## Results
### The Relationship between Carbon Fluxes and the Soil Microenvironment  
Irrespective of the experimental treatment, the temporal variation in CO\({}_{2}\) fluxes and CH\({}_{4}\) flux could be primarily explained by the soil water and temperature (Figure 7). GPP (R\({}^{2}\)\(\geq\) 0.54, \(p\) < 0.01) and R\({}_{e}\) (R\({}^{2}\)\(\geq\) 0.25, \(p\) < 0.01) were both positively correlated with the soil water content at 5 cm, 10 cm, and 20 cm soil depths, and NEE was negatively correlated with the soil moisture content (R\({}^{2}\)\(\geq\) 0.52, \(p\) < 0.01; Figure 6a-c). There was a significant quadratic relationship between CH\({}_{4}\) flux and soil water content (R\({}^{2}\)\(\geq\) 0.25, \(p\) < 0.05), with CH\({}_{4}\) flux reaching a maximum value when the soil water content was more than 40% (Figure 7d-f). With the decrease in soil water content, the flux of GPP, R\({}_{e}\), and CH\({}_{4}\) were continuously decreased, and the decrease in GPP was more sensitive than that of R\({}_{e}\), which resulted in an increase of the NEE in the ecosystem and a significant decrease in the CO\({}_{2}\) fixing ability of the ecosystem (\(p\) < 0.01); however, the ability to absorb CH\({}_{4}\) increased significantly (\(p\) < 0.05). In addition, the best correlation between CO\({}_{2}\) fluxes, CH\({}_{4}\) fluxes, and soil water content occurred at a depth of 5 cm, and this correlation decreased with the increase in soil depth. Moreover, there were several thresholds for the CH\({}_{4}\) source/sink transition, which were an SWC\({}_{5\rm cm}\) equal to 23%, an SWC\({}_{10\rm cm}\) equal to 25%, and an SWC\({}_{20\rm cm}\) equal to 32%. NEE was also positively correlated with soil temperature at 5 cm, 10 cm, and 20 cm depths (R\({}^{2}\)\(\geq\) 0.21, \(p\) < 0.05; Figure 7g-i). With the increase of soil depth, the correlation between soil temperature and NEE was gradually increased. However, there was no relationship between CH\({}_{4}\) and soil temperature.  
Figure 6: Effects of extreme drought on CH\({}_{4}\) fluxes in 2016.  
To test whether the drought effects on carbon budgets can be explained by changes in the microenvironment, the interaction between soil temperature and soil water content on NEE, GPP, \(\mathrm{R_{e}}\) and \(\mathrm{CH_{4}}\) fluxes was also analyzed using generalized additive regression models with soil temperature and soil water content as smooth terms. In general, the strength of \(\mathrm{CO_{2}}\) sink was low when soils were dry (soil water content below 15%) and wet (soil water content above 40%), and high when soil temperature and soil water content ranged from 14-22 \({}^{\circ}\)C and 20-40%, respectively (Figure 8a,b). Drought-induced soil water stress weakened the carbon sink capacity of the ecosystem, but within a certain range of soil temperature. Furthermore, the emission of \(\mathrm{CO_{2}}\) was highest when soil temperature and soil water content ranged from 18-22 \({}^{\circ}\)C and 20-40%, respectively (Figure 8c). Additionally, when extreme drought led to soil water stress, the source and sink functions of \(\mathrm{CH_{4}}\) for the ecosystem will change with the fluctuation of soil temperature. The function of \(\mathrm{CH_{4}}\) source in ecosystem was stronger when the soil water content ranged from 20% to 40% and the soil temperature ranged from 15-21 \({}^{\circ}\)C (Figure 8d). These results also revealed an interactive effect of soil temperature and soil water content on the carbon dynamics to some extent, especially under drought stress.  
Figure 7: Relationships between (**a–c**) NEE, \(\mathrm{R_{e}}\), GPP and soil water content; (**d–f**) \(\mathrm{CH_{4}}\) fluxes and soil water content; (**g–i**) NEE, \(\mathrm{CH_{4}}\) fluxes and soil temperature in 2016. Legend: *, significant level at the 0.05 level; **, significant level at the 0.01 level; NEE, net ecosystem \(\mathrm{CO_{2}}\) exchange; \(\mathrm{R_{e}}\), ecosystem respiration; GPP, gross primary productivity; SWC, soil water content; \(\mathrm{T_{s}}\), soil temperature.