Copper contamination of soil and vegetation in coffee orchards after long-term use of Cu fungicides

Abstract

The repeated use of Cu fungicides to control coffee berry disease can result in increased Cu content in soils and vegetation and thus raising the pollution levels and a concern for potential effects on human health. Therefore a field survey of coffee orchards of the Kilimanjaro and Arusha regions of Tanzania, where Cu fungicides are frequently used, was conducted to determine the possible Cu contamination of soils and vegetation. Soil samples were collected from 0–5, 5–15 and 15–30 cm depth at a distance of 0.5 m from the trunk of coffee trees. Plant leaf samples of beans and maize crops intercropped with coffee trees as well as leaves of coffee trees, were also collected at the same time and places. Soil and plant samples were collected both from small farms and commercial estates. Copper in the soil samples was extracted with CaCl₂ (CCu), whereas for the total Cu, samples were digested with aqua regia (HNO₃ and HCl(1:3) (ACu)). Plant samples were drying ashed and digested with HNO₃. Copper concentration in the extracted and digested solutions was determined with atomic absorption spectrophotometry. A significant Cu enrichment of the soils was observed and the Cu concentration in the 0–5 cm soil depth was about three times higher than in the 15–30 cm depth. A large variability in the soil Cu concentration between fields of the same farm and within the same orchard was observed. The differences in the total Cu concentration (ACu) between small farms and commercial estates were not statistically significant, but the extractable Cu (CCu) was higher and soil pH was lower in the estates of the Kilimanjaro region than at small farms. The soil organic carbon (SOC) content increased significantly with increasing altitudes, and it was also higher on small farms than on commercial estates. The SOC content showed a close and positive correlation with total Cu. At soil pH below 5.3, the CCu fraction increased steeply. Bean plants from coffee fields showed a high concentration of Cu, suggesting a possible Cu toxicity problem in bean plants. Coffee leaves also showed extremely high Cu concentration.