Copper fertilizer effects on copper distribution and vertical transport in soils

The fate of Cu fertilizer in soils is important for its plant availability and environmental significance. Little research has been done to investigate the status of Cu in soil under repeated application of Cu fertilizer. A field study was conducted to determine the distribution and transport of Cu in soil profiles after 17 years' applications of Cu fertilizer. Soil samples obtained at different application times and soil depths were analyzed for labile Cu by a diethylenetriaminepentaacetic acid (DTPA) extraction procedure. A modified sequential extraction technique was used to separate soil Cu into exchangeable (Exe), organic matter weakly bound (Wbo), carbonate bound (Carb), manganese oxides bound (MnOx), organic matter specifically bound (Sbo), and mineral bound (Min) fractions. The results showed that long-term application of Cu fertilizer caused the accumulation of Cu in the surface soil. The accumulation of soil Cu in the 0-15 cm and 0-60 cm soil layers accounted for 41.3% and 57.3% of the Cu applied over 17 years. Calculations indicated that little Cu was removed from the plots by crop harvest, which meant that about 40% of fertilizer Cu leached below the 60 cm soil depth. Additional evidence suggested that some Cu leached below 400 cm. Total labile Cu and the increase in labile Cu in the surface layer soil followed a linear relationship with the amount of Cu added and with the application time of the Cu fertilizer. Copper bound to minerals and weakly bound to organic matter accounted for 76% to 92% of the total Cu in soil. In addition, the two fractions had the same distribution trend in the soil profile. The Cu content of different fractions declined in a similar order, i.e. Min-Cu>Wbo-Cu>Exe-Cu>Sbo-Cu>Carb-Cu>MnOx-Cu, regardless of Cu treatment. Most Cu added to soil entered into Min, Exe, Wbo, and Carb fractions. The Exe-Cu, Wbo-Cu, and Carb-Cu fractions are readily mobile and can be used to predict Cu transport. The transport ability of Cu fractions followed the order of Wbo-Cu>Exe-Cu>Carb-Cu and increased with increasing soil depth. These results indicate that the accumulation of Cu in the soil resulting from the long-term application of Cu fertilizer could pose an environmental risk due to the downward mobility of Cu.