This improved quantitative process knowledge will contribute to a better predictive power of land-use change impacts on SOM in these vulnerable tropical soils, thus setting the base for strategies preventing SOM losses for sustainable land use.