Effects of biochars combined with ferrous sulfate and pig manure on the bioavailability of Cd and potential phytotoxicity for wheat in an alkaline contaminated soil

Abstract Reducing the content of Cd in wheat grains is necessary for human health, especially in alkaline farmland in northern China where a large area of soils is heavily polluted with Cd. A field experiment was conducted to investigate the effects of wheat straw biochar (WB) and maize straw biochar (MB) combined solely with ferrous sulfate (FeSO4) or combined with FeSO4 and pig manure (PFWB and PFMB) on Cd bioavailability in soil and the phytotoxicity of Cd in wheat in an alkaline contaminated soil. The results showed that the application of these treatments decreased the concentrations of CaCl2-extractable Cd by 32.8–60.5% and increased the soil organic matter by 36.8–106.7% compared with those of the control treatment. The acid-soluble fraction of Cd was decreased by 3.7–16.8% but the residual fraction was increased by 1.7–13.7%. Furthermore, the addition of WB and MB increased the soil pH by 0.29 and 0.16 units, respectively, while the other treatments decreased the soil pH by 0.16–0.37 units. The concentrations of Cd in different wheat tissues were decreased in amended soils, and these decreases were positively correlated with CaCl2-extractable Cd and the acid-soluble fraction of Cd. The ability to accumulate Cd was in the order of root>straw>grain. In addition, there was a downward trend in the bio-concentration and translocation factors under different amendments. The amendments of PFWB and PFMB had better efficiency in the immobilization of Cd in soil and the inhibition of Cd uptake of wheat than WB and MB. In addition, the treatments of PFWB and PFMB increased the wheat yield by 14.3–16.2%, and thus have a great application prospect based on the cost-benefit analysis.