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.

Biochar is a soil amendment that has the potential to effectively improve soil salinization. However, there is a paucity of studies on sodium adsorption using biochar, and the adsorption mechanism remains unclear. To better understand the adsorption mechanism of Na⁺ on the surface of biochar, both pyrochar and hydrochar were produced at different temperatures. The capacity and influencing factors of Na⁺ adsorption in biochar were analyzed via batch adsorption experiments. Pore filling dominated the Na⁺ adsorption in the concentration of the NaCl solution when it was ≤100 mg/L, where wheat straw pyrochar (WB, 3.95–4.94 mg/g Na) and poplar wood chip pyrochar (PB, 0.62–0.70 mg/g Na) presented the release and adsorption of Na⁺, respectively. When the concentration of the NaCl solution was >100 mg/L, the adsorption capacity of WB (25.44–36.45 mg/g) was significantly higher than PB (4.46–6.23 mg/g). Both the adsorption and release of Na⁺ in hydrochar was insufficient. In a high concentration of NaCl solution, ion exchange became the key mechanism determining the adsorption of Na⁺ in pyrochar, in which K⁺ contributed to more than 94% of the Na⁺ adsorption. The findings proposed strategies for the structural design of biochar used for Na adsorption. These will promote the utilization of solid biowaste for sodium adsorption and the potential of soil salinization amendment for agriculture in arid lands.

Salinization is a growing global problem, particularly in arid and semi-arid areas, where salt concentration interferes with the soil structure, altering natural cycling, decreasing agricultural outputs, and threatening food security. Although many soil amendments have been studied, there is still a limited understanding of their interaction with soil after mixture application and the geochemical processes and long-term sustainability that govern their effects. To address this knowledge gap, this review elucidated the effectiveness and sustainability of soil amendments, biochar, humic substances, and mineral additives in restoring saline and sodic soils of arid and semi-arid region to explore the geochemical processes that underlie their impact. A systematic search of 174 peer-reviewed studies was conducted across multiple databases (Web of Science, Google Scholar, and Scopus) using relevant keywords and the findings were converted into quantitative values to evaluate the effects of biochar, gypsum, zeolite, and humic substances on key soil properties. Biochar significantly improved cation exchange capacity, nutrient retention, microbial activity, and water retention by enhancing soil porosity and capillarity, thereby increasing plant-available water. Gypsum improved phosphorus availability, while zeolite facilitated the removal of sodium and supported microbial activity. Humic substances enhanced soil porosity, water retention, and aggregate stability. When applied together, these amendments improved soil health by regulating salinity, enhancing nutrient cycling, while also stabilizing soil conditions and ensuring long-term sustainability through improved geochemical balance and reduced environmental impacts. The findings highlight the critical role of multi-functional amendments in promoting climate-resilient agriculture and long-term soil health restoration in saline-degraded regions. Further research and field implementation are crucial to optimize their effectiveness and ensure sustainable soil management across diverse agricultural environments.

Biochar hydrophobicity is crucial for understanding its interaction with environmental substances (e.g., soil, water, pollutants). Contact angle (CA) and water droplet penetration time (WDPT) are commonly used methods for assessing biochar hydrophobicity. However, occasional inconsistencies between CA and WDPT measurements introduce uncertainties, emphasizing the need for more accurate evaluation. This study addressed these temporal inconsistencies by proposing a new method using the dynamic contact angle (DCA) to evaluate the hydrophobicity of 17 standard materials and 18 types of biochars. The DCA method, which considers droplet diffusion recorded CA changes over 90 s and compared the significance ( p < 0.05) between initial CA (CA_0) and CA after 90 s (CA_90). Based on this, a new classification of hydrophobicity was established, encompassing super-hydrophobic, strongly hydrophobic, ‘pseudo'-hydrophobic and hydrophilic categories. ‘Pseudo'-hydrophobic materials exhibited a significant decrease in CA within 90 s, where the CA transitioned from hydrophobic (CA > 90°) to hydrophilic (CA < 90°) within this period, revealing their hydrophilic nature. The combination of CA_0 and the rate of CA change over time (| k |) was considered as a new criterion for hydrophobicity evaluation. Through a 90-day incubation experiment of biochar and soil, most biochars significantly increased the water repellency of the biochar-amended soil, as evidenced by increases in both CA_0 and CA_90. Our DCA method, along with the definition ‘pseudo'-hydrophobicity, resolves contradictions between CA and WDPT measurements for both soil and biochar, enhancing the accuracy of hydrophobicity assessments. Graphical Abstract

Sodium ions (Na⁺) are typically the dominant ions in soil salts, where their excessive accumulation forms sodic saline soils with degraded structure that harms crops and soil biota. However, the adverse effects of Na⁺ on soil functions and microorganisms have not yet been systematically investigated, and the application of conventional and emerging remediation methods with regard to Na⁺ dynamics remains unclear. This review synthesizes the current knowledge on natural and anthropogenic sources, distribution, and transport of soil Na⁺ and critically discusses its adverse impacts on soil function and crop performance. Long-used remediation techniques and recently developed emerging methods have been compared, and their mechanisms, reported effectiveness and practical applicability have been highlighted. Our study revealed that traditional reclamation methods through agronomic practices, ion exchange amendments (e.g., Ca- or K-rich conditioners), and plant–microbial interventions can effectively reduce soil Na⁺ and sodicity. Emerging saline–alkali soil amelioration methods (e.g., engineered/nano amendments, electrochemical stimulation, and salt-tolerant plant–microbe consortia), especially when combined in composite strategies, can more selectively and efficiently regulate Na⁺ and ion transport. Moreover, durable soil Na⁺ reduction is most consistently associated with integrated management strategies that couple optimized irrigation, water quality, and targeted amendments. Finally, we outline research needs for mechanistic, field-scale, long-term validation of the fate of Na⁺ and its transport for robust assessments of safety, durability, and cost to support scalable implementation.