## 4 Discussion
### Silicon Antitranspirant and Subirrigation's Impact on Net Carbon Balances  
Researchers are still looking for unambiguous relationships to understand the influences of individual factors on NEE fluxes from grasslands. Previous studies have shown that the intensity of grassland use and cutting events are important aspects impacting NEE variability. Measurements indicated that NEE increases (is less negative, or positive) with the number of cuts. This is mainly because GPP is reduced to almost zero for a few days after harvesting the meadow sward [51; 54]. Depending on the year and the prevailing meteorological conditions, meadows can either act as a net sink or a source of carbon dioxide. In a study by [PERSON] et al. [62] on Mediterranean grassland, precipitation was the main determinant of interannual variation in NEE. During a dry year, the grassland was a net source of CO\({}_{2}\) to the atmosphere, whereas during a normal year, CO\({}_{2}\) uptake prevailed. Furthermore, it has been shown that climate change, warmer conditions, and intensive grassland management can negatively affect grassland carbon balances and result in a less-negative annual NEE [48]. However, [PERSON] et al. [63] found that water availability is more important than temperature for shaping carbon fluxes from alpine meadows. Their results demonstrated that soil water content directly influenced NEE, GPP, and Reco fluxes.  
Figure 15: Relationship between cumulative GPP fluxes in each cut and yield obtained from HWL_Si and LWL_Si plots.  
This study shows that the HWL plot in the first year of the study had higher (positive) NEE cumulative fluxes when compared to the LWL plot, representing a greater excess of CO2 emission over uptake at HWL. In contrast, in the following year, the HWL plot had more negative NEE values than LWL, representing greater uptake (Table 3). A study of [PERSON] et al. [51] conducted in a three-cut meadow showed a significant correlation between net annual CO2 balance and mean annual groundwater level. It showed that a meadow system combined with a higher WTD (about 20 cm below the surface) achieved the lowest yield-related greenhouse gas emissions.  
In this study, a positive effect on cumulative annual NEE fluxes was only observed in combination with high groundwater levels for the silicon application. The AT treatment in the HWL_Si plot resulted in lower positive NEE fluxes in 2021 than at the HWL plot and more negative NEE fluxes than at the HWL plot in 2022. These results clearly show the positive effect of this treatment on the net carbon dioxide balance. However, a different result was obtained for the lower WTD, where the Si application either increased CO2 emissions or reduced CO2 uptake, depending on the year. Therefore, it can be concluded that this AT effect is beneficial when combined with a higher WTD.  
There is an interesting aspect, which has been found while comparing relationships between NEE fluxes and the biomass yield cumulated in each cut for the plots with and without AT treatments (Figure 16). Although the yield of biomass on AT-treated plots was smaller, the yield at which the meadow turns from being a net source to a net sink of CO2 in single cuts shifts from around 3800 kg-ha\({}^{-1}\) at plots without Si application to 3200 kg-ha\({}^{-1}\) at the AT-treated plots. Also, in more cases, the cumulated NEE was more negative at the same biomass yield for the plot with silicon application.