The Impact of Remediation Through Stabilizing Amendments on Taxonomic and Metabolic Patterns of Bacteria and Archaea in Cadmium-Contaminated Paddy Fields in Southwestern China

The in-situ immobilization of heavy metal contamination in soils using stabilizing amendments is a cost-effective remediation technique. However, previous research on mediating cadmium polluted fields using amendments has focused mainly on the absorption and accumulation of cadmium by plants, rather than the response of soil microorganisms to amendments. In our study, five compounds with different pH values and concentrations of carbonate minerals, clay minerals, sulfur, and phosphorous were selected to investigate their effect on the soil microorganisms and metabolic patterns through metagenomic analysis over three months in cadmium-contaminated paddy fields (CCPFs) in southwestern China. The results showed that the pH value of the amendments was the major factor determining the microbial diversity and communities. In weak acidic paddy fields, the appropriate use of an alkaline amendment composed mainly of calcium oxide increased the pH value of the soil, which helped to improve the soil microbial diversity, promote the growth of azotobacter, nitrate-reducing bacteria (such as Pseudomonas), and metabolisms of nitrogen fixation and reduction, which contributing to the decrease of available cadmium in soils. Acid amendments which significantly reduced the soil pH value, had lowest removal rate of available cadmium and showed significant restrictive effects on bacterial and arhaeal diversity and growth. In addition, the effects differed between different alkaline amendments. Alkaline amendment composed of magnesium oxide promoted the growth of Pseudomonas, but also inhibited Nitrosospira and the metabolism of soil nitrogen fixation. In conclusion, when applying amendments to remediate cadmium-contaminated paddy soils, we need to take into account the pH value of the amendment and the content of each component, and ensure the efficiency of amendments while at the same time maintaining a positive effect on soil microorganisms.\n\nImportancePrevious research on amendment applications in cadmium-contaminated paddy fields (CCPFs) has mainly focused on the availability of cadmium to plants, the identification of the best functional additive proportion of immobilization remediation, and the absorption and accumulation of heavy metals by plant, etc. However, research on the taxonomic and metabolic patterns of bacteria and archaea in amended soils, especially purple paddy soil, which is typical of the Sichuan area (4.601 million hectares), is insufficient. Secondly, the plough layer of paddy soil would be in a reduction state during the period of irrigation, but be in an oxidation state when fields were in draining and drying periods. The periodical alternation of wetting and drying forms unique physical, chemical, and biological properties, and it is vital to understanding the taxonomic and functional dynamics of microbiomes in amended soils in order to improve the process performance.