Soil bacterial co-occurrence patterns and community assembly along a gradient of heavy metal and explosive pollution at ammunition demolition sites

• Soil bacterial diversity declined as levels of combined pollution increased. • High combined pollution decreased bacterial community complexity and stability. • Deterministic processes intensified with increasing pollution stress. • Cr, Pb, Cd, Cu, TNT, and soil texture were the main factors driving the determinism. • Fermentation and nitrogen respiration functions were enriched under high pollution. Soil microbial communities play an essential role in ecological processes. Pollution from anthropogenic activities can disrupt interactions and assembly processes within soil bacterial communities. However, these processes have not been extensively studied at military-contaminated sites. This study explored bacterial community compositions and functions, the co-occurrence networks, and the assembly processes by analyzing soil samples collected along a heavy metal and explosive pollution gradient. The results showed that with the increase of pollution, the bacterial α-diversity indices decreased significantly, while the abundance of metal- and explosive-tolerant bacterial taxa increased, such as Bacteroidetes and Firmicutes. Lead, cadmium, total organic carbon, and 2,4,6-trinitrotoluene were key factors affecting the diversity and composition of bacteria. High combined pollution reduced the complexity and robustness of the bacterial network, resulting in a decrease in community stability. The deterministic process dominated the community assembly and became more pronounced as pollution increased. Chromium, lead, cadmium, copper, 2,4,6-trinitrotoluene, and soil texture were considered important driving factors for the determinism. Functional prediction showed that the anaerobic metabolism processes (fermentation and nitrogen respiration) were abundant under high pollution levels, and metabolic pathways associated with drug resistance were more prominent, which suggested alterations in biogeochemical cycles and potential ecological risks. This study confirms that the combined pollution from ammunition demolition reshapes bacterial communities by strengthening environmental filtration and altering bacterial interactions, providing a theoretical basis for the ecological restoration of military-impacted soils in the future.