## 3 Results
### Effects of Environmental Factors on CO2 Exchange in the Wetland Ecosystem
#### 3.4.2 Seasonal Scale  
The effects of environmental control variables (Tair, Tsoil, PAR, PPT, WL, and AGB) on NEE are often complex and interrelated. To investigate the mechanisms influencing NEE, a correlation analysis was conducted between the monthly average NEE and the environmental factors at the seasonal scale (Table 2). Apart from the WL, significant linear correlations between the monthly average NEE and the main environmental and biological factors were found (Figure 14). They could be ranked in decreasing order of significance as follows: Tsoil > Tair > PAR > PPT > biomass (AGB). Larger values of Tsoil, Tair, PAR, and AGB indicated a stronger carbon uptake capacity of the ecosystem. Tair significantly influenced Tsoil, indicating that Tair indirectly affected NEE flux by influencing Tsoil. Concurrently, the monthly carbon uptake capacity significantly increased with an increase  
Figure 12: Relationship between nighttime net ecosystem exchange (NEE) and temperature of soil (Tsoil) at depth of 5 cm. *** represents a significance level of \(p\) < 0.001.  
Figure 13: Relationship between nighttime net ecosystem exchange (NEE) and air temperature (Tair). *** represents a significance level of \(p\) < 0.001.  
in Tai. The dynamics of AGB were more dependent on the variations in PAR. This was primarily because the monitoring area experienced prolonged waterlogged conditions, where the anaerobic environment inhibited the decomposition of detritus, leading to the accumulation of organic matter in the soil. As a result, the wetland became a carbon sink that suppressed the increase in the atmospheric CO\({}_{2}\) concentration. The impact of rainfall on biomass accumulation was relatively weak. Under waterlogged conditions, the reed biomass was relatively high, with the maximum biomass reaching up to 2.5 kg.m\({}^{-2}\). The plant height could exceed 270 cm, and the ground cover density was relatively high.  
\begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline  & **NEE** & **Tsoil** & **Tair** & **PAR** & **PPT** & **WL** & **AGB** \\ \hline NEE & 1 & \(-\)0.811 *** & \(-\)0.783 *** & \(-\)0.629 *** & 0.476 *** & 0.635 & \(-\)0.469 * \\ Tsoil & - & 1 & 0.950 *** & 0.676 *** & 0.436 ** & \(-\)0.741 & 0.104 \\ Tair & - & - & 1 & 0.812 *** & 0.525 *** & \(-\)0.737 & \(-\)0.068 \\ PAR & - & - & - & 1 & 0.545 *** & \(-\)0.969 *** & 0.904 * \\ PPT & - & - & - & - & 1 & 0.030 & 0.042 \\ WL & - & - & - & - & - & 1 & 0.154 \\ AGB & & & & & & 1 \\ \hline \hline \end{tabular} Note. * represents a significance level of \(p<0.05\); ** represents a significance level of \(p<0.01\); *** represents a significance level of \(p<0.001\). AGB, aboveground biomass; NEE, net ecosystem exchange; PAR, photosynthetically active radiation; PPT, daily precipitation; Tair, air temperature; Tsoil, soil temperature; WL, water level.  
\end{table}
Table 2: Pearson’s correlation coefficients for effects at the monthly scale.  
Figure 14: Net ecosystem exchange (NEE) correlations with environmental and biological factors at the monthly scale for 2015-2017 and 2021. (**a**) Air temperature (Tair), (**b**) soil water content at depth of 5 cm (Tsoil), (**c**) photosynthetically active radiation (PAR), (**d**) rainfall (PPT), (**e**) water level (WL), and (**f**) aboveground biomass (AGB). The error line represents the standard deviation of the means.