Plant Growth-Promoting Rhizobacteria Microbial Fertilizer Changes Soils' Microbial Structure and Promotes Healthy Growth of Cigar Tobacco Plants

Cigar tobacco, an economically important cash crop, holds a substantial role within the fiscal framework of the national economy. This crop, however, is characterized by a marked vulnerability to pathogenic bacteria, culminating in consequential financial loss throughout its cultivation phase. Plant growth-promoting rhizobacteria (PGPR), a salient class of advantageous bacterial flora, are recognized for their ability to enhance plant growth, inhibit deleterious pathogens, and synthesize compounds that either have a direct impact on plant morphogenesis or activate otherwise ineffectual soil components. Through these mechanisms, PGPR augments the soil's nutritional profile, making it more receptive to plant uptake, thus stimulating vegetative growth. The Bacillus subtilis microbial fertilizer, the prime exemplar of PGPR, demonstrates not only a pathogen-suppressive effect but also an induction of the plant's innate disease resistance mechanism. This bolsters the plant's resilience to disease fosters a probiotic milieu within the soil, and catalyzes the formation of agglomerate structures, all of which contribute to enhanced soil fertility and moisture retention, increased soil friability, and the facilitation of root expansion. In this study, a controlled pot experiment was conducted to elucidate the mechanism through which inter-root probiotics rehabilitate the soil's ecosystem and foster crop growth in cigar tobacco seedlings afflicted with root black rot bacteria. Four treatments were instituted, including CK: a blank control (no microbial application); A: probiotic only (Bacillus subtilis microbial fertilizer); B: both pathogenic and probiotic (the Bacillus subtilis microbial fertilizer together with root black rot pathogen); C: pathogenic only (the root black rot pathogen). Our empirical findings delineate that the presence of pathogenic bacteria deteriorates the soil environment, thereby constraining the transmutation of soil nutrients and their subsequent assimilation by plants. This severely impedes the vegetative development of cigar plants. By contrast, the application of a PGPR microbial fertilizer modified the soil microbial community structure, exhibiting an antagonistic interaction with the indigenous pathogenic bacterial species. Relative to the CK treatment, the application of the Bacillus subtilis microbial fertilizer was found to invigorate the catalytic conversion of soil enzymes, incrementing the peroxidase, acid phosphatase, urease, and sucrase activity by 12.98%, 19.55%, 13.57%, and 17.91%, respectively. Meanwhile, it was observed to ameliorate the soil's physicochemical attributes, enhancing the available content of nitrogen, phosphorus, and potassium by 4.52%, 6.52%, and 15.14%, respectively, along with the augmentation of soil organic matter content by 17.33%. The fortification of soil physicochemical properties and the enrichment of soil fertility, as a result of the PGPR microbial fertilizer application, translated into a robust 57.23% enhancement of root vigor and a 60.47% extension of the root length of cigar tobacco seedlings. These soil amendments subsequently fueled an uptick in the growth parameters of cigar plants, including increases in plant height, stem girth, leaf count, maximal leaf dimensions, and both the fresh and dry weight of cigar tobacco.