## 4 Discussion
### Spatiotemporal Variations in \(\text{FCO}_{2}\) and Its Driving Factors
The continuous decline in SM without precipitation replenishment can intensify soil hydrological drought and thus diminish the \(\text{FCO}_{2}\) emission during the dry season [8; 86]. [PERSON], et al. [24] indicated the \(\text{FCO}_{2}\) emissions during the dry season in the subtropical forests of southern China are determined by precipitation amount. Because the precipitation in the dry season can be the major determinant of SM, and changes in SM caused by precipitation will directly influence \(\text{FCO}_{2}\) emissions. Similarly, [PERSON] et al. [86] also indicated that precipitation strongly affects the SM and thus determines the \(\text{FCO}_{2}\) release from three subtropical forests in southern China during the dry season. Nevertheless, further research is needed to consider the effects of interaction between precipitation and temperature on the \(\text{FCO}_{2}\) emissions in subtropical forests. The abiotic factors play a crucial role in regulating the \(\text{FCO}_{2}\) variations in the subtropical forest areas. However, changes in ST and SM may not fully explain the variation of the \(\text{FCO}_{2}\) we observed in this study (Figure 4) and the previous study [43]. Many previous studies reported that biotic factors such as plant productivity can significantly affect the variations of \(\text{FCO}_{2}\)[5, 8, 14, 34, 37, 59], especially for the subtropical forests [20, 24]. Our further analysis suggested that the growth of vegetation can also affect the \(\text{FCO}_{2}\) variation as demonstrated by the significant and positive relationships between the \(\text{FCO}_{2}\) and vegetation-related indices (Table 3). The reasons are that plant productivity determines the production of litter and fine root biomass, which in turn affects soil respiration [24]. The positive relationship between plant productivity and the \(\text{FCO}_{2}\) changes also coincides with many previous studies in the subtropical and temperate forests [5, 20, 24, 39, 58]. Moreover, the variable importance ranking derived from the optimal RF model also indicated that both abiotic (ST and SM) and biotic factors (VIs) significantly affect the soil \(\text{CO}_{2}\) emissions in subtropical forests in this study (Figure 6) and as indicated in other studies [13, 24]. The \(\text{FCO}_{2}\) varies across space (Figures 8 and 9) and is also subject to topographic factors (Figure 10). [PERSON], et al. [40] highlighted the significant effect of topography on the \(\text{FCO}_{2}\) in a forested watershed. Previous studies reported that the higher soil \(\text{CO}_{2}\) flux was found in the bottomlands with abundant soil moisture in the subalpine watershed [87] and semiarid Loess Plateaus [88]. However, the opposite phenomenon occurred in tropical rainforest areas [89], which showed that higher soil \(\text{CO}_{2}\) flux emissions were located in higher altitude areas. In this study, we also observed that the high values of \(\text{FCO}_{2}\) mainly occurred in the high altitudinal areas and the slopes of 10-25\({}^{\circ}\) of the subtropical forested watershed. Similar results were reported in other studies [16, 20]. One of the reasons is that these areas, with high elevations and 10-25\({}^{\circ}\) slopes, are the key regions under the construction and protection of the Chinese government's ecological project [90] and are less disturbed by human activities. Moreover, the absolute altitude of this study area is not very high, and the higher vegetation coverage, species richness, and plant productivity are mainly located in these areas. Therefore, the higher vegetation cover and plant productivity, which is links with fine roots biomass and the litterfall, can significantly induce higher soil \(\text{CO}_{2}\) flux emission in the higher subtropical forest areas [16, 20]. Besides, the \(\text{FCO}_{2}\) emission is subject to aspects, and the \(\text{FCO}_{2}\) emission in sunny slopes is higher than in the shady slopes (Figure 9c).