Effect of amendments on bioavailability of cadmium in soil-rice system: a field experiment study

The field experiment study investigated the effect of lime (L), manure compost (M), combination of lime and manure (LM), and combinations of lime with four kinds of passivators (LP1, LP2, LP3, and LP4) on the bioavailability of cadmium (Cd) in soil and Cd accumulation in rice plants. These four passivating products were composed of organic and inorganic compounds such as silicon-sulfhydryl group, CaO, SiO₂, and so on. The results indicated that the application of these amendments improved soil pH, organic matter content, and cation exchange capacity (CEC) by 0.19-0.73 unit, 0.6-8.2%, and 5.7-38.9%, respectively; meanwhile, decreased soil acid-extractable Cd by 4.0-13.9% compared with before remediation. Alleviating Cd stress to rice also resulted in a significant increase in rice grains yield, whereas the LP4 showed an increment of 15.8-27.6%. Among these amendments, LP4 had a relatively high effectiveness, it promoted the decrease of extractable Cd by 13.9% and the increase of residual Cd by 8.1%; meanwhile, the bioconcentration factor of rice grain in LP4 decreased by 71.3%. The high pH, CEC, and rich functional groups in amendments might cause soil Cd transform from mobile fraction to residual fraction, and the cation ions in amendments also competed with Cd ions due to the antagonism. Taken all of these effects, the amendments alleviated Cd pollution in soil-rice system, decreasing Cd migration from soil to grain. In future, the long-term field experiment will need to be done for verify the long-term effect of soil amendments.