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.