Exogenous Carbon Type Determines the Structure and Stability of Soil Organic Carbon in Dryland Farmlands Under a Continental Semi-Arid Climate

The effects of different exogenous carbon types on the chemical structural characteristics and stability of soil organic carbon in dryland farmland remain unclear. Based on a four-year fixed-site experiment in a typical dryland farmland on China’s Loess Plateau, this study systematically analyzed the impacts of different carbon sources on soil enzyme activities, organic carbon content, chemical structural characteristics, and their interrelationships under five treatments: (i) no fertilization (T0); (ii) 100% chemical nitrogen, phosphorus, and potassium fertilizers (CK); (iii) 50% CK + fermented cattle manure (T1); (iv) 50% CK + corn straw (T2); (v) 50% CK + mixed fermented cattle manure/corn straw (T3). The results showed that the activities of β-glucosidase and N-acetylglucosidase ranked in the order T1 > T2 > T3 and T3 > T2 > T1, respectively. Specifically, β-glucosidase activity under T1 increased by 35.26% compared to CK, while N-acetylglucosidase activity under T3 increased by 30.78% relative to CK. Compared to CK, the T1, T2, and T3 treatments increased soil organic carbon by 26.84%, 11.27%, and 18.63%, and alkyl carbon content by 7.67%, 2.91%, and 5.57%, respectively. Additionally, T1 and T3 treatments elevated aromatic carbon content by 20.59% and 176.47% relative to CK. The organic carbon activity index under T1 was the lowest, decreasing by 10.04% compared to CK. Structural equation modeling (SEM) path analysis revealed that the addition of different exogenous carbon sources in dryland farming primarily influenced the structure and stability of soil organic carbon by directly or indirectly enhancing the activities of glucosidase, β-acetylglucosidase, and alkaline phosphatase, with T1 demonstrating the most significant improvement.