Jin-Sheng He and Hao Wang

　　　　Department of Ecology, College of Urban and Environmental Sciences, Peking University, 5 Yiheyuan Rd., 100871 Beijing, China

　　　　Alpine wetland and grassland, representing ~61% of the land surface of the Tibetan Plateau, play an important role in global methane (CH4) budgets. Generally, alpine wetlands is a CH4 source and alpine grassland is a CH4 sink, but so far the source/sink strength and temporal pattern remain highly uncertain because of technical limitations and the harsh environments. In addition, rapid climate changes and intensifying human activities have resulted in water table lowering and enhanced nitrogen deposition. These changes may alter the magnitude and direction of greenhouse gas emissions, affecting the climate impact of these fragile ecosystems. To quantify the CH4 source/sink strength, we conducted the first in situ year-round CH4 flux observation using eddy covariance method in a fen (2011.7-) and a mesic meadow (2016.8-). Meanwhile, a mesocosm experiment combined with a metagenomics approach (GeoChip 5.0) was established to elucidate the effects of water table lowering (-20 cm relative to control) and nitrogen deposition (30 kg N ha-1 yr-1) on CO2, CH4 and nitrous oxide (N2O) fluxes as well as the underlying mechanisms. We found that the annual CH4 emissions were 26.4 and 33.8 g CH4 m-2 in 2012 and 2013, respectively, and a two-peak seasonal variation in CH4 fluxes was observed, with a small peak in the spring thawing period and a large one in the peak growing season. Furthermore, the non-growing season CH4 emissions accounted for 43.2-46.1% of the annual emissions, highlighting an indispensable contribution that was often overlooked by previous studies. Results from mesocosm experiment showed that water table lowering largely reduced CH4 emissions but did not affect net CO2 uptake and N2O fluxes, while N deposition increased net CO2 uptake and N2O emissions but had little influence on CH4 emissions. As a result, both water table lowering and N deposition reduced the overall global warming potential. GeoChip analysis revealed that decreased CH4 production potential, rather than increased CH4 oxidation potential, led to the reduction in net CH4 emissions, and decreased nitrification potential and increased denitrification potential affected N2O fluxes under WTL conditions. Our study highlights the importance of microbial mechanisms in regulating ecosystem-scale greenhouse gas responses to environmental changes.