Aqueous compost extracts with stabilized biofertilizing microbiota promote plant root growth and drought resilience

The excessive use of agrochemicals has caused significant negative environmental impacts, highlighting the growing need for more sustainable alternatives. Among these, aqueous composts extracts enable less harmful intensive agricultural practices. The objective of this study was to explore methods for stabilizing the biofertilizing microbiota of compost extracts and to evaluate their effects on drought and oxidative stress. For this, an aqueous extract was prepared from agri-food waste compost by suspending it in water at a 1:5 ratio and incubating the mixture for 14 days at room temperature. The physicochemical properties of the extract were analyzed. In addition, microorganisms associated with the biofertilizing capacity of the extract, which was formulated with various compounds were monitored over the course of one month storage at different temperatures. The bioformulations showing better biofertilizing potential were selected for testing on cucumber seedlings to evaluate their capability for promoting plant growth and alleviating oxidative stress. Additionally, a drought stress test was conducted on grass to evaluate the effect of applying the extract. The results of the physicochemical characterization and bacterial abundance showed a good nutritional composition and a rich microbiota with biofertilizing activity. In terms of microorganism counts under storage conditions, the most stable formulations were those formed by the extract supplemented with 2 % glycerol, as well as the extract without supplement (as extracted). Cucumber seedlings treated with the more stable extracts exhibited enhanced agronomic traits, particularly improved root development, and reduced oxidative stress. The root-promoting effect was also observed in the drought stress test, where grass seedlings subjected to 30 % soil moisture and treated with a combination of the extract with chemical fertilizer presented greater root development (around 5.50 g cm^-3) compared to treatments lacking the extracts (around 2-3.30 g cm^-3). These results suggest that aqueous compost extracts provide drought resistance and increased root development, offering a promising alternative to conventional mineral fertilization.