Soil Organic Carbon Stocks Under Daylily Cultivation and Their Influencing Factors in the Agro-Pastoral Ecotone of Northern China

Perennial crops are crucial for enhancing soil organic carbon (SOC) stocks to mitigate climate change, yet the effects of planting duration on SOC stocks remain inconsistent. In the agro-pastoral ecotone of northern China, where soil degradation is a growing concern, daylily, a perennial crop cultivated for over 600 years, presents both ecological and agricultural potential. This study evaluates the impact of long-term (LD, >10 years) and short-term (SD, ~5 years) daylily cultivation on SOC stocks and identifies key drivers. Paired soil samples (0–100 cm) from eight sites under LD, SD, and long-term maize cultivation (CK) were analyzed using analysis of variance (ANOVA), correlation analysis, random forest, and structural equation modeling (SEM). LD significantly increased SOC stocks by 19.63% compared to CK, while SD showed no significant difference. The sampling site had a greater impact on SOC stocks than the treatment across different geographic locations. At the same location, SEM revealed that soil factors influenced SOC differently across treatments: for LD, soil total nitrogen (TN) > pH > soil water content (SWC); for SD, TN > SWC > soil available phosphorus (AP); for CK, TN > soil available potassium (AK) > SWC. This study provides insights for regional soil management and carbon sequestration strategies, highlighting the role of LD in enhancing soil quality and promoting ecological restoration.