## 3 Results and Discussion  
### Diurnal Variations in CH\({}_{4}\) Fluxes and Driving Factors  
During the vegetative stage of all four rice growing seasons and the reproductive stage of early rice, the CH\({}_{4}\) fluxes showed a distinct diurnal pattern: the CH\({}_{4}\) fluxes started to increase at approximately 10:00, reached a peak at 14:00-16:00, decreased to low values after 20:00, and then maintained low values without significant fluctuations at night (Figure 2). The CH\({}_{4}\) fluxes in the vegetative stage of late rice were much greater than those during the reproductive stage and ripening stage. They were the greatest in the reproductive stage of early rice, followed by the vegetative stage and the ripening stage.  
Many previous studies have shown that in flooded ecosystems, CH\({}_{4}\) fluxes fluctuate regularly at diurnal timescales; these fluctuations are related to cyclical changes in temperature, solar radiation and the physiological state of rice plants during the day [9; 11; 29]. Many scholars have reported that the CH\({}_{4}\) fluxes in paddy fields worldwide exhibit a single-peak diurnal pattern, with the peak value occurring in the afternoon [2; 9; 12; 41; 63]. However, it has been reported by some scholars that CH\({}_{4}\) fluxes in paddy fields do not change regularly [35; 64]. [PERSON] et al. [64] reported that two intraday peak CH\({}_{4}\) fluxes occurred at 12:00 and 0:00 during the early growing season, while only one peak occurred at midnight later in the growing season. In this study, a single-peak diurnal pattern was observed during the reproductive stage of early rice but not during the three seasons of late rice, which may be the result of inadequate mid-season drainage of the early rice caused by rain.  
The net CH\({}_{4}\) flux is the result of the simultaneous microbial production, oxidation and transport of CH\({}_{4}\) and is thus controlled by meteorological and biophysical factors. Previous studies have shown that temperature strongly affects CH\({}_{4}\) fluxes in paddy fields [65; 66]. The activity of methanogens is enhanced at the optimum temperature [67], while the plant-mediated CH\({}_{4}\) transport, molecular diffusion and ebullition of CH\({}_{4}\)[65] also increase with increasing temperature. The GPP represents the CO\({}_{2}\) assimilation capacity of a region and is an indicator of plant physiology. The air temperature (T\({}_{\rm a}\)) and GPP changed regularly in the span of one day and represent typical meteorological and biophysical factors driving diurnal CH\({}_{4}\) fluxes. As shown in Figure 3, all wavelet coherence had high periodic coherence at the 24 h timescale, which was consistent with the cyclical and regular changes in CH\({}_{4}\), T\({}_{\rm a}\) and GPP throughout the day. In Figure 3a,c,e,g, the arrows point to the right and have a very low positive inclination, while in Figure 3b,d,f,h, the arrows point to the right and have a very low negative inclination. The phase angles of GPP and T\({}_{\rm a}\) with CH\({}_{4}\) fluxes were very small. The correlations between the two time series were strong and GPP, rather than T\({}_{\rm a}\), was the driver of diurnal CH\({}_{4}\) emissions. The phenomenon was also reported by [PERSON] et al. [44], which could be explained by the fact that the main methanogenic substrates are obtained through photosynthesis; thus, it would take several hours to transport them from leaves to roots [68; 32].