Soil degradation through salinization, sodicity, organic carbon depletion, and chemical contamination severely threatens agricultural productivity in arid and semi-arid regions. Central to this degradation is the disruption of the sulfur-organic matter nexus. In healthy ecosystems, organic sulfur constitutes over ninety-five percent of total soil sulfur, providing a stable reservoir that buffers long-term biological demand. In degraded soils, loss of organic binding sites and altered microbial activity accelerate sulfate leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More, leading to persistent nutrient deficits and poor soil structure. Addressing these multifaceted limitations requires evaluating engineered sulfur-based materials, designer biochars, and hybrid composites.

Industrial byproduct gypsum, particularly flue gas desulfurization gypsum from coal scrubbing, serves as a primary feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More for reclaiming sodic soils. Containing at least ninety-five percent calcium sulfate dihydrate and minimal radioactivity, flue gas desulfurization gypsum provides soluble calcium ions that displace exchangeable sodium from soil colloids via ligand and ion exchange mechanisms. Subsequent leaching of sodium sulfate restores soil aggregation, decreases soil compactness, and improves subsurface saturated hydraulic conductivity. While phosphogypsum is produced in much higher global quantities, its application remains constrained by regulatory limits concerning radium-226 radioactivity, despite column studies showing leachate concentrations below drinking water safety standards.

Elemental sulfur provides a slow-release mechanism for in situ soil acidification and sulfate generation in alkaline, calcareous soils. Because elemental sulfur is not directly bioavailable, chemoautotrophic bacteria such as Thiobacillus species must oxidize it into sulfate. Reaction kinetics are governed by particle size distribution, with smaller particles expanding specific surface area to accelerate bacterial colonization. Controlled studies indicate that elemental sulfur loadings between 0.5 and 3.0 percent by weight induce significant pH reductions and substantial sulfate generation. Over eighty percent of total sulfate yield occurs within the first nine weeks of incubation, coinciding with peak agricultural demand windows, although sandy loams exhibit twenty-three percent higher sulfate mobility compared to clay soils.

To enhance carbon storage and structural functionality, designer biochars are produced through physical, chemical, or biological modifications. Ball milling and steam activation increase specific surface area and microporosity, whereas acid or alkali treatments increase oxygen-containing surface functional groups such as carboxyl and hydroxyl moieties. Hydrogen peroxide oxidation selectively grafts carboxyl groups to boost heavy metal binding without damaging pore structure, while chitosan coatings introduce amine functional groups that capture anionic pollutants. Furthermore, incorporating sulfonic acid groups creates sulfonated biochars and polymers that act as solid acid catalysts and strong cation exchange resins, facilitating targeted ion exchange and localized pH adjustment.

Hybrid composites combine these distinct material classes to achieve synergistic benefits. Gypsum-biochar composites blend the calcium release and clay-flocculating ability of gypsum with the high porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More, moisture retention, and cation exchange capacity of biochar. Adding fifty percent biochar by weight reduces material bulk density by over fifty percent while expanding nutrient retention. Similarly, layered double hydroxide-biochar composites utilize synthetic anionic clays on biochar supports to provide simultaneous cation sorption and anion exchange. Incorporating two percent calcium-aluminum layered double hydroxide-biochar composite passivates copper by 47.85 percent and lead by 37.95 percent in contaminated soils while enriching beneficial microbial phyla such as Actinobacteriota.

Selecting appropriate remediation amendments requires matching material performance characteristics to site-specific soil constraints. Flue gas desulfurization gypsum represents the benchmark technology for sodicity reclamation and sodium displacement. Elemental sulfur-biochar composites offer superior multifunctionality across pH adjustment, structure improvement, and slow-release nutrient delivery in alkaline soils. Sulfonated polymers like polyacrylamide excel at aggregate stabilization and erosion control in coarse soils. Long-term field trials, predictive kinetic modeling of sulfur oxidation, and standardized life cycle assessments remain essential priorities to optimize field-scale implementation.

Source: Abd Zaid, A. (2026). A critical review of the valorization of industrial byproducts and designer biochar for restoring sulfur and organic matter interactions in degraded soils. Discover Soil, 3, Article 151.