4. DISCUSSIONS
4.1. Seasonal variations of GPP, RES and NEE
The result showed that GDD was the primary control factor affecting the
change of monthly NEE and GPP in the alpine wetland (Figs. 3 and 4). In
addition, to a certain extent, Ts had a relatively strong control over
monthly GPP and monthly NEE in the alpine wetland (Figs. 3). In the
alpine ecosystem of the QTP, photosynthetic radiation is relatively
strong, and vegetation contains a large amount of aboveground biomass.
As a result, temperature in the growing season has a strong impact on
photosynthesis and carbon sequestration of vegetation in alpine wetland
ecosystem (Marcolla et al., 2011; Shen et al., 2015). Furthermore, the
accumulation effect of temperature had important effects on dormancy,
leaf phenology and late growth of plants (Kato et al., 2006; Ueyama et
al., 2013). Therefore, the metabolism of alpine plants, especially in
response to temperature, had rich phenotypic plasticity (Körner, 1999).
In addition, temperature could affect microbial activity, enzyme
activity and decomposition of soil organic matter, so that the thermal
condition was indirectly positively correlated with the supply of
available nutrients in the soil of the ecosystem, thus making
temperature a key control factor for the photosynthetic and carbon
fixation capacity of alpine vegetation in the wetland of QTP (Street et
al., 2007).
Ts played the most important role in the change of monthly RES in the
alpine wetland of QTP (Fig. 3b). Many studies have shown that low soil
temperature limits the soil enzyme activity and soil microbial biomass,
and had an important impact on soil respiration and carbon balance in
the alpine ecosystem (Van den Bos., 2003). This might be because the
alpine wetland contained a large amount of soil organic matter, which
was extremely sensitive to the change of soil temperature. Higher soil
temperature could stimulate the activity of microorganisms and the
decomposition of soil organic matter, so that soil temperature was the
dominant control factor of RES (Moore et al., 1997; Updegraff et al.,
2001). Furthermore, during the growing season, monthly NEE had
significant correlation with monthly GPP (r2= 0.88, p
< 0.001) and monthly RES (r2= 0.44, p
< 0.001). Therefore, GPP played a more controlling role on the
variation of NEE compared with RES during the growing season, which was
consistent with other studies on temperature-restricted ecosystems
(Groendahl et al., 2007; Ueyama et al., 2013).
4.2. Inter-annual variations of GPP, RES and NEE
There was a significant positive correlation between Ts of the
non-growing season and annual GPP of the following year (p
<0.05). This might be because the higher Ts in the non-growing
season stimulated the activities of microorganisms, enhanced the soil
enzyme activity, accelerated the decomposition of litters and soil
organic matter, and thus produced rich nutrients in the soil, promoted
the growth of vegetation in the following year, and improved the
photosynthesis and carbon fixation capacity of vegetation in alpine
wetland (Yu et al., 2003; Shen et al., 2011). In addition, the increase
of Ts in the non-growing period promoted the melting of snow and ice,
thus reducing the surface albedo, which was conducive to the early
re-greening of vegetation and ultimately improved the photosynthetic
productivity of vegetation in alpine wetland (Li et al., 2014; Reverter
et al., 2010; Yu et al., 2003; Zhang et al., 2013). Meanwhile, the
findings suggested that annual GPP had significant negative correlation
with the growing season PPT (p <0.05). One possible was that
PPT raised the water level of the wetland, thereby limiting the
diffusion of oxygen in the atmosphere to the soil, thus inhibiting the
activity of microorganisms and reducing the decomposition rate of soil
organic matter, thus reducing the nutrients in the soil, and
consequently reducing the photosynthetic and carbon fixation capacity of
vegetation in alpine wetland (Chimner and Cooper., 2003).
Annual RES had a significant positive correlation with the non-growing
season Ts (r2 = 0.44, p = 0.037), which had no
significant correlation with growing season Ts (r2 =
0.006, p = 0.83) and annual Ts (r2 = 0.01, p = 0.74).
Furthermore, the non-growing RES had significant correlation with
non-growing season Ts (r2 = 0.45, p = 0.034). This
might due to the long cold weather, the wetland contained a large amount
of soil carbon that was extremely sensitive to temperature changes (Hao
et al., 2011; Kang et al., 2014). The higher temperature during the
non-growing season was conducive to the decomposition of soil organic
matter and the improvement of soil respiration (Zhao et al., 2005; Kang
et al., 2014). These results also suggested that the non-growing RES was
crucial for annual RES. In addition, the non-growing RES had significant
correlation with non-growing season PPT (r2 = 0.54, p
= 0.015). This might due to rainfall and snowfall cause snow and ice
cover on the surface of wetland, which had a certain insulation effect
on soil temperature and was conducive to promoting soil respiration (Hao
et al., 2011; Song et al., 2018; Zhang et al., 2008).
Annual PPT was significant positive correlated with annual NEE (Table
1). This might be due to the increase of PPT in the growing season would
reduce GPP, while the increase of PPT in the non-growing season would
increase RES. As a result, the increase of inter-annual PPT leaded to
accelerate the carbon loss in wetland ecosystem. The increase of the
non-growing season temperature promotes soil respiration, lead to have
significant positive correlation with annual NEE. In this study, the
non-growing season RES was significantly correlated with annual RES and
NEE (p <0.05), which might be because the alpine wetland
ecosystem on the QTP had a long non-growing period due to high altitude
and low temperature, and the alpine wetland was covered by snow and ice
for nearly half one year, so CO2 emission in the
non-growing season had a crucial impact on the variations of annual
CO2 fluxes (Hao et al., 2011; Song et al., 2018; Zhao et
al., 2005). Our results also suggested that under the context of global
warming, especially the increase of temperature in the non-growing
season of the QTP, thus alpine wetland will might have a positive
feedback on carbon loss caused by climate warming. In addition, the
water table depth in the alpine wetland could also affect the soil
thermal conductivity, thus impact on the change of soil temperature. In
the context of climate change with increasing temperature and
precipitation on the QTP, which is likely to through the interactive
effects of Ts and water table depth on CO2 production
(Gao et al., 2012; Zhu et al., 2013). Further research is needed to make
this clearly.