Biochemical and Microbiological Soil Effects of a Biostimulant Based on <i>Bacillus licheniformis</i>-Fermented Sludge

Biostimulants are substances and/or microorganisms that are applied to plants or to the rhizosphere in order to enhance the natural process improving the absorption of nutrients and the quality of crops as well as the tolerance to abiotic stresses. A new biostimulant was developed from sewage sludge through its fermentation with <i>Bacillus licheniformis</i> as a plant growth-promoting bacteria (PGPB). The fermented product includes three classes of biostimulant components: the <i>B. licheniformis</i> biomass; the enzymatic secretion of said microorganism, which are mainly peptidases and amidases related to nitrogen metabolism and glucanases, related to carbohydrate metabolism; and finally, the hydrolyzed sludge organic matter, with a high content of protein hydrolysates. The biostimulant was evaluated in soil at the biochemical (enzymatic activities) and microbiological levels (metabarcoding analysis). Metabarcoding analysis revealed that the biostimulant complex, mainly the soluble fraction containing the <i>Bacillus</i> multienzyme complex and protein hydrolysate, induced PGPB soil bacteria, and it was detected that the inoculation in the soil of <i>B. licheniformis</i> remained active throughout the study. These results show the fermentation process with <i>B. licheniformis</i> as an interesting option for the total valorization of activated sewage sludge aimed at obtaining products of agronomic/environmental interest.