## 4 Discussion  
### Temporal Trends in Carbon Fluxes  
The fluctuations in daily net ecosystem exchange (NEE) underscore the highly dynamic behavior of carbon fluxes within wetland ecosystems. During periods of inundation, the wetland alternated between acting as a carbon sink and a carbon source, with daily NEE values ranging from \(-\)7.57 to 22.49 g C m\({}^{-2}\) d\({}^{-1}\). This high variability underscores the sensitivity of wetland carbon dynamics to hydrological conditions, particularly groundwater level fluctuations. An average NEE of 0.40 g C m\({}^{-2}\) d\({}^{-1}\) during flood conditions indicates that the system functioned as a modest carbon source overall, aligning with observations from other inundated ecosystems [43; 44].  
In contrast, gross primary production (GPP) exhibited a strong seasonal pattern, with peaks aligned with the warm growing season (May-August). This pattern reflects the temperature and light dependence of photosynthetic activity, commonly reported in temperate wetlands [45]. While GPP reached up to 27.45 g C m\({}^{-2}\) d\({}^{-1}\), ecosystem respiration (RE) frequently exceeded GPP, particularly during periods of prolonged flooding. The average daily RE during flooded conditions was 4.51 g C m\({}^{-2}\) d\({}^{-1}\), occasionally peaking at nearly 30 g C m\({}^{-2}\) d\({}^{-1}\). Increased RE during non-flooding may be attributed to enhanced root respiration and anaerobic microbial processes in waterlogged soils [46; 47].  
Non-flooded periods, which were rarely observed after 2016, were associated with higher average daily GPP (6.54 g C m\({}^{-2}\) d\({}^{-1}\)) and RE (6.95 g C m\({}^{-2}\) d\({}^{-1}\)) compared to flooded periods, with average daily GPP and RE of 4.06 g C m\({}^{-2}\) d\({}^{-1}\) and 4.51 g C m\({}^{-2}\) d\({}^{-1}\), respectively. This result indicates that drier conditions supported more active vegetation growth and microbial decomposition. The larger flux magnitudes during non-flooded periods suggest enhanced oxygen availability in the rhizosphere, which can stimulate both autotrophic and heterotrophic respiration [48]. This shift underscores the sensitivity of carbon cycling in wetland ecosystems to hydrological fluctuations and emphasizes the critical need for long-term monitoring of water regime dynamics, particularly in the face of climate variability and land-use changes.  
Importantly, a notable shift in ecosystem function was observed post-2015: GPP and RE became markedly higher under non-flooded conditions, while flooded conditions increasingly suppressed GPP but maintained elevated RE. This divergence indicates a possible regime shift in the wetland's carbon cycling processes, potentially due to increased inundation frequency or altered plant community structure caused by increased tree mortality (Table 1). As RE began to consistently outpace GPP, the ecosystem transitioned toward a persistent net carbon source. This phenomenon aligns with previous research indicating that prolonged flooding can degrade ecosystem productivity and carbon sequestration capacity [44].  
Our findings illustrate how hydrological conditions significantly influence the carbon dynamics of wetland ecosystems. A delicate balance exists. While periodic flooding can promote carbon storage through reduced decomposition and organic matter accumulation, chronic inundation may inhibit photosynthesis and enhance anaerobic respiration, leading to long-term carbon losses. Understanding this balance is critical in light of projected climate-driven changes in precipitation, sea level rise, and water table dynamics.