## 4 Discussion  
### Energy Dynamics and Carbon Flux  
The energy closure values obtained were generally within the range (0.6-0.9) of natural grassland ecosystems reported by other studies [24; 25], which is typically lower in natural complex ecosystems such as pastures compared with row-cropping systems [26]. The remaining energy fraction can be retrieved from energy storage in the top soil layer above the heat flux plates, and from the vegetation canopy between the soil surface and the IRGA sensor head; which primarily consists converted energy by photosynthesis. The values found in this study (0.74 and 0.62 for the first and the second growing seasons, respectively) were close to the lower bound within the normal range for C3 plants [27], indicating that the EC systems properly captured and recorded energy fluxes on an ecosystem level. The slightly lower energy closure values observed in this study could be attributed to the fact that global warming gradually pushes the climate transition zones towards the north, causing significant encroachment of summer annual/perennial species (e.g., large crabgrass (_Digitaria sanguinalis_ (L.) Scop.) purpletop, johnsongrass, dalliggrass, etc.) during late summer/early fall growing seasons. This warm-season encroachment scenario  
Figure 9: Benchmarking results of filling artificial gaps of the latent heat flux (LE) data based on the optimal prediction models and cross-validation settings. Regressions of predicted NEE by the REddyProc software using global radiation, ambient air temperature (Tair), and vapor pressure deficit (VAP, **top-left**); support vector machine (SVM, **top-right**) and artificial neural network (ANN, **bottom-left**) using 7- and 14-day window training data flanking each missing/testing value, respectively; and the least-square linear model (LSLM) based on a 10-fold cross validation (10-fold CV, **bottom-right**); against the eddy-covariance (EC) measured LE values are presented. The red line indicated 1:1 perfect prediction line. complemented the depleted production of cool-season species during hot summer months, therefore, significantly increased overall photosynthetic activity and consequentially increased biomass production and eco-efficiency. Additionally, this could also be caused by the increased heat storage portion in open canopy dominated grassland sites as reported by [PERSON] et al. (2018). Unlike warm-season annual/perennial grasses, which produce the majority of biomass during the late summer-early fall of each year; the growth of cool-season grasses typically peaks at the spring and fall seasons of each year with summer (summer slump) and winter dormancy periods spliced in between ([PERSON] et al., 2018). This bimodal growth distributions of cool-season grasses were also observed in this study (indicated by the GPP pattern in Figure 4) and are generally well-known in grassland ecosystem community. However, what is less known is the large day-by-day variation of GPP and NEE caused by different environmental and managerial factors. Particularly, hay bailing events on 17 July and 6 September 2017, as well as on 15 July 2018 rapidly transitioned the ecosystem from a net carbon sink to carbon source for a short period of time as indicated by the shifting of NEE from negative to positive numbers. Similar findings were also reported by [PERSON] et al. (2018) from a warm-season, old world bluestem dominated grassland in Oklahoma. It was also noteworthy that GPP responded more rapidly than NEE after bailing, turning the entire ecosystem into a carbon emission source for approximately 12 days after the September cutting in 2017 and about 5 days after the July cutting in both 2017 and 2018. Additionally, the July cutting removed a small portion of senesced warm-season grasses and very little cool-season grasses that were still in summer dormancy, thus, resulting in shorter duration of responses. Following a short duration of carbon emission period, both GPP and NEE increased rapidly triggered by vigorous regrowth of young plant tissues.