Accumulation and Fractionation of Copper in Citrus-growing Soils of Indian River Area.

Copper (Cu) accumulation in agricultural soils has been increasingly reported, particularly under citrus and grape production due to repeated application of Cu-containing fungicides.  Fractionation of trace metals has been considered as a useful tool to evaluate metal bioavailability in relation to transformation in soils. In this study, soil samples were collected from representative commercial citrus groves in the Indian River area and analyzed for total recoverable, and extractable Cu by three most commonly used procedures. Meanwhile Cu fractionation was also conducted to evaluate Cu bioavailability relative to transformations in sandy soils under citrus production for varying periods.

Total recoverable Cu in the surface soils ranged from 4.7 to 228.5 mg kg^-1, with an average of 60.0 mg kg^-1, which is much higher than background level of agricultural soils (20-30 mg kg^-1). Soil available Cu as estimated by Mehlich 3 was in the range of 2.2 to 119.1 mg kg^-1, with an average of 31.3 mg kg^-1, and was significantly correlated with total recoverable Cu (P<0.001).  The biggest proportion of total recoverable Cu was present as organic matter-bound, ranged from 25.2% to 58.1%. Both the recoverable Cu and Mehlich 3 extractable Cu were significantly correlated with the soil organically bound Cu (P<0.001). The mean percentages of the total recoverable Cu as various Cu fractions in the citrus soils were organically bound (48.2%) > oxides-bound (29.8%) > residual (16.5%) > carbonate-bound (4.2%) > exchangeable (1.3%).  These results indicate that Cu accumulation in the sandy soils under citrus production primarily occurs in organic fraction, and the organically bound fraction may control the mobility and availability of Cu in the citrus-growing soils.