Enhancing soil fertility, nutrient recovery and carbon sequestration: the role of biochar, composted biochar, and biochar-compost mixtures in sustainable agriculture

• Biochar sequesters CO 2 , increases nutrient uptake, and removes soil contaminants. • Biochar-compost improves plant growth and plant physiological output. • Combined effect of biochar and compost offer sustainable soil restoration. Soil degradation, caused by salinity, nutrient depletion, and trace metal pollutants, is a major problem for agricultural productivity. Organic amendments, such as biochar, have shown potential to improve soil health, water retention, and nutrient availability. However, biochar-based blended amendments, such as biochar compost mixture and composted biochar, may offer greater benefits even under harsh soil environments. This systematic review evaluates the limitations and advantages of biochar, compost, and biochar-compost mixtures in restoring degraded and polluted soils. The review assesses their roles in improving soil fertility, nutrient availability, microbial activity, and key physiochemical properties relevant to sustainable agriculture. Following PRISMA guidelines, we systematically searched major scientific databases and screened studies based on predefined inclusion and exclusion criteria. Eligible studies included laboratory, greenhouse, and field experiments assessing soil restoration outcomes using biochar-based amendments. Biochar applications enhanced biomass production, CO 2 capture, soil quality, and immobilization of inorganic ions and organic waste material due to promising properties like advanced cation exchange capacity, high permeability, and large surface area. Biochar-compost further improved nutrient availability, biomass uptake, water retention, carbon sequestration, and plant physiological responses under stress. Composted biochar offered more sustained and long-lasting improvements in soil quality and plant productivity under salinity stress, thus solving multiple biochar-associated challenges, although further long-term field evidence is still needed. Overall, biochar and biochar-compost represent promising tools for soil restoration, pollutant remediation, and crop productivity. Although their benefits are widely reported, variability in outcomes and gaps in long-term evidence underscore the importance of optimizing application strategies for different soil systems.