## 4 Discussion
### Controls on the Carbon Fluxes of Riparian Forest  
Ecosystem NPP is widely known to be controlled by meteorological and hydrological regimes. Given the similar meteorological variables except precipitation (Table 1), we suggest the temporal pattern of CO\({}_{2}\) fluxes of the riparian forest was mainly controlled by the soil hydrological regime  
Figure 8: Responses of gross primary production (GPP) to photosynthetically active radiation (PAR) from May to October across two years, 2015–2016, at the study site. GPP used in curve fitting was averaged in 50 \(\upmu\) mol m\({}^{-2}\) s\({}^{-1}\) PAR bins. including precipitation and groundwater. [PERSON] et al. [43] showed that the aboveground NPP of riparian and desert vegetation in the middle HRB was strongly associated with annual precipitation. However, we found that the annual precipitation was not related to the CO\({}_{2}\) fluxes of riparian forest due to the paucity of rainfall in the hyper-arid region within the lower HRB. Since it was demonstrated that the higher \(P\) (i.e., 52.1 mm, 2016) corresponded to the lowest NEP (278 g C m\({}^{-2}\) year\({}^{-1}\)), and conversely, the lowest \(P\) (17.3 mm, 2014) corresponded to the second lower NEP (297 g C m\({}^{-2}\) year\({}^{-1}\)) (Table 1), this data may suggest that the annual precipitation could not be used to explain the year-to-year variations in CO\({}_{2}\) fluxes. In search for plausible explanations, one could consider that for a single rainfall event, the maximum precipitation at the site is likely to be less than 9 mm, and therefore, the water cannot reach the soil layer of 60 cm, where is the primarily distributed range of fine roots of the dominant forests [44]. And some larger diameter roots are likely to be distributed below the groundwater depth level in this study site [44]. Deep rooting can thus reach the groundwater and thus redistribute such waters into the upper soil profile, enabling the tree to utilize both water sources [8; 44], so that the riparian forest can grow continuously under hyper-arid climates. In fact, some studies have shown that the annual precipitation could not explain the inter-annual variations of CO\({}_{2}\) fluxes in arid ecosystems [45; 46], which has also been confirmed by the present study. In the present study area, the soil textures were generally coarse, and the salt content was high and concentrated on the surface (Figure 4). In this regard, the salinization issues are considerably serious. In addition, the salinity of the groundwater was high, and this varied from about 800 to 3000 mg L\({}^{-1}\)[47]. After decades of ecological water conveyance in this study region, the groundwater depth has become quite shallow and exhibits significant fluctuations, resulting in the accumulation of salts in the ground water and the surface soils [48]. The groundwater and salinity can both synthetically affect the growth of the desert riparian forests. Indeed, the study of [PERSON] et al. [48] indicated that the accumulation of salinity is likely to offset the vegetation restoration process that could happen due to alleviated water stress, through the increase in the salt stress to plants. In this study, the groundwater depth fluctuated from 0.28 to 2.42 m within the three years of the observation period. In fact, the values were well above the rational depth (i.e., 3.5-4.5 m) for the growth of _P. euphratica_[49] and the critical depth of ground water which was greater than 5 m. During the growing season, evapotranspiration is normally intensive due to the extremely dry climate, thus the changes in groundwater depth are likely to directly affect the accumulation of surface salinity that could eventually lead to the disturbance of the growth and development of the riparian forest. This is because the shallower is the groundwater depth, the greater is the evapotranspiration and the more serious is the soil salinization.