Greenhouse gas fluxes in a boreal peatland under experimental warming, nitrogen addition, and vegetation composition change

Although peatlands cover only 3% of land surface over the world, they have stored a large amount of carbon due to the relatively higher rate of net primary production than decomposition. The Intergovernmental Panel on Climate Change (IPCC) shows that net zero carbon dioxide (CO₂) emissions should be reached around 2050 to limit warming to 1.5°C above pre-industrial levels. The carbon sink function of peatlands could help to reduce global warming (cooling function). However, it is unclear whether this carbon sink function of peatlands will be altered under future global changes such as climate warming, elevated nitrogen (N) deposition, and vegetation composition change. Moreover, methane (CH₄) and nitrous oxide (N₂O) are two potent greenhouse gases with 25 and 298 times higher global warming potential than CO₂, respectively. Their responses to the three global changes (climate warming, elevated nitrogen deposition, and vegetation composition change) are poorly known in peatland ecosystems, especially the interaction of the three global changes, which leads to an uncertainty in evaluating the cooling function of peatlands in the future. In this thesis, the three global changes were mimicked in a boreal peatland located in western Newfoundland, Canada. The fluxes of greenhouse gases (CO₂, CH₄, and N₂O) and environmental variables were measured. The results showed that a change in vegetation composition played an essential role in net CO₂ uptake. With graminoid removal, net CO₂ uptake was significantly decreased, and combined warming and N addition (WN) further decreased CO₂ uptake owing to the detrimental effect of N on Sphagnum mosses. Shrub removal also decreased net CO₂ uptake, but CO₂ uptake could recover in the seventh year owing to the growth of graminoids. Warming and N addition could promote graminoid growth, which might offset the loss of Sphagnum moss cover. Consequently, the net CO₂ uptake was not altered under the condition of shrub removal. Graminoid removal significantly decreased CH₄ emissions due to the reduction of available carbon for CH₄ production and aerenchyma (air channels of some plants) for CH₄ transport from soil to the atmosphere. However, this negative effect was not observed under WN conditions, possibly owing to the alteration of temperature sensitivity. Shrub removal significantly decreased CH₄ emissions under warming treatment, but this negative effect was also not observed under WN conditions, which could be attributed to the growth of graminoids. The positive impact of graminoid growth on CH₄ emission could offset the negative effect of shrub removal. Nitrogen addition significantly promoted N₂O emissions due to the increase of nitrogen availability for N₂O production. Warming could mitigate the positive effect of N addition under intact vegetation in the middle growing season, which could be attributed to the stimulation of N uptake by plants and less N for N₂O production. With the removal of graminoids or shrubs, WN significantly increased N₂O emissions in the early growing season owing to the alleviation of carbon and nitrogen limitation for N₂O production. In summary, if the dominant vegetation shifts to shrubs, the net CO₂ uptake in peatlands would be decreased under climate warming and elevated N deposition. If the dominant vegetation shifts to graminoids, the net CO₂ uptake in peatlands would also be decreased in a short time (~ 3 years) under climate warming and elevated N deposition, but could recover in a long time (~7 years). No matter whether the dominant vegetation shifts to shrubs or graminoids, their impacts on CH₄ emissions would be negligible under climate warming and elevated N deposition. In contrast, no matter whether the dominant vegetation shifts to shrubs or graminoids in the future, the N₂O emissions were significantly promoted under climate warming and elevated N deposition. Overall, taking all three greenhouse gases into account, the cooling function of peatlands would be reduced under future climate warming, elevated N deposition, and vegetation composition change. Besides peatland conservation, other effective measures should be taken in order to slow down the global temperature increase.

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