Co-Composting of Green Waste and Dredged Sediments Can Reduce the Environmental Impact of the Potted Nursery without Affecting Plant Growth

The ornamental nursery industry is steadily growing in Europe, and a consequent increase in the demand for substrates related to container plant cultivations is expected in the coming years. Currently, substrates consist in part or entirely of peat, a non-renewable resource with concerns about its environmental impact due to extraction, transport, and use. Therefore, it is essential to focus on alternative materials, particularly waste by-products to be recycled as components of substrates to achieve more sustainable cultivations. In this study, substrates obtained by mixing co-composted dredged sediments (S) and green waste (GW) in different ratios (1:3; 1:1;3:1) were tested for cultivation, and plant growth was compared with a control growing media (peatand pumice in a 1:1 ratio). The cultivation trial lasted for one year and was carried out on two potted ornamental evergreen shrubs (Photinia × fraseri and Viburnum tinus). The results showed that the plant growth parameters of both species, occurring in substrates with co-composted materials, were not significantly affected compared to the control, with the exception of below-ground biomass inV. tinus. Moreover, a Life Cycle Assessment (LCA) analysis was carried out to quantify thegreenhouse gas emissions (GHG) deriving from the replacement of peat with the other proposedsubstrates. The functional unit was 10 L (Ø 24 cm) potted plants and the results were expressed inkg of CO 2 equivalent (kg CO 2eq). We demonstrated that the replacement of peat-based substrateswith the alternative substrates was able to reduce the GHG emission by an average of 11.56 to23.13%. Higher GHG emissions were related to the cultivation phase (0.9 kg CO2eq/plant), and whilecomparing substrates, we obtained an average percentage reduction of 28.1% to 59.6%. Thus, ourresults suggest that co-composted mixtures of dredged sediments with green waste could be usedas sustainable techno-soils for pot nursery cultivation of ornamental species with reducedenvironmental impact.