Organic matter cycling along geochemical, geomorphic and disturbance gradients in forests and cropland of the African Tropics – Project TropSOC Database Version 1.0

Abstract

Abstract. The African Tropics are hotspots of modern-day land-use change and are, at the same time, of great relevance for the cycling of carbon (C) and nutrients between plants, soils and the atmosphere. However, the consequences of land conversion on biogeochemical cycles are still largely unknown as they are not studied in a landscape context that defines the geomorphic, geochemically and pedological framework in which biological processes take place. Thus, the response of tropical soils to disturbance by erosion and land conversion is one of the great uncertainties in assessing the carrying capacity of tropical landscapes to grow food for future generations and in predicting greenhouse gas fluxes (GHG) from soils to the atmosphere and, hence, future earth system dynamics. Here, we describe version 1.0 of an open access database created as part of the project “Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to variability in soil geochemistry and land use” (TropSOC). TropSOC v1.0 contains spatial and temporal explicit data on soil, vegetation, environmental properties and land management collected from 136 pristine tropical forest and cropland plots between 2017 and 2020 as part of several monitoring and sampling campaigns in the Eastern Congo Basin and the East African Rift Valley System. The results of several laboratory experiments focusing on soil microbial activity, C cycling and C stabilization in soils complement the dataset to deliver one of the first landscape scale datasets to study the linkages and feedbacks between geology, geomorphology and pedogenesis as controls on biogeochemical cycles in a variety of natural and managed systems in the African Tropics. The hierarchical and interdisciplinary structure of the TropSOC database allows for linking a wide range of parameters and observations on soil and vegetation dynamics along with other supporting information that may also be measured at one or more levels of the hierarchy. TropSOC’s data marks a significant contribution to improve our understanding of the fate of biogeochemical cycles in dynamic and diverse tropical African (agro-)ecosystems. TropSOC v1.0 can be accessed through the supplementary material provided as part of this manuscript or as a separate download via the websites of the Congo Biogeochemistry observatory and the GFZ data repository where version updates to the database will be provided as the project develops.

Soil geochemistry – and not topography – as a major driver of carbon allocation, stocks, and dynamics in forests and soils of African tropical montane ecosystems

“…Namely, the potential of soils to stabilize SOC through various mineral-related stabilization mechanisms that are present or lacking in a given geochemical context (Khomo et al, 2017;Rasmussen et al, 2018;Traoré et al, 2020;von Fromm et al, 2021). Indeed, data for the top 50 cm reported for our study sites shows that the sum of organically complexed, amorphous and crystalline Al and Fe (hydro-)oxides were high in soils developed on mafic compared with their counterparts in the felsic and sedimentary regions (Doetterl et al, 2021a).…”

The role of geochemistry in organic carbon stabilization against microbial decomposition in tropical rainforest soils

“…4). Additionally, auxiliary data on the overall net primary productivity of the investigated systems (Doetterl and Fiener, 2021a, b) point towards relatively comparable C inputs across the three regions, at least in topsoil. Subsoils of the sediment region, however, may receive significantly less C input, which is the subject of future investigations.…”

“…In the framework of the project TropSOC (Doetterl et al, 2021a, b), soil sampling took place from March to June 2018, applying a stratified random sampling design with triplicate plots of 40 m × 40 m across three topographic positions (i.e., plateau, slope and valley; Fig. 1a) in each geochemical region.…”

“…A wide range of soil physical and chemical parameters were analyzed in the framework of project TropSOC (Doetterl et al 2021a, b), from which the following were used in this study as potential covariates for controls on SOC: bulk density, total elements of base cations (Ca, Mg, K, Na), total phosphorus, metal oxides with relevance to C stabilization against microbial decomposition (Al, Fe, Mn), elements where concentrations relate strongly to weathering (Si, Ti, Zr) and additional soil properties that relate to soil fertility (texture, pH, effective and potential cation exchange capacity, base saturation, bioavailable phosphorus). Generally, each analysis was performed with 20 % of the samples analyzed in triplicate to assess analytical error.…”

“…Parent material geochemistry and weathering stage Parent material, from which soils in the three geochemical regions have developed, showed distinct differences in elemental composition (Doetterl et al, 2021a, b), with generally low concentrations of Ca, Mg and Na base cations (0.01-0.58 wt %). Al and Fe concentrations were significantly higher in the mafic (Al: 6.27 ± 2.84; Fe: 8.98 ± 1.84 mass%) than sedimentary rocks region (Al: 0.62 ± 0.41; Fe: 2.32 ± 1.73 mass%) and the felsic region (Al: 0.52 ± 1.21; Fe: 1.09 ± 1.58 mass%).…”