Soil erosion and land-use management influence bioavailable selenium in a highland agricultural landscape in Kenya

Selenium (Se) is an essential micronutrient whose availability in agricultural systems is influenced by geochemical, pedological, and anthropogenic factors. The study aimed to investigate the effects of soil erosion, land-use, and management practices on the bioavailability and redistribution of selenium within a soil-plant continuum in a highland agricultural landscape in Nandi County, Kenya. A stratified sampling approach was applied across three plots representing distinct land-use management practices and varying soil erosion rates. Selenium concentrations were measured both in soils and crops (maize, beans, and pumpkin) collected from the experimental sites. Total and bioavailable Se concentrations in soils were evaluated using a mixed acid digestion (HF, HNO₃, and H₂O₂) and an ammonium chloride (NH₄Cl) extraction, respectively, followed by ICP-MS analysis. Structural equation modelling and regression analyses revealed that soil pH (β = - 0.41, p < 0.001), organic matter (β = 0.29, p < 0.01), and total Se (β = 0.30, p < 0.001) significantly influenced bioavailable Se, although plant uptake showed weak correlation. Temporal comparisons showed a significant increase in bioavailable Se in Plot 3 from 6.74 µg kg^-1 (2021) to 10.03 µg kg^-1 (2023), representing a 42% rise. In contrast, Plot 1 experienced a significant decline (p < 0.05) from 14.72 µg kg^-1 to 9.56 µg kg^-1 over the same period, linked to an increase in erosion rate by 0.58 t ha^-1 yr^-1. Maize showed significantly higher Se accumulation in Plot 1 (187 µg kg^-1) compared to Plot 3 (87 µg kg^-1) (p < 0.05), while beans and pumpkin showed no significant differences across plots (p > 0.05). These findings suggest that appropriate land management and erosion mitigation may help maintain soil Se bioavailability, while targeted agronomic strategies could be considered to enhance crop Se concentrations. However, the relatively low Se concentrations observed in several crops suggest a potential concern regarding Se adequacy, and assessment of dietary intake and human biomarkers would be required to establish nutritional risk.