Co-Application of Modified Biochar and Functional Amendments for Salt-Affected Soil Amelioration: Performance, Mechanisms, and Prospects

Soil salinization constrains agricultural productivity and sustainable land use by simultaneously impairing soil structure, nutrient availability, biological activity, and plant growth. Modified biochar (MBC) has attracted increasing attention because modification can regulate biochar alkalinity, surface chemistry, pore structure, and mineral composition, thereby improving its suitability for contrasting salt-affected soils. However, the interacting ionic, structural, nutritional, and biological limitations of these soils are unlikely to be resolved by a single material. This review synthesizes evidence on the co-application of MBC with functional amendments, including mineral and ionic materials, organic and nutrient inputs, polymers, and microbial agents, while using studies of pristine biochar or single materials only as contextual evidence. The available literature indicates that treatment performance depends on matching amendment function to the dominant soil constraint. Calcium- or magnesium-supplying and acidifying materials may be better matched to sodic and saline-sodic soils where alkalinity and exchangeable Na+ restrict infiltration; nutrient, organic, and biological amendments may be more relevant where fertility and rhizosphere limitations dominate; and polymers may complement MBC where aggregate stability and water retention are primary constraints. These combinations act through coordinated regulation of ion exchange and salt transport, nutrient retention and release, pore connectivity, aggregate formation, and microbial processes. Nevertheless, functional complementarity should not be equated with synergy because few factorial studies include MBC alone, the functional amendment alone, and their co-application. Field application is further limited by uncertain dose reduction, variable material quality, residual salinity, environmental risk, cost, and insufficient long-term validation. Future research should prioritize factorial dose–response experiments, multi-season field trials, and integrated agronomic, environmental, and economic assessment. This review provides a constraint-based framework for selecting and evaluating MBC-based co-application strategies in salt-affected soils.