Biostimulants from waste: humic-like substances from municipal organic fractions enhance growth, nitrogen acquisition and antioxidant activity of rocket (Eruca sativa Mill.) plants

Background: The integration of mineral fertilizers with biostimulants offers a valuable solution to improve nutrient use efficiency and reduce environmental impact, especially considering the increasing prices of synthetic fertilizers and of the growing ecological concerns. This work explores the valorisation of low-value materials, derived from the organic fraction of municipal solid waste (OFMSW), to produce potential biostimulants. Results: OFMSW was processed using different treatments and extraction methods to obtain several products containing humic-like substances. Three of these extracts, selected through germination tests, were evaluated for their biostimulant effects on rocket (Eruca sativa Mill.) plants grown under soilless conditions. All treatments resulted efficient in enhancing leaf and root development and significantly increasing nitrogen acquisition and assimilation by the plant. The biostimulant mechanism of the humic-like materials was investigated under both high and low nitrogen conditions, not only considering biomass production and nitrogen uptake, but also evaluating nitrate content in the leaves, activity of enzymes involved in N assimilation, and total antioxidant activity. Conclusions: The results confirm the potential of waste-derived humic-like products to improve plant growth and nutrient use efficiency, supporting their use as effective, sustainable biostimulants in line with circular economy principles.