# Long-Term Nitrogen Addition Eliminates the Cooling Effect on Climate in a Temperate Peatland
## 2 Results  
### CO\({}_{2}\) Fluxes  
Long-term N addition had a significant impact on the gross primary productivity (GPP) (\(p<0.05\)) and net ecosystem exchange (NEE) (\(p<0.01\)) (Table 1). As N addition increased, both GPP and NEE exhibited a declining trend, especially the N2 treatment significantly reduced the GPP and NEE of the peatland. Compared to the control group, GPP decreased by approximately 56.9% (Figure 1A), while NEE declined by approximately 203.0%, ultimately shifting the peatland from a carbon sink to a carbon source (Figure 1C). Although long-term N addition did not show a significant effect on the respiration rate (ER) (Table 1), ER still showed an increase with higher N addition. Specifically, ER increased by approximately 42.8% and 51.9% under the N1 and N2 treatments, respectively, compared to the control group (Figure 1B). Figure 1: The effects of nitrogen (N) addition (Control: 0 g N m\({}^{-2}\) yr\({}^{-1}\), N1: 5 g N m\({}^{-2}\) yr\({}^{-2}\) and N2: 10 g N m\({}^{-2}\) yr\({}^{-2}\)) on gross primary productivity (GPP, **A**), ecosystem respiration (ER, **B**), and net ecosystem exchange (NEE, **C**) (Mean \(\pm\) SE, n = 4). Positive values indicate CO\({}_{2}\) uptake and negative values indicate CO\({}_{2}\) emission. Different lowercase letters represent significant differences among treatments ([PERSON]’s HSD test, \(p<0.05\)). Regarding the monthly average variation in GPP, the data on May 9 showed that the GPP under the N1 treatment reached approximately 58.93 mg m\({}^{-2}\) h\({}^{-1}\), which was significantly higher than the GPP in the control group (3.23 mg m\({}^{-2}\) h\({}^{-1}\), \(p<0.05\)) and N2 treatment (2.27 mg m\({}^{-2}\) h\({}^{-1}\), \(p<0.05\)). This suggests that the N1 treatment advanced the onset of peatland plant photosynthesis. In contrast, the data on May 22 showed that the GPP under the N2 treatment still remained at approximately 2.27 mg m\({}^{-2}\) h\({}^{-1}\), which was significantly lower than that of the control (134.50 mg m\({}^{-2}\) h\({}^{-1}\), \(p<0.05\)), indicating that the N2 treatment delayed the onset of peatland plant photosynthesis. Throughout the growing season, GPP in the N2 treatment consistently remained lower than that in both the control and N1 treatment groups, with the control group exhibiting the highest GPP among all treatments. Notably, GPP in the peatland peaked in mid-July, which is consistent with the typical growing season pattern (Figure 2A). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Vegetation Cover** & \(F\) & \(p\) & **Greenhouse Gas** & \(F\) & \(p\) & **Enzyme Activity** & \(F\) & \(p\) \\ \hline To-C & 4.302 & 0.049 & GPP & 3.594 & 0.031 & PHO & 3.297 & 0.090 \\ \(Sp\)-C & 3.314 & 0.083 & ER & 1.482 & 0.233 & NAG & 6.388 & 0.026 \\ Gr-C & 1.628 & 0.249 & NEE & 10.996 & 0.000 & BDG & 1.750 & 0.228 \\ Sh-C & 1.392 & 0.297 & CH\({}_{4}\) & 4.913 & 0.009 & PPO & 3.894 & 0.060 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The effects of nitrogen (N) addition on the total vegetation cover (To-C), _Sphagnum_ cover (\(Sp\)-C), graminoid cover (Gr-C), shrub cover (Sh-C), ecosystem respiration (ER), net ecosystem exchange (NEE), gross primary productivity (GPP), methane (CH\({}_{4}\)) flux, \(\beta\)-D-glucose(BDG), N-acetyl-\(\beta\)-glucosidase(NAG), phosphatase (PHO), and phenol oxidase (PPO) activities. Figure 2: The effects of N addition (Control: 0 g N m\({}^{-2}\) yr\({}^{-1}\), N1: 5 g N m\({}^{-2}\) yr\({}^{-1}\), and N2: 10 g N m\({}^{-2}\) yr\({}^{-1}\)) on the monthly average gross primary productivity (GPP, **A**), ecosystem respiration (ER, **B**), and net ecosystem exchange (NEE, **C**) (Mean \(\pm\) SE, n = 4) of the peatland in the 2019 growing season. Positive values indicate CO\({}_{2}\) uptake and negative values indicate CO\({}_{2}\) emission. Different lowercase letters represent significant differences among the different treatments ([PERSON]’s HSD test, \(p<0.05\)).