Dynamics of Soil Organic Carbon Mineralization Under Straw Addition: Evidence from a Controlled Incubation Experiment

Returning straw to the soil is increasingly recognized as a sustainable practice that enhances soil fertility and promotes carbon sequestration. However, it can also accelerate the decomposition of soil organic carbon (SOC) and CO2 emissions, raising concerns about carbon loss. This study aimed to clarify the biological and environmental drivers of SOC mineralization across soil depths in a semi-arid system. A 79-day incubation experiment was conducted using wheat straw applied at four rates (0, 3500, 7000, and 14,000 kg ha−1) to soils from 0–10, 10–20, and 20–30 cm. Cumulative CO2 release, SOC, dissolved organic carbon (DOC), and extracellular enzyme activities were quantified, and relationships were analyzed using correlation and structural equation modeling. Compared with the control, straw return increased cumulative CO2 emissions by 48–126%, SOC by 9–21%, and DOC by 17–32%. Enzyme activities of β-glucosidase and N-acetylglucosaminidase were 25–64% higher under straw treatments. Structural modeling revealed that enzyme activity had a stronger direct effect on SOC mineralization than chemical properties. These results support the co-metabolism theory, stimulating microbial metabolism to enhance both straw- and native-SOC decomposition. Overall, straw return improves nutrient cycling but increases CO2 emissions, underscoring the need for optimized management to balance soil fertility with carbon mitigation.