Assembly processes of bacterial community and characteristics of keystone taxa in farmland soils under single cadmium and combined cadmium-chromium pollution.

Cadmium (Cd) and chromium (Cr) pollution in soil has become a global environmental concern due to their high biological carcinogenicity and persistence. The impact of Cd, Cr, and their combined pollution on soil bacterial community is unclear. This study assessed the effects of single Cd and combined Cd and Cr pollution on the structure, assembly processes, interactions, and functions of topsoil bacterial communities. Gemmatimonas is sensitive to Cd, and along with Intrasporangium, Marmoricola, Flavobacterium, and Lysobacter can be considered potential biomarkers reflecting the degree of Cr contamination. Compared with single Cd pollution, combined Cd and Cr pollution decreased the abundance of carbon cycle-related functional genes, including glyoxylate, dicarboxylate and glycolysis/gluconeogenesis metabolisms. Deterministic processes dominated the assembly of bacterial communities and the combined Cd and Cr pollution enhanced the dominance of dispersal limitation. In combined Cd and Cr pollution, Cr is the primary pollutant, leading to higher biodiversity and network complexity compared to single Cd contamination. Keystone taxa structure is significantly correlated with water-extractable chromium concentrations. The potential biomarkers and other bacteria related to Cr content become keystone taxa that interact with other species to adapt to and mitigate heavy metal stress. These findings provide novel insights into mitigating contamination pressures and preserving soil health.