Effects of Ca-WTRs Amendment On Available Cu in a Sandy Soil: Incubation Study.

Copper (Cu) is one of the heavy metals which cause contamination to agricultural soils at a large scale due to repeated application of Cu-containing fungicides. Laboratory incubation and analysis were conducted to determine the effect of calcium enriched water treatment residuals (Ca- WTRs) application on Cu availability in acidic sandy soil under citrus production in south Florida.  Two soils (an Alfisol: fine loamy siliceous hyperthermic Typic Glossaqualfs and a Spodosol: sandy siliceous hyperthermic Alfic Haplaquods) used in the incubation study were collected from two citrus groves in the Indian River area of south Florida. Ca-WTRs were applied at the rates of 0, 5, 10, 20 g kg^-1 soil to the Alfisol soil and 0, 5, 50, 100 g kg^-1 soil to the Spodosol soil. There were two levels of Cu in this study: the original soil with or without enriched with 400g kg^-1 Cu in the form of CuNO₃. The amended soils were then incubated at room temperature after the moisture was adjusted to 70% of water holding capacity (WHC). At the intervals of 0, 1, 3, 7, 14, 28, 42, 70 d of incubation, subsamples of the soil were collected for analyses of pH, electrical conductivity (EC), dissolved organic carbon (DOC), and extractable Cu  determined by (i) 0.01 M CaCl₂; (ii) 1M ammonium acetate (NH₄OAc); and (iii) Mehlich-3.  At the end of incubation (70 d), soils were also analyzed for Cu fractionation into exchangeable, carbonate-bound, organically bound, oxide-bound, and residual fraction. Ca-WTRs application increased soil pH, Ca, and DOC, but deceased extractable Cu as estimated by the three extraction methods. For the amended soils, the decrease in extractable Cu was in the order of 0.01 M CaCl₂> 1M NH₄OAc> Mehlich-3. Lower exchangeable Cu in soil was generally associated with higher WTR application rate. Carbonate- and organic- bound Cu fractions decreased while oxide-bound, and residual fraction increased. For both soil types, amending WTRs to soil at the lowest Ca-WTRs rate reduced the exchangeable Cu by more than 63% as compared with soils without WTRs. For both soils enriched with 400g kg^-1 Cu, exchangeable Cu was reduced by more than 92% by WTRs amendment. These results indicate that application of adequate amount of WTRs can reduce labile Cu in soils, thus reducing Cu bioavailability and mobility. However, further study is needed to examine the effect of increased DOC on Cu losses by leaching and/or surface runoff.