# Diversity of nifH Gene in Culturable Rhizobia from Black Locust (Robinia pseudoacacia L.) Grown in Cadmium-Contaminated Soils

## TL;DR

This study explores rhizobia diversity in black locust plants from cadmium-contaminated soils to find metal-tolerant strains useful for phytoremediation.

## Contribution

The study identifies metal-tolerant rhizobia strains and their resilience in cadmium-contaminated soils for phytoremediation applications.

## Key findings

- Cadmium exposure did not significantly affect rhizobia abundance, diversity, or evenness but altered community structure.
- Dominant genera like Mesorhizobium, Sinorhizobium, and Rhizobium remained stable under all cadmium levels.
- Soil pH, total Cd, DTPA-Cd, and C/H ratio were key factors influencing the rhizobia community composition.

## Abstract

This study investigated culturable rhizobia diversity in black locust (Robinia pseudoacacia L.) grown in cadmium (Cd)-contaminated soils to identify metal-tolerant strains for enhancing phytoremediation. Using high-throughput sequencing of the nifH gene, 16 genera and 26 species were identified across five Cd treatments. Cd exposure did not significantly alter rhizobia abundance, diversity, or evenness but reshaped the nifH-containing community structure. Dominant genera (Mesorhizobium, Sinorhizobium, Rhizobium) persisted under all Cd levels, while the relative abundance of Azohydromonas, Xanthobacter, and others shifted significantly. Soil pH, total Cd, DTPA-Cd, and C/H ratio were key factors influencing community composition. Although Cd negatively affected the nifH gene community, the resilience of dominant rhizobia highlights their potential for symbiotic remediation systems. These findings aid in selecting stress-tolerant rhizobia to optimize plant growth and metal detoxification in contaminated soils.

(1) Background: Rhizobia can promote plant growth by providing essential nutrients such as NH4+ and PO43−; thus, rhizobia that can tolerate the stress of heavy metals will be conducive to the phytoremediation of heavy-metal-contaminated soils. Therefore, understanding the dominant heavy-metal-tolerant rhizobia that can be cultured is important for the establishment of an indigenous legume–rhizobia symbiotic remediation system; (2) Methods: Here, we investigated nifH gene diversity in culturable rhizobia from black locust (Robinia pseudoacacia L.) grown in cadmium (Cd)-contaminated soils using high-throughput sequencing.; (3) Results: A total of 16 genera and 26 species were identified from the cultures of root nodules of black locust exposed to five Cd levels. Cadmium did not show a significant effect on the abundance, diversity, and evenness of the culturable rhizobia community. However, Cd significantly affected the community structure of culturable rhizobia containing nifH. Mesorhizobium, Sinorhizobium, and Rhizobium were the absolute dominant genera present in the cultures under five Cd treatments. Additionally, Cd significantly affected the relative abundance of Azohydromonas, Xanthobacter, Skermanella, Bradyrhizobium, Paenibacillus, and Pseudacidovorax in the cultures. Soil pH, total Cd, DTPA-Cd, and C/H ratio were the significant factors on culturable rhizobia community.; (4) Conclusions: Cd showed a negative effect on nifH gene community of culturable rhizobia from black locust, which will provide insight into the selection of excellent strains that can promote phytoremediation of heavy-metal-contaminated soils.