Valorisation of cost-effective wastes and underused soils through Technosols construction for the Suaeda vera Forssk. ex J. F. Gmel. cultivation: a valued marine halophyte

Salinization is one of the major causes of soil degradation and salt-affected soils have been underused due to their physical and chemical properties. Although most conventional crops are non-salt tolerant plants (glycophytes), halophytes can develop under high salinity concentrations and provide food, fodder and other uses, such as remediation and pharmaceuticals. Among them, Suaeda vera can be used for human consumption as forage and in phytoremediation. This study aims to optimize the cultivation of this species by producing a Technosol (TEC) using wastes as amendments to improve the properties of a Fluvisol (FLU). The amendments were constituted by cost-effective wastes: sludge and waste kieselguhr from breweries, medium sand, gravel limestone and biomass obtained from pruning. The FLU used in a microcosm assay was collected in the Tagus Estuary, close to Lisbon, Portugal. The soils properties, plants growth in Fluvisol and Technosol under the estuarine water (Ew) irrigation also collected in Tagus Estuary, were assessed. The microcosm assay was set up with four replicates and the substrata were incubated at 70% of the maximum water-holding capacity for 28 days, in the dark. After, seedlings obtained by germination in wet filter paper with deionised water were transplanted. The amendments, the FLU and the TEC were analysed for: pH, electrical conductivity (EC), concentrations of C_organic, P_extractable, K_extractable and N_total. The chemical characteristics of Ew were also analysed for pH, EC, Cl^–, HCO₃^–, Na, Ca, Mg, and the SAR was calculated. The plant growth parameters like stem length and biomass were determined. The FLU was slightly alkaline (pH 8), with a high EC (5.6 mS/cm) and low values of C_organic (20 g/kg), N_total (1.7 g/kg), P_extractable (1x10^-3 g/kg) and K_extractable (0.9 g/kg). Due to the wastes’ properties, the TEC showed a significant increase in C_organic (28 g/kg), N_total (2.5 g/kg), P and K_extractable (0.1 and 1.2 g/kg, respectively), in comparison with the FLU. The Ew was strongly saline (EC 22.1 mS/cm), had a neutral pH (7.78) and high concentration of Cl^- (7330 mg/L), HCO₃^- (267 mg/L), Na⁺ (4305 mg/L), Ca²⁺ (210 mg/L) and Mg²⁺ (538 mg/L). The Ew SAR value was high (157) but tolerated by halophytes. The individuals grown in the TEC presented a larger stem (18 cm) and a higher fresh biomass value (23 g) than individuals grown in the FLU. These results indicate that the wastes improved the Fluvisol properties being S. vera cultivation favoured if carried out in TEC. In a circular economy perspective, this study reveals that it is possible to cultivate a halophyte species with economic potential from underused resources as saline soils, wastes and brackish water.