Lignin–Carrageenan Biohydrogel‐Coated Urea Granules for Controlled Nutrient Release in Soil Systems

ABSTRACT The low utilization efficiency of conventional fertilizers (≤ 30%) and increasing environmental stressors necessitate advanced materials for sustainable agriculture. Herein, we report a synthetic chemical‐free biodegradable lignin–carrageenan (50:50) bio‐composite hydrogel engineered as a conformal coating for urea granules, enabling coupled water retention and controlled nutrient release. Lignin, derived from mustard stalks, reinforces the carrageenan network via hydrogen bonding and π–π interactions, forming a mechanically robust, physically crosslinked matrix with a high swelling ratio (~753%) and gel fraction (~48%). FTIR analysis confirms strong intermolecular interactions, while thermal characterization (TGA/DSC) indicates enhanced stability of the composite system. Rheological measurements reveal a pronounced elastic response (G′ ≫ G″; G′ ~29 kPa) with a well‐defined linear viscoelastic region and yield behavior, evidencing a percolated and resilient network. SEM imaging demonstrates a uniform, porous hydrogel coating (~20 μm) onto urea granules (3 mm in diameter), acting as an effective diffusion barrier. Soil studies show significant improvement in water retention, with maximum water holding capacity increasing by up to 32%, and extended moisture retention durations relative to control soil. Release kinetics exhibit sustained urea delivery (~85% in 26 days in water; ~92% in 66 days in soil), well described by the Peppas–Sahlin model ( R 2 = 0.99), indicating diffusion‐dominated transport with minimal polymer relaxation. The hydrogel maintains structural integrity during swelling, enabling stable, long‐term release. Collectively, this lignin‐reinforced hydrogel platform provides a scalable, bio‐based strategy to enhance fertilizer efficiency, reduce environmental losses, and improve soil moisture management under challenging agricultural conditions.