Soil Organic Carbon dynamics in the eroding uplands of the Kasai Basin (DRC)

The net terrestrial C exchange from both vegetation and soils accompanying land use change is relatively well constrained. In contrast, C exchange associated with accelerated soil erosion following the degradation of natural habitats is an uncertain component of the carbon budget 1,2,3, particularly in tropical settings 10,11,12. The Kasai Basin, located in the southwestern part of the Congo River Basin, displays a unique array of climatic conditions, mineralogical compositions and land use trajectories. It is currently experiencing an explosive demographic expansion, which leads to drastic deforestation for cropland expansion. These changes are accelerating soil erosion on hillslopes, which plays an important role in the carbon cycle: most studies 1, 2, 3, 4, 5, … consider the eroding uplands an important sink of atmospheric CO2, while others conclude it could be a sink or a source 6, 7, 8, 9, …. To remedy this lack of knowledge, we conducted a robust sampling campaign and collected more than 6000 soil samples on 24 hillslope transects alongside the Kasai River across its climatic, mineralogic, and human disturbance gradients. Multiple analyses were carried out on a selection of these samples (carbon and nitrogen content and stable isotopes, soil texture, clay mineralogy, and fallout radionuclide inventory), and MIR spectroscopy was used to get high-depth resolution data. The preliminary results show a variation in SOC concentration with depth, topographic position, climatic gradients, and land use types. Across the upper 40 cm of the soil profile, SOC content ranged from 0.30% to 2.5%. Notably, a distinct 137Cs signal was detected at all sampling sites, an unexpected and encouraging result given the study’s location in the Southern Hemisphere. The results of this investigation of SOC dynamics provide insights into the different contributions of climate, land use intensity, and clay mineralogy as drivers of OC recovery in tropical eroding upland soils. ACKNOWLEDGEMENTS This research is supported by the Sinergia scheme of Swiss National Science Foundation (Grant number: 205998). REFERENCES 1 Stallard (1998) https://doi.org/10.1029/98GB00741 2 Berhe et al. (2007) https://doi.org/10.1641/B570408 3 Van Oost et al. (2007) https://doi.org/10.1126/science.1145724 4 Li et al. (2015) https://doi.org/10.1016/j.epsl.2014.11.036 5 Wang et al. (2017) https://doi.org/10.1038/nclimate3263 6 Harden et al. (1999) https://doi.org/10.1029/1999GB900061 7 Billings et al. (2010) https://doi.org/10.1029/2009GB003560 8 Kirkels et al. (2014) https://doi.org/10.1016/j.geomorph.2014.07.023 9 Lal (2019) https://doi.org/10.1016/j.still.2018.02.001 10 Don et al. (2011) https://doi.org/10.1111/j.1365-2486.2010.02336.x 11 Reichenbach et al. (2023) https://doi.org/10.1111/gcb.16622 12 Wilken et al. (2020) https://doi.org/10.5194/SOIL-7-399-2021