Marine waste-derived soil amendments via combined biological treatments and mineral enrichment: process characterisation and potential for ecological restoration

Posidonia oceanica litter accumulates along shorelines forming deposits with important ecological functions but also challenging management requirements. With a view to propose an effective management strategy, P. oceanica litter was combined with fish market wastes at a 3:1 ratio to optimise the C:N:P balance. The feedstock mixture was subjected to 6 months of aerobic composting followed by 1 month of vermicomposting to yield a mature solid matrix. In parallel, biomethane production potential was evaluated through thermo-acidic pretreatment and anaerobic digestion of the same feedstock. The resulting liquid digestate was then added to the mature vermicompost, either untreated or enriched with zeolite-rich tuff, to assess nutrient stabilisation, phytotoxicity mitigation, and overall product quality. During the stabilisation process, significant temporal changes in pH, electrical conductivity, water content and C/N ratio were observed, reflecting the progressive transformation and maturation of the organic amendment. Overall, all the chemical elements showed marked variations over the 7 months of composting and vermicomposting, indicating differential mobility driven by coupled biological and chemical processes. Zeolite addition lowered the organic matter content and the concentrations of most macronutrients through dilution, but increased the concentrations of several geogenic elements. It also selectively removed ammonium from the soluble fraction, markedly reducing phytoinhibition. Overall, the combination of anaerobic digestion, composting, vermicomposting, and zeolite enrichment proved to be an effective approach for the stabilisation and valorisation of marine organic wastes, leading to the production of mature soil amendments with high potential for use in ecological restoration strategies for degraded substrates.