# Long-Term Nitrogen Addition Eliminates the Cooling Effect on Climate in a Temperate Peatland
## 1 Introduction  
Peatlands cover only about 3% of the global land area, but store about \(\sim\)600 Gt of the global soil C and play a vital role in the global carbon (C) cycle [1, 2]. A peatland is both a C sink, which can effectively mitigate climate change, and a C source, which can exacerbate global warming under certain conditions [3, 4, 5, 6]. However, the C balance of peatlands is highly susceptible to environmental changes, especially the significant increase in nitrogen (N) deposition caused by anthropogenic activities [7]. Over the past 150 years, the amount of reactive N in the atmosphere has increased more than 10-fold, driven by agricultural fertilization, fossil fuel combustion, and industrial activities, and global N deposition levels are expected to increase another 2-3 times by 2050 [8, 9, 10, 11]. This anthropogenically driven N enrichment has profound impacts on nutrient-limited peatland ecosystems, potentially not only weakening C storage capacity but also significantly altering greenhouse gas emission fluxes [12, 13, 14]. Peatland ecosystems are extremely sensitive to N deposition, which is closely related to the characteristics of their dominant plant--_Sphagnum_ moss, which absorbs atmospheric N through its capitulum branches [15]. N addition affects the production and consumption ofgreenhouse gases (CO\({}_{2}\) and CH\({}_{4}\)) by regulating plant and microbial activity and changing related biogeochemical reactions. In most ecosystems, N addition generally increases plant growth, thereby increasing C storage in plant biomass [16; 17; 18; 19]. However, this positive effect is less common in non-forest ecosystems [14; 20]. In peatlands, N addition may inhibit the growth of _Sphagnum_ moss or even cause their death, significantly reducing net ecosystem exchange (NEE) [7; 14]. At the same time, increased N deposition generally promotes the growth of vascular plants [21; 22]. Although the expansion of vascular plants may increase carbon absorption capacity by enhancing photosynthesis in the short term, the increase in vegetation N content will also accelerate the decomposition of litter [23; 24; 25; 26], leading to higher CO\({}_{2}\) emissions. In addition, studies have shown that higher levels of atmospheric N deposition may further exacerbate peatland C loss by enhancing heterotrophic respiration and dissolved organic carbon (DOC) leaching [12]. Therefore, the promotion of soil organic C decomposition by N addition may exceed its stimulating effect on plant productivity, ultimately leading to a net reduction in peatland C storage. The mechanism of the effect of N addition on CH\({}_{4}\) emissions is relatively complex, involving the joint regulation of multiple biogeochemical processes [27; 28; 29; 30]. CH\({}_{4}\) is mainly produced by methanogenic archaea decomposing organic matter under anaerobic conditions and is oxidized by methanotrophic bacteria under aerobic conditions [28]. Nitrate (NO\({}_{3}\)\({}^{-}\)) can effectively inhibit methanogenesis by increasing the redox potential [27; 31], while ammonium (NH\({}_{4}\)\({}^{+}\)) can inhibit CH\({}_{4}\) oxidation by competing with methane-oxidizing bacteria for the activity of methane monooxygenase (MMO), resulting in increased CH\({}_{4}\) emissions [27; 32; 33]. In addition, nitrite (NO\({}_{2}\)\({}^{-}\)) produced during nitrification or denitrification has a certain toxicity, which can significantly reduce the activity of methanotrophic bacteria and further weaken CH\({}_{4}\) oxidation [34; 35; 36]. Plant functional type plays an important role in the regulation of CH\({}_{4}\) emissions, especially under N addition conditions. The increase in the proportion of grass plants (such as sedge) may have an important impact on the production and transmission of CH\({}_{4}\)[30; 37; 38; 39]. The aerenchyma of this type of plant can transport CH\({}_{4}\) generated in the deep anaerobic environment directly to the atmosphere, bypassing the aerobic zone.