Geochemical composition of suspended sediments generated after erosive rainfall events in nested agroforestry catchments with Mediterranean climate

• Suspended sediments are enriched in clay particles and depleted in soil organic carbon. • Trace elements mainly attached to fine particles are enriched in the suspended sediments. • Enrichment in the finer fraction occurs after high and low intensity events. • Significant higher As, Be and P contents recorded in cropland than in woodland soils. • Suspended sediments show enrichment in As, Cu, Mn, Pb, P, Sr, and Tl compared to soils. Erosion results not only in the loss of soil but also of elements that act as macro and micronutrients for plants, as well as elements considered toxic that can have negative impacts on the environment. Most erosion studies assess total soil and nutrient losses, but fewer address the loss of trace metals and even less the fine material transported in suspension in ephemeral streams. To fill this gap, this study on the geochemical composition of suspended sediments (SS) was carried out in three nested agroforestry catchments drained by intermittent flows after 19 seasonal campaigns including events of different characteristics. We aimed to assess if land use and lithology affect suspended sediment composition and how runoff triggered by different rainfall events changes sediment composition. Soil and sediment properties were characterised by assessing soil particle distribution, soil organic carbon and stable elements. The difference in elemental content between land uses and sediment enrichment compared to soil was analysed. Clay and SOC were higher in sandstones while silt content was higher in mudstones, which dominated in the catchment with a higher proportion of cultivated areas that was situated closer to the outlet, receiving more runoff. Consequently, this catchment exported the most suspended sediments, enriched in clay but depleted in SOC. Most elements (Al, As Be, Bi, Cd, Cu, Fe, K, Li, Mn, Pb, Se, V and Zn) were more concentrated in sandstones, and associated with the clay fraction while only P, S, Sr and Ti were higher in mudstones and linked to the silt fraction. Boron and Ni correlated with SOC, whereas As and P showed significant correlations with clay and SOC. Arsenic, P and Tl had the highest contents in croplands suggesting an anthropogenic origin. Cr, Al and Sr were most in forests, mainly associated with clay. Metal enrichment in the finer fraction of suspended sediments occurs not only during high intensity events, but also in continuous, low intensity rainfall. Among the enriched elements (Cd, Cu, Mn, Pb, P, Sr, Tl, V and Zn), As, Pb and Tl showed greater enrichment during high erosive events, while Sr, Mn, and K, were more enriched during low erosive events. No significant differences in Cu, Pb, V and Zn were observed between event types. This research provides valuable insights into soil and metal export to water bodies, highlighting the risk posed by the enrichment of several harmful elements the environment.