# _atmosphere_
## 4 Discussion
### Carbon Budgets of the Two Wetlands  
The distinct carbon function of the two wetlands is explained more by RES, than by GPP, specifically by nongrowing RES (Figures 4 and 6). The smaller annual carbon use efficiency (about 6%) and little difference in growing GPP/RES (1.20) also confirmed the important role of nongrowing RES in carbon budgets. The nongrowing season daily CO\({}_{2}\) effluxes were 1.07 g C/(m\({}^{2}\) d) in LHZ and 0.56 g C/(m\({}^{2}\) d) in XBH, which lie within the reported values of other alpine wetlands [4; 40]. The higher nongrowing season RES at LHZ compared to XBH thus contributed to a net carbon source at LHZ. Soil temperature was similar at both sites (Figure 1) and was still high enough to support organic matter decomposition and root respiration due to the snow insulation effect, despite the air temperature being below \(-10\)\({}^{\circ}\)C [4; 16]; hence, greater root biomass and more soil organic carbon at LHZ, rather than thermal conditions, probably led to more ecosystem respiration at LHZ compared to XBH during the nongrowing season [1; 4]. Meanwhile, the easily degradable labile carbon from plant litter, induced by higher vegetation coverage (e.g., two-fold higher EVI at LHZ), will stimulate enhanced microbial growth, carbon mineralization, and soil respiration at LHZ [11]. However, the temperature sensitivity of RES was much higher at XBH (Figure 8), which also indicated lower stability of soil organic matter at this site [21]. The nonsignificant difference in GPP possibly stems from the higher productivity of swampland plants compared to peatland plants [9]. This appears to be confirmed by a three-fold higher sensitivity of GPP to EVI (slope = 40.5 g C/(m\({}^{2}\) d)) at XBH compared to that at LHZ (slope = 14.5 g C/(m\({}^{2}\) d)). The growing season RES was closely correlated with GPP (\(R^{2}\) > 0.90, \(p\) < 0.001; Figure S4) at both sites, through substrate supply [13]. Therefore, nonsignificant GPP results in little difference in growing season plant autotrophic respiration and the consequent RES, specifically, nongrowing season RES contributes to the distinct carbon functions in the two alpine wetlands. Higher carbon sink capacity appears to coincide with higher precipitation, which is in agreement with previous reports from alpine wetlands [14; 19; 40]. However, the underlying responses of the two sites are clearly different. Greater precipitation inhibited carbon dynamics at LHZ while stimulated carbon exchanges at XBH (Figure S5). Specifically, at LHZ, the higher precipitation (536.1 mm versus 439.6 mm) decreased annual GPP less by 27.5 g C/m\({}^{2}\) (645.2 g C/(m\({}^{2}\) year) versus 617.7 g C/(m\({}^{2}\) year)), compared to a decrease in RES of 52.6 g C/m\({}^{2}\) (699.3 g C/(m\({}^{2}\) year) versus 646.7 g C/(m\({}^{2}\) year)). Decreased carbon fluxes might be caused by more waterlogged patches and lower oxygen availability for soil respiration under higher precipitation [14; 17; 23]. At XBH, the greater precipitation (238.8 mm versus 344.7 mm) stimulated increased GPP by 21.8 g C/m\({}^{2}\) (584.2 g C/(m\({}^{2}\) year) versus 606.0 g C/(m\({}^{2}\) year)), compared to an increase in RES of 11.1 g C/m\({}^{2}\) (548.7 g C/(m\({}^{2}\) year) versus 559.8 g C/(m\({}^{2}\) year)). Such a phenomenon indicates that GPP and RES at XBH should be to some extent water-stressed, which agrees well with previously reported positive relationships between CO\({}_{2}\) fluxes and water availability at the same site from 2012 to 2013 [29]. Indeed, recent studies have suggested that the response of soil carbon decomposition to water-table manipulation in LHZ is controlled by ferrous iron, in contrast to the classic \"enzyme latch\" theory seen in other wetlands [21; 48]. Therefore, the different responses of CO\({}_{2}\) fluxes to promoted precipitation between peatlands and swamplands should be taken into full consideration in predicting the feedback of alpine wetlands to climate change on the QTP.