Understanding carbon cycling in tropical soil systems As the population surrounding the African great lakes and the Congo basin continues to grow, increasing amounts of land are being converted into agricultural use. A deeper understanding of how carbon is sequestered and released from soils in tropical regions will both inform models of earth system dynamics and help farmers in the region use land effectively, as Dr Sebastian Doetterl explains. Little is known
about biogeochemical cycling and the underlying mechanisms in tropical Africa, where soils function very differently from temperate zones. As the Principal Investigator of the DFG funded Tropsoc project, Dr Sebastian Doetterl aims to help build a deeper understanding of carbon sequestration and release in the area. “We’re looking at sites in Uganda, Rwanda and the Congo,” he says. While the study sites in these countries are broadly similar in terms of climate and population pressure, the way land is used and the geology are very different. “For example, in Rwanda there are a lot of sedimentary rocks and often terraced, very old cropland systems to limit erosion, while we find in our sites in the Congo an abundance of basalt and nearly unconsolidated cropland expansion into primary rain forest. The severity of the impact of land degradation on the fertility of cropland soils depends on the geochemistry of the bedrock, as some rocks can weather more quickly and release more nutrients than others,” explains Dr Doetterl.
Carbon stabilisation potential The geology of an area is therefore an important issue in terms of the project’s overall agenda, as soil fertility and the carbon stabilisation potential of soils - which constrains greenhouse gas emissions from soils - are closely related to mineral availability. Carbon stabilisation is controlled by a very complex interplay between different factors, with Dr Doetterl and his colleagues focusing on topographical and geological drivers. “We are looking at systems that have a comparable amount of rainfall and a comparable topography,” he says. “We want to understand how much carbon is stabilized in soils under different environmental systems (i.e. cropland versus forests) along a geochemical and topographic gradient.” “Soil is mobilized on slopes in each of the areas we are looking at, leading to a lot of erosion, and later deposited in valley systems as sediments. The environmental conditions between slopes and valleys then drive the stability of the mobilized carbon.” This lateral transport of soils is an important factor with respect to carbon storage. Vast areas
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Impact of the study
TropSOC fieldwork in the Eastern Congo basin, a region troubled by conflict and violence, scarcity of fertile land and a fast growing popluation.
of cropland suffer from degradation and erosion, which affects the properties of soil on these slopes. “That also drives differences in the carbon stabilisation potential,” explains Dr Doetterl. In this context, the researchers distinguish between stable and dynamic landscapes to gain deeper insights into carbon sequestration. “Plateaus are the stable landscapes, as there is basically no loss of soil. The dynamic part of the landscape are these slopes, where soil can be lost through erosion,” says Dr Doetterl. “As topsoil is lost through erosion, important plant nutrients and stabilized carbon are lost as well.” Changes in soil properties will also have a significant impact on forest vegetation. Even though erosion is much less pronounced than on cropland, it will change nutrient availability in some areas, a topic that Dr Doetterl and his colleagues are addressing. “In our sites we investigate functional traits of plants and changes in plant strategies. For example, we analyze if plants increasingly invest in roots in nutrient depleted envrionments than they do in a nutrient-enriched environment, and can this be tied to the geochemistry of soils on the one hand and to topography and erosion on the other?” he outlines. The wider aim here is to link the insights on soil carbon with the functional traits of plants in forests, in terms of carbon sequestration. “Hence, it’s not an exclusively soil-focused project. We also want to raise awareness of questions around tropical biogeochemical cycling and how it drives plantsoil interactions,” continues Dr Doetterl.
This project aims to bring a wider relevance to modelling carbon in earth system dynamics by taking into account soil dynamics in less studied areas across the globe. Many current models use observations on carbon dynamics gathered from temperate zones, which do not fully reflect soil functioning in tropical Africa. Dr Doetterl hopes to make an important contribution in this respect. “We aim to help reduce the uncertainties around tropical carbon release and stabilisation in large-scale models,” he outlines. The project’s research will also have an impact on the local level, helping farmers to use land more effectively. “We’re working with the International Institute of Tropical Agriculture (IITA), which is specifically interested in improving farming techniques and the livelihood of subsistence farmers in developing countries,” says Dr Doetterl. “We will provide information on crop yields and how they are affected by erosion specifically in relation to mineralogical properties.”
TropSOC Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to variability in soil geochemistry and land use TropSOC is funded by the German Research Foundation DFG as an Emmy Noether Junior research group hosted at Augsburg University (Germany). Funding was approx. € 1.7 Million including overheads. Additional € 300,000 came from structural funds of the hosting university. T: +41 44 633 60 20 E: doetterl@geo.uni-augsburg.de W: https://www.congobiogeochem.com/tropsoc
Dr Sebastian Doetterl is an Assistant Professor for Soil Resources at ETH Zurich’s Department of Environmental Systems Science. He holds a PhD in Geosciences and a Diploma in Physical Geography, his main research focuses in on investigating geoecosystems at different temporal and spatial scales.
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