Enhancing global agricultural sustainability critically requires improving the physicochemical properties of saline–alkali soil. Biochar has gained increasing attention as a strategy due to its unique properties. However, its effect on the physicochemical properties of saline–alkali soil varies significantly. This study uses psychometric meta-analysis across 137 studies to synthesize the findings from 1447 relatively independent data sets. This study investigates the effects of biochar with different characteristics on the top 20 cm of various saline–alkali soils. In addition, aggregated boosted tree (ABT) analysis was used to identify the key factors of biochar influencing the physicochemical properties of saline soils. The results showed that biochar application has a positive effect on improving soil properties by reducing the sodium adsorption ratio (SAR) and the exchangeable sodium percentage (ESP) by 30.31% and 28.88%, respectively, with a notable 48.97% enhancement in cation exchange capacity (CEC). A significant inverse relationship was found between soil salinity (SC) and ESP, while other factors were synergistic. Biochar application to mildly saline soil (0.4%). However, the reduction in SC in highly saline soil was 4.9 times greater than in moderately saline soils. The enhancement of soil physical properties positively correlated with higher biochar application rates, largely driven by soil movements associated with the migration of soil moisture. Biochar produced at 401–500 °C was generally the most effective in improving the physicochemical properties of various saline–alkali soils. In water surplus regions, for mildly saline soil with pH < 8.5, mixed biochar (pH 6–8) at 41–80 t ha⁻¹ was the most effective in soil improvement. Moreover, in water deficit areas with soil at pH ≥ 8.5, biochar with pH ≤ 6 applied at rates of >80 t ha⁻¹ showed the greatest benefits. Agricultural residue biochar showed superior efficiency in ameliorating highly alkaline (pH ≥ 8.5) soil. In contrast, the use of mixed types of biochar was the most effective in the amelioration of other soil types.

The influences of feedstocks on biochar properties are widely reported. However, the influence of the transformation of biomass components (mainly cellulose, hemicellulose, and lignin) during feedstock pyrolysis on the obtained biochar has not been clearly stated. Here, biochar was pyrolyzed from four biomass types with different fractions of the three main components, of which surface area, pore structure, functional group, and thermogravimetric analyses were conducted. Further, we investigated the links among the physicochemical properties and water-holding capacity (WHC) of the biochar by measuring the WHC of a biochar–silica-sand (SS) mixture. Cellulose and hemicellulose were considered the easily pyrolyzable components of the feedstock owing to their low thermal stabilities. Additionally, the thermal decomposition of the easily pyrolyzable components caused the disappearance of most functional groups from the biochar that was synthesized at >350 °C. Moreover, the WHC of the biochar–SS mixture correlated significantly with the surface area and pore volumes of the biochar. Notably, the thermal residual mass and the WHC of the biochar–SS mixture exhibited the strongest correlation. Poplar wood sawdust (PT), which accounted for the highest mesopore volume of the biochar sample, contained the highest amount (86.09%) of the easily pyrolyzable components. The PT-derived biochar exhibited superior WHC than other biochar types, indicating that the dehydration, deoxygenation, and condensation of the easily pyrolyzable components of biomasses promoted gradual pore formation, further contributing to the increased WHC of the mixture. Rather than high-temperature-pyrolyzed biochar, PT350 demonstrated the highest WHC (599 mg/g), revealing that attention should be drawn to the contribution of low-temperature-pyrolyzed biochar to soil water retention in future research.

To address rising global food demand, improving phosphorus (P) use efficiency in agriculture is crucial. Organic fertilizers and biochar are recognized for their potential to improve soil phosphorus availability and reduce environmental losses. However, the synergistic effects of their combined application on phosphorus retention and transformation have received insufficient attention. This study investigated the synergy between cow dung-derived biochar (produced at 400 °C and 700 °C) and organic fertilizer using P fractionation, leaching, and extraction tests. Results indicated that the H₂O-P content in organic fertilizer as high as 42.17 mg·g⁻¹, resulting in a cumulative leaching loss of up to 11.62 mg·g⁻¹. In contrast, biochar exhibited lower leaching due to more stable C–P compounds, as confirmed by X-ray photoelectron spectroscopy (XPS). When biochar and organic fertilizer were co-applied, complexation with Ca²⁺ on their surfaces reduced phosphorus leaching from the mixture by 83.69%. The formation of Ca₂P₂O₇ crystals, detected through X-ray diffraction (XRD), indicates a strong synergistic effect between biochar and organic fertilizer. Additionally, the porous structure of biochar adsorbed phosphorus from organic fertilizer, further inhibiting leaching losses. This synergy enhances P retention, offering an effective strategy to improve P use efficiency and support sustainable soil management.