Synthesis of a calcium alginate hydrogel-modified phosphorus-enriched Douglas fir biochar controlled-release fertilizer

Enhanced fertilizer efficiency is crucial for sustainable crop production and food security. Conventional soluble fertilizer formulations suffer from low use efficiency and rapid nutrient release. Here, P-enriched Douglas fir biochar (PBC) was synthesized by reacting CaCl 2 and KH 2 PO 4 , precipitating Ca 3 (PO 4 ) 2 onto biochar. This was further modified with calcium alginate hydrogel (CA). This combination (CA@PBC) was studied for use as a potential controlled-release fertilizer. Ca 2+ -crosslinked alginate forms a resilient hydrogel integrated with biochar, synergistically enhancing plant nutrient and soil water retention, reducing nutrient dissolution, and improving soil health. PBC and CA@PBC were characterized by FTIR, XRD, SEM, XPS, elemental analyses, and BET surface area measurements. Calcium phosphate and minor hydroxyapatite precipitated at pH 10 on PBC. Fixed-bed column leaching experiments in water performed for 20 days at ambient temperature showed prolonged phosphate release from CA@PBC-treated soil versus PBC-treated soil. PBC-treated soil leaches phosphate slower than TSP-treated soil. Greenhouse pot experiments using corn as a test crop were conducted for 35 days, comparing plant growth in soil promoted by CA@PBC, PBC, BC, and triple superphosphate (TSP) reference fertilizer versus unamended soil. Plant growth at a 50 kg P 2 O 5 ha −1 nutrient rate application showed ∼16.1% higher height and ∼16.4% greater P uptake with PBC than TSP reference fertilizer, whereas CA@PBC showed ∼1.2% lower height and ∼8.1% lower P uptake compared to TSP. A series of phosphate-leaching experiments exhibited rapid to sustained phosphorus release, which can be modulated over a longer time by tuning CA's pH and Ca 2+ crosslinking density, and by combining CA with various P sources. • Ca 3 (PO 4 ) 2 -amended biochar (PBC) modified with Ca-alginate hydrogel (CA@PBC). • PBC & CA@PBC each controlled P release, valorizing biomass for sustainable agriculture. • CA@PBC controls P release in two discrete modes creating a large design space. • Greenhouse growth plus batch and column leaching proved controlled P release. • CA@PBC and PBC were characterized by EE, FT-IR, XRD, SEM, XPS, and BET analysis.