Effects of External Copper (Cu) Loading On Forms of Cu In Sandy Soils Under Citrus Production.

Copper (Cu) is essential to plant growth and animal health (including human), but toxic at excessive levels. Due to repeated use of Cu-based fungicides, Cu has been reported to accumulate in citrus grove soils in Florida. In this study, laboratory incubation and soil Cu characterization were conducted to examine the effects of external Cu loading on the forms and bioavailability of Cu in soils. A representative Alfisol and Spodosol, dominant soil types under citrus production in South Florida, were collected at the 0-20 cm from commercial citrus groves. The soils were treated with external Cu at the levels of 0, 200, 600, and 1000 mg kg^-1, and incubated at room temperature at 70% field holding capacity for 12 months before subsamples were taken and analyzed for available Cu by CaCl₂, NH₄OAc, and Mehlich 3 extraction methods, fractionation of Cu, and related soil properties. External Cu loading generally decreased soil pH, particularly for the Spodosol, and such pH decrease may, in turn, enhance Cu availability and movement. Chemical speciation data show that activity of organically-bound Cu (i) is greater than that of inorganic Cu species and (ii) increases with Cu concentration and pH for both soils. Available Cu determined by the three extraction methods dramatically increased with external Cu loading rate. The two soils yielded similar results although total recoverable and extractable forms of Cu were higher in the Spodosol than in the Alfisol. Soil Cu fractionation revealed that most increases in soil Cu occurred in the exchangeable and oxides-bound fractions, which are positively related to external Cu loading rate and correlated with available Cu. These results indicate that most Cu accumulated in the citrus grove soils is highly mobile and bioavailable, and thus may have potential impacts on citrus production and the environment.