# Long-Term Nitrogen Addition Eliminates the Cooling Effect on Climate in a Temperate Peatland
## 3 Discussion
### Effects of Long-Term Nitrogen Addition on CH\({}_{4}\) Fluxes in Peatlands  
Extensive research has demonstrated that N fertilization influences both CH\({}_{4}\) production and consumption, but the extent and direction of this effect vary widely [32; 57]. Some studies have demonstrated that N addition could enhance CH\({}_{4}\) emissions [20], whereas others have found no significant effect [58] or even a suppressive effect on CH\({}_{4}\) emissions [13].  
In this study, we found that long-term N addition significantly increased CH\({}_{4}\) flux, aligning with our third hypothesis. Several mechanisms may contribute to the enhancement of CH\({}_{4}\) emissions by N addition. First, NH\({}_{4}\)\({}^{+}\) serves as an N source that promotes the growth of methanogenic archaea, thereby increasing methane production. Second, NH\({}_{4}\)\({}^{+}\) may inhibit CH\({}_{4}\) oxidation by competing with methane-oxidizing bacteria for methane monooxygenase [20; 28; 29], leading to elevated CH\({}_{4}\) emissions. Additionally, the application of NH\({}_{4}\)NO\({}_{3}\) fertilizer can induce nitrification or denitrification processes, resulting in the accumulation of nitrite (NO\({}_{2}\)\({}^{-}\)), which is toxic and may suppress methane-oxidizing bacterial activity, further reducing CH\({}_{4}\) oxidation [36].  
N addition also indirectly affects CH\({}_{4}\) production and transportation by altering peatland vegetation composition [28; 30; 39]. After 12 years of N addition, graminoid cover increased significantly. Moreover, a significant positive correlation was observed between graminoid cover and CH\({}_{4}\) flux, with the highest values for both occurring in the N1 treatment, further emphasizing the regulatory role of graminoid growth in peatland CH\({}_{4}\) emissions. Graminoids absorb C through photosynthesis and allocate a portion of it to methanogenic archaea via root exudates, thereby stimulating CH\({}_{4}\) production [28; 40]. Additionally, graminoids and other emergent macrophytes possess aerenchyma tissues, which facilitate CH\({}_{4}\) transport to the atmosphere, reducing the opportunity for CH\({}_{4}\) oxidation in the aerobic layer and further increasing CH\({}_{4}\) emissions [37; 38; 59]. Thus, long-term N addition may drive a shift in peatland vegetation composition, from _Sphagnum_ mosses- to vascular plants-dominated communities. This transformation not only enhances deep CH\({}_{4}\) production but also suppresses CH\({}_{4}\) oxidation and increases CH\({}_{4}\) transport efficiency, ultimately leading to higher overall CH\({}_{4}\) emissions.  
Our study revealed a significant positive correlation between CH\({}_{4}\) emissions and water table depth (WTD). WTD is one of the key factors determining CH\({}_{4}\) emissions [28; 60; 61]. Long-term N addition led to a significant reduction in WTD, meaning the distance between the soil surface and the water surface decreased. Field in situ observations show that this reduction in WTD is primarily due to accelerated peat decomposition, inhibited _Sphagnum_ growth, and even its mortality, ultimately resulting in a decline in hummock height. CH\({}_{4}\) is primarily produced by methanogenic archaea under anaerobic conditions, while a portion of it is oxidized by methanotrophic bacteria in aerobic environments [29]. Consequently, a thicker potential CH\({}_{4}\) production zone and a thinner oxidation zone led to higher CH\({}_{4}\) emissions into the atmosphere [60; 61; 62]. This implies the increase of CH\({}_{4}\) emissions as the surface-to-water distance decreases. Additionally, under low water table depth conditions, the solubility of CH\({}_{4}\) in soil water rises. As CH\({}_{4}\) accumulates in soil pores, it can eventually reach a critical concentration, forming bubbles that rapidly escape to the atmosphere through ebullition [63; 64].