Natural saline–alkaline soils are widespread in Central Asia, yet microbial responses to salinity gradients and ionic composition remain poorly resolved. We profiled bacterial communities (16S rRNA V3–V4, Illumina MiSeq) in 20 topsoil (0–20 cm) samples from four regions of Kazakhstan spanning non-saline to highly saline conditions. Soil chemistry included pH, total mineralization (dry residue), and major ions (Na⁺, Cl⁻, SO₄²⁻, HCO₃⁻, Ca²⁺, Mg²⁺, K⁺). Alpha (Chao1, Shannon, observed ASVs) and beta diversity (Bray–Curtis; ANOSIM; PCoA) were evaluated across salinity classes. Soils were alkaline (pH 7.91–10.47) and covered a broad salinity range (256–26,312 mg/L), driven mainly by Na⁺ with chloride and/or sulfate. Alpha diversity remained stable across salinity classes, though dispersion increased under high salinity. Community composition differed significantly among classes (ANOSIM R = 0.428, p = 0.005), with partial PCoA separation and overlap, indicating gradual turnover along the salinity gradient. In contrast, fungal communities showed no significant response to salinity, with stable alpha and beta diversity across all samples and consistent dominance of Ascomycota. Communities were dominated by Actinomycetota (formerly Actinobacteriota), Bacteroidota, and Pseudomonadota (formerly Proteobacteria). Bacteroidota increased in highly saline soils (FDR q = 0.036), whereas Acidobacteriota decreased (FDR q = 0.052). Thermodesulfobacteriota (formerly Desulfobacterota) correlated positively with sulfate, and Cyanobacteriota negatively with chloride. Overall, Kazakhstan’s saline–alkaline soils show stable bacterial alpha diversity but moderate, ion-linked compositional shifts with enrichment of halotolerant taxa.

The decline of soil organic matter in Kazakhstan due to long-term monoculture and intensive fertilizer use has increased the need for sustainable soil management strategies. This study aimed to develop a new composting technique to produce more performant organic fertilizers from cattle manure to contribute to the broader strategy to increase soil organic matter and to improve fertility in northern Kazakhstan soils. Composting experiments were conducted using oxidized coal at 5%, 10%, and 20% dosages combined with two microbial consortia, and physicochemical and microbiological parameters were monitored throughout the process. The addition of microbial consortia and oxidized coal significantly enhanced composting dynamics by accelerating the transition to the thermophilic phase and increasing microbial activity. Treated systems exhibited higher temperatures (up to 48–49 °C), greater microbial abundance, and improved nitrogen transformation, characterized by increased NH₄⁺–N accumulation followed by NO₃⁻–N formation. Organic matter degradation was more efficient in amended treatments, while oxidized coal contributed to improved moisture retention and pH stabilization. However, excessive coal addition (20%) reduced microbial activity at later stages. Among the treatments, the combination of Consortium 3 with 10% oxidized coal showed the most balanced performance, with enhanced microbial activity, efficient nutrient transformation, and stable compost maturation. These findings demonstrate that the integration of microbial consortia with oxidized coal is an effective strategy for improving composting efficiency and producing high-quality organo-mineral fertilizers, with potential for improving compost quality and nutrient stabilization, and supporting sustainable and regenerative soil management in degraded agricultural systems.