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).