The Importance of Humic Acids in Shaping the Resistance of Soil Microorganisms and the Tolerance of Zea mays to Excess Cadmium in Soil

Contamination with cadmium (Cd²⁺) poses a severe threat to the soil environment due to its toxic effect on bacteria, being of key importance to soil fertility and plant health. The present study aimed to evaluate the effect of a humic preparation, Humus Active (HA), on the structure, diversity, and functional potential of soil bacteria under conditions of cadmium stress during <i>Zea mays</i> cultivation. A model study was conducted to analyze the response of bacteria to soil contamination with 60 mg Cd kg^-1 under conditions of soil fertilization with humic acid at doses of 2 g (HA2) and 4 g (HA4) kg^-1 of soil. Microbiological analyses were carried out with both culture and non-culture (16S rRNA gene amplicon sequencing method) methods. Bacteria function prediction was also performed using FAPROTAX software. The study results demonstrated that Cd caused a 92% reduction in <i>Zea mays</i> biomass and a significant decrease (by 52%) in the abundance of organotrophic bacteria. The NGS analysis showed that it also reduced the population of the <i>Neobacillus</i> bacteria in the soil (by 50%), simultaneously causing an over twofold increase in the population of the <i>Nocardioides</i> genus bacteria. The application of HA (particularly in the HA4 dose) substantially mitigated Cd phytotoxicity. In the Cd-contaminated soil, HA4 stimulated the growth of culturable actinobacteria. The soil bacteria community was predominated by chemoheterotrophic bacteria and the nitrogen cycle bacteria, driven by tolerant, Cd²⁺-resistant bacteria from the following genera: <i>Bacillus</i>, <i>Nocardioides</i>, and <i>Arthrobacter</i>. The study results enable concluding that even though Humus Active does not restore the original microbiome structure, it promotes the development of a new stress-resistant bacterial community exhibiting high bioremediating potential, thereby directly translating into improved plant condition. Subsequently, humic acids provide an innovative approach that not only extends knowledge about the mechanisms behind bacterial resistance but also enables developing practical methods for diminishing cadmium mobility in the soil.