A critical review of the valorization of industrial byproducts and designer biochar for restoring sulfur and organic matter interactions in degraded soils

Abstract

Soil degradation expressed through salinization, organic carbon depletion, and contamination by persistent pollutants is intrinsically linked to the disruption of sulfur-organic matter interactions. This review critically evaluates engineered sulfur-based and organic amendments, focusing on industrial byproduct gypsum, elemental sulfur, designer biochar, and their composites, as tools for restoring these interactions. We synthesize mechanistic pathways governing sulfate retention, microbial sulfur cycling, and contaminant interference, and we assess field-scale performance using quantifiable engineering metrics. The central insight emerging from this synthesis is that no single amendment addresses all degradation pathways; rather, a fit-for-purpose selection guided by soil-specific constraints such as sodicity, pH, texture, and contaminant profile is essential. Key findings reveal that elemental sulfur oxidation achieves more than 80% sulfate yield within nine weeks under optimal conditions, while gypsum-biochar composites reduce bulk density by over 50% and enhance nutrient retention. However, significant contradictions persist sulfur oxidation rates vary markedly between soil textures, and the long-term stability of metal sulfide precipitates remains unconfirmed. We conclude that sustainable soil restoration requires an integrated framework that couples’ material properties, microbial ecology, and site-specific engineering design. This review provides such a framework and identifies five research priorities including multi-year field trials, predictive kinetic modeling, and harmonized life cycle assessment to accelerate the translation of laboratory findings into field-scale implementation.