Changes in Soil Bacteriobiome in Response to Organic Amendments and Cd²⁺ Stress

Cadmium contamination of soils poses a global threat to food security and ecosystem stability. Soil bacteria play a key role in mitigating Cd-induced stress, and their adaptive capabilities can be modulated by the application of organic amendments such as compost, fermented bark, or preparations containing humic acid. This article presents the results of studies on soil bacterial communities using culture-dependent and next-generation sequencing approaches. Based on the obtained data, colony development indices and ecophysiological diversity indices were determined for organotrophic bacteria and actinobacteria. Alpha and beta diversity of bacteria were also assessed, common and unique genera occurring in the studied soils were identified, and the predicted metabolic functions of microorganisms were determined. It was found that cadmium reduced the abundance of organotrophic bacteria and actinobacteria by 54.5% and 12.9%, respectively, compared to the control, resulting in a shift in the bacterial community structure from r-strategists toward K-strategists. Humic acid increased the abundance of organotrophic bacteria and actinobacteria by 42.8% and 57.3%. Compost most effectively mitigated cadmium effects by stabilizing the colony development index and bacterial ecophysiological diversity. Cadmium strongly altered the soil bacterial microbiome, reducing the abundance of Actinomycetota while increasing that of Pseudomonadota and Bacteroidota. The application of organic amendments influenced the bacterial response to Cd²⁺-induced stress. Fermented bark was associated with an increased abundance of Sphingomonas, whereas compost was associated with an increased abundance of Cellulosimicrobium. Although none of the organic amendments affected the overall diversity index under these conditions, compost improved the evenness and ecological stability of the bacterial community. The dominance of aerobic chemoheterotrophs involved in the carbon cycle and the degradation of organic compounds was demonstrated. Compost most effectively supported biogeochemical processes.