Phosphorus dynamics in the soil-sunflower (Helianthus annuus L.) system as affected by P-enriched biochar from real treated wastewater

This study reports on the characteristics of phosphorus-enriched biochar, and on its effect when used as slow released fertilizers, on the soil-sunflower (Helianthus annuus L.) system. The biochar was enriched using real treated wastewater at a pilot-scale wastewater treatment plant of the Water Resource Recovery Facility of Palermo University. Two types of biochar, B440 and B880, obtained from the same biomass by pyrolysis at 440°C and 880°C, respectively, were used. The experimental design involved the use of two soil types: one rich in calcium and one rich in iron, but both deficient in phosphorus (P). Four treatments were performed: a control without P, a control with P applied as KH2PO4 at a dose of 50 mg of P per kg of dry soil, a treatment with biochar B440, and another one with biochar B880, both applied to add 50 mg of P per kg of dry soil. The experiment lasted (122 days) and was carried out in a growth chamber. Sunflower plants were irrigated daily with 20 mL of P-free Hoagland nutrient solution. Sunflower was chosen as a test plant due to its high P requirement and susceptibility to nutritional deficiencies. Before beginning the experiment, extensive biochar characterization was performed, including the determination of Olsen P, total P, iron, zinc, carbon, nitrogen, and sulphur content. Germination and phytotoxicity tests were conducted, and a P release study was carried out using an anionic resin to assess the mode and amount of P released from the materials. At the end of the study, sunflower plants were explanted, and soil and plant samples were stored for further analysis. Different forms of P were analysed in the soil, along with the amounts of mineral N and enzyme activities (acid phosphatase, alkaline phosphatase, and β-glucosidase). Total P concentrations in roots, stems, and flowers were quantified in plant samples, along with biometric parameters such as fresh weight and dry weight. Finally, P uptake and Phosphorus Fertilizer Replacement Value (PFRV) were assessed.