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Article

Effects of Planting Density of Poaceae Species on Slope Community Characteristics and Artificial Soil Nutrients in High-Altitude Areas

by
Dayuan Sun
1,
Junzhuo Li
2 and
Yuanbo Gong
1,*
1
College of Forestry, Sichuan Agricultural University, Chengdu 611130, China
2
Sichuan Highway Planning, Survey, Design and Research Institute Ltd., Chengdu 610041, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(10), 8321; https://doi.org/10.3390/su15108321
Submission received: 22 February 2023 / Revised: 3 May 2023 / Accepted: 9 May 2023 / Published: 19 May 2023
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Abstract

:
Ecological restoration of slopes in high-altitude areas is usually difficult. Gramineae species are widely used in slope vegetation restoration due to their strong adaptability and rapid growth. In the process of ecological slope protection, increasing the seeding rate of gramineous species usually improves the success rate of slope vegetation restoration, but the long-term effect is not obvious. Therefore, choosing an appropriate planting density of grass species is beneficial to the sustainable restoration of slopes in high-altitude areas. This study evaluated the effects of different planting densities of Poaceae species on community characteristics and artificial soil nutrients on high-altitude slopes. The slope ecological protection engineering experiment was carried out in Jiuzhaigou County, Sichuan Province. Commercial seed mixtures of five grasses and legumes were sown at three different planting densities of Poaceae species (10, 5, and 1 g/m2). Plant community species composition, community diversity index, and soil-available nutrients were determined annually. The results showed that there were differences in the species composition of the slope plant community under different planting densities. There was a significant negative logarithmic correlation between the community diversity indices and the planting density of grass species, and it changed with the recovery time. There were significant differences in hydrolyzed nitrogen, available phosphorus and available potassium in artificial soil, and they decreased with a logarithmic function of the recovery time. There was a positive correlation between the community diversity indices and the soil nutrient content. Overall, our study shows that low planting densities of Poaceae species are beneficial to the long-term stability of ecological restoration when ecological slope protection works are performed on slopes in high-altitude areas.

1. Introduction

The rapid development of China’s highways, railways, hydropower and other engineering projects has led to the formation of a large number of exposed slopes, which has not only destroyed the original natural landscape but also promoted a series of environmental problems, such as vegetation damage, soil erosion, and landslides [1,2]. In particular, in the high-altitude areas in southwestern China, due to the particularity of the topography, climate and vegetation, the ecological environment is very fragile [3,4]. Once the surface is damaged by all kinds of projects, it is extremely difficult to restore, and slope instability is more likely to cause secondary disasters such as soil erosion, landslides, and mudslides, which are serious threats to the safety of the local population and constrain local economic development [5,6,7]. Therefore, it is necessary to rebuild and restore vegetation on exposed slopes after construction, improve slope stability, prevent soil erosion, and protect biodiversity.
Most high-altitude areas have arid and cold climates, with diverse and complex ecosystems [8]. The exposed slopes generated by infrastructure construction are often steep, and the topsoils upon these kinds of slopes are scarce, making it difficult to maintain vegetation growth [9,10]. Therefore, it is extremely important to develop appropriate slope vegetation restoration techniques to improve the stability of bare slopes in high-altitude areas. In recent years, an increasing amount of research has focused on the use of ecological protection technology to improve the exposed slopes caused by the infrastructural projects [11,12,13,14]. Ecological protection of the exposed slopes refers to slope stabilization measures that use plants alone or in combination with earth and rock works and nonliving plant materials. The use of ecological protection technology to prevent and control exposed slopes can improve the soil layer of the slope, restore vegetation coverage, stabilize the slope, and prevent soil erosion [15,16,17]. The construction of a long-term stable slope community is necessary for the success of ecological protection technology.
There is an accumulating body of literature showing that vegetation types significantly affect the structure and function of ecosystems after slope restoration [18,19,20,21]. Although a single vegetation species can quickly stabilize slopes, there are challenges associated with the continuous protection of slopes, such as vegetation degradation and a limited ability to adapt to environmental changes [22]. Compared with monoculture seeding, mixed plant seeding is more conducive to the success of slope ecological restoration [23,24]. Mixed planting of grass and legume plants can quickly cover exposed slopes in the early stage of vegetation establishment, increase soil infiltration and soil fertility, strengthen soil erosion resistance, and lead to the formation of a long-term stable plant community structure, which is conducive to the long-term stability of slopes [25]. Generally, grasses are used as pioneer species [26,27]. To increase the mulching effect in the early stage of colonization, a higher planting density is recommended on slopes under harsh conditions [28]. However, some studies have shown that the effects are not durable and have little impact on long-term vegetation restoration [29,30]. Additionally, some researchers found that grass species increased vegetation cover but hindered the establishment of native species and reduced species diversity [31,32]. Although the effects of grass planting density on ecological restoration are known, most of the studies on grassland ecological restoration in alpine areas lack the effect of grass planting density on the ecological restoration process under slope ecological restoration conditions [33,34].
To explore the effects of grass planting density on the ecological restoration process under exposed slopes in high-altitude areas, we compared changes and differences in the community characteristics and artificial soil nutrients after planting Poaceae species (Festuca elata Keng ex E. B. Alexeev) at different densities on typical exposed slopes damaged by infrastructure engineering in the western Sichuan Plateau. We started the field experiments in 2019 and made the following assumptions during the initial restoration phase (the first three years): (i) in the process of slope ecological restoration in high-altitude areas, the lower planting density of the Poaceae species is more conducive to the establishment of slope vegetation; (ii) with increasing recovery time, soil nutrients gradually improve, and the effect of low sowing density improves; and (iii) we demonstrate that the soil-available nutrients are closely related to the development of plant communities and their succession. The results allow us to provide useful suggestions for the ecological restoration of slopes in high-altitude areas.

2. Materials and Methods

2.1. Study Site

The study site is located in Jiuzhaigou County, southwestern Sichuan Province (32°53′~33°20′ N, 103°27′~104°26′ E; Figure 1a), at the edge of the transition from the northeastern Qinghai-Tibet Plateau to the Sichuan Basin. The study area has a plateau cold temperate zone-subarctic zone monsoon climate, with an average annual temperature of 8.05 °C. The average temperature in January, the coldest month, is approximately −2.25 °C, and the average temperature in July, the hottest month, is approximately 15.85 °C. The annual rainfall is 789.97 mm, with distinct and wet seasons. The rainfall is concentrated from April to October, with the rainfall in this period accounting for 89.61% of the total annual rainfall. Due to the construction of the Chuanzhusi-Jiuzhaigou Highway, the soil and vegetation along the route were severely damaged, resulting in a large number of exposed slopes. In this study, three exposed slopes with soil and stone adjacent to each other on the same road were selected as test slopes (Figure 1b). If effective slope protection measures are not taken in time, serious soil erosion will occur in the rainy season, threatening the safety of local transportation [35]. Therefore, artificial ecological restoration is urgently needed to enhance the stability of slopes.

2.2. Experimental Design

Because the topsoil of the three exposed slopes was thin and unstable, which was not conducive to the growth of plants, a method of covering the soil with foreign soil was used for vegetation restoration [36]. Foreign soil covering technology helps to create appropriate matrix conditions for vegetation growth and slope stability, and the foreign soil thickness is 12 cm. The physical and chemical properties of the foreign soil are shown in Table 1.
Because the soil seed bank of the slope has been damaged, the vegetation must be restored by artificial planting. The selection of plant species is critical for vegetation restoration [37]. In this study, five kinds of commercial grass legume seed mixes commonly used in regional vegetation were used, in which the grass F. elata Keng ex E. B. Alexeev was used as the pioneer species, with three different planting density gradient treatments, namely, 10 g/m2 (HD), 5 g/m2 (MD) and 1 g/m2 (LD) (the HD and MD treatments are commonly used in the design of regional vegetation restoration projects), applied to the three test slopes. The other plant seeds included Melilotus officinalis (L.) Pall (2 g/m2), Lespedeza bicolor Turcz. (5 g/m2), Caragana microphylla Lam. (10 g/m2) and Sophora davidii (Franch.) Skeels (8 g/m2). The planting densities on the three experimental slopes were the same.

2.3. Vegetation and Soil Survey Methods

The slope vegetation restoration project was carried out in early May 2019, and management was not carried out in the later period after sowing. Vegetation and soil surveys were carried out in early June 2019 (one month after sowing), early May 2020 (one year), early May 2021 (two years), and early May 2022 (three years). Three 4 × 4 m plant quadrats were set up in each plot, and the plant species and number of plants were counted at each site. Each plant species was photographed and identified based on Plant Plus of China (www.iplant.cn (accessed on 6 June 2022)). At the same time, soil samples (0–15 cm) were collected using the five-point method, placed into sealed plastic bags, labeled, and brought back to the laboratory for further processing. Three days before sampling, there was no rain at the study site, and during sampling, care was taken to remove weeds and larger gravel. The soil samples brought back were air-dried, ground, and passed through a 20-mesh (0.9 mm) screen. The hydrolyzed nitrogen content was measured by the alkaline solution diffusion method; the available phosphorus content was measured by spectrophotometry using sodium bicarbonate as the extractant; and the available potassium content was measured by flame photometry using ammonium acetate as the extractant [38]. Three replicates were used for all analytical assays.

2.4. Data Processing

The formulas used for calculating the community diversity indices are as follows:
Shannon-Wiener   index   ( H ) :   H = − ∑ i = 1 S P i l n P i
Simpson   index   ( D ) :   D = 1 − ∑ i = 1 S P i 2
Evenness   index   ( E ) :   E = H / l n S  
where S is the total number of species in the quadrat; Pi is the percentage of the number of individuals of the ith species in the quadrat among the total number of individuals of all species in the quadrat [39].
The differences in the community diversity indices and soil nutrients were analyzed by SPSS Statistics 19.0 using variance analysis. When ANOVA showed statistically significant differences, Duncan’s multiple-range test was used for separation of means (p < 0.05). Spearman correlation coefficients were used to evaluate correlations between the community diversity indices and soil nutrients.

3. Results

3.1. Plant Community Composition

All plant species in each slope in the investigation are listed in Table A1. According to the division of plant functional groups, the plant species in the plot community were divided into three functional groups: graminoids, legumes and other species [40]. The plant community species composition of each plot under different planting densities was dominated by graminoids and legumes, both of which accounted for more than 85% of the total, and other species showed an upward trend (Figure 2). With the increase in the recovery time of the slope community, the proportion of grasses under the HD treatment gradually increased, accounting for approximately 99.76% after three years, and legumes only accounted for 0.24%; the proportions of grasses and legumes under the MD and LD treatments first increased and then gradually stabilized. At the same time, other species, such as Artemisia gmelinii Web. ex Stechm. and Leontopodium leontopodioides (Willd.) Beauv., appeared. Among them, the proportion of other species was the highest after three years of LD treatment, reaching 11.44%.

3.2. Community Diversity Indices

The regression analysis showed that the Shannon-Wiener index, Simpson index and evenness index showed a significant logarithmic decreasing trend with the increasing planting density of grass species (p < 0.001, Figure 3a–c). With increasing recovery time, the community diversity indices first decreased and then increased under the LD treatment, and the indices were the lowest after one year of recovery; the diversity indices first decreased and then stabilized under the MD treatment and HD treatment (Figure 3d–f).

3.3. Soil-Available Nutrients

Planting density had a significant effect on the soil-available nutrient content (Figure 4). In the initial stage of recovery, the soil-available nutrient content in the HD and MD treatment plots decreased significantly relative to the LD treatment; with increasing recovery time, the soil-available nitrogen content in the MD and LD treatment plots increased significantly compared with that in the HD treatment; however, the soil-available phosphorus increased significantly with decreasing planting density. In addition, after two years of restoration, the soil-available potassium increased significantly with decreasing planting density. Regression analysis showed that the available nitrogen, available phosphorus, and available potassium in the soil decreased with a logarithmic function of recovery time (p < 0.001).

3.4. Correlation Analysis between the Community Diversity Indices and Soil Nutrients

Correlation analysis showed that there was a significant positive correlation between the community diversity indices and soil-available nutrient content. The correlations between available phosphorus content and the community diversity indices were the highest, while the correlations between the evenness index and soil-available nutrients were the highest (Table 2).

4. Discussion

4.1. Vegetation Community Characteristics

The species composition and renewal of plant communities can reflect the structure and nature of the restored community and indicate the potential succession trend of the community [41]. We evaluated the species composition of slope plant communities under different planting densities of Poaceae species and observed that graminoids and legumes dominated the slope plant communities in the sample plots, accounting for over 85% of the total proportion. This indicates that graminoids and legumes play an important role in the ecological restoration of slope communities [42]. In addition to the high planting density, the proportion of graminoids and legumes increased at first and then gradually stabilized with the succession process of the slope plant community, and other species appeared. This is because graminoids and legumes can widely adapt to harsh soil conditions [43], and rapidly cover while increasing the soil nitrogen content, improving the soil environment and providing nutrients for other species [44]. However, the high planting density of the Poaceae species led to a rapid increase in the abundance of gramineous plants, and non-gramineous plants were inhibited by competition, leading gradually to the formation of a single-species dominant community, and the potential ecological risk of community degradation was high [45,46].
Community diversity is an important feature of an ecosystem and is closely related to the ability of an ecosystem to resist adversity and disturbance [47]. The increase in diversity increases the stability of the system and reflects the ecological significance of restoring the community. Research on community diversity has indicated that the diversity indices decreased as the planting density of Poaceae species increased. The dominance of graminoids in the seed mixture has increased the dominant graminoids abundance, which will interfere with the establishment and growth of legumes and further affect the invasion of other species, leading to a negative impact on community diversity and stability [48]. In addition, the community diversity indices showed a trend of first decreasing and then increasing under low planting density, which may be related to the replacement of the dominant position of graminoids during the succession process, being partially replaced by legumes, which has a significant impact on soil nutrient cycling and provides space and nutrient competitive advantages for invasion by other species; the community diversity and evenness increase with natural succession [49,50]. Therefore, appropriately reducing the planting density of gramineous plants is conducive to the formation of more diverse biological communities and ecosystems, thereby regulating the functions of the ecosystem, preventing soil erosion, and ultimately achieving the effect of slope protection. When comparing restoration success in terms of community species composition and diversity indices, sowing a low density of Poaceae species was very clearly more successful than using a high density, thus confirming our first hypothesis.

4.2. Soil-Available Nutrients

Soil nutrients are an important factor affecting the ecosystem structure and plant growth of slope restoration communities, and changes in the state of available nutrients are indicators of soil health maintenance [51]. A reduction in the soil-available nutrients can limit the growth of dominant species, which in turn affects community development and stability [52]. Our study showed that there were differences in soil-available nutrients under the three planting densities, especially under the natural succession process. The soil-available nutrient concentrations at the high sowing density were the lowest, and were significantly lower than those at the medium and low planting densities. The increase in community diversity at low planting density leads to the full utilization of limited nutrients in the soil, reduces the leaching loss of available nutrients in the ecosystem, and improves soil biodiversity, helping to maintain ecosystem stability [53].
Although other studies have shown that the soil available nutrients increased continually with recovery time, but decreased logarithmically in this study, the unique environmental conditions at high altitude may weaken the input of soil nutrients and reduce the effective nutrients of artificial soils at high altitude [54,55]. These results do not fully conform to our second hypothesis, and indicate that reducing the planting density of pioneer species is beneficial for stabilizing artificial soil nutrients, whereas the long-term availability of nutrients need to be improved as a crucial limiting factor for slope ecological restoration.
Community composition and diversity can change in response to environmental fluctuations, especially with a variation in soil physiochemical properties, whereas the effects of soil nutrients on diversity or richness may increase with time [56]. The results showed that there were significantly positive correlations between community diversity indices and soil-available nutrients, which may not be consistent with other relevant research [57]. During the early restoration period, plants rapidly accumulate biomass to establish vegetation cover by absorbing large amounts of nutrients, which leads to a decrease in the soil-available nutrients [58]. With the progression of the restoration time, the change in plant community diversity led to maintenance of the soil-nutrient balance of slopes. Additionally, soil-nutrient availability has a positive feedback effect on plant growth, and then affects community diversity and stability [59,60].

5. Conclusions

It is important to determine the changes in the response of community diversity and artificial soil nutrients at different planting densities of the Poaceae species. The results of this study showed that the plant community species composition and diversity on slopes were significantly different under different planting densities: the lower the planting density was, the richer was the community species composition and the higher the diversity indices were, which increased with natural succession. We demonstrated that the contents of soil hydrolyzed nitrogen, available phosphorus and available potassium under a low planting density of Poaceae were higher than those under a high planting density and changed with the change in vegetation restoration time. In addition, through correlation analysis we established a significant positive correlation between the community diversity indices and soil-available nutrients. Therefore, in the process of slope vegetation reconstruction and ecological restoration, rapid coverage with grass should not be blindly pursued, and an appropriate sowing rate of gramineous plants should be selected to improve community diversity and soil quality, which is beneficial to ecosystem restoration and persistence. Although this study clarified the plant community characteristics and soil properties of different Poaceae planting densities on typical slopes of the Chuanzhusi–Jiuzhaigou Highway, the long-term effects of the restoration process should be observed and studied to obtain definitive conclusions.

Author Contributions

Conceptualization: Y.G.; methodology, D.S. and Y.G.; formal analysis, D.S. and J.L.; investigation, D.S. and J.L.; writing—original draft preparation, D.S.; writing—review and editing, Y.G., J.L. and D.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Transportation Science and Technology Project of Sichuan Province (2022-A-5).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Table
Table A1. List of plant species encountered on each slope during the experimental period.
Table A1. List of plant species encountered on each slope during the experimental period.
SlopeGraminoidsLegumesOthers
Slope 1Festuca elata Keng ex E. B. Alexeev, Elymus dahuricus Turcz.Melilotus officinalis (L.) Pall., Lespedeza bicolor Turcz., Caragana microphylla Lam., Sophora davidii (Franch.) Skeels
Slope 2F. elata Keng ex E. B. Alexeev, E. dahuricus Turcz., Deyeuxia scabrescens (Griseb.) Munro ex DuthieM. officinalis (L.) Pall., L. bicolor Turcz., C. microphylla Lam., S. davidii (Franch.) SkeelsArtemisia gmelinii Web. ex Stechm.
Slope 3F. elata Keng ex E. B. Alexeev, E. dahuricus Turcz., D. scabrescens (Griseb.) Munro ex Duthie, Berberis wilsoniae HemsleyM. officinalis (L.) Pall., L. bicolor Turcz., C. microphylla Lam., S. davidii (Franch.) SkeelsLeontopodium leontopodioides (Willd.) Beauv., A. gmelinii Web. ex Stechm., Corydalis edulis Maxim., Ajania potaninii (Krasch.) Poljak.

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Figure 1. Geographical location of the study area. Note:(a-1) shows Jiuzhaigou Country loction in Sichuan Province; (a-2) shows the study areas loction in Jiuzhaigou Contry; (b-1) Slope 1, area 1745 m2, slope 42°, east–south 30°; (b-2) Slope 2, area 1356 m2, slope 43°, east–south 32°; (b-3) Slope 3, area 1478 m2, slope 41°, and east–south 31°.
Figure 1. Geographical location of the study area. Note:(a-1) shows Jiuzhaigou Country loction in Sichuan Province; (a-2) shows the study areas loction in Jiuzhaigou Contry; (b-1) Slope 1, area 1745 m2, slope 42°, east–south 30°; (b-2) Slope 2, area 1356 m2, slope 43°, east–south 32°; (b-3) Slope 3, area 1478 m2, slope 41°, and east–south 31°.
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Figure 2. Distribution of plant functional groups in the plot community under the three planting densities of Poaceae species. Note: The years 2019–2022 are years 1, 2 and 3 for the ecological restoration of the sample plot; HD represents that the planting density of F. elata Keng ex E. B. Alexeev is 10 g/m2; MD represents that the planting density of F. elata Keng ex E. B. Alexeev is 5 g/m2; LD represents that the planting density of F. elata Keng ex E. B. Alexeev is 1 g/m2.
Figure 2. Distribution of plant functional groups in the plot community under the three planting densities of Poaceae species. Note: The years 2019–2022 are years 1, 2 and 3 for the ecological restoration of the sample plot; HD represents that the planting density of F. elata Keng ex E. B. Alexeev is 10 g/m2; MD represents that the planting density of F. elata Keng ex E. B. Alexeev is 5 g/m2; LD represents that the planting density of F. elata Keng ex E. B. Alexeev is 1 g/m2.
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Figure 3. Changes in the community diversity indices of the plots under different grass planting densities. Note: (a) represents the relationship between the Shannon-Wiener index and planting density; (b) represents the relationship between the Simpson index and planting density; (c) represents the relationship between the Evenness index and planting density; (d) represents the Shannon-Wiener index with recovery change in years; (e) represents the change in the Simpson index with recovery years; (f) represents the change in the Evenness index with recovery years.
Figure 3. Changes in the community diversity indices of the plots under different grass planting densities. Note: (a) represents the relationship between the Shannon-Wiener index and planting density; (b) represents the relationship between the Simpson index and planting density; (c) represents the relationship between the Evenness index and planting density; (d) represents the Shannon-Wiener index with recovery change in years; (e) represents the change in the Simpson index with recovery years; (f) represents the change in the Evenness index with recovery years.
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Figure 4. Variation in soil nutrients in the plot under different planting densities of Poaceae species. Note: (a) represents available nitrogen; (b) represents available phosphorus; (c) represents available potassium; (d) represents the change in soil-available nitrogen, available phosphorus and available potassium with recovery time. Different lowercase letters in the same year with different planting densities indicate significant differences (p < 0.05).
Figure 4. Variation in soil nutrients in the plot under different planting densities of Poaceae species. Note: (a) represents available nitrogen; (b) represents available phosphorus; (c) represents available potassium; (d) represents the change in soil-available nitrogen, available phosphorus and available potassium with recovery time. Different lowercase letters in the same year with different planting densities indicate significant differences (p < 0.05).
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Table
Table 1. Physical and chemical properties of the foreign soil.
Table 1. Physical and chemical properties of the foreign soil.
Bulk Density
(g/cm3)		Organic Matter
(g/kg)		Extractable N
(mg/kg)		Extractable P
(mg/kg)		Extractable K
(mg/kg)
1.3323.98156.3371.22124.41
Table
Table 2. Correlation analysis of the community diversity indices and soil nutrients.
Table 2. Correlation analysis of the community diversity indices and soil nutrients.
VariablesShaSimEveANAPAK
Sha1
Sim0.991 **1
Eve0.950 **0.969 **1
AN0.374 *0.410 *0.569 **1
AP0.486 **0.530 **0.662 **0.960 **1
AK0.365 *0.401 *0.557 **0.978 **0.974 **1
** Correlation is significant at the 0.01 level (2-tailed). * Correlation is significant at the 0.05 level (2-tailed). The highest correlation coefficients with significant p values are in bold. Sha, Shannon-Wiener index; Sim, Simpson index; Eve, Evenness index; AN, soil-available nitrogen; AP, soil-available phosphorus; AK, soil-available potassium.
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Sun, D.; Li, J.; Gong, Y. Effects of Planting Density of Poaceae Species on Slope Community Characteristics and Artificial Soil Nutrients in High-Altitude Areas. Sustainability 2023, 15, 8321. https://doi.org/10.3390/su15108321

AMA Style

Sun D, Li J, Gong Y. Effects of Planting Density of Poaceae Species on Slope Community Characteristics and Artificial Soil Nutrients in High-Altitude Areas. Sustainability. 2023; 15(10):8321. https://doi.org/10.3390/su15108321

Chicago/Turabian Style

Sun, Dayuan, Junzhuo Li, and Yuanbo Gong. 2023. "Effects of Planting Density of Poaceae Species on Slope Community Characteristics and Artificial Soil Nutrients in High-Altitude Areas" Sustainability 15, no. 10: 8321. https://doi.org/10.3390/su15108321

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