Ecological risk of copper and zinc and their different bioavailability change in soil-rice system as affected by biowaste application

A large amount of organic fertilizer application could be accompanied by soil contamination caused by trace heavy metals. A field experiment was carried out in this study to examine the accumulation and availability of copper (Cu) and zinc (Zn) in soil, and their uptake by rice under continuous application of chicken manure, pig manure and sewage sludge. Results showed that after four years of chicken manure, pig manure and sewage sludge application, the soil Cu accumulation rates were 0.15–1.17 mg kg−1 yr−1, 1.01–4.22 mg kg−1 yr−1 and 0.13–1.15 mg kg−1 yr−1, respectively; Zn accumulation rates were 0.54–5.46 mg kg−1 yr−1, 1.51–9.65 mg kg−1 yr−1 and 1.13–10.47 mg kg−1 yr−1, respectively. Compared to the control, the chicken- and pig manure treatments significantly decreased the DTPA-extractable Cu, but increased the DTPA-extractable Zn in soils; thus decreased the Cu contents in rice grain by 2.2–40.6% and increased the grain Zn by 2.6–30.9%, respectively, with increasing application rates and number of years. The addition of sewage sludge significantly increased bioavailability of Zn in soil and its accumulation in rice, while had limited effect on Cu bioavailability. Results suggested that the continuous application of organic fertilizer with elevated Cu and Zn contents at high application rates can induce their accumulation in soil and affect their bioavailability differently.