Dissolved Organic Matter (DOM) is one of the important components controlling metal dissolution and mobility and hence the aim of this study was to reveal the interaction between DOM of different origins and Cu in a well-aged Cu-contaminated orchard sandy loam, with an ultimate goal to select an appropriate organic amendment for this calcareous sandy loam. Dissolved organic matter derived from five different organic materials (green manure, rice residue, pig manure, peat, and sewage sludge) were selected in this study. Dissolved organic matter of green manure and pig manure consisted of higher amounts of -COOH and -NH2 groups, but a lower affinity with the tested soil as compared to DOM from sewage sludge and peat. The sorption of DOM onto the soil was negatively correlated with the amount of small molecular size fraction and carboxyl-containing substance occurring in DOM. Addition of DOM derived from these organic materials to the selected soil caused an increase in soil-soluble Cu and Cu bioavailability of ryegrass . Chemical speciation showed that most Cu in soil solution existed in the form of DOM-Cu complexes. The ability of DOM derived from different organic materials in facilitating Cu dissolution follows the given sequence: green manure „l pig manure ƒ® rice residue ƒ® peat „l sewage sludge, which was positively correlated with the amount of the lower molecular size or hydrophilic fraction in DOM, and negatively with the affinity of DOM itself onto the soil. However, apparent biodegradation of DOM increased with increasing time of incubation of organic materials, which reduced Cu dissolution significantly, especially for green manure DOM. In aerobic incubation for 63 days, almost 90% of green manure DOM and 25% of sewage sludge DOM were decomposed within one day, and nearly 100% and 55% after one week following the addition of DOM, respectively. It took about 7 weeks or more for sewage sludge DOM to be decomposed completely in the soil (Fig. 1). Nevertheless, the Cu contents in soil receiving DOM still exceeded that of the control (with no DOM addition) 20 to 50 days after incubation, depending on the sources of DOM. It can be concluded that sewage sludge and peat moss would likely be the more suitable amendment materials for the Cu- contaminated orchard soil in view of the lower mobility of Cu.

Fig. 1. The kinetics of (a) biodegradation of DOM and (b) Cu dissolution from the Cu contaminated sandy loam incubated with DOM from green manure (GM) and sewage sludge (Slu) at 22„b1„aC