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

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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.