Abstract:
Grassland degradation can affect plant eco-physiological properties and thus influence ecosystem productivity and ecosystem function. However, how land degradation affects the relationship between plant biomass and eco-physiological properties of active restoration grasslands and native grasslands in alpine meadow is less understood. A series of degraded grasslands (non-degraded, slightly degraded, moderately degraded, heavily degraded and extremely degraded grasslands) and a series of active restoration grasslands (active restored grasslands with different growth time, 5 years, 9 years, 11 years, 14 years and 17 years) were selected to investigate the relationship between eco-physiological properties and aboveground biomass (AGB) of grasses alongside degradation. Results showed that the AGB, net photosynthetic rate (Pn) and plant nitrogen (N) concentration decreased significantly with increased levels of degradation in native grasslands. Plant photosynthetic capacity, plant N and phosphorus (P) concentration significantly decreased at 9th year or longer than 9 years of replanting time in active restoration grasslands. Plant eco-physiological properties in active restoration grasslands shows stronger association with its AGB than native grasslands. In native grasslands, degradation affect AGB directly and through Pn indirectly. In active restoration grasslands, degradation affect AGB directly and through Pn, plant N and P indirectly. Our results indicated that through improving plant nutrients to restore degraded active grasslands may be more effective than to restore degraded native grasslands.
Key words: Active restoration; Grassland degradation; Photosynthesis; Plant nitrogen concentration; SEM modeling
Introduction
Alpine grasslands cover more than 60% area of the Qinghai-Tibetan Plateau (QTP) (Li et al., 2014; Shen et al., 2015; Dong et al., 2020) and play an integral role as livestock production, biodiversity maintenance and water conservation in this region (Qiu et al., 2008; Wang et al., 2009; Miehe et al., 2019; Shen et al., 2021). However, due to climate change and human activity, these grasslands have been degraded in recent decades (Shang et al., 2008; Dong et al., 2020; Peng et al., 2020). This not only influenced local economic development but also altered the relationship between plant productivity and eco-physiological properties, leading to ecosystem instability (Andrade et al., 2015; Guo et al., 2019; Bai et al., 2020). To prevent grasslands degradation and to restore these degraded grasslands, the active restoration means has been applied widely in this region (Shang et al., 2008; Feng et al., 2010; Li et al., 2018; Bai et al., 2020). However, the active restoration grasslands degraded again and have led to a continuous undermine soil texture, plant productivity and plant eco-physiological properties (Shang et al., 2016; Li et al., 2018; Gao et al., 2019; Shen et al., 2019). Therefore, understanding the difference effect of land degradation on the relationship between plant biomass and eco-physiological properties alongside degradation is important for guiding the rebuilt of degraded grasslands.
Plant photosynthetic capacity is very sensitive to varying environmental conditions and can reflect ecological strategies of plants to its survival habitat (Skelton et al., 2017; Wang et al., 2018; Trueba et al., 2019; Shen et al., 2021). Nitrogen (N) and phosphorus (P) are important component of photosynthesis pigments and photosynthetic enzyme that may be affected by grassland degradation, and thus to impact on plant photosynthetic capacity and biomass accumulation (Li et al., 2018; Guo et al., 2019; Shen et al., 2021). It has been suggested that plant photosynthetic capacity, plant N and P concentration could be used to identify the level of grassland degradation (Angassa, 2014; Giangiacomo et al., 2014). However, litter is known on how plant eco-physiological properties (including plant photosynthetic capacity, plant N and P concentration) can act as an indicator of degradation level, and how the differential relationship between plant biomass and the eco-physiological properties of native and active restoration grasslands of alpine grasslands.
Many studies have reported that grasses in native grasslands typically have higher belowground biomass and higher foliar N concentrations compared with active restoration grasslands (Shang et al., 2008, Li et al., 2014, 2018), this leading to plants with a competitive advantage in native grasslands. However, other research has also demonstrated that higher soil nutrient availability or higher degradation level could eliminate this advantage and negatively impact on photosynthetic activity, especially in N-richer ecosystems or in heavily degraded grasslands (Xu et al., 2015, 2018; Ye et al., 2018; Shen et al., 2021). Based on the above background, we hypothesized that: 1) plant eco-physiological properties of grasses would show differential responses between native grasslands and active restoration grasslands, and 2) aboveground biomass of grasses in active restoration grasslands shows stronger association with its eco-physiological properties than native grasslands alongside degradation, and 3) the different declining mechanisms of aboveground biomass exist between native grasslands and active restoration grasslands.
Materials and methods
Study site
The field experiment was carried out in an alpine meadow located the Dawu village, Maqu county of Qinghai Province, China. The geographic site is 100º 12´E, 34º 28´N and 4200 m ASL. The mean annual precipitation and mean annual temperature is 510 mm and -0.6℃, respectively. The soil is clay soil and the vegetation is alpine meadow.
We selected a series of degraded grasslands (non-degraded (ND), slightly-degraded (SLD), moderately degraded (MD), heavily degraded (HD) and extremely degraded (ED) grasslands) of native grasslands (Guo et al., 2019; Peng et al., 2020) and a series of active restoration grasslands (active restored grasslands with different growth time, five years (5Ys), nine years (9Ys), eleven years (11Ys), fourteen years (14Ys) and seventeen years (17Ys)). The 17Ys, 14Ys, 11Ys, 9Ys and 5Ys grasslands site were the severely degraded grasslands and were replanted with E. mutans in 2000, 2003, 2006, 2008 and 2012, respectively. Each site area were more than 10000 m2.
Plant photosynthetic capacity measurements
Plant photosynthetic capacity (net photosynthetic rate (Pn), intercellular CO2concentration (Ci ), transpiration rate (Tr ) and stomatal conductance (gs )) of all key plants of grasses were measured using a portable photosynthesis system (Li-6400, Lincoln, NE, USA) between 10:00 am-11:00 am in July, 2018. Six leaves of each grass species at each site were chosen for replication measurement. Instantaneous water use efficiency (WUEi ) =Pn/Tr .
Field sampling and laboratory analysis
In early August of 2018, a field survey was conducted and six sampling quadrats (0.5 m×0.5 m) were randomly selected at each site. In each quadrat, the coverage and species richness of community and grasses were recorded and plant aboveground biomass (AGB) of community and grasses were clipped and oven-dried at 70℃ to constant weight to get the AGB. Meanwhile, we collected the aboveground parts of grasses in each site for nutrient analysis. Plant nitrogen (N) and phosphorus (P) concentration were measured by a flow injection auto-analyzer (FIAstar 5000 Analyzer) (Bao, 2000; Guo et al., 2019).
Statistical analysis
One-way ANOVA was used to test the differences in plants eco-physiological properties and AGB in native grasslands and active restoration grasslands, respectively. A multi-comparisons of a least significant difference (LSD) test in analysis of ANOVA was used to test the effect of grasslands degradation of native grasslands and the effect of replanting time in active restoration grasslands of plant AGB, photosynthetic capacity and nutrient concentration. Principal component analysis (PCA) was used to assess the relationships between plant AGB and eco-physiological properties of grasses and two structural equation model (SEM) were used to identify the direct and indirect pathways that determined grasses’ AGB in the two types of grasslands.
Results