Dynamic response of cadmium immobilization to a Ca-Mg-Si soil conditioner in the contaminated paddy soil

Soil conditioners are often used to immobilize soil heavy metals. Understanding the transfer of Cd in soil-plant system to different application rates and modes of soil conditioners application is essential for food safety. The stabilization persistence of soil conditioners in immobilizing Cd, to date however, is still limited. In this study, the stabilization persistence of a Ca-Mg-Si soil conditioner (SC) was assessed based on a six-year Cd-contaminated paddy field study with growth of two rice local main varieties (Yongyou17-YY and Xiushui14-XS) and four application rates (1500 kg ha^-1 (low), and 3000 kg ha^-1 (high) for the first year only, and 1500 kg ha^-1 and 3000 kg ha^-1 every year). Results showed that continuous SC application with high rate increased soil pH, simultaneously with more water soluble and exchangeable Cd was transferred to Fe-Mn oxides bound and carbonate-bound Cd in the first 3-4 years; while the low rate was only effective with growth of YY that were applied for a shorter period of time. Statistical analysis indicated that the stability effect of SC was integratedly affected by soil pH, SC application rate, and meteorological factors (precipitation and temperature). Especially, soil fractionation contributed the most changes of Cd availability in soil, while meteorological factors, SC application rate and crop varieties altogether exhibited the great effect on Cd accumulation in grain. Our finding demonstrated the potential long-term stabilization of SC in soil Cd immobilization, with the performance needed for further verification on the basis of different soil types.