Response of soil respiration to environmental and photosynthetic factors in different subalpine forest-cover types in a loess alpine hilly region

Soil respiration (Rs) is important for transporting or fixing carbon dioxide from the atmosphere, and even diminutive variations can profoundly influence the carbon cycle. However, the Rs dynamics in a loess alpine hilly region with representative sensitivity to climate change and fragile ecology remains poorly understood. This study investigated the correlation and degree of control between Rs and its photosynthetic and environmental factors in five subalpine forest cover types. We examined the correlations between Rs and variables temperature (T10) and soil moisture content at 10 cm depth (W10), net photosynthetic rate (Pn) and soil properties to establish multiple models, and the variables were measured for diurnal and monthly variations from September 2018 to August 2019. The results showed that soil physical factors are not the main drivers of Rs dynamics at the diel scale; however, the trend in the monthly variation in Rs was consistent with that of T10 and Pn. Further, Rs was significantly affected by pH, providing further evidence that coniferous forest leaves contribute to soil acidification, thus reducing Rs. Significant exponential and linear correlations were established between Rs and T10 and W10, respectively, and Rs was positively correlated with Pn. Accordingly, we established a two-factor model and a three-factor model, and the correlation coefficients (R2) was improved to different degrees compared with models based only on T10 and W10. Moreover, temperature sensitivity (Q10) was the highest in the secondary forest and lowest in the Larix principis-rupprechtii forest. Our findings suggest that the control of Rs by the environment (moisture and temperature) and photosynthesis, which are interactive or complementary effects, may influence spatial and temporal homeostasis in the region and showed that the models appropriately described the dynamic variation in Rs and the carbon cycle in different forest covers. In addition, total phosphorus (TP) and total potassium (TK) significantly affected the dynamic changes in Rs. In summary, interannual and seasonal variations in forest Rs at multiple scales and the response forces of related ecophysiological factors, especially the interactive driving effects of soil temperature, soil moisture and photosynthesis, were clarified, thus representing an important step in predicting the impact of climate change and formulating forest carbon management policies.