Salt stress-induced changes in microbial community structures and metabolic processes result in increased soil cadmium availability

Salt stress can cause significant changes to soil microbial community structure, metabolic processes, and the surrounding micro-environments. However, how these processes affect the availability of cadmium (Cd) remains poorly understood. Changes in soil microbial communities within different soil micro-environments (large-, macro-, small-aggregate and silt plus clay fractions) and their metabolic responses to saline stress in Cd contaminated soils were analyzed. Salt addition negatively affected the stability of soil aggregates and promoted Cd translocation from large soil aggregates into smaller clay/silt sized fractions. Comparatively, soil bacteria were more sensitive than fungi in their responses to salt addition, as more prominent variations in bacterial community structure were observed between AS (added salt) and CK (no added salt) treatments, determined via non-metric multidimensional scaling analysis. Both bacteria and fungi showed greater diversity and richness in microaggregate fractions than in large aggregates, and salt addition decreased overall alpha diversity and altered microbial community composition. Among the node taxa, Sphingomonadaceae, Pyrinomonadaceae, Nitriliruptoraceae, Bacillaceae, Halomonadaceae and Pseudomonadaceae likely played critical roles in mediating Cd availability in soil micro-environments under saline stress. Furthermore, exposure to salt stress resulted in soil microbe metabolic reprogramming. The accumulation of tricarboxylic acid (TCA) cycle components and membrane lipid metabolites potentially contributed to observed increases in soil Cd availability and microbial Cd resistance.