# A Typical Small Watershed in Southwestern China Is Demonstrated as a Significant Carbon Sink
## 4 Discussion  
### Effects of Environmental Factors on Carbon Fluxes  
Ecosystem carbon fluxes are affected to varying degrees by various environmental factors (e.g., radiation, temperature, and moisture) [17; 34; 35]. In the Reshuihe River watershed, PAR is the most important environmental factor influencing the carbon-sink capacity of forest, cropland, and non-timber forest ecosystems. Among them, the strong correlation between NEE and radiation in forest ecosystems has been widely recognized in previous studies [16; 18; 36]. Similar results show that PAR is the dominant factor controlling the variation in daily NEE in cropland ecosystems. This was verified in a wheat-maize rotation system in the Huaibei Plain of China [37], and a maize field in the arid areas upstream of the Yellow River Basin [38]. In addition, the carbon-sink capacity of the non-timber forest ecosystem (_Zanthoxylum bungeannum Maxim_ plantation) in this study area was closely related to PAR, which is consistent with the light preference of _Zanthoxylum bungeannum Maxim_ growth [39]. Soil volumetric water content regulates photosynthesis and respiration processes by affecting plant transpiration and soil respiration [40; 41], thus determining carbon cycle. In different ecosystems of our study area, SVWC is significantly positively correlated with GPP. The reason for this is that more SVWC enhances the photosynthetic capacity of plants (Figure 7 and Table 2). Since soil drought inhibits the activities of plant roots and soil microorganisms, leading to decreased soil respiration [42], more SVWC is beneficial for reducing drought stress. Moreover, increases in SVWC correspond to smaller NEE values,  
Figure 9: Conceptual diagram of the estimated annual carbon budget of forest, cropland, and non-timber forest ecosystems, and the sum of the total inter-annual carbon sink of the three ecosystems in the Reshuihe River watershed. Numbers after the \(\pm\) symbols represent the standard errors. i.e., stronger carbon sinks, suggesting that SVWC promotes GPP more than Re, resulting in greater carbon absorption than carbon release. The effects of air temperature on daily NEE were small (Figure 6); however, its effects on GPP and Re were larger (Figures 6 and 8). In the coniferous forest ecosystems of our study area, the effect of T\({}_{\text{air}}\) on the variation of daily NEE was weak (path coefficient was 0.08). However, [PERSON] et al. found through long-term observations in the Yunnan Province, Southwestern China that temperature had a large effect on the monthly NEP in a subalpine coniferous forest ecosystem [17]. This discrepancy may be due to the different timescales used in the analyses. Here, it was found that carbon fluxes in cropland ecosystems in this study area were more susceptible to high temperatures. On the one hand, higher temperatures (T\({}_{\text{air}}\) > 21\({}^{\circ}\)) inhibit GPP in the cropland ecosystem. This phenomenon may be the result of plants adapting to greater evaporative demands and less soil moisture, where stomatal conductance is reduced at high temperatures, thus reducing GPP [43]. In addition, T\({}_{\text{air}}\) is positively correlated with VPD, leading to a high VPD at high temperatures, and a high VPD causes GPP to decrease [44; 45]. On the other hand, Re increases with T\({}_{\text{air}}\) in the appropriate temperature range, but high temperatures (T\({}_{\text{air}}\) > 21\({}^{\circ}\)) suppress Re in the cropland ecosystem. Analyzing the effects of high temperatures on respiration is further complicated by the fact that ecosystem respiration comprises the respiration of plant roots, stems, and foliage, as well as the heterotrophic respiration of soil microorganisms [46]. Moreover, in the three ecosystems, GPP and Re are suppressed when temperatures are below 0 \({}^{\circ}\)C because low temperatures lead to processes such as reduced water and nutrient uptake, as well as stomatal closure [47], resulting in limited plant production [32; 48].