# Long-Term Nitrogen Addition Eliminates the Cooling Effect on Climate in a Temperate Peatland
## 3 Discussion  
### Effects of Long-Term Nitrogen Addition on CO2 Fluxes in Peatlands  
Net ecosystem exchange (NEE) is the difference between gross primary production (GPP) and ecosystem respiration (ER), driven by the combined effects of GPP and ER. Long-term N addition (\(\sim\)12 years) led to a decline in NEE, particularly under the N2 treatment, where the peatland's C sink function disappeared, turning into a C source, primarily driven by the reduction in GPP. The results indicate that long-term N addition significantly  
Figure 8: Relationships between the ecosystem respiration (ER) and the soil temperature (Correlation analysis, \(p<0.05\)). \begin{table}
\begin{tabular}{l l l l} \hline \hline \multirow{2}{*}{**Treatments**} & \multicolumn{2}{c}{**GWP (g CO2 m\({}^{-2}\) yr\({}^{-1}\))**} \\ \cline{2-4}  & \(\mathbf{CO_{2}}\) & \(\mathbf{CH_{4}}\) & **Net GWP** \\ \hline Control & \(-1492\) & \(130\) & \(-1361\) \\ N1 & \(-582\) (0.0) & \(893\) (100.0) & \(311\) \\ N2 & \(1537\) (71) & \(632\) (29) & \(2169\) \\ \hline \hline \end{tabular} Positive values represent warming and negative values represent cooling. The numbers in brackets indicate the contribution of each greenhouse gas to the net GWP.  
\end{table} 
Table 3: The average global warming potential (GWP) of the emitted greenhouse gases (CO2 and CH4) under the different treatments (Control: 0 g N m\({}^{-2}\) yr\({}^{-1}\), N1: 5 g N m\({}^{-2}\) yr\({}^{-1}\), and N2: 10 g N m\({}^{-2}\) yr\({}^{-1}\)) during the 2019 growing season. reduced GPP and NEE, while its impact on ER was not significant. In the N2 treatment, GPP decreased by more than half compared to the control, which contradicts our initial hypothesis. In this study, the decline in GPP was mainly attributed to the suppression of _Sphagnum_ growth by long-term N addition, leading to a reduction in _Sphagnum_ cover and consequently decreasing CO\({}_{2}\) uptake through photosynthesis. Structural equation modeling results showed a significant positive correlation between GPP and _Sphagnum_ cover, supporting our explanation for the decline in GPP (Figure 7). In nutrient-limited peatland ecosystems, short-term N addition can lead to N saturation in the _Sphagnum_ layer, causing any excess N to infiltrate into the rhizosphere zone, thereby promoting the growth of vascular plants [31; 42]. A comparative study on peat C storage at our research site revealed that N fertilization led to peat decomposition while simultaneously stimulating the growth of vascular plant roots and rhizomes. The increase in underground biomass even compensated for the organic matter loss caused by peat decomposition, ultimately enhancing the entire ecosystem's C sink function [43]. However, there is still a lack of long-term simulation experiments to assess the profound impacts of N addition on the C sink function of peatlands. Nevertheless, N fertilization over a timescale of approximately a decade has been widely documented to cause excessive NH\({}_{4}\)+ accumulation, leading to the production of toxic compounds that inhibit plant growth and ultimately result in _Sphagnum_ mortality [7; 14; 31; 44]. Therefore, it is evident that long-term N addition suppresses _Sphagnum_ growth, significantly reducing its contribution to peatland C storage. Given that more than half of the global peat originates from _Sphagnum_-dominated peatlands [15], the role of peatlands in the global carbon pool could be severely weakened. The increase in ER is also one of the factors driving the significant decline in NEE. The current study found that the long-term N addition showed a nonsignificant increase in ER. Previous studies have demonstrated that under high atmospheric N deposition, ER can increase significantly [12; 44; 45]. Nitrogen addition can enhance N availability in nutrient-poor peatlands, alleviating microbial N limitation and stimulating organic matter decomposition by reducing the C:N ratio of peatland litter [12; 46; 47; 48; 49].