# A Typical Small Watershed in Southwestern China Is Demonstrated as a Significant Carbon Sink
## 3 Results
### Carbon Fluxes in Relation to Environmental Factors
#### 3.3.2 Responses of Carbon Fluxes to PAR, SVWC and T\({}_{\rm air}\)  
In this study, the Michaelis-Menten equation (Equation (3)) was used to fit the relationships between NEE and PAR during the daytime (shortwave incoming global radiation > 10 W m\({}^{-2}\)) under different SVWC conditions for different sites (Figure 7). The results showed that all daytime NEE decreased with increases in PAR, and the rate of change of daytime NEE decreased with increases in PAR. Furthermore, the effects of PAR on daytime NEE were also regulated by SVWC. The light response curves of different sites differed under different SVWC conditions. In general, the values of A\({}_{\rm max}\), \(\varepsilon\), and Re of the light response curves were higher under high SVWC conditions (>29%, 33%, and 26%, respectively) (Table 3). Figure 6: Path diagrams illustrating the effects of PAR, SVWC, and T\({}_{\rm air}\) on NEE (left panels), GPP (middle panels), and Re (right panels) in (**a**) forest, (**b**) cropland, and (**c**) non-timber forest sites. The values on the arrows indicate the path coefficients. The width of the arrows is proportional to the strength of the path coefficients, and the green and red colors indicate positive and negative effects, respectively. Data were available from October 2021 to September 2023 for forest and cropland sites, and from October 2022 to September 2023 for the non-timber forest site. \({}^{*}p<0.05\); \({}^{**}p<0.01\); \({}^{***}p<0.001\). Figure 8 illustrated the relationships between carbon fluxes with daily mean T\({}_{\rm air}\) at different sites. GPP and Re tended to zero when T\({}_{\rm air}\) was below 0 \({}^{\circ}\)C. When T\({}_{\rm air}\) < 21 \({}^{\circ}\)C in cropland site, GPP and Re increased with increasing T\({}_{\rm air}\); however, when T\({}_{\rm air}\) > 21 \({}^{\circ}\)C, GPP and Re decreased significantly (\(p\) < 0.01) compared to 18 \({}^{\circ}\)C \(<\) T\({}_{\rm air}\) < 21 \({}^{\circ}\)C. Overall, GPP and Re increased with T\({}_{\rm air}\) in forest and non-timber forest sites, and no significant changes were observed in GPP and Re when T > 21 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sites**} & \multicolumn{2}{c}{\(\varepsilon\)} & \(\mathbf{A_{max}}\) & **Re** & \\  & & **(\(\mathbf{\mu mol}\) CO\({}_{2}\)** & **(\(\mathbf{\mu mol}\) CO\({}_{2}\)** & **(\(\mathbf{\mu mol}\) CO\({}_{2}\)** & **R\({}^{2}\)** \\  & & \(\mathbf{\mu mol^{-1}}\) PAR)** & **m\({}^{-2}\) s\({}^{-1}\))** & **m\({}^{-2}\) s\({}^{-1}\))** & \\ \hline \multirow{3}{*}{Forest} & SWWC \(<\) 22\% & 0.0200 \(\pm\) 0.0002 a & 19.96 \(\pm\) 3.02 a & 2.43 \(\pm\) 1.94 a & 0.94 \\  & 22\% \(<\) SWC \(<\) 29\% & 0.0485 \(\pm\) 0.0125 a & 23.80 \(\pm\) 3.04 a & 4.56 \(\pm\) 2.09 a & 0.98 \\  & SWWC \(>\) 29\% & 0.0608 \(\pm\) 0.0307 a & 33.41 \(\pm\) 1.99 b & 6.38 \(\pm\) 0.21 a & 0.95 \\ \hline \multirow{3}{*}{Cropland} & SWWC \(<\) 27\% & 0.0091 \(\pm\) 0.0011 a & 10.38 \(\pm\) 0.39 a & 1.77 \(\pm\) 1.43 a & 0.95 \\  & 27\% \(<\) SWC \(<\) 33\% & 0.0258 \(\pm\) 0.0067 ab & 16.48 \(\pm\) 3.83 a & 3.51 \(\pm\) 1.45 ab & 0.88 \\  & SWWC \(>\) 33\% & 0.0527 \(\pm\) 0.0148 b & 28.49 \(\pm\) 3.48 b & 6.66 \(\pm\) 1.02 b & 0.97 \\ \hline \multirow{3}{*}{Non-timber forest} & SWWC \(<\) 11\% & 0.0050 \(\pm\) 0.0017 a & 17.42 \(\pm\) 2.12 a & 0.49 \(\pm\) 0.38 a & 0.98 \\  & 11\% \(<\) SWWC \(<\) 26\% & 0.0256 \(\pm\) 0.0039 ab & 15.96 \(\pm\) 4.18 a & 2.57 \(\pm\) 1.98 ab & 0.85 \\ \cline{1-1}  & SWWC \(>\) 26\% & 0.0455 \(\pm\) 0.0163 b & 18.50 \(\pm\) 3.59 a & 4.47 \(\pm\) 0.32 b & 0.96 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Fitting parameters, standard errors, and coefficients of determination (R\({}^{2}\)) of Equation (3) for forest, cropland, and non-timber forest sites under different SVWC conditions in Figure 7. Significant differences in a given parameter among the three levels of SVWC are indicated by different letters (\(p\) < 0.05, one-way ANOVA and Tukey’s HSD test).