1. INTRODUCTION
The ongoing global warming have caused wide spread changes the carbon cycle in terrestrial ecosystems (Ganjurjav et al., 2015). The wetland accounts for only 5%–8% of land surface area, but it accounts for 25%–30% soil organic carbon (SOC) in terrestrial ecosystem, which is crucial for global carbon balance (Mitsch et al., 2007). The Qinghai-Tibetan Plateau (QTP) contains approximately 33% of wetland area in China, which accounts for 30%–40% soil organic matter of Chinese wetland (Zheng et al., 2013), where is experiencing an intense increase in surface temperature (IPCC, 2013). Due to the special environment of high altitude and low temperature in QTP, the carbon dynamics in alpine wetland are sensitive to global climate change (Barichivich et al., 2013). Therefore, understanding the carbon budgets from alpine wetland ecosystems has become increasingly important in accurately projecting global carbon cycling in future climatic change (Hirota et al., 2006; Shen et al., 2015).
Past studies have not reached a consistent conclusion about the wetland ecosystem is carbon sink or carbon source. Due to the difference of environmental factors, the CO2 fluxes of wetland ecosystem have large inter-annual variability (Griffis et al., 2000; Heinsch et al., 2004; Lafleur et al., 1999; Liikannen et al., 2005;). Many investigations indicated that wetland CO2 fluxes are affected by a variety of ecological factors, such as air and soil temperature, PPFD, precipitation, and average water depth, which are usually non-linear associations (Hao et al., 2011; Hirota et al., 2006). Furthermore, changes in hydrothermal conditions and other key environmental factors, along with the increased concentrations of CO2, will influence plant community characteristics and photosynthetic productivity (Song et al., 2018). Therefore, due to different environmental factors in different wetland ecosystem, the carbon source or sink of wetland ecosystems are not consistent (Heinsch et al., 2004; Song et al., 2018). However, the studies of carbon balance on the QTP were mainly focused on alpine shrub and alpine meadow (Kang et al., 2014; Saito et al., 2009; Zhao et al., 2010; Zhu et al., 2015). In recent, there are few studies on the carbon sources/sinks and their influencing mechanisms in alpine wetland ecosystems (Kato et al., 2006; Zhao et al., 2005; Zhang et al., 2008). There is still no unified result about the source or sink of alpine wetland in QTP (Hao et al., 2011; Zhao et al., 2005). Furthermore, previous studies on carbon balance of wetland ecosystem were limited to short time series data, which was difficult to fully explain the dynamics of carbon balance and its influence mechanism (Kang et al. 2014; Zhang et al., 2011). Therefore, the study based on the field measurement for many years could more accurately understand the CO2 fluxes of the alpine wetland ecosystems (Chen et al., 2016; Griffis et al., 2000; Marcolla et al., 2011; Song et al., 2018).
Under the background of global climate change, a large number of studies have shown that the increase of temperature promotes the extension of growing season and the early greening of vegetation (Li et al., 2015), and promote alpine ecosystem to absorb carbon and improve the vegetation productivity (Zhang et al., 2013; Shen et al., 2015). In addition, the increase of temperature is beneficial to the activity of microorganisms and would accelerate the decomposition of soil organic matter, thus stimulating soil C emission (Chen et al., 2016). However, the in situ observational response of carbon balance in alpine wetland under the current climate change remains limited (Hao et al., 2011). As permafrost melts and glaciers recede, the area of wetlands on the QTP might increase, which will have an important impact on the carbon dynamic of the alpine ecosystem (Sturm et al., 2005; Zhang et al., 2011). Therefore, we used ten consecutive years of flux data from January 2007 to December 2016 to study an alpine wetland in QTP. The purposes of the research are to (1) investigate the variation patterns of the seasonal and inter-annual CO2 fluxes (gross primary production (GPP), ecosystem respiration (RES) and net ecosystem CO2exchange (NEE)); and (2) clarify the environmental drivers for the change of seasonal and annual CO2 fluxes. We proposed the following hypotheses: (1) the hydrothermal condition is the dominant factor controlling the change of seasonal CO2 fluxes; and (2) CO2 emission during the non-growing season are crucial to annual CO2 fluxes in alpine wetland (Hao et al., 2011; Song et al., 2018; Zhao et al., 2005).