Time effects of rice straw and engineered bacteria on reduction of exogenous Cu mobility in three typical Chinese soils

Globally, copper (Cu) accumulation in soils is a major environmental concern. Agricultural organic waste and some bacterial species can readily absorb metals in an eco-friendly manner, and thus are commonly used in metal-contaminated soil remediation. This study investigates the change in Cu fractions during the aging process and the time effects of rice straw (RS) and engineered bacteria (EB) (Pseudomonas putida X4/pIME) on reduction of Cu mobility. Three typical Chinese soils (red, cinnamon, and black soils) were incubated with RS or RS + EB in the presence of exogenous Cu for 24 months. The soil physicochemical properties, reactive soil components, Cu fractions, and Cu mobility were determined over time. The Cu mobility factor (MF) values were the lowest in the black soil (6.4-9.2) because of its high organic carbon and clay contents. The additions of both RS and RS + EB accelerated Cu stabilization during the aging process in all three soils. The Cu MF values decreased with time during the initial 20 months; however, the MF values increased thereafter in all soils, which might be due to the reduction of humic substances and amorphous iron oxides and the increase in iron oxides complexed on the organic matter. The reduction rates of Cu MF were similar after 16, 24, and more than 24 months in the red, cinnamon, and black soils, respectively, indicating that RS and RS + EB could limit Cu mobility at different times in various soils. The RS treatment showed the greatest efficiency in reducing Cu mobility in the red, cinnamon, and black soils after 12, 12, and 8 months of incubation, respectively. The RS + EB treatment was more efficient than the RS treatment in the red soil during the initial 8 months of the incubation period. Our study provides theoretical support for Cu risk assessments and RS supplementation for Cu remediation in different soils.