Response of Soil Organic Carbon in Citrus Orchards at Different Slope Positions to Citrus Peel Biochar and Field Snail 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.