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

从自然生态系统到农业生态系统的土地利用变化对土壤有机质（SOM）的储存和稳定的影响尚不清楚，在高度风化的酸性热带土壤，虽然这些土壤在很大程度上受到这种扰动。由于这些热带土壤的功能高度依赖于SOM，因此有关SOM损失和负责机制的不确定性产生了巨大影响。因此，强烈需要确定负责在这种高度风化的热带土壤中的SOM的稳定的最重要的属性和机制。这是至关重要的缓解土壤有机质的损失后，土地利用变化。金属氧化物和粘土矿物是这些土壤中有机质储存和稳定的最重要的代理人。然而，既不是可用的信息，如果金属氧化物的最佳比例，无氧化物粘土存在SOM存储和稳定，也不是如果形成的矿物有机协会（MOA）和聚集体的关键机制具有相同的最佳。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对非洲热带潮湿的高度风化的酸性土壤中有机质的储存和稳定的影响。我们将使用一个独特的梯度在东乌桑巴拉山脉（坦桑尼亚东北部）的金属氧化物粘土的比例，研究两个关键的稳定机制的相对重要性-MOA的形成和物理保护的SOM在聚集体中的闭塞。与天然森林相比，农田土壤含有特别稳定的SOM，因为它抵抗干扰和增强的微生物分解。因此，在金属氧化物的粘土梯度将结合从天然林到农田的土地利用梯度，允许稳定的SOM的结论。初步的实地调查显示，在地形、气候和母质方面的条件具有高度可比性。土壤主要是以高金属氧化物含量（赤铁矿、三水铝石）和主要粘土矿物高岭石为特征的Acrisol。将沿着金属氧化物和粘土梯度采集森林和农田表层土样品。样品将进入组合密度和聚集体尺寸分级，以区分MOA和聚集体提供的SOM储存。基本的土壤表征结合先进的方法来表征土壤矿物质及其与SOM的相互作用（XRD，XRF，XPS，FTIR;成像技术，如SEM，NanoSIMS）将应用于这些部分。此外，孵化实验，包括使用13 C和15 N标记的化合物和14 C分析将被用来确定金属氧化物和粘土对SOM的稳定和负责的机制的影响。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的热带土壤中SOM的影响，从而为防止SOM损失的可持续土地利用战略奠定基础。

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.