Assessing Copper Mobility and Bioavailability over Time (Ageing) in Vineyard Soils Following Bordeaux Mixture Application: Influence of Soil Organic Matter and pH

Abstract Cu-based pesticides are widely used in vineyards to control fungal diseases, but their repeated application leads to Cu accumulation in soils, posing environmental and ecological risks. Since Cu mobility, bioavailability, and potential toxicity depend not only on total concentrations but also on chemical fractionation and ageing processes, this study investigated Cu speciation—including water-soluble Cu (Cu W ), exchangeable Cu (Cu CaCl2 ), and potentially bioavailable Cu (Cu DTPA ) in 10 vineyard soils from northern Iberia. Soils were analyzed both unspiked and after laboratory spiking with Bordeaux mixture. Total Cu contents in unspiked soils ranged from 46 to 500 mg kg −1 , whereas Cu W and Cu CaCl2 remained below 14.2 mg kg −1 , showing that only a small fraction of total Cu is immediately mobile or bioavailable. Spiking significantly increased Cu W and Cu CaCl2 (up to 89.8 and 164.6 mg kg −1 , respectively), with the highest levels observed in acidic soils with low organic matter, demonstrating that soil properties strongly modulate Cu mobility. Cu DTPA contents were higher, ranging from 3.9 to 94.3 mg kg −1 in unspiked soils, and reached 956.1 mg kg −1 after spiking, reflecting medium- to long-term potential bioavailability and ecological risk. Incubation over time led to decreases in Cu W and Cu DTPA , indicating that ageing progressively shifts Cu from labile to more stable fractions, reducing mobility and bioavailability. These results highlight that assessments based on freshly spiked soils may overestimate environmental risks, whereas long-term accumulation in field conditions poses lower hazards, underscoring the importance of considering soil properties and ageing effects in vineyard Cu management and risk evaluation.