Ethyl cellulose-alginate/poly(vinyl alcohol) dual-layer coating incorporating biochar-immobilized PGPR for controlled urea release, water retention, and plant stress resilience.

Abstract

Coated controlled-release fertilizers can synchronize nutrient supply with crop demand, yet single-layer systems are often limited by poor structural integrity and restricted functionality. Here, salt-tolerant plant growth-promoting rhizobacteria (PGPR) were immobilized on biochar and embedded in a dually crosslinked poly(vinyl alcohol)/sodium alginate hydrogel (PSBP). A dual-layer controlled-release urea fertilizer (PSBPEU-CRF) was fabricated using an ethyl cellulose inner diffusion barrier and a PSBP outer hydrogel coating. PSBPEU-CRF enabled sustained viable-cell release and improved bacterial immobilization stability. PSBP hydrogel reached an equilibrium swelling ratio of 424%, improved soil water retention and showed 70.65% mass loss after 28 d. At 2% (w/w), soil water retention remained 20.96% on day 10, compared with 2.35% in unamended soil. PSBPEU-CRF markedly retarded urea release in water and soil; the release profiles fitted the Korsmeyer-Peppas model, indicating anomalous transport governed by diffusion and polymer-network swelling/relaxation. In saline-alkali pakchoi pots, PSBPEU-CRF increased chlorophyll accumulation and antioxidant enzyme activities while reducing lipid peroxidation and the Na+/K+ ratio. Overall, the integration of controlled nutrient release, soil water retention, and rhizosphere functional regulation provided by the PSBP hydrogel layer offers a promising approach for developing multifunctional fertilizers for salt-affected soils. Nevertheless, its long-term field performance, manufacturing scalability, and economic feasibility require further evaluation.