The Decomposition Dynamics and Substrate Component Potential of Biomass from the Seagrass Posidonia oceanica (L.) Delile

Posidonia oceanica, a Mediterranean Sea seagrass, generates significant litter debris in coastal regions, adversely affecting tourism. To manage this debris, we explored two strategies: (i) promoting in situ decomposition by introducing ligninolytic microbes from forest microbiomes and (ii) utilizing P. oceanica debris as a substrate for ornamental and horticultural species. To achieve this goal, we conducted a one-year experiment using litter bags in mesocosms that simulate in situ conditions, evaluating the second strategy with different application rates (10%, 30%, 50%, and 100%) of fresh and decomposed P. oceanica debris mixed with peat or soil, with or without mineral fertilizer. The results revealed the resistance of P. oceanica necromass to decomposition; in fact, the introduction of forest microbiomes led to a major decomposition rate, albeit with constrained practical applicability. Utilizing P. oceanica debris as a substrate without fertilizer resulted in a modest growth response relative to the application rate, particularly accentuated in horticultural species compared to ornamental ones. Mineral fertilizer alleviated adverse effects at lower application rates; however, a notable decline in growth was observed at the 100% application rate. At application rates of 10% and 30%, certain crops demonstrated improved growth compared to the control. In summary, our study suggests the potential use of raw P. oceanica debris as a growth substrate within the 10% to 50% dosage range.