Digestate as a source of biostimulants for winter rye growth

The growing use of anaerobic digestion to treat organic waste and generate biogas has led to the production of large amounts of digestates. Digestates, nowadays valued as organic fertilizers and soil amendments, could also be a source of bioactive molecules with biostimulant characteristics. In this study, the digestates (raw and liquid), the humic and fulvic acids derived from raw digestate by alkali-acid extraction, their mixture (humic substances), as well as the water-soluble fraction of raw digestate, were evaluated for their biostimulant properties on winter rye. Potential biostimulant effects were compared to a commercial biostimulant and a nutrient solution during germination and growth of winter rye plants under hydroponic and soil conditions. The digestates did not show any significant biostimulant effect compared to the nutrient solution, while the water-soluble and fulvic acid extracts significantly increased root dry matter, root carbon and nitrogen quantities and projected leaf area of winter rye under hydroponic conditions. The root system architecture was differently modified by digestate and extract applications. While the raw digestate more than doubled the length of the thickest roots (diameter > 2 mm), the water-soluble extract promoted elongation of the thinnest roots (diameter < 0.3 mm) and enhanced total root length compared either with nutrient solution, commercial reference or digestates. The effects were more moderate on soil-grown rye plants but still existed. This work highlights water-soluble and fulvic acid extracts obtained from digestates as effective biostimulants for winter rye and, therefore, suggests a potential interest in using digestates as a source of biostimulants. • The digestate from territorial methanizer was used to obtain humic substances. • Fulvic acids and water-soluble extract from digestate improved winter rye growth. • The biostimulation effects on winter rye were mainly expressed in root architecture. • Biostimulation responses were superior under hydroponics and nutritional stress.