Biochar changes iron-reducing bacteria community in paddy soils and promotes the bacterial iron reduction

The iron reduction in paddy soils, which is mainly driven by iron-reducing bacteria (IRB) communities is vital for iron and other nutrient supplies and contamination management. This study aimed to investigate biochar effects on the community structure of IRB in paddy soils and the iron reduction driven by them in terms of iron reduction dynamic and secondary iron mineral composition.

We isolated bacterial iron reduction in paddy soils in a constructed system with the bacteria source from paddy soils and IRB-selective media, with decoupling the effects of complicated geochemical factors from natural environments. Rice straw biochar was added into the iron-reduction system at 1 and 3% rates (referred to as BC1 and BC3). The putative IRB community was analyzed using 16 S rRNA gene sequencing, the dynamic of Fe(II) were tested by ferrozine test method, and the products from bacterial iron reduction were analyzed by XRD and a sequential extraction procedure.

Biochar at 1 and 3% (BC1 and BC3) rates notably increased the relative abundance of Bacillota phylum of paddy soils by 52.2 and 55.0% in comparison with control, and decreased Pseudomonadota and Thermodesulfobacteriota significantly. Moreover, BC1 and BC3 markedly increased the relative abundance of a typical hydrogen (H₂)-producing genus (Clostridium sensu stricto) by 64.8 and 62.2%, respectively. PCA analysis also showed that putative IRB communities in biochar treatments were significantly different from those in the control. Fe(II) concentrations in BC1 (1.92–10.0 mg/L) and BC3 (3.80–12.4 mg/L) were much higher than those in the control (0.27–0.83 mg/L), implying the enhancement of bacterial iron reduction related to the biochar addition. XRD analysis and sequential extraction showed that siderite was the major secondary iron mineral of bacterial iron reduction due to biochar addition, whereas considerable amounts of ferrihydrite, akaganeite, and magnetite were also observed in the control.

It suggests that the increase in Bacillota and Clostridium sensu striction in paddy soils owing to biochar addition is responsible for bacterial iron reduction enhancement. The promotion of iron reduction could facilitate the formation of iron plaque in the rice rhizosphere. Moreover, the high Fe(II) content induced by biochar tends toward Fe(II) oversaturation, resulting in the rapid formation of siderite via precipitation pathways; hence, it is beneficial for heavy metal immobilization in paddy soils through co-precipitation with siderite.