In order to investigate the carbon sequestration potential of biochar on soil, citrus orchard soils with a forest age of 5 years was taken as the research object, citrus peel biochar (OBC) and magnesium-modified citrus peel biochar (OBC-mg) were selected as additive materials, and organic carbon mineralization experiments were carried out in citrus orchard soil. OBC and OBC-Mg were applied to citrus orchard soils at four application rates (0, 1, 2, and 4%), and incubated at a constant temperature for 100 days. Compared with CK, the cumulative mineralization of soil organic carbon decreased by 5.11% with 1% OBC and 2.14% with 1% OBC-Mg. The application of OBC and OBC-Mg significantly increased the content of soil organic carbon fraction, while the content of soil organic carbon fraction was higher in OBC-Mg treated soil than in OBC treated soil. Meanwhile, the cumulative mineralization of soil organic carbon was significantly and positively correlated with the activities of soil catalase, urease and sucrase. The enzyme activities increased with the cumulative mineralization of organic carbon, and the enzyme activities of the OBC-Mg treated soil were significantly higher than those of the OBC treated soil. The results indicated that the OBC-Mg treatment inhibited the organic carbon mineralization in citrus orchard soils and was more favorable to the increase of soil organic carbon fraction. The Mg-modified approach improved the carbon sequestration potential of biochar for citrus orchard soils and provided favorable support for the theory of soil carbon sink in orchards.

Concrete is a multiphase composite material composed of coarse aggregate, cement mortar, and interface transition zone (ITZ). It is of great significance to study the effect of ITZ and coarse aggregate on chloride microscopic diffusion behavior for predicting the service life of reinforced concrete (RC) structures. By introducing the random distribution function, a random coarse aggregate model considering the randomness of the thickness of the ITZ was established. Furthermore, a two-dimensional (2D) chloride ion diffusion mesoscopic model was developed by specifying different diffusion properties for different phase materials of concrete. Moreover, the effects of coarse aggregate rate, ITZ thickness, and ITZ diffusion property on chloride ion diffusion behavior were investigated in this paper. The research showed that the aggregate has hindrance and agglomeration action on chloride ion diffusion. Although the volume content of the ITZ was very small, less than 0.2% of the total volume of concrete, the effect of the ITZ on the chloride diffusion in concrete cannot be ignored. More importantly, the mechanism of promoting chloride diffusion in the ITZ was revealed through the chloride diffusion trajectory. The research revealed the transmission mechanism of chloride ions in the meso-structure of concrete and provides theoretical support for the design of RC structures in coastal areas.

Background: In recent years, there have been several studies on the remediation of heavy metal pollution in soil by the application of biochar. However, little attention has been paid to understanding the effects and underlying mechanisms of biochar on soil carbon sequestration in manganese-contaminated farmlands. Therefore, in this study, bagasse biochar was applied to the soil in different proportions (0%, 0.5%, 2%, and 5%) and the test was conducted indoors for 100 days at a constant temperature. Soil physical and chemical properties, organic carbon mineralization, organic carbon components, and enzyme activities were analyzed in this study. Results: In this study, when compared with the control, the application of 0.5%, 2%, and 5% bagasse biochar to the manganese-contaminated sugarcane field soil effectively reduced the cumulative CO₂ emissions, i.e., decreased by 123.18 mg·kg⁻¹, 208.28 mg·kg⁻¹, and 287.79 mg·kg⁻¹, respectively. Among the different treatment groups, the highest decrease in cumulative CO₂ emissions was observed in the 5% bagasse biochar-treated soil when compared with the control. The application of bagasse biochar increased the soil microbial biomass carbon content by 12.72 mg·kg⁻¹, 13.71 mg·kg⁻¹, and 15.10 mg·kg⁻¹, respectively when compared with the control. The soil nutrients and enzyme activities significantly increased with the increase in biochar application amount. Conclusions: The application of bagasse biochar to manganese-contaminated sugarcane soil field effectively inhibited the mineralization of soil organic carbon, improved the carbon sequestration potential of manganese-contaminated sugarcane field soil, and provided a theoretical basis for the carbon sequestration mechanism in manganese-contaminated farmland soil. Graphical

In order to study the mechanism of soil organic carbon transformation and fixation in manganese-contaminated farmland, 100-day indoor constant temperature incubation experiments were conducted to analyze the characteristics of soil physical and chemical properties, organic carbon mineralization, organic carbon components and enzyme activities by applying different proportions (0%, 0.5%, 2%, 5%) of biochar. The results showed that the cumulative CO₂ emission decreased by 15.78~36.87% compared with the control when different proportions of biochar were applied in the manganese-contaminated soil. The cumulative CO₂ emissions were reduced by 35.29~57.29% compared with the control when different proportions of biochar were applied to the uncontaminated sugarcane field soil. The microbial carbon content of uncontaminated sugarcane soil was 28.49 mg·kg^-1(0.5%),37.92 mg·kg^-1(2%) and 39.42 mg·kg^-1(5%), respectively, which were higher than that of manganese contaminated sugarcane soil 12.72 mg·kg^-1 (0.5%),13.71 mg·kg^-1(2%) and 15.10 mg·kg^-1 (5%). The catalase activities of manganese-contaminated sugarcane soil were 0.13 mg·g^-1·min^-1 (0.5%), 0.21 mg·g^-1·min^-1 (2%) and 0.27 mg·g^-1·min^-1 (5%), respectively, which were lower than those of uncontaminated sugarcane soil 9.41 mg·g^-1·min^-1 (0.5%), 10.01 mg·g^-1·min^-1 (2%), 10.91 mg·g^-1·min^-1(5%). The results showed that compared with uncontaminated sugarcane field soil, the application of biochar in manganese-contaminated sugarcane soil had little effect on microbial biomass carbon content and catalase activity. The application of exogenous biochar had an auxiliary effect on carbon sequestration in manganese-contaminated soil, and the application of 5% biochar had the best carbon sequestration effect.