Metal distribution and biological diversity of crusts in paddy fields polluted with different levels of cadmium.

The paddy-crusts (PCs) play an important pole in the transformation and transfer of heavy metal in paddy. Different PCs were collected from paddy fields whose soils contained cadmium (Cd) at four concentration levels (0.61, 0.71, 1.53, and 7.08 mg/kg) in Hunan Province, China P.R. at Sep 2017. This metal's distribution among and biological community structures of PCs were both measured. Our results indicated that PCs were able to accumulate Cd from irrigation water and soil. With greater Cd levels in paddy fields, the weak EPS-binding Cd fraction decreased whereas the non-EDTA-exchangeable Cd fraction increased. The sorbed Cd fraction was initially enhanced at low-to mid-level Cd concentrations, but then gradually declined. Biomineralization was shown to function as the dominant Cd accumulation mechanism in non-EDTA-exchangeable fractions. The biological diversity of soil microbes decreased with more Cd in soil, and the Proteobacteria, Bacteroidetes, and Cyanobacteria were the dominant phyla in all the sampled PCs. Canonical correspondence analysis (CCA) between the composition of microbial communities and soil chemical variables in the PCs clustered all samples based on the Cd-contaminated level, and demonstrated that Cd, Mn, and Fe all significantly influenced the microbial communities. In particular, the Alphaproteobacteria and Chloroplast classes of bacteria may play a significant role in Cd accumulation via the bio-mineralization process. Taken together, our results provide basic empirical information to better understand the heavy metal speciation transformation mechanisms of PCs upon Cd-contaminated paddy fields.