Graphitic carbon nitride alleviates cadmium toxicity to microbial communities in soybean rhizosphere.

Cadmium (Cd) contamination has led to various harmful impacts on soil microbial ecosystem, agricultural crops, and thus human health. Nanomaterials are promising candidates for reducing the accumulation of heavy metals in plants. In this study, graphitic carbon nitride (g-CN), a two-dimensional polymeric nanomaterial, was applied for ameliorating Cd phytotoxicity to soybean (Glycine max (L.) Merr.). Its impacts on rhizosphere variables, microorganisms, and metabolism were examined. It was found that g-CN increased carbon/nitrogen/phosphorus (C/N/P) content, especially when N contents were averagely 4.2 times higher in the g-CN-treated groups. g-CN significantly induced alterations in microbial community structures (P < 0.05). The abundance of the probiotics class Nitrososphaeria was enriched (on average 70% higher in the g-CN-treated groups) as was Actinobacteria (226% higher in the g-CN group than in the CK group). At the genus level, g-CN recruited more Bradyrhizobium (122% higher) in the Cd + g-CN group than in the Cd group and more Sphingomonas (on average 24% higher) in the g-CN-treated groups. The changes of microbial clusters demonstrated the potential of g-CN to shape microbial functions, promote plant growth, and enhance Cd resistance, despite observing less pronounced modifications in microbial communities in Cd-contaminated soil compared to Cd-free soil. Moreover, abundance of functional genes related to C/N/P transformation was more significantly promoted by g-CN in Cd-contaminated soil (increased by 146%) than in Cd-free one (increased by 32.8%). Therefore, g-CN facilitated enhanced microbial survival and adaptation through the amplification of functional genes. These results validated the alleviation of g-CN on the microbial communities in the soybean rhizosphere and shed a new light on the application of environmental-friendly nanomaterials for secure production of the crop under soil Cd exposure.