Hydroponic rocket (Eruca sativa Mill.) microgreens grown with digestates from anaerobic digestion of maize co-digested with oxidized and non-oxidized biochars: nutrient dynamics and plant performance

Sustainable food production increasingly relies on circular strategies aimed at recovering and reusing nutrients from organic residues. This study assessed the agronomic performance and suitability of digestates obtained from maize silage anaerobic digestion co-digested with oxidized and non-oxidized biochars as nutrient supplements for the hydroponic cultivation of rocket (Eruca sativa Mill.) microgreens. Microgreens were cultivated for 18 days in a floating hydroponic system under controlled conditions. Six treatments were tested: a standard nutrient solution (control), and five solutions enriched by maize-silage digestates, either without biochar or co-digested with unoxidized or oxidized biochars derived from poplar and wheat straw. Digestates were applied at three concentration levels (1.5%, 3%, 6% v/v) in the standard nutrient solution. Plant responses were evaluated in terms of dry biomass, pigments, sugars, and antioxidants. Linear mixed-models and the Partial Least Squares Regression (PLS-R) were used to identify key response drivers. Digestates co-digested with oxidized biochar, applied at moderate doses (1.5–3%) significantly increased plant performance, pigment content and antioxidants, with fresh biomass increasing up to 74% and. In contrast, high application rates (6%), particularly of unoxidized wheat-straw-derived biochar co-digested digestate, reduced biomass by ca 40%, primarily due to increased electrical conductivity (EC) and associated osmotic stress. Multivariate analysis highlighted EC and nutrient balance as major factors influencing growth and quality traits. Overall, oxidized biochar co-digested digestates applied at moderate doses proved compatible with hydroponic microgreen production, enhancing crop performance and nutritional quality while supporting nutrient recycling within circular soilless cultivation systems.