Paula Alvarenga

The combined use of soil amendments and beneficial microorganisms represents a promising strategy to restore degraded mining soils by improving soil properties, immobilizing contaminants, and supporting plant production. A pot experiment was carried out with soils from the Aljustrel mining area (Iberian Pyrite Belt, pH 3.9, contaminated with As, Cu, Pb and Zn in quantities of 4782, 1596, 8652, and 938 mg kg⁻¹, respectively). The amendments tested included biochar (64 g kg⁻¹), municipal solid waste compost (64 g kg⁻¹), their combination, and ground oyster shell (11.3 g kg⁻¹, with liming capacity similar to CaCO₃), combined with compost. Eucalyptus globulus Labill. was selected because of its major economic importance in Portugal, and an arbuscular mycorrhizal fungus (AMF) (Rhizophagus irregularis) was inoculated. After five months, the combined application of compost and biochar increased soil organic matter from 1.4% to approximately 6% and resulted in the highest aboveground biomass production (53.8 g plant⁻¹ dry weight). The combination of compost and ground oyster shell increased soil pH to approximately 6.5 and was associated with lower Cu and Zn concentrations in aboveground plant tissues, by 49% and 70%, respectively, compared with the corresponding unamended treatment. This treatment also increased soil dehydrogenase activity, while AMF inoculation had a significant effect on this parameter despite the absence of detectable root colonization. The available water capacity increased significantly in the biochar and in the biochar–compost combination. Results show that the combined use of waste- and biomass-derived organic amendments and liming materials can improve selected soil properties in acidic, multi-element contaminated mine soils, supporting eucalyptus establishment.

Agricultural valorization of animal slurry is considered a priority in some countries, but that practice needs to be carried out minimizing potential environmental impacts, allowing an improvement in soil quality, without a reduction in productivity. A greenhouse experiment used a Haplic Arenosol, fertilized with raw pig slurry, and the materials obtained by its solid-liquid separation and acidification (pH 5.3–5.4), to evaluate the basal- P fertilization of maize ( Zea mays L.), only via organic materials (140 kg P ₂ O ₅ /ha), leveling the N application with a mineral fertilizer. Slurry-derived fertilizers, which are richer in other essential nutrients, produced significantly greater biomass and promoted faster early plant growth than the mineral fertilizer. The best performance was achieved with raw slurry, acidified slurry, and the liquid fraction of acidified slurry, highlighting the benefits of acidification for nutrient availability. Up to the evaluated growth stage (approximately 100 cm), the P supplied exclusively by the organic materials was sufficient to sustain plant growth. Although leaf P concentrations were significantly lower than in the mineral- P control, they remained within the normal range. However, because of the higher biomass achieved in the organic treatments, total P exported by the plants fertilized with the pig slurry fractions were, as an absolute value, higher, or identical, to the control with mineral- P fertilization. Both fertilizers created nutrient-rich conditions, that appeared to inhibit the establishment of a functional symbiosis with the native arbuscular mycorrhizal fungi during that growth phase, whereas evidence of such a symbiosis was observed in the non-fertilized control plants. The single application of slurry-derived materials did not effectively increase the soil organic matter (OM) content, but it was possible to observe a stimulation in some soil enzymes with the organic fertilizer application, namely dehydrogenase, β-glucosidase and acid phosphatase activities, especially reflecting the combined positive effect of solid OM and slurry acidification on overall microbial metabolism.

Eucalyptus (Eucalyptus globulus Labill.) plays a fundamental role in the Portuguese forestry sector, highlighting the need for sustainable practices that contribute to maintain soil fertility and biodiversity, without compromising productivity. An experiment was conducted, with a low extractable-P soil (2O₅ kg⁻¹), to evaluate the use of biofertilizers as a strategy to reduce mineral fertilizers dependency. Eucalyptus trees were inoculated at transplantation, with two commercial inoculants based on Glomus iranicum (an arbuscular mycorrhizal fungus, AMF) and inorganic phosphate-solubilizing bacteria (Pseudomonas putida and Pseudomonas fluorescens), under differential P fertilization conditions. Reducing P fertilization to 70% and 40% of the reference dose did not significantly reduce final aboveground biomass, under the studied conditions. Biofertilizer application enhanced soil P availability, particularly when AMF and Pseudomonas were co-inoculated, suggesting synergistic effects on P mobilization and uptake. Plant physiological performance, assessed through vegetation indices, remained stable across treatments, indicating effective acclimation to reduced P supply, while stomatal conductance suggested improved tolerance to P deficiency in Pseudomonas-inoculated plants. The widespread presence of a well-established native mycorrhizal community, which led to a higher root colonization under low P conditions, highlights the importance of plant–microbe interactions in nutrient-limited environments. Overall, these findings underscore the potential of integrating reduced P fertilization with microbial inoculants to improve P use efficiency and support sustainable eucalyptus production.

Almond hulls and shells are abundant by-products of the almond industry that could be valorized as bio-based absorbent polymers (BAP), offering a promising alternative to synthetic materials to improve water management in the agricultural setting. In this study, almond hulls and shells were pelletized in different proportions to assess pelletization feasibility and physical properties, followed by industrial-scale production of an industrialized formulation (80% hulls, 20% shells). Ecotoxicological risk was assessed using direct bioassays with whole pellets (germination with Lactuca sativa and Zea mays; acute toxicity with Eisenia fetida) and indirect bioassays with pellet water extracts (germination with L. sativa, immobilization with Daphnia magna, and bioluminescence inhibition with Vibrio fischeri). Field trials were conducted in an irrigated almond orchard to evaluate soil moisture dynamics and plant water status under different BAP application rates and irrigation regimes. Pelletization increased the soil’s water-holding capacity in the laboratory test and soil moisture in the field, even under reduced irrigation. However, ecotoxicological assays revealed significant to high acute toxicity at higher concentrations, depending on the organism and exposure pathway. Almond hull and shell pellets show potential to improve soil water retention and reduce irrigation demand but require cautious application and further testing to mitigate ecotoxicological risks.

Environmental impacts of urban wastewater treatment plants (WWTPs) can be reduced by recovering nutrients and organic matter (OM) from their streams for agricultural use, decreasing dependence on conventional fertilizers. This study evaluated dehydrated sewage sludge (SS) as an organic amendment and the partial replacement of mineral P fertilizers in lettuce cultivation. Struvite, a byproduct of WWTPs, was also investigated as a sustainable P source. A 43-day greenhouse pot experiment assessed SS (12 t/ha) and struvite (at two P rates: 30 and 60 kg P₂O₅/ha), both alone and combined. SS significantly increased soil OM (p < 0.001), though long-term applications would be required to enhance this effect. The highest struvite rate (60 kg P₂O₅/ha) yielded the greatest extractable soil-P levels (150 ± 8.1 mg P₂O₅/kg), while its combination with SS further increased extractable P (>250 mg P₂O₅/kg), indicating a stable soil P pool. The highest plant dry biomass (8.9 ± 1.1 g, p < 0.05) also occurred under the highest struvite dosage. Complementary effects between SS and struvite were observed in foliar K, Ca, Mg, and S contents, although no significant interaction between both was found for P content. Adequate foliar P levels (0.40–0.52%) were achieved only in treatments containing SS, indicating its essential role in improving plant P nutrition.

This study proposes and evaluates a two-step phosphorus (P) recovery strategy that combines chemical precipitation with adsorption to comply with the updated EU Urban Wastewater Treatment Directive (Directive (EU) 2024/3019), which sets stricter limits on nutrient discharge and promotes resource recovery. The objective was to enhance the P removal efficiency beyond that achieved by conventional precipitation. A laboratory-scale design of experiments was conducted using real wastewater with an initial P concentration of 10 mg P/L post-precipitation and was extended to 1 and 40 mg P/L to assess broader applicability. The optimal lab-scale conditions (30 cm bed height and 5 mL/min flow rate) resulted in a saturated bed fraction (FSB) of 0.425 and a breakthrough time of 126 min. The process was successfully scaled up to a column with a height of 60 cm and a diameter of 4 cm, achieving a higher FSB (0.764), improved adsorption capacity (84.1 mg P/kg), and reduced unused bed (40%). The integrated system maintained effluent P levels below 0.5–0.7 mg P/L for over 400 min, demonstrating regulatory compliance and operational reliability. These findings confirmed the feasibility and scalability of combining precipitation with adsorption for enhanced P recovery in wastewater treatment systems.

Phosphorus (P) is a critical element for life, and wastewater treatment systems can be strategic points for its recovery, thereby avoiding eutrophication pollution in nature. The aim of this research was to investigate P recovery via struvite, namely in terms of the influence of operating parameters, coexisting interfering ions, and seeding. This paper focuses on synthetic solutions, although an assessment was performed on wastewater. The results of the assessment indicated that, in the synthetic solution, the minimum concentration for struvite precipitation is about 30 mg P/L, and that the Mg/P molar ratio of 1 promotes P removal efficiency with less contribution from other minerals. In order to assess the results in terms of real-world scenarios, the influence of coexisting ions (calcium and sodium) was investigated. Calcium was shown to have the greatest impact on the process, as 80% was removed for an initial concentration of 200 mg Ca/L. Indeed, these experiments generated an amorphous precipitate that did not contain struvite. The utilization of biomass ash (size < 63 µm) as seeding in crystallization increased the P removal efficiency compared to the sample without seed and helped to control the pH. The precipitation experiments with wastewater demonstrated good P removal efficiencies (over 90%) but indicated a reduction in the purity of the final product (struvite was a minor crystalline phase identified in XRD—15%wt).

The use of agricultural drainage water (ADW) in irrigation is a great challenge, improving water use efficiency, nutrient circularity, and avoiding surface and ground-water contamination. The objective of this study was to evaluate the chemical and ecotoxicological characteristics of an ADW to analyze the safety of its reuse. An irrigated area with maize crops was selected (Tejo Basin, Portugal), where a subsurface structure for the recovery of ADW was installed, collecting the drainage in a pond and recycling it for crop irrigation. Water was collected monthly during the irrigation campaign of 2021 (April to August). Three herbicides and two metabolites were quantified, reaching a maximum concentration of 0.74 µg L⁻¹ for S-metolachlor and 0.48 µg L⁻¹ for terbuthylazine. The lethal bioassays did not detect toxicity, except for the sample collected in August toward Vibrio fisheri (EC₅₀ = 25.2%). The samples were not toxic to Pseudokirchneriella subcapitata, with a growth inhibition rate of less than 10%. The low lethal and sublethal effects may be ascribed to the high nutrient concentration (e.g., 1.76 mg P L⁻¹ and 98.9 mg NO₃⁻ L⁻¹, in July) that could have masked toxic effects. Ecotoxicological responses support the option of ADW reuse in irrigation, offering a safe and sustainable solution for water and nutrient management.