## 4 Discussion
### Energy Dynamics and Carbon Flux
The large negative daily GPP values (strong carbon sink) observed in this study were primarily caused by the warm-season grass production in summer. Additionally, greater abundance of summer-season precipitation and SVWC in the first growing season had also resulted in more negative GPP in the first growing season (Figures 1 and 4). However, the first season's NEE indicated a less carbon sink than the second season, mainly because of the increased summer respiration intensity, caused by the higher night-time temperatures associated with more precipitation (Figures 1 and 4). Therefore, unlike the semiarid environment, precipitation might actually reduce carbon sequestration potential at an ecosystem level in the humid temperate regions. The seasonal cumulative GPP/NEE values obtained in this study were more negative, indicating strong biomass productivity and carbon sequestration capacity. [PERSON] et al. (2018) also reported that GPP for a warm-season old world bluestems-based pasture only reached up to 21 g C m\({}^{-2}\) day\({}^{-1}\), which was very similar to those values reported by [PERSON] et al. (2018) and [PERSON] et al. (2018), but only about half of what we found in our grassland ecosystem. This magnitude differences were likely caused by the fact that the majority of the warm-season grass species found in the Southeastern U.S. feature great biomass production capacity (e.g., purpletop, john-songrass, and dallisgrass), which are well-adapted to the humid southeastern weather and typically have much larger canopy architecture than those warm-season short-bunchy grasses found in the semiarid western U.S. regions. Additionally, in an ecosystem carbon exchange comparison study, [PERSON] et al. (2018) pointed out that the extended growing seasons of perennial warm-season grasses offer excellent carbon sequestration capacity compared to many annual species. As expected, the seasonal cumulative GPP/NEE values were substantially greater than those reported values from other managed cool-season monoculture pastures or warm-season row-cropping systems. For example, [PERSON] et al. (2018) reported that the annual maximum cumulative GPP of a cool-season reed canary grass (_Phalaris arundinaceae_ L.) pasture in eastern Finland only reached \(-689\) g C m\({}^{-2}\). In another study, a kentucky bluegrass monoculture pasture only yielded an annual NEE value of \(-1.1\) kg C m\({}^{-2}\)([PERSON] et al., 2018). [PERSON] et al. (2018) found that a perennial warm-season hay pasture in Fort Valley of Georgia provided an annual NEE value of \(-1.62\) kg C m\({}^{-2}\), which was similar to our results from the first growing season (\(-1.78\) kg C m\({}^{-2}\)) but still substantially lower than the second growing season (\(-2.68\) kg C m\({}^{-2}\)). In our study, we attributed the low seasonal GPP/NEE values (strong carbon sink) to the fact that the pasture was partially managed as a natural ecosystem with few biomass production gaps and almost whole-year-long growing season involving succession of dominance between cool and warm-season grass species. Based on NEE data, the entire ecosystem remained a net carbon sink during the majority of the time within each growing season (Figure 4). This contradicts the findings reported by [PERSON] [9], indicating that a cool-season pasture ecosystem in Pennsylvania with very similar grass species composition could become a carbon source during the winter months. However, previous studies also indicated that transitioning from row-crop to perennial grass systems could greatly enhance GPP and NEE, but it might take more than one or two years before turning the entire ecosystem into a carbon sink [8]. The longer time period since the establishment of this pasture ecosystem might play a role in causing the differences between our study and the one reported by [PERSON] [16].