Scalable Production of Hygroscopic Gel Spheres to Enhance Seedling Growth via Atmospheric Water Harvesting and Controlled Fertilizer Release

Atmospheric water harvesting (AWH) technologies offer potential solutions for alleviating agricultural water scarcity. However, it faces significant challenges, including environmental compatibility, scalability, and multifunctional integration. Therefore, there is an urgent need for novel strategies to improve agricultural water use efficiency. Herein, natural polysaccharides sodium alginate (SA) and thermoresponsive hydroxypropyl cellulose (HPC) are selected as the matrix, with SA cross-linking with Ca²⁺ to enable rapid gelation and continuous production of shape-controllable spheres. Additionally, carbon black (CB) and urea are incorporated through physical doping and chemical interactions to enable solar energy collection and slow-release fertilization. The synergistic effect of the hygroscopic salt and porous matrix resulted in water uptake of 0.64–3.38 g g^–1 at 30–90% relative humidity. The photothermal of CB, combined with the low-temperature phase transition of HPC, facilitated low-temperature desorption and rapid solar-driven water release, achieving maximum desorption rate of 4.07 g g^–1 h^–1. Notably, as water was released from the material and the matrix degraded, urea was gradually discharged, with release of 224.6 ± 5.2 mg kg^–1 over 30 days, providing sustained support for crop growth. This innovative approach provides sustainable and geographically unconstrained solution for agricultural water management in arid regions.