## 4 Discussion  
Generally, the results showed that the physical and chemical properties of growing media are suitable for plant growth, in line with Italian legislation [27], with some exceptions (i.e., GM4-5-6) (see Figure S3). However, the addition of peat as a source of organic matter to co-composts allowed us to reach Italian law limits [26]. In fact, the presence of 50% peat reduced the EC and BD as well as increased the TOC in the co-composts. The high NO\({}_{3}\)\({}^{-}\) concentrations in the substrates composed of co-compost and peat could be related to the increase in pH; in fact, nitrification in peat is limited because of the acidic pH [50]. In addition, the lower C/N in the substrates composed of co-compost compared to peat suggested nitrogen mineralization and a higher availability of nitrogen for plant uptake [51]. In all substrates, a GI > 100% indicates the absence of phytotoxicity and the presence of nutrients that stimulate seed germination and seedling root elongation. Enzyme activities are often used as indicators of soil health (in this case, growing media) and nutrient cycles [52]. In particular, the addition of peat to co-composts improved the microbial activities, as measured by the butyrate esterase assay [53], as well as the enzymes related to the C (\(\beta\)-glucosidase), S (arylsulfatase), and P (phosphatase) cycles. In addition, the combination of peat and co-composts in the growing media enhanced arylsulfatase activity, which was not detected in peat. In fact, the high content of organo-sulfur molecules in the sediment and the high organic matter in peat stimulates enzyme activities involved in sulfate release [54; 55]. The cultivation trial carried out for _P. \(\times\) fraseri_ did not highlight any statistically significant difference in the plant growth parameters for any treatment when compared to the standard substrate (CGM). However, it is interesting to underline that plants grown on substrates entirely made up of compost (GM4, GM5, GM6), despite their chemical-physical properties, did not fully comply with the Italian legislative standard [27] and did not show substantial growth differences compared to the other substrates. Our results are consistent with those of [PERSON] et al. [56], where differences in plant elongation were not detected at the end of the growing period in _P. \(\times\) fraseri_ seedlings cultivated in co-composted substrates rather than in the peat and pumice control mix (ratio 1:1). Furthermore, for _V. tinnitus_, the measured parameters (height and above-/below-ground dry biomass) did not show any statistical difference among the plants grown in different media with exception of the root apparatus in GM1 and GM5. Nevertheless, these differences observed in the Laurustius plants did not alter their total growth (above- and below-ground biomass, data not shown), confirming overall balanced plant development and their marketability regardless of the substrates tested. Furthermore, the same growth results in terms of plant height and above-ground biomass were obtained in a different experimental scenario, with our target species cultivated for two years in the field adding dredged sediments to alluvial soil [21]. [PERSON] et al. [23] also reported that there were no differences in maximum plant height over time in another evergreen shrub (i.e., _Prunus laurocerasus_) grown on similar substrates. From an environmental point of view, LCA analysis allowed us to estimate the CO\({}_{2}\)eq emissions linked to both the production of substrates and the subsequent cultivation phase. The standard peat-pumice growing mix (CGM) was confirmed to be the most impactful substrate because of the extraction and transport of peat, as stated by [PERSON] et al. [5; 57]. On the other hand, as confirmed in a previous study [58], the use of waste by-products in growing mixtures led to lower total emissions of CO\({}_{2}\)eq into the atmosphere.