Nitrogen-mediated distinct rhizosphere soil microbes contribute to Sorghum bicolor (L.) Moench and Solanum nigrum L. for phytoremediation of cadmium-polluted soil

Aims Nitrogen is correlated with plant biomass and Cd concentration, collectively influencing phytoextraction of Cd-polluted soil. The Cd phytoextraction by hyperaccumulating plants and high biomass plants as affected by nitrogen remains elusive. Methods Sweet sorghum (Sorghum bicolor (L.) Moench), a high biomass energy plant, and Solanum nigrum L., a Cd hyperaccumulator were investigated. Plant growth, Cd accumulation and soil properties as affected by different forms and dosages of nitrogen supply in Cd-polluted soil were studied. Additionally, rhizosphere soil bacterial and fungal community changes of the two species and their relationships with plant Cd accumulation were determined. Results A high level of nitrate resulted in the highest and equal Cd accumulation in shoots of the two plants. Sweet sorghum exhibited high photosynthesis rates with capacity for bioethanol production, enabling the re-utilization of post-harvested Cd materials. Consequently, sweet sorghum demonstrated a greater potential for Cd phytoextraction than S. nigrum. Regarding microbial community, clay was the driving factor influencing rhizosphere soil bacterial community in sweet sorghum as affected by nitrogen under Cd stress, while soil available Cd played a vital role in S. nigrum. Moreover, Pseudomonas and AKYG1722 bacteria showed a positive correlation with shoot Cd accumulation in sweet sorghum. AKYG1722, Sphingobium herbicidovorans, Sphingobium chlorophenolicum, Streptomyces scabiei, Devosia sp. I507, Bacillus simplex and Ensifer meliloti were positively correlated with shoot Cd accumulation in S. nigrum. Conclusions Nitrate performs better than ammonium and urea for improving phytoremediation of Cd-polluted soil using sweet sorghum and S. nigrum, but their rhizosphere soil microbial communities were species-dependent.