Nutrients and carbon losses due to wind erosion in Sahelian Senegal

Abstract

Land degradation appears today as a major issue for feeding humans, whose population on Earth is still increasing. Such issues are particularly acute in semi-arid regions like the Sahel, where, in addition, soils are already poor in nutrients and carbon. Wind erosion is one of the processes likely to cause soil depletion of nutrients and carbon in this region, thus potentially leading to land degradation. However, there is little scientific literature providing quantitative estimates of soil losses of nutrients and carbon through the windblown sediments, particularly for the Sahel. We monitored the horizontal flux of windblown sediments (collected every 2 weeks in MWAC sand-traps) for the main land use types of Western Sahel in the Peanut Basin of Senegal: a field (bare, then cropped with groundnut) (2020-2021), 4 fallows (2022/2023), and 4 millet fields (2023/2024) to characterize differences in windblown sediment fluxes due to land management (e.g. management of crop residues, grazing pressure, …). Each plot was about 1 ha, and had 5 masts of 5 MWAC each, from 5 cm to 80 cm above ground level. During these experiments, we also monitored vegetation characteristics (every week) and meteorological variables (at 5-minutes resolution). We then carried out analyses on the windblown sediments collected in the sand traps for the bare/groundnut field (the amounts of sediments collected in the fallows were too low to perform such analyses, and the erosive period, which extends from January to April in Western Senegal, just started in early 2024 for the millet fields experiment). Our first results show that the horizontal flux of windblown sediments is much larger for a bare/groundnut field (around 40 t/ha/year) than for fallows (0.03 to 0.80 t/ha/year). The results also suggest that differences in land management among fallows (e.g. age, woody cover) may have an impact on this horizontal flux. Additionally, the orders of magnitude of the organic C, total N and available (Olsen) P concentrations of the horizontal (saltation) flux from the bare/groundnut field are respectively approximately 0.22%, 0.02%, and 8 ppm, thus larger than the concentrations of the topsoil, that are respectively 0.15 %, 0.01 % and 7 ppm. These values compare well with those existing in the literature (e.g. for Niger), and confirm an enrichment of the horizontal flux (by a factor of 1.1 to 1.8) in soil carbon and nutrients compared to the topsoil. A rough estimate of organic C losses from the monitored field of 1 ha would be about 80 kg/year, and 8 kg/ha and 0.32 kg/ha for N and available P, respectively. To better characterize the composition of windblown sediments, we also plan to carry out microbiological analyzes to determine the composition of the microbial communities (bacteria and fungi) they contain. Indeed, these microbial communities play a role in the biogeochemical cycle of nutrients and in the storage of carbon in the soil, thus potentially impacting their fertility. We thank the IMAGO/LAMA analytical laboratory of Dakar for the analyses of the composition of the sediment samples.