Ageing of copper, zinc and synthetic pesticides in particle-size and chemical fractions of agricultural soils

Abstract

The transformation and mobility of heavy metals and synthetic pesticides in soil depend on ageing, involving their chemical and physical distributions among soil fractions over time. Heavy metals and synthetic pesticides often co-occur in soil, although their ageing is usually evaluated separately and in bulk soil. Here, contrasting vineyard and crop soils were spiked with copper (Cu; 700 mg kg−1) and zinc (Zn; 200 mg kg−1) a and/or synthetic pesticides (5 mg kg−1), i.e., the fungicide metalaxyl (MTY) and herbicide S-metolachlor (SMET), to evaluate within 200 days their distribution among soil physical and chemical fractions. More than 90% of MTY and SMET in soil was released into the water phase, even 200 days after spiking. This emphasizes the potential mobilization of MTY and SMET from the soil following field application. MTY, SMET, Cu and Zn were associated mainly with the silt fraction and to a lesser extent (<30%) with the sand and clay fractions. Overall, the ageing of MTY, SMET, Cu and Zn in agricultural soil was affected mainly by the soil type and sterilization and only to a minor extent by their co-occurrence. Sorption controlled the dissipation of MTY and SMET in soil, while biodegradation contributed to less than 10%. A large fraction (37 ± 2%) of Cu was associated with Fe oxides after 200 days of ageing, while Zn was found (33 ± 2%) in the residual soil fraction. The silt fraction of the nonsterile vineyard soil became enriched in 65Cu over time (Δδ65Cu = 0.25 ± 0.07‰), whereas the clay fraction was depleted in 65Cu (Δδ65Cu = −0.20 ± 0.07‰). Cu isotope fractionation mirrored the Cu distribution in soil chemical fractions, suggesting that Cu stable isotopes may help to follow-up Cu ageing. In contrast, no significant Zn isotope fractionation was observed among soil experiments or over time. Overall, our study emphasizes the variability in ageing of synthetic pesticides and heavy metals co-occurring in agricultural soils and their interplay in physical and chemical fractions of the soil.