## 3 Results
### Effects of Environmental Factors on CO2 Exchange in the Wetland Ecosystem  
#### 3.4.1 Daily Scale  
Pearson correlations were used to analyze the correlations between the factors and the dependent variable. A matrix of the correlation coefficients between the independent variables is shown in Table 1. NEE represents the daily average CO2 exchange, Tsoil represents the temperature at a soil depth of 5 cm at the daily scale, Tair represents the daily average air temperature, PAR represents the daily average photosynthetically active radiation, PPT represents the daily precipitation, and WL represents the water level. From the table, the Tsoil, PAR, PPT, and WL all significantly influenced the net CO2 exchange. Specifically, the Tair, Tsoil, and PAR exhibited negative correlations with net CO2 exchange. As the temperature and PAR increased, the net CO2 exchange of the ecosystem decreased, indicating a trend towards an enhanced carbon absorption capacity. The PPT was negatively correlated with the net CO2 exchange. On rainy days, lower solar radiation resulted in reduced CO2 absorption. The WL showed a positive correlation with the net \begin{table}
\begin{tabular}{l c c c c c} \hline  & **NEE** & **Tsoil** & **Tair** & **PAR** & **PPT** & **WL** \\ \hline NEE & 1 & \(-\)0.488 *** & \(-\)0.452 *** & \(-\)0.327 *** & \(-\)0.182 ** & 0.463 *** \\ Tsoil & - & 1 & 0.932 *** & 0.564 *** & 0.136 ** & \(-\)0.914 *** \\ Tair & - & - & 1 & 0.667 *** & 0.153 ** & \(-\)0.901 *** \\ PAR & - & - & - & 1 & \(-\)0.022 & \(-\)0.603 *** \\ PPT & - & - & - & - & 1 & \(-\)0.076 \\ WL & - & - & - & - & - & 1 \\ \hline \end{tabular} Note. *** represents a significance level of \(p<0.01\); *** represents a significance level of \(p<0.001\). NEE, net ecosystem exchange; PAR, photosynthetically active radiation; PPT, daily precipitation; Tair, air temperature; Tsoil, soil temperature; WL, water level.  
\end{table} 
Table 1: Pearson’s correlation coefficients for effects at the daily scale. CO\({}_{2}\) exchange. As the WL rose, the proportion of plants exposed to the surface decreased, leading to a reduction in the wetland's carbon absorption capacity. The role of PAR on the net exchange of CO\({}_{2}\) in wetlands was mainly reflected in the daytime of the growing season. In Figure 11, the net CO\({}_{2}\) exchange evidently increased rapidly with the rise in PAR. However, as the PAR intensity surpassed a certain threshold, the rate of the increase in the net CO\({}_{2}\) exchange slowed, eventually reaching a steady-state value. The influence of PAR was highly significant during different growth stages of the needs. The phenomenon of net CO\({}_{2}\) absorption was closely related to the growth activities of the reeds. During the sprouting to flowering period from 4 May to 18 August, the wetland exhibited a strong photosynthetic assimilation capacity, with the maximum carbon absorption reaching up to 1.9 mg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). During the early and late stages of reed growth, the carbon absorption capacity was relatively weak, with the maximum carbon absorption reaching only 0.94 mg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). The temperature during the growing season had a significant impact on the photosynthetic physiology of the vegetation and was the most important meteorological factor affecting the respiration intensities of various components within the ecosystem. Figure 12 demonstrates a significant exponential correlation between the nighttime CO\({}_{2}\) flux and Tsoil at a depth of 5 cm. A comparative analysis of the relationship between the nighttime carbon flux and the 2.5 m air temperature during the growing and non-growing seasons (Figure 13) revealed an exponential growth trend in the response of the nighttime carbon flux to Tair. This suggests that the intensity of nighttime carbon release is directly controlled by thermal conditions. Both Tair and the upper Tsoil affected the respiration rate of the ecosystem.