THE USE OF BY-PRODUCTS AS SOIL AMENDMENTS IN THE AGRICULTURAL/FORESTRY SECTOR

Sewage sludge is a solid or semi-solid residue generated during wastewater treatment, rich in macro- and micronutrients, and potentially useful as an agricultural or forestry input. In addition to providing nutrients, it can improve soil physical characteristics, such as aeration and water retention capacity. However, its application should be done with caution, considering the possible presence of potentially toxic elements that can harm plant development. To deepen our understanding of the effects of sewage sludge application on seedling production, this thesis was structured in two phases. The first consisted of a systematic literature review followed by a meta-analysis, based on the PRISMA 2021 protocol, analyzing recent articles with quantitative data on the effects of sewage sludge on soils and plants. The second phase involved two pot experiments conducted for 120 days under greenhouse conditions, with different tree species used in urban landscaping, grown in substrates with varying proportions of domestic sewage sludge. The experiments were conducted to evaluate the effects of sewage sludge on soil chemical parameters, plant growth through non-destructive analyses (monthly stem height and diameter), and nutrient concentrations accumulated in plants through final chemical analyses of tissues separated between shoots and roots. In both experiments, statistical analyses included ANOVA and Tukey's test at a 5% significance level, as well as multivariate analyses (PCA and PFA) to identify the factors that most contributed to the variations between treatments. The first experiment was conducted in Sassari, Sardinia, Italy, using Nerium oleander as the species. The treatments consisted of T0 (100% commercial substrate), T1 (25% centrifuged sewage sludge (SS) + 50% urban soil (US) + 25% perlite), T2 (50% SS + 25% US + 25% perlite) and T3 (75% SS + 25% perlite), each with six replicates (pots). The control treatment (T0) showed the best initial performance in height, stem diameter and aerial biomass, followed by T1, T2 and T3. In the treatments with ≥50% sludge (T2 and T3), leaf chlorosis was observed in the seedlings during the first 60 days, possibly associated with the high initial electrical conductivity (T3 = 25.9 mS/cm) representing substrate salinity, which may have limited nutrient uptake. Despite this, from the third monthly assessment onwards, a gradual recovery in growth was observed in T2 and T3, possibly due to salt leaching and progressive nutrient release. Total and aboveground biomass were significantly higher in T0, while nutrient concentrations showed distinct patterns, with macronutrients exhibiting greater translocation to the shoots, while micronutrients accumulated primarily in the roots, with the exception of Mn, Zn, and B, which were more abundant in the aboveground tissues. Heavy metal levels in soil and plants remained below the limits established by European Directive 86/278/EEC and Italian Legislative Decree No. 99/1992. continues...