Quantifying nutrient loss across particle size fractions in eroded tropical soils using 239+240Pu fallout radionuclides

In sub-Saharan Africa, where livelihoods depend on agriculture, steep valley slope cultivation intensifies soil erosion, threatening agricultural sustainability by depleting fertile topsoil of nutrients. This study measured soil erosion and deposition using 239+240 Pu fallout radionuclides and associated macro- and micronutrient loss across particle size fractions in the Oroba Valley, Nandi Hills, Kenya. Three experimental plots were assessed: Plot 1 (conventional tillage) and Plot 2 (terraced agricultural system), both of which were cleared for cultivation in 1940, while Plot 3 (historically shrubland) was recently cleared in 2023. A stratified sampling design was used to collect soil samples, which were analysed for particle size distribution, organic matter (OM) content, pH, and total elemental composition using ICP-MS/MS. The MODERN model to estimate erosion and deposition rates using 239+240 Pu inventories integrates nutrient losses across specific particle sizes to estimate the nutrients lost. There was severe soil erosion in Plots 1 and 2, with net losses of 13.68 t ha - ¹ yr⁻¹ and 6.09 t ha - ¹ yr⁻¹ , respectively, while Plot 3 showed minimal loss (0.32 t ha - ¹ yr⁻¹), reflecting the protective effect of vegetative cover. Fine particles (<100 µm), rich in OM and nutrients (Se, Zn, P, I) were disproportionately lost downslope, particularly in intensively cultivated plots. Terracing in Plot 2 was ineffective due to poor maintenance, with erosion observed at terrace sections. Micronutrient loss, notably Mo, Cu, and Se, was linked to < 50 µm particle size erosion, while the large fractions (>100 µm) retained K and Mg. This study shows how 239+240 Pu isotopes can be used as sensitive indicators of soil loss and fertility degradation, offering land management and conservation insights. • Agricultural soil redistribution measured using 239+240 Pu. • Nutrient loss was quantified in specific soil particle size fractions. • Land management practices influence soil redistribution rates.