Soil erosion in the remote Heart of Africa

Soils can store high quantities of carbon, but erosion can cause these carbon sinks to flip into sources. An international research team is looking to unravel this erosional transfer of carbon in the Kasaï Basin, the largest tributary of the Congo River.

by Lissie de Groot
river
River in Kasaï Basin, which was sampled for TropSEDs project (Image: Nathan Carlier).

Soils are an important reservoir of carbon; stocks of carbon build up over time via the weathering of rocks, biological activity in the soil, and plant decomposition. Over long periods of time, soils can contain high quantities of carbon that again can be released via erosional processes1-6. This release may cause carbon sinks to change into sources. One way to study erosion is to measure the release of particulate and dissolved materials from the land into river networks. Both climate and land-use change can increase soil erosion and lead to observable changes in these particulate and dissolved materials mobilized into rivers. Tropical regions may be considered frontiers of land-use change due to high population growth, increasing dependencies on shifting cultivations and climate change7–9.

An international research team is now looking to unravel this erosional transfer of carbon from source to sink in the Kasaï Basin. This basin is the largest tributary to the mighty Congo River and contributes high sediment loads compared to other rivers. Consisting of a large gradient in land-use, precipitation, and mineralogy, this river basin provides the ideal environment to test how each gradient controls tropical soil erosion.

This project focusing on the Kasaï Basin is called TropSEDs (Tropical Soil Erosion Dynamics). The research team brings together different expertise from researchers from the Environmental Systems Science Department (Johan Six, Professor of Sustainable Agroecosystems) and the Earth Science Department of ETH Zurich (Jordon Hemingway, Professor of Surface Earth Evolution) with researchers from UC Louvain Belgium, the Congo Atomic Energy Commission, and the Woods Hole Research Centre.

On a recent sampling trip along the Kasaï, the research team was comprised of four individual investigative groups responsible for upland soils, floodplains, lakes, and rivers. Each group covered locations along the entire basin to carefully collect representative samples.

For example, ETH Zurich doctoral student Lissie de Groot focused on river samples. Rivers are an excellent source to capture the carbon signature of a biome. Like arteries, they integrate processes and transport material over large distances, distilling them into a convenient, single location. Erosion can vary depended on the spatial location and season. The wet season brings higher erosion due to more intense and higher frequency of precipitation. To understand the quantity, source, and fate of these eroded materials, Lissie and colleagues relied on many different techniques ranging from radiocarbon dating to analysing specific compounds in trace materials.

With stable and radiocarbon isotopes, the sources of the material can be traced, as deeper soil is older and more enriched in 13C compared to surface soils. Moreover, isolating and examining the isotopic signatures of specific compounds in the soils such as leaf waxes offers a window into the original source material before secondary processes obscured the signature. To perform these specialized analyses, large amounts of carbon are needed and, thus, large quantities of filtered material. For example, during the trip, the researchers filtered more than 2300 litres of water from 32 rivers. The samples were then processed on site and prepared for shipment to either Switzerland or Belgium.

The team continues to process such samples from the trip. In June, the project team we will return to Kasaï Basin to sample the same rivers in the dry season. The data obtained for this project will be used to improve Earth-system models. Another aim of the project is to reconstruct how carbon cycling responded to past climate conditions, which are known to have shifted dramatically throughout the Holocene peroid, when large areas of rainforest disappeared and re-established in central Africa.

Lissie de Groot
“In June, we will return to sample the dry season signature for the same rivers in Mbuji-Mayi. Conditions are expected to be much dustier and warmer. Which may mean we would be less stuck in the wet soils. I am curious how the environment changes and the rivers will look like.  ”
Lissie de Groot
Lissie de Groot, ETH Zurich doctoral student
soils

More on project: https://sae.ethz.ch/research/N_Cycling_and_Isotopes/tropseds--tropical-soil-erosion-dynamics---unravelling-the-roles.html

Funding: Swiss National Science Foundation Synergia Program; International Institute of Tropical Agriculture (Democratic Republic of Congo); Régie des Voies Fluviales (Democratic Republic of Congo)

References

1. Doetterl S, Stevens A, Six J, et al. Soil carbon storage controlled by interactions between geochemistry and climate. Nat Geosci. 2015;8(10):780-783. doi:10.1038/ngeo2516
2. Drake TW, Podgorski DC, Dinga B, Chanton JP, Six J, Spencer RGM. Land-use controls on carbon biogeochemistry in lowland streams of the Congo Basin. Glob Chang Biol. 2020;26(3):1374-1389. doi:10.1111/gcb.14889
3. Batjes NH. Total carbon and nitrogen in soils of the world. 1996;(June).
4. Schefuß E, Eglinton TI, Spencer-Jones CL, et al. Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin. Nat Geosci. 2016;9(9):687-690. doi:10.1038/ngeo2778
5. Schlesinger WH. Carbon sequestration in soils : some cautions amidst optimism. 2000;82:121-127.
6. Ramesh T, Bolan NS, Kirkham MB, et al. Soil organic carbon dynamics: Impact of land use changes and management practices: A review. Adv Agron. 2019;156:1-107. doi:10.1016/bs.agron.2019.02.001
7. Don A, Schumacher J, Freibauer A. Impact of tropical land-use change on soil organic carbon stocks - a meta-analysis. Glob Chang Biol. 2011;17(4):1658-1670. doi:10.1111/j.1365-2486.2010.02336.x
8. Bauters M, Drake TW, Verbeeck H, et al. High fire-derived nitrogen deposition on central African forests. Proc Natl Acad Sci U S A. 2018;115(3):549-554. doi:10.1073/pnas.1714597115
9. Moore S, Evans CD, Page SE, et al. Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes. Nature. 2013;493(7434):660-663. doi:10.1038/nature11818

 

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