The effect of climatic and edaphic factors on soil organic carbon turnover in hummocks based on δ13C on the Qinghai-Tibet Plateau

Hummocks (thúfur, pounus) are peculiar landforms usually formed by repeated freeze-thaw processes and differential frost heave, and are common in frost soil regions, especially in the Qinghai-Tibet Plateau. However, little is known about the response of δ13C in soil organic carbon (δ13CSOC) to soil and climate properties in hummocks. The β value indicates the decomposition rate of soil organic carbon (SOC) in soil, and was obtained from the slope of the regression between the log10-transformed SOC concentration and δ13CSOC in soil depth profiles. In this study, we investigated δ13CSOC and SOC contents along a soil profile (0-60 cm), together with edaphic and climatic properties, both in hummocks and control plots (alpine grasslands) on the northeastern Qinghai-Tibet Plateau. Then, the variations in δ13CSOC and β values, and the main factors affecting them, were analyzed. The results show that δ13CSOC increases with soil depth, while SOC decreases both in the hummocks and control plots. However, β values in the hummocks were significantly (P < 0.05) higher than in the control plots while δ13CSOC showed no difference between hummock and control. Redundancy analysis showed that altitude is the main control factor for δ13CSOC and β in the hummocks. Climate type was the main factor affecting δ13CSOC in the control plots, while mean annual precipitation and soil fractal dimension were the main factors controlling β. Overall, climate, rather than soil, is the key factor that affects the carbon turnover rate in the hummock in the northeastern QTP. The findings of this study will expand our understanding of the soil carbon cycle and δ13CSOC changes, especially in the case of hummocks.