# A Typical Small Watershed in Southwestern China Is Demonstrated as a Significant Carbon Sink
## 4 Discussion
### Carbon Flux Variability  
In the Reshuihe River watershed, forest, cropland, and non-timber forest ecosystems serve as significant carbon sinks. The annual carbon sequestration capacity (per unit area) is in the following order: forest (\(-\)592.8, \(-\)488.1 gC m\({}^{-2}\) a\({}^{-1}\)) > non-timber forest (\(-\)371.0 gC m\({}^{-2}\) a\({}^{-1}\)) > cropland (\(-\)92.5, \(-\)71.6 gC m\({}^{-2}\) a\({}^{-1}\)). In terms of seasonal variations, the factors that control NEE (including PAR, T\({}_{\text{air}}\), and SVWC) are more abundant during the wet seasons, resulting in a significant carbon sink during this period [15; 17]. Forest ecosystems make up over 60% of the study area, which is dominated by coniferous forests. Compared to coniferous forests in other areas and different types of forests (such as coastal plain, dry tropical, temperate, and boreal, etc.) (Table 4), the forest in this study area was found to have a strong carbon sink capacity. This corroborates previous research that has shown that Southwestern China's forests are considered to have a strong carbon sink capacity [1; 2]. Additionally, we observed a carbon sink in the forest site every month for two years. This phenomenon has been found in numerous forest ecosystems with strong carbon sinks, such as broadleaved forest ecosystems and subalpine coniferous forest ecosystems in the Yunnan Province, China [17], and evergreen forest ecosystems in South China [50]. During the crop growth stage, cropland ecosystems exhibit a rapid increase in GPP and Re [61]. By comparing the annual NEE of cropland ecosystems in different locations (Table 5), we found that the carbon sink capacity of cropland in the Reshuihe River watershed was relatively weak. This is mainly due to the long fallow period of local cropland (about 6 months per year), during which carbon release from soil respiration reduces the annual carbon sink. Non-timber forest ecosystems have a strong carbon sink capacity. Moreover, the period of net carbon uptake after harvesting of _Zanthoxylum bungeanum Maxim_ fruits was due to two main reasons: (1) The growth of _Zanthoxylum bungeanum Maxim_ trees increased carbon fixation due to sufficient light and moisture, and (2) due to the lack of garden management  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Country** & **Area** & \begin{tabular}{c} **Functional** \\ **Type** \\ \end{tabular} & \begin{tabular}{c} **Mean Annual** \\ **NEE (gC m\({}^{-2}\))** \\ \end{tabular} & \begin{tabular}{c} **Mean Annual** \\ **GPP (gC m\({}^{-2}\))** \\ \end{tabular} & \begin{tabular}{c} **Mean Annual** \\ **Annual Re** \\ **(gC m\({}^{-2}\))** \\ \end{tabular} & **Reference** \\ \hline China & Liangshan & \begin{tabular}{c} Potaco \\ and maize \\ \end{tabular} & \(-\)82 & 1437 & 1354 & This study \\ \hline China & \begin{tabular}{c} Heihe \\ river basin \\ \end{tabular} & maize & \(-\)536 & - & - & [62] \\ \hline China & \begin{tabular}{c} North \\ Plain \\ \end{tabular} & \begin{tabular}{c} wheat and \\ maize \\ \end{tabular} & \(-\)258 & - & - & [63] \\ \hline Canada & Breton & \
\begin{tabular}{c} secale \\ \end{tabular} & \(-\)89 & 1242 & 1153 & [25] \\ \hline US & Ponca & wheat & \(-\)155 & 1395 & 1240 & [64] \\ \hline US & Nebraska &
\begin{tabular}{c} maize and \\ soybean \\ \end{tabular} & \(-\)225 & 1201 & 976 & [65] \\ \hline Germany & Thuringia & wheat & \(-\)185\(\cdots\)245 & - & - & [66] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of carbon fluxes in different cropland ecosystems. after harvesting, weeds (mainly ragweed) grew rapidly. Weed growth has been widely discussed as an important source of carbon fluxes in agroecosystems. For example, two years of observations in cotton fields in Texas, USA, showed that carbon sinks were stronger in years that were more conducive to weed growth [67]. [PERSON] et al. found that the complete removal of weeds from an olive orchard resulted in a significant reduction in carbon uptake [68].