## 3 Results and Discussion
### Driving Factors of Seasonal CH\({}_{4}\) Fluxes
Unlike in the other three growing seasons, SWC and T\({}_{55}\) were the two most important controlling factors regulating daily CH\({}_{4}\) fluxes in early rice in 2020. GPP and SWC were the factors controlling seasonal CH\({}_{4}\) fluxes in P1 and P2, respectively. In general, daily CH\({}_{4}\) fluxes were strongly explained by T\({}_{55}\) in the paddy fields from 2018 to 2020 in this study. The mechanism by which temperature affects CH\({}_{4}\) has been described above in Section 3.1. Adding GPP or SWC to the model somewhat increased its R\({}^{2}\), implying that photosynthesis and the soil water content also regulate daily CH\({}_{4}\) emissions to a certain extent. The efficiency of CH\({}_{4}\) flux transplanting will increase with the growth of rice plants, as plant-mediated CH\({}_{4}\) transport is the main pathway conveying CH\({}_{4}\) from the soil to the atmosphere [35; 69]. Meanwhile, [PERSON] et al. [44] reported that the increase in oxygen produced by photosynthesis and transferred to roots would result in an increase in methane emission fluxes. Therefore, GPP was able to explain the seasonal variability in the CH\({}_{4}\) flux. Standing water above the soil provides an anaerobic environment that is favourable for methanogenic bacteria; this anaerobic environment has been reported to be an indispensable condition for CH\({}_{4}\) production [41; 50]. Many previous studies have indicated that the water table operates like a general 'on-off switch' for CH\({}_{4}\) emissions. When the water layer disappeared, SWC regulated the balance between CH\({}_{4}\) production and oxidation by influencing the depth of anaerobic and aerobic zones in the soil. Additionally, as explained in Section 3.4, the daily CH\({}_{4}\) fluxes probably increased slightly and then declined sharply after drainage, and the longer drainage periods could strongly affect the subsequent daily CH\({}_{4}\) fluxes. RH and U\({}^{*}\) played minor roles in regulating daily CH\({}_{4}\) fluxes compared with T\({}_{55}\), GPP and SWC over the course of the growing season. RH was reported to influence CH\({}_{4}\) fluxes by the occurrence of convective throughflow caused by humidity-induced pressurization [70]. Similarly, [PERSON] et al. [41] observed the dependence of daily CH\({}_{4}\) fluxes on relative humidity in the post-heading period. CH\({}_{4}\) fluxes were enhanced by increased turbulent mixing [41; 44]; therefore, U\({}^{*}\) mainly affected short-term CH\({}_{4}\) fluxes and had less influence on daily CH\({}_{4}\) fluxes. The main controlling factors identified in this study are slightly different from those identified in previous studies. [PERSON] et al. [44] analysed 6.5 years of eddy covariance measurements of fluxes of CH\({}_{4}\) from a flooded rice paddy in northern California, USA, and reported that the seasonal variability in CH\({}_{4}\) fluxes was strongly regulated by GPP, followed by T\({}_{55}\). However, [PERSON] et al. [41] reported that before the heading stage of rice, CH\({}_{4}\) emissions were largely controlled by wind speed and T\({}_{55}\), while RH and water temperature had a greater effect on daily CH\({}_{4}\) emissions after heading. [PERSON] et al. [41] explained that the shift in the environmental dependence of CH\({}_{4}\) emissions between cultivation stages occurred due to the shift in transport pathways, as well as changes in the rate and quality of root exudates. It is reported by [PERSON] et al. [50] that seasonal variations of soil temperature, air temperature, and GPP had strong effects on daily CH\({}_{4}\) fluxes during pre-panicle initiation of the rice growing season, while the soil temperature and leaf area index had very strong effects on daily CH\({}_{4}\) fluxes during the post-panicle initiation stage from a rice-wheat rotation agroecosystem in eastern China. The variability in the results of these studies and our study can be attributed to differences in the study areas.