Anaerobic digestates from sewage sludge (SSADs) are a by-product of the wastewater treatment process that still preserves a certain agronomic interest for its richness in plant nutrients and organic matter. Fertilizing properties of two liquid and two dewatered SSADs were tested on tomato plants (Solanum lycopersicum L.). Pot experiments were performed on sandy soil and peat substrate under greenhouse conditions with a SSADs application rate of 170 kg N/ha over a period of three months. Beneficial effects of SSADs were reported on different growth parameters, revealing an increase in biomass and height up to 37.5 and 6-folds over untreated control. No phytotoxic effect occurred on SSAD-exposed plants. Chemical analysis of soils treated with SSADs showed enrichment of macroand micro-nutrients as well as organic matter. In some cases, the chemical characterization of leaves revealed an enhancement of uptaken macronutrients. This study contributed in general to deepen the knowledge on the short-term growing season fertilizing effects of SSAD. Despite the treatment dosage was calculated only on nitrogen requirements, the study highlighted the importance of the other nutrients and organic matter on plant growth.

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Nowadays, phosphorus natural reserves are being depleted, while P fertilizers demand is increasing. Phosphorus is well contained in waste materials such as sewage sludge. Only a small amount (1–3%) of the soil total phosphorus is bioavailable for plant nutrition. More in detail, the present study focuses on the determination of the kinetics of bioavailable phosphorus concentrations in a sandy calcareous soil after the application of sewage sludge. A centrifuged (C) and dried (D) anaerobic digestate from sewage sludges obtained from the same wastewater treatment plan were separately tested to fertilize a calcareous sandy soil. Falcon tubes (50 mL) containing negative control (T) and soil treated with C and D were incubated from 1 to 90 days. Soil phosphorus fractionation was performed with the SMT method and bioavailable-P was extracted through the Olsen method. Phosphorus was spectrophotometrically quantified by the molybdovanadate method. Lastly, kinetics of bioavailable-P on soils were evaluated using four kinetic models. Phosphorus fractions were constant throughout the experiment. Conversely, the bioavailable-P significantly decreased from day 1 to day 90 in C treatment (from 34.9 ± 2.9 to 23.8 ± 1.5 ppm) and T treatment (from 4.2 ± 1.2 to 0.3 ± 0.6 ppm). This decrease might be due to the precipitation of P with calcium; in fact, high concentration of Ca2+ ions and the alkaline soil pH can induce the sequential formation of calcium phosphates, even less soluble over time. Whereas D treatment showed a peak of bioavailable-P concentration on day 14 (26.6 ± 3.0). This trend could be due to organic carbon compounds, competitive sorption and metal bridging. The fitting of experimental data revealed that the Elovich model best described the adsorptive-precipitate process of bioavailable-P in T (r2 = 0.90) and C (r2 = 0.93). Conversely, none of the models satisfactorily described the behavior of bioavailable-P in D samples. This study increases the knowledge on P-related phenomena for designing and optimizing fertilizers and reducing their drawbacks such as eutrophication.

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