3.4. Influencing way of land degradation on AGB of grasses
A principal component analysis of
photosynthetic capacity,
nutrient content and
AGB showed that
PC1 and PC2 explained % of the
variance of 69.2% and 86.5% in native and active restoration
grasslands, respectively. In native grasslands, AGB was mainly
positively related with Pn and plant N (Fig. 6a). In active restoration
grasslands (Fig. 6b), AGB was mainly positively related with Pn, plant N
and P, gs and WUEi .
# Insert Figure 6 #
Two structural equation models (SEM) were used to explore the effects of
degradation on the AGB of grasses in the two types of grasslands (Fig.
7). In native grasslands,
degradation affect AGB directly
and through Pn indirectly (R2 =0.90,P <0.001) (Fig. 7a). In active restoration grasslands,
degradation affect AGB directly and through plant N
(R2 =0.91, P <0.001), plant P
(R2 =0.93, P <0.001) and Pn
(R2 =0.85,P <0.001) indirectly
(Fig. 7b).
# Insert Figure 7 #
Discussion
4.1 Plant AGB in
response to grassland degradation
Generally, AGB is a one of most important index and can be used to
assess the growth status of plants and stability of plant community to
their surrounding environment (Liu et al., 2018; Xu et al., 2018; Shen
et al., 2019). In our study, there were negative changes in AGB of
community and grasses with increased levels of degradation, this result
is consistent with previous studies (Shang et al., 2008; Benaya et al.,
2009; Wang et al., 2010; Li et al., 2014, 2018; Zhou et al., 2021).
Livestock selective palatable grasses, soil nutrients and soil water
loss were mainly responsible for the decreasing of AGB of community and
grasses alongside degradation (Shang et al., 2008, 2016; Guo et al.,
2019).
The
establishment of active
restoration grasslands increased AGB
both of grasses and community, it
was reasonable to conclude that the artificial replanting in ED
grassland is the most effective approach to restore degraded grasslands
(Wu et al., 2010; Li et al., 2018; Guo et al., 2019; Bai et al., 2020).
However, the AGB of grasses and community
in those grasslands have decreased
at 9th year or longer than 9 years of replanting time. Therefore, in
order to prevent grassland
degeneration of the active restoration, management intervention such as
weeding and fertilization should be taken into consideration at 9th
restoration year (Shang et al., 2008; Li et al., 2014, 2018; Gao et al.,
2019).
4.2 Plant photosynthetic
capacity in response to grassland degradation
In this study, grasses showed
higher Pn in ND and SLD relative
to MD, HD and ED in native grasslands, and showed higher Pn in 5Ys and
9Ys relative to 11Ys, 14Ys and
17Ys in active restoration
grasslands, respectively. Degradation of both the native grasslands and
active restoration grasslands can decrease soil
total carbon (C), soil total N
content and soil water content (Li et al., 2014; Xu et al., 2015; Liu et
al., 2020), and which could limit plant photosynthesis activity by
reducing the supplements of C, N and water to photosynthetic tissues
(Chaves et al., 2009; Jilling et al., 2018; Quan et al., 2019 Shen et
al., 2019). Our results also indicated that the different responses of
grass’ photosynthetic capacity to land degradation were exist between
the two types of grasslands, which means different mechanisms may be
responsible for these differential responses (Skogen et al., 2011, Shen
et al., 2021). In native
grasslands, land degradation significantly decreased
Pn,
but Ci (Fig.2g) andgs (Fig. 2c) kept relatively stable, we can confirm
that
the decline of Pn mainly caused by
non-stoma limitation. However, in active restoration grasslands, with
the decrease of Pn, Ci(Fig.2h) and gs (Fig.2d) were significant decreased, which
indicated that the decline of Pn mainly caused by stoma limitation (Xu
et al., 2010; Carriquí et al., 2015; Bartlett et al., 2017; Brodribb et
al., 2020).
Our study also indicated that
plant
photosynthetic capacity of grasses
in active restoration grasslands shows
stronger association with its AGB
than native grasslands, this would suggest that
plant photosynthetic capacity were
very sensitive to soil degradation and should be considered as an
indicator of degradation level. In
native grasslands, the relationship between plant photosynthetic
capacity and its AGB of grasses is less stronger, this may attribute to
its higher belowground biomass (Li
et al., 2014; Crouzeilles et al., 2017; Guo et al., 2019), higher soil
water content and higher nutrient content (Shang et al., 2008; Jensen et
al., 2020) as compared to the active restoration grasslands, which maybe
can modulate the relationship. So, the relationships among plant
photosynthetic capacity, soil water content and nutrient content of
native and active restoration grasslands need to be further
investigation.
4.3 Plant N and P concentration in response to
grassland degradation
Consistent with other studies (Li et al., 2014; Guo et al., 2019; Zhou
et al., 2021), plant N concentration of grasses decreased with
degradation intensify both in native grasslands and active restoration
grasslands. In alpine grassland, soil N concentration is relatively low
and the ecosystems are usually N-limited (Guo et al., 2017;
Xu et al., 2018; Shen et al.,
2019). Continuous grassland
degeneration altered soil carbon and N availability and decreased soil
carbon and N storage (Chen et al., 2016; Wang et al., 2018; Peng et al.,
2020), which not only had negative feedbacks on plant N concentration,
but also would decrease plant carbon gain and biomass accumulation
(BassiriRad, 2015; Carriquí et al., 2015; Bartlett et al., 2017; Shen et
al., 2019).
Our result indicated that there were no significant difference in P
concentration in grasses alongside degradation in native grasslands.
This not only suggested that plant P was not or less sensitive to
degradation than N, but also confirmed that alpine soil were less
P-limited than N-limited (Xu et al., 2015; Liu et al., 2018; Zhou et
al., 2021). The main reasons are the different relationships of plant N
and P content with soil water content, difference cycle processes of N
and P and difference nutrients resorption mechanism of N and P by plants
(Pastor et al., 1984; Reich and Oleksyn, 2004; Rui et al., 2012; Yan et
al., 2018; Zhou et al., 2021). In this study,
in active restoration grasslands,
plant P concentration of grasses decreased with the increase of
replanting time, and positively related with its AGB. This means that a
possible change in the balance of P nutrient alongside degradation,
which had negative feedbacks on plant photosynthetic activity and plant
productivity (Dijkstra et al., 2012; Pistocchi et al., 2018; Zhou et
al., 2020). The above results indicated that through improving plant N
and P concentration to restore degraded active grasslands may be more
effective than to restore degraded native grasslands.