Soil organic carbon stability and its influencing factors during grassland enclosure on the Loess Plateau, China

Taking grasslands with different closure durations (13, 22, 30, and 39 years) on the Loess Plateau as the research subjects, with grazed grasslands serving as the control, we explored the characteristics and regulating factors of changes in SOC mineralization (C_min), temperature sensitivity (Q₁₀), SOC turnover rate (K), and turnover time (T) during grassland enclosure, using the space-for-time substitution method. The results showed that C_min significantly increased with enclosure duration, reaching 2.1 times that of grazed grassland after 39 years of enclosure. Q₁₀ significantly decreased with enclosure duration, falling to 0.9 times that of grazed grassland after 39 years of enclosure. The changes in K and T with enclosure duration were regulated by temperature. At 15 ℃, K significantly increased and T significantly decreased with enclosure duration, reaching 8.8 times and 0.3 times those of grazed grassland, respectively, after 39 years of enclosure. At 25 and 35 ℃, the trends of K and T with enclosure duration were opposite to that observed at 15 ℃. C_min, K, and T were significantly positively correlated with labile SOC fractions and negatively correlated with recalcitrant SOC fractions, whereas the opposite pattern was observed for Q₁₀. Stepwise regression analysis revealed that the key SOC fraction influencing C_min was particulate organic carbon. In contrast, the key SOC fractions influencing K and T were mineral-associated organic carbon (at 15 ℃) and dissolved organic carbon (at 25 and 35 ℃). In conclusion, enclosure promoted SOC mineralization but suppressed Q₁₀, and its effect on SOC turnover shifted from positive to negative as temperature increased. SOC fractions were key regulators of SOC stability, with increased labile SOC fractions directly promoting SOC mineralization and turnover while indirectly suppressing Q₁₀.