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.

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 Abstract