# Characteristics of Carbon Fluxes and Their Environmental Control in Chenhu Wetland, China
## 3 Results and Discussion
### Dynamics of CO\({}_{2}\) Flux over Various Temporal Scales  
#### 3.2.1 Diurnal Variations in NEE  
Utilizing data collected throughout the year 2022, the hourly flux measurements were averaged on a monthly basis from 00:00 to 24:00 (Beijing Standard Time) during both the growing season (April to September) and the non-growing season (January to March and October to December). The diurnal variation of NEE during these two distinct periods is depicted in Figure 3. In the growing season, there was a significant increase in CO\({}_{2}\) exchange activity, characterized by enhanced CO\({}_{2}\) absorption during daylight hours and CO\({}_{2}\) release at night, in comparison to the non-growing season. NEE values ranged from \(-\)12.42 \(\pm\) 8.00 to 4.28 \(\pm\) 5.45 \(\mu\)molCO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). Conversely, during the months of January, February, and December, the wetland ecosystem exhibited characteristics of a weak CO\({}_{2}\) source, as indicated by positive flux values, with NEE ranging from \(-\)1.26 \(\pm\) 2.67 to 1.56 \(\pm\) 3.53 \(\mu\)molCO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). Figure 2: The monthly variation of environmental factors in the Chenhu wetland during the year 2022. (**a**) Air temperature (T\({}_{\rm a}\)), soil temperature (T\({}_{\rm s}\)), and average T\({}_{\rm a}\) in Wuhan from 2011 to 2021. (**b**) Wind speed (WS), photosynthetically active radiation (PAR). (**c**) Relative humidity (RH), vapor pressure deficit (VPD). (**d**) Soil water content (SWC), total precipitation (PPT), and average PPT in Wuhan from 2011 to 2021. Throughout the growing season, the average NEE for each month indicated that the ecosystem functioned as a source of CO\({}_{2}\) during nighttime and as a sink, characterized by negative flux values, during daytime, as illustrated in Figure 3. The average diurnal variation of CO\({}_{2}\) flux during the growing season exhibited a U-shaped curve. Although the diurnal patterns of NEE across various growth stages displayed similar shapes, they differed significantly in amplitude. Specifically, the range of NEE variation in May was recorded at 15.76 umolCO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), while in September it was 6.55 umolCO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). These findings are consistent with the variations in carbon flux observed in other wetland ecosystems during the growing season, as reported in prior research [30; 31]. In contrast, during the non-growing season, the CO\({}_{2}\) flux exhibited minimal variation at night, with some fluctuations occurring during the daytime. In summary, from March to October, NEE displayed negative values during daylight hours, indicating net carbon absorption. Conversely, it exhibited positive values at night, signifying net carbon release. NEE values were recorded as less than zero following sunrise (when PAR exceeded 200 umol\(\cdot\)s\({}^{-1}\cdot\)m\({}^{-2}\)), indicating that the photosynthetic activity of the needs surpassed the combined rates of autotrophic respiration and soil heterotrophic respiration. Consequently, the wetland ecosystem functioned as a carbon sink during this period. Peak NEE values were recorded between 11:00 and 14:00 h across various growth stages. Subsequently, the net CO\({}_{2}\) absorption displayed a pattern of decreasing fluctuations. Notably, between 17:00 and 18:30, NEE transitioned from negative to positive, reflecting a shift from a carbon sink to a carbon source as photosynthesis ceased at night. During nocturnal hours, the CO\({}_{2}\) flux was positive, attributed to the respiration processes of both plants and soil. Variations in radiation exposure resulted in differing durations of the negative NEE values throughout the months. The average duration of carbon sink activity from March to October was recorded as 9.5 h, 10.5 h, 11.5 h, 11 h, 9.5 h, 9 h, and 8.5 h, respectively, demonstrating an initial increase followed by a subsequent decrease in duration.