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 (peat and 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 in V. tinus. Moreover, a Life Cycle Assessment (LCA) analysis was carried out to quantify the greenhouse gas emissions (GHG) deriving from the replacement of peat with the other proposed substrates. The functional unit was 10 L (Ø 24 cm) potted plants and the results were expressed in kg of CO2 equivalent (kg CO2eq). We demonstrated that the replacement of peat-based substrates with the alternative substrates was able to reduce the GHG emission by an average of 11.56 to 23.13%. Higher GHG emissions were related to the cultivation phase (0.9 kg CO2eq/plant), and while comparing substrates, we obtained an average percentage reduction of 28.1% to 59.6%. Thus, our results suggest that co-composted mixtures of dredged sediments with green waste could be used as sustainable techno-soils for pot nursery cultivation of ornamental species with reduced environmental impact.