The influence of liming on cadmium accumulation in rice grains via iron-reducing bacteria

Cadmium (Cd) in soil is attracting worldwide attention, and many valuable measures and suggestions of minimizing the rice grain Cd are available. Among these methods, liming can increase the soil pH and decrease the rice grain Cd content. Here, we report that soil pH was negatively and significantly correlated with the concentration of soil extractable Fe and Cd. In addition, the iron concentration on root surface was significantly and positively associated with the available metals in soil and the rice grain Cd. However, the return of contaminated rice straw significantly increased the Cd accumulation in the rice grain, although the returned straw did not significantly influence the concentration of extracted soil Cd. Furthermore, an analysis of the functional microbe community was performed, and the response of iron-reducing bacteria (IRB) under the six treatments provides valuable insights for reducing the available Cd concentration in soil. A LEfSe (LDA coupled with effect size measurement for significant differences) analysis showed that the application of liming reduced the abundance of IRB. The results of a redundancy analysis (RDA) indicated that soil pH was significantly and negatively associated with the abundance of Proteobacteria and Geobacter and the concentration of bioavailable Fe and Cd in the soil, which could explain the reduced accumulation of bioavailable Cd in rice grain. Collectively, our results demonstrated that liming in Cd-polluted paddy soil is a reasonable strategy for minimizing rice grain Cd by increasing the soil pH, which reduces the soil available iron and Cd concentration by shifting the diversity and composition of IRB, thus ultimately resulting in decreased Cd in rice.