# Characteristics of Carbon Fluxes and Their Environmental Control in Chenhu Wetland, China
## 2 Materials and Methods
### Data Processing  
The original data obtained from the eddy covariance system may contain outliers or exhibit missing values due to various factors, including system maintenance, voltage instability, and power failures. The flux data were processed in accordance with the standard procedures for flux data processing as recommended by ChinaFLUX. It is essential to emphasize that the calculated CO\({}_{2}\) flux (F\({}_{\rm c}\)) values underwent rigorous quality control measures, which included (1) the elimination of data recorded during equipment malfunctions; (2) the exclusion of daytime data that deviated from the expected trend of light radiation; (3) the replacement of flux data classified with a quality grade of 9, indicating poor data quality; (4) the removal of nighttime data exhibiting negative NEE; (5) the exclusion of nighttime data collected under conditions of weak turbulence, specifically with wind speeds (\(u^{\star}\)) less than 0.1 m\(\cdot\)s\({}^{-1}\); and (6) the exclusion of NEE values that fell outside the instrument's measurement range, specifically when F\({}_{\rm c}\) exceeded \(-\)2.64 to 1.32 mg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\). Following the data processing and quality control procedures, 86.8% of the daytime and 43.8% of the nighttime F\({}_{\rm c}\) data were deemed suitable for further analysis.  
In the process of interpolating missing data, a linear interpolation method that utilizes adjacent measurement data was employed for gaps with a duration of less than 2 h. Conversely, for data gaps that exceeded 2 h, the Michaelis-Menten model [27] was applied to estimate the missing NEE during the growing season:  
\[\rm NEE=-\frac{\alpha P_{max}\cdot PAR}{\alpha\cdot PAR+P_{max}}+R_{eco,day} \tag{1}\]  
where PAR represents the photosynthetically active radiation (\(\rm\mu mol\cdot m^{-2}\cdot s^{-1}\)), \(\alpha\) denotes the ecosystem's apparent initial radiation use efficiency (\(\rm\mu molCO_{2}\cdot\mu mol^{-1}\)), P\({}_{\rm max}\) refers to the maximum photosynthetic capacity of the ecosystem (\(\rm\mu mol\ CO_{2}\cdot m^{-2}\cdot s^{-1}\)), and \(R_{eco,day}\) denotes the daytime ecosystem respiration (\(\rm\mu mol\ CO_{2}\cdot m^{-2}\cdot s^{-1}\)).  
The absent nighttime NEE was estimated using an exponential function [28] that characterizes the relationship between soil respiration and temperature:  
\[R_{eco,night}=a\cdot e^{bT} \tag{2}\]  
where R\({}_{eco,night}\) refers to nighttime F\({}_{\rm c}\) (\(\rm\mu mol\cdot m^{-2}\cdot s^{-1}\)), T is soil temperature (\({}^{\circ}\) C), the coefficient a is the intercept of R\({}_{eco,night}\) when the temperature is zero (\(\rm\mu mol\ CO_{2}\cdot m^{-2}\cdot s^{-1}\)), and the coefficient b represents the temperature sensitivity of R\({}_{eco,night}\).  
The integral daily NEE was partitioned into ecosystem respiration (Reco) and gross primary productivity (GPP) utilizing Equations (3) and (4):  
\[\text{R}_{\text{eco}}=\text{R}_{\text{eco,day}}+\text{R}_{\text{eco,night}} \tag{3}\]  
\[\text{GPP}=\text{R}_{\text{eco}}-\text{NEE} \tag{4}\]  
where daily ecosystem respiration \(\text{R}_{\text{eco}}\) is the sum of daytime respiration (\(\text{R}_{\text{eco,day}}\)) and nighttime respiration (\(\text{R}_{\text{eco,night}}\)). The nighttime net exchange flux is considered indicative of nighttime ecosystem respiration. Within the framework of this study, a negative value of NEE is traditionally interpreted as the wetland sequestering CO2 from the atmosphere, while a positive NEE value signifies the release of CO2 from the wetland into the atmosphere.