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.

Soil organic carbon (SOC) loss in sloping farmland is a critical challenge for agricultural sustainability. This study investigated how citrus peel biochar (CPB), field snail shell powder (SSP), and their composite (CPB + SSP) differentially regulate SOC dynamics across slope positions (upper, middle, lower) in Guangxi’s citrus orchards. Key findings revealed: CPB significantly increased SOC content (up to 5.5 g·kg⁻¹ at lower slopes) via high carbon input but suppressed mineralization amount in lower slope position (reduction of 17.9%) due to its high C/N ratio. SSP neutralized soil acidity (pH 3.95 to 7.5), stimulating microbial activity and raising mineralization rates by 58.95% (lower slope), yet minimally enhanced SOC (only +0.7 g·kg⁻¹). CPB + SSP effectively balanced carbon stability and active release: dissolved organic carbon (DOC) and readily oxidizable organic carbon (ROC) increased by 14.4 mg·kg⁻¹ and 0.22 g·kg⁻¹ (middle slope), while SOC rose significantly (e.g., +2.2 g·kg⁻¹ at lower slope). Slope position effects strongly influenced outcomes: the lower slope (highest initial SOC) responded most strongly to CPB for carbon stabilization, while middle slopes benefited from CPB + SSP to reconcile carbon loss with fertility. These results provide slope-specific strategies for SOC management by integrating amendment synergy and machine learning-driven insights in citrus orchards.