In view of the continuous decline in organic carbon content in citrus orchard soil, to explore the effects of biochar and farmland waste on the transformation of organic carbon in citrus orchard soil. In this study, the soil of a citrus orchard in Yangshuo County, Guilin, Guangxi, China, was collected. The citrus peel and the Cipangopaludina chinensis shell were used as raw materials, the citrus peel was used to prepare biochar, and the Cipangopaludina chinensis shell was powdered. The materials were added to the soil in different proportions. A 30-day indoor incubation soil was conducted to investigate the effects of adding different proportions of citrus peel biochar and Cipangopaludina chinensis shell powder on the transformation of organic carbon in citrus orchard soil. Compared with the control group, the addition of 4% orange peel biochar, 2% orange peel biochar + 2% Cipangopaludina chinensis shell powder, 2.6% orange peel residue biochar + 1.3% Cipangopaludina chinensis shell powder, and 3% orange peel biochar + 1% Cipangopaludina chinensis shell powder increased soil organic carbon by 22.49%, 20.06%, 19.81%, and 21.35%, respectively. Compared with the control, 2.6% orange peel biochar + 1.3% Cipangopaludina chinensis shell powder had the best effect on the improvement of soil organic carbon components, and microbial biomass carbon (MBC), dissolved organic carbon (DOC) and readily oxidized organic carbon (ROC) increased by 19.81%, 64.88%, 67.81%, and 19.44%, respectively. Different proportions of orange peel residue biochar and Cipangopaludina chinensis shell powder were applied to the soil of the citrus orchard to effectively increase the soil organic carbon component content and enzyme activities. This study provides a theoretical basis for the carbon sequestration mechanism of citrus orchard soil using different proportions of orange peel residue biochar and Cipangopaludina chinensis shell powder.

Introduction: Susbtantial agricultural wastes are produced globally which need urgent management policies. To explore the effective utilization of agricultural waste in enhancing soil quality and carbon sequestration capacity, straw and its biochar can be applied as soil ameliorants.Methods: This study was designed to investigate the impact of different return-to-field methods of rice straw on the transformation between different carbon components in the soil of Siraitia grosvenorii fields. We hypothesize that rice straw and its biochar, as soil amendments, can influence the transformation and cycling of different carbon components in the soil of S. grosvenorii fields through various return-to-field methods. Rice straw, rice straw biochar, and “rice straw + rice straw biochar” were applied as additives in a 2-year field experiment.Results: The results showed that the field application of rice straw and its biochar increased the content of soil organic carbon, the amount of organic carbon mineralization, particulate organic carbon, mineral-associated organic carbon, dissolved organic carbon, and readily oxidizable organic carbon content, while reducing the content of soil microbial biomass carbon. The combined application of rice straw and biochar in S. grosvenorii cultivation fields had a more significant effect on various soil carbon fractions compared to the use of either rice straw or biochar alone. The co-application of rice straw and its biochar to the soil increased the content of soil organic carbon by 117.4%, enhanced the mineralization of organic carbon by 100.0%, and reduced the content of soil microbial biomass carbon by 61.6%. The metabolic entropy and microbial entropy of rice straw and its biochar mixed application in the field were 5.2 and 0.18 times higher than of the control group, respectively.Discussion: In summary, the return of rice straw and biochar to the field improves soil structure and the content of recalcitrant organic carbon, providing a habitat for microorganisms, thereby promoting the stability and cycling of soil organic carbon.

In order to study the effects of wildfires on soil carbon dioxide (CO₂) emissions and microbial communities in planted forests, Pinus massoniana Lamb. and Cunninghamia lanceolata (Lamb.) Hook. forests were selected as the research subjects. Through a culture test with 60 days of indoor constant temperature, the soil physical and chemical properties, organic carbon mineralization, organic carbon components, enzyme activity, and microbial community structure changes of the two plantations after fire were analyzed. The results showed that wildfires significantly reduced soil CO₂ emissions from the Pinus massoniana forests and Cunninghamia lanceolata forests by 270.67 mg·kg⁻¹ and 470.40 mg·kg⁻¹, respectively, with Cunninghamia lanceolata forests exhibiting the greatest reduction in soil CO₂ emissions compared to unburned soils. Bioinformatics analysis revealed that the abundance of soil Proteobacteria in the Pinus massoniana and Cunninghamia lanceolata forests decreased by 6.00% and 4.55%, respectively, after wildfires. Additionally, redundancy analysis indicated a significant positive correlation between Proteobacteria and soil CO₂ emissions, suggesting that the decrease in Proteobacteria may inhibit soil CO₂ emissions. The Cunninghamia lanceolata forests exhibited a significant increase in soil available nutrients and inhibition of enzyme activities after the wildfire. Additionally, soil CO₂ emissions decreased more, indicating a stronger adaptive capacity to environmental changes following the wildfire. In summary, wildfire in the Cunninghamia lanceolata forests led to the most pronounced reduction in soil CO₂ emissions, thereby mitigating soil carbon emissions in the region.

In order to investigate the effect of biochar on the strength characteristics of red loam, a number of experiments were carried out in the red loam of northern Guangxi Province, including a direct shear test, a scanning electron microscopy (SEM) test, and an X-ray diffraction (XRD) test of red loam, bagasse biochar–red loam, and rice platycodon biochar–red loam. The aim of this study was to determine the effects of different biochar contents, types, and particle size ranges on the shear strength of the improved soils. The results showed that both bagasse biochar and rice platycodon biochar could effectively improve the shear strength of soil, and the shear strength increased first and then decreased with the increase in biochar content and reached the peak value when the content of biochar reached 6%. Under vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa, the shear strength of the two improved soils increased by 53.39%, 52.52%, 43.43%, and 47.08%, respectively, and 54.56%, 23.89%, 33.71%, and 47.78%, respectively, compared with that of plain soil. In addition, the grain size was negatively correlated with the shear strength, and the shear strength of the sample increased with a decrease in the grain size, in which the biochar in the range of 0~0.5 mm had the best effect on the strength improvement of the red loam. The results of this study provide theoretical and technical support for revealing the evolutionary mechanism of red loam strength and coping with soil erosion in red soil areas.