Biodiversity and Soil Reinforcement Effect of Vegetation Buffer Zones: A Case Study of the Tongnan Section of the Fujiang River Basin

Abstract

The riparian vegetation buffer zone is an important component of riverbank ecosystems, playing a crucial role in soil consolidation and slope protection. In this study, the riparian vegetation buffer zones in the Tongnan section of the Fujiang River Basin were selected as the research object. Surveys and experiments were conducted to assess the species composition and the soil and water conservation effectiveness of the riparian vegetation buffer zone. There are a total of 35 species, mainly comprising angiosperms and ferns. The dominant species include Cynodon dactylon, Setaria viridis, Phragmites australis, Erigeron canadensis, and Melilotus officinalis. The Patrick richness index (R) and Shannon–Wiener diversity index (H) are more significantly influenced by the types of land use in the surrounding area, whereas the impact on the Simpson diversity index (D) and Pielou uniformity index (E) is comparatively less pronounced. When the root diameter is less than 0.2 mm, the tensile strength of Cynodon dactylon roots is the highest. For root diameters larger than 0.2 mm, Melilotus officinalis roots exhibit the highest tensile strength. The presence of plant root systems significantly reduces erosion, delaying the time to reach maximum erosion depth by 1–4 min, decreasing erosion depth by 9–38 mm, and reducing the total amount of erosion by 20.17–58.90%. The anti-scouribility effect of Cynodon dactylon is significantly better than that of Setaria viridis. The root system notably enhances soil shear strength, delaying the shear peak by 0.26–4.8 cm, increasing the shear peak by 4.76–11.37 kPa, and raising energy consumption by 23.76–46.11%. Phragmites australis has the best resistance to shear, followed by Erigeron canadensis, with Melilotus officinalis being the least resistant. Therefore, to balance the anti-scouribility effect and shear resistance of plant roots, it is recommended to use a combination of Cynodon dactylon and Phragmites australis for shallow-rooted and deep-rooted planting. This approach enhances the water and soil conservation capacity of riverbanks.

1. Introduction

The vegetation buffer zone is a transitional area made up of vegetation located between water bodies and land. This zone forms a dynamic and interconnected ecosystem that includes plants, animals, and microorganisms, making it one of the most complex ecosystems on Earth [1,2]. The vegetation buffer zone acts as a filter, intercepting and filtering out sediments, organic matter, pesticides, and agricultural pollutants that might enter rivers or reservoirs. This filtration helps achieve various goals, including soil and water conservation, flood control and embankment stabilization, water source conservation, flood storage and drought prevention, sediment accumulation and land formation, as well as maintaining biodiversity and ecosystem integrity [3,4,5,6].

The utilizing of vegetation buffer zones can be traced back to Europe in the 15th and 16th centuries. After continuous development, standardized designs and application standards for buffer zones appeared in the United States in the 1930s. It was not until the late 1970s that the definition of vegetation buffer zones was first proposed, and people gradually started to understand, manage, and protect them. Subsequently, buffer zones were applied in the prevention and control of agricultural non-point source pollution in the United States, becoming an effective ecological engineering measure to intercept surface pollutants [7]. Canada incorporated vegetation buffer zones into relevant environmental planning and soil and water management measures, while many European countries conducted related research and promoted the use of buffer zones as a water purification measure [8]. In the 1990s, countries such as Denmark, Costa Rica, the United States, and Brazil required the establishment of vegetation buffer zones on both sides of rivers through legislation and other means [9,10]. Research on vegetation buffer zones in China started relatively late, and before the 1990s, it was considered as part of wetland research. After introducing research findings from abroad, studies on vegetation buffer zones in China began [11].

Currently, research on vegetation buffer zones primarily focuses on soil and water conservation, pollutant reduction, and biodiversity protection [12,13,14,15]. These buffer zones enhance the soil and water conservation capacity of slopes and have proven to be an effective measure for slope management [16]. The reinforcement provided by plant roots includes both the anchoring effect of vertical roots and the stabilizing effect of horizontal roots [16]. The effectiveness of plant roots in reinforcing soil largely depends on the morphological characteristics of the roots, their tensile strength, and the interaction between the roots and soil [17]. Regarding erosion resistance, Xu et al. [18] indicated that the root-soil composite of Miscanthus sacchariflorus exhibits strong anti-scouribility during both the early growth and vigorous growth stages in the Three Gorges Reservoir area. Chen et al. [19] showed that dominant herbaceous plants along the lower reaches of the Puyang River can enhance soil anti-scouribility, with Phragmites australis and Phragmites australis being particularly effective. In terms of shear resistance, Shen et al. [20] found that the root–soil composite of Kobresia pygmaea and Stipa purpurea in the source area of the Yellow River has the highest cohesion. Hao et al. [21] determined through vegetation investigation and model calculation that Artemisia sieversiana, Alopecurus aequalis, and Leonurus japonicus had the best soil reinforcement effect in the Haizhou mine soil. Previous studies have mainly focused on the erosion or shear resistance of plant roots and have not fully explored the soil and water conservation effects of multiple plant configurations.

The Tongnan section of the Fu River Basin features flat terrain and concentrated rainfall during the rainy season. The soil structure in this area is loose, leading to severe soil erosion in some places. Riparian vegetation buffer zones can effectively mitigate soil erosion. However, there is a lack of research on the soil and water conservation capacity of vegetation along the Fu River. Previous studies have predominantly focused on individual properties such as root tensile strength, soil erosion resistance, and shear strength without fully exploring the comprehensive soil and water conservation effects of multiple plant configurations. This study aims to address this gap by employing plot surveys, anti-scouribility tests, direct shear tests, and other methods to identify the dominant species and their soil stabilization and slope protection effects within the vegetation buffer zones along the riverbanks. The findings of this study are expected to provide a theoretical basis for the selection of species for the protection of soil and water resources along riverbanks. The Tongnan section of the Fujiang River Basin is a typical shallow, hilly river basin, and the research results can support ecological protection and restoration in shallow, hilly river basins in the southwestern region of China.

2. Materials and Methods

2.1. Study Area

The Tongnan section of the Fu River Basin is located in the northwest of Chongqing and belongs to the Jialing River system of the Yangtze River Basin (Figure 1). It traverses the northern part of Tongnan District from northwest to southeast. The study area has a subtropical monsoon climate with abundant water and heat resources. It has a mild winter and hot summer, a short spring and autumn, and is often foggy with little frost. The annual average temperature ranges from 13.8 to 18.8 °C, and the annual average rainfall ranges from 1000 to 1400 mm. The Fu River and its tributaries create a clear grid or branched water system. The terrain consists mainly of gentle hills and river terraces. The soil types are mainly alluvial soils, followed by yellow earths, paddy soil, and purplish soil. The study area belongs to the subtropical evergreen broad-leaved forest zone; the primary tree species are Cupressus funebris and Bambusoideae, with a single tree species and simple structure. The vegetation on the river terraces is dominated by herbaceous plants, with some areas featuring extensive stands of Bambusoideae and Eucalyptus.

2.2. Field Quadrat Survey Sampling and Biodiversity Analysis

Eight different plant communities along the river were selected for a quadrat survey (sample plot information is shown in

Table 1. The Basic Situation of Sample Plots.

No.	Name of Sample Plots	Geographical Position	Elevation (m)	Site Type	Soil Type	Surrounding Land Use
1	Lujiaba	105.941, 30.155	231.27	Soft slope	Loam	Arable
2	Yuxingba	105.899, 30.187	231.60	Soft slope	Loam	Wasteland
3	Zhangjiawan	105.871, 30.190	231.10	Soft slope	Loam	Village
4	Zhongba	105.798, 30.228	238.87	Soft slope	Sandy	Arable
5	Caojiagou	105.774, 30.314	261.85	Soft slope	Loam	Forest
6	Yuxi Town	105.774, 30.324	247.81	Soft slope	Sandy	Village
7	Yinglong Street	105.751, 30.321	250.44	Soft slope	Sandy	Wasteland
8	Hujiawan	105.734, 30.326	254.48	Soft slope	Sandy	Forest

). Five plots were established for each community, with each quadrat measuring 1 m × 1 m (as shown in Figure 2). The plant species and their quantities within each quadrat were counted using visual estimation, and plant samples were collected.

The importance value is used as a comprehensive indicator for each plant species in the plot survey. If the importance values of sample plots exceed 15%, it is considered as a dominant species [22]. Species richness and species diversity reflect the organizational characteristics of the plant community. In this study, Patrick richness index, Simpson diversity index, Pielou uniformity index, and Shannon–Wiener diversity index were used as indicators of community diversity. The formulas are as follows:

Relative height = (Height of a specific species/Sum of heights of all species) × 100%

Relative coverage = (Coverage of a specific species/Sum of coverages of all species) × 100%

Patrick richness index: R_p = S

(1)

where S is the number of species appearing in the quadrat.

$$\mathrm{Simpson} \mathrm{diversity} \mathrm{index}\text{:} D_s=1-{\displaystyle \sum _{i=1}^s{n}_i}(n_i-1)/N(N-1)$$

(2)

where S is the number of species; N is the sum of the individual numbers of all species in the community; n_i is the number of individuals of the i-th species;

$$\mathrm{Shannon}–\mathrm{Wiener} \mathrm{diversity} \mathrm{index}\text{:} H=-{\displaystyle {\sum }_{i=1}^S{P}_i\ln }{P}_i$$

(3)

where C is the constants, generally set to C = 1; P_i is the probability of species i.

$$\mathrm{Pielou} \mathrm{uniformity} \mathrm{index}\text{:} E=H/\ln S$$

(4)

2.3. Determination of Root Mechanics and Morphological Characteristics

We first measured the morphological characteristics of the root using tools such as a ruler, protractor, vernier caliper, and balance. The formula for calculating the root surface area ratio is as follows:

$$RAR={\displaystyle \frac{{\displaystyle \sum \frac{\mathsf{\pi }{D}^2}{4}}}{A}}={\displaystyle \frac{A_r}{A}}$$

(5)

where RAR is the root area ratio, %; D is root diameter, mm; Ar is the total area of the roots, mm²; and A is the reference area mm².

Root tensile strength was measured by an S9M Universal Mechanical Testing Machine [23] (Figure 3). According to previous research [24,25], the formula of root ultimate tensile strength is as follows:

$$T_r={\displaystyle \frac{4F_{max}}{\mathsf{\pi }{D}^2}}=a{D}^{-b}$$

(6)

where Tr is root tensile strength, MPa; F_max is the maximum tensile force, N; a and b are constants.

2.4. Measuring Soil Anti-Scouribility

In order to test the influence of root networks on soil anti-scouribility, 15 underwater jet tests were conducted on plots with different vegetation cover of Cynodon dactylon and Setaria viridis at the riverbank (the test equipment refer to Hanson and Simon [26]).

Prior to each test, the above-ground biomass was removed, and the spraying device was placed on the soil surface with grass stems kept approximately 1 cm above the ground to distinguish different root density penetration areas for the experiment. The tests were conducted under a water head of 1.8 m until no further vertical scouring occurred, usually within 10 to 15 min. Scour depth was measured every minute [27]. After each test, the volume and depth of the scour holes were measured, and the exposed roots in the scour holes were collected for laboratory analysis of root morphology (Figure 4).

2.5. Large Box Shear Test

We chose species with well-developed roots and planted them in a shear box that measures 0.4 m in length, width, and height (as shown in Figure 5). The shear box is divided into upper and lower sections, with a large number of drainage holes uniformly distributed at the bottom of the lower section. During planting, soil was layered to maintain the original root shape in the soil, ensuring the roots were distributed near the shear plane. Different species (Melilotus officinalis, Erigeron canadensis, and Phragmites australis) were used as treatments, with bare soil serving as the control group. After 2 months of plant growth, a custom, large-scale, strain-controlled shear apparatus was used to conduct the shear test. Stress–strain curves of different root–soil composites were obtained using stress sensors and a data acquisition unit. The energy consumed by various root–soil composites and the bare soil during the shearing process was calculated using an energy model.

The energy model was proposed by Ekanayake and Phillips [28] to simplify the complex soil–root interactions during shear failure based on the principle of energy conservation. Assumptions in the energy model are as follows: The stress–strain curve of the bare soil reaches a peak shear stress followed by a rapid decrease, while the stress–strain curve of the root–soil composite shows a slow decrease after reaching the peak shear stress. The energy consumed during the direct shear test at each time point is the product of shear force and shear displacement, summed over all time points. The area difference between the curves representing the root–soil composite and bare soil represents the energy difference generated by the presence of roots, indicating the reinforcing effect of roots on soil.

The expression for the enhanced shear strength of soil due to plant roots is given by the following equation:

$$E(x_p)={\displaystyle {\int }_0^{x_p}F(x)dx}$$

(7)

where F(x) represents the stress–strain curve, and x_P represents the shear displacement at the peak stress.

3. Result

3.1. Species Composition and Biodiversity Analysis of Vegetation Buffer Zones

The field survey results indicated that along the river, there are 2 classes, 3 orders, 16 families, 19 genera, and 35 species of plants, including 33 angiosperms and 2 pteridophytes (

Table 2. Vegetation List (Herbaceous) of the Riparian Buffer Zone along the Fu River.

No.	Phylum	Class	Order	Family	Genus	Species
1	Angiospermae	Pteridophyta	Asterales	Asteraceae	Erigeron	Erigeron canadensis L.
2	Angiospermae	Pteridophyta	Asterales	Asteraceae	Erigeron	Erigeron annuus (L.) Pers.
3	Angiospermae	Pteridophyta	Asterales	Asteraceae	Artemisia	Artemisia selengensis Turcz. ex Besser
4	Angiospermae	Pteridophyta	Asterales	Asteraceae	Artemisia	Artemisia annua L.
5	Angiospermae	Pteridophyta	Asterales	Asteraceae	Artemisia	Artemisia indica Willd.
6	Angiospermae	Pteridophyta	Asterales	Asteraceae	Artemisia	Artemisia argyi H. Lév. & Vaniot
7	Angiospermae	Pteridophyta	Asterales	Asteraceae	Lactuca	Lactuca serriola L.
8	Angiospermae	Pteridophyta	Asterales	Asteraceae	Symphyotrichum	Symphyotrichum subulatum (Michx.) G. L. Nesom
9	Angiospermae	Pteridophyta	Asterales	Asteraceae	Xanthium	Xanthium strumarium L.
10	Angiospermae	Pteridophyta	Asterales	Asteraceae	Carpesium	Carpesium abrotanoides L.
11	Angiospermae	Pteridophyta	Poales	Poaceae	Cynodon	Setaria viridis (L.) P. Beauv.
12	Angiospermae	Pteridophyta	Poales	Poaceae	Cynodon	Cynodon dactylon (L.) Persoon
13	Angiospermae	Pteridophyta	Poales	Poaceae	Phragmites	Phragmites australis (Cav.) Trin. ex Steud
14	Angiospermae	Pteridophyta	Poales	Typhaceae	Typha	Typha orientalis C. Presl
15	Angiospermae	Pteridophyta	Caryophyllales	Amaranthaceae	Chenopodium	Chenopodium album L.
16	Angiospermae	Pteridophyta	Caryophyllales	Amaranthaceae	Dysphania	Dysphania ambrosioides (L.) Mosyakin & Clemants
17	Angiospermae	Pteridophyta	Caryophyllales	Amaranthaceae	Alternanthera	Alternanthera sessilis (L.) R. Br. ex DC.
18	Angiospermae	Pteridophyta	Caryophyllales	Polygonaceae	Rumex	Rumex crispus L.
19	Angiospermae	Pteridophyta	Lamiales	Lamiaceae	Clinopodium	Clinopodium chinense (Benth.) Kuntze
20	Angiospermae	Pteridophyta	Lamiales	Lamiaceae	Clinopodium	Clinopodium polycephalum (Vaniot) C. Y. Wu & S. J. Hsuan ex P. S. Hsu
21	Angiospermae	Pteridophyta	Lamiales	Lamiaceae	Leonurus	Leonurus japonicus Houtt.
22	Angiospermae	Pteridophyta	Fabales	Fabaceae	Melilotus	Melilotus officinalis (L.) Pall.
23	Angiospermae	Pteridophyta	Fabales	Fabaceae	Glycine	Glycine soja Siebold & Zucc.
24	Angiospermae	Pteridophyta	Rosales	Rosaceae	Duchesnea	Duchesnea indica (Andr.) Focke
25	Angiospermae	Pteridophyta	Rosales	Cannabaceae	Humulus	Humulus scandens (Lour.) Merr.
26	Angiospermae	Pteridophyta	Gentianales	Apocynaceae	Cynanchum	Cynanchum auriculatum Royle ex Wight
27	Angiospermae	Pteridophyta	Boraginales	Boraginaceae	Cynoglossum	Cynoglossum furcatum Wall. in Roxb.
28	Angiospermae	Pteridophyta	Boraginales	Brassicaceae	Raphanus	Raphanus raphanistrum L.
29	Angiospermae	Pteridophyta	Contortae	Loganiaceae	Buddleja	Buddleja asiatica Lour.
30	Angiospermae	Pteridophyta	Asparagales	Asparagaceae	Anemarrhena	Anemarrhena asphodeloides Bunge
31	Angiospermae	Pteridophyta	Myrtales	Onagraceae	Oenothera	Oenothera rosea L’Hér. ex Aiton
32	Angiospermae	Pteridophyta	Solanales	Solanaceae	Solanum	Solanum americanum Mill.
33	Angiospermae	Pteridophyta	Acorales	Araceae	Acorus	Acorus calamus L.
34	Pteridophyta	Filicopsida	Polypodiales	Pteridaceae	Pteris	Pteriscretica L. var.nervosa (Thunb.) Ching et S. H. Wu
35	Pteridophyta	Equisetopsida	Equisetales	Equisetaceae	Equisetum	Equisetum arvense L.

). Photos of some plant samples are shown in Figure 6. Among them, angiosperms dominate the composition of riverbank vegetation, including 17 families such as Asteraceae, Poaceae, Poaceae, Amaranthaceae, Polygonaceae, Lamiaceae, Fabaceae, Rosaceae, Cannabaceae, Apocynaceae, Boraginaceae, Brassicaceae, Loganiaceae, Asparagaceae, Onagraceae, Solanaceae, and Araceae, with two Pteridophytas families, including Pteridaceae and Equisetaceae. The dominant species in each community with a relative importance value of 15% or above are considered as dominant species, totaling five species, namely Cynodon dactylon, Setaria viridis, Melilotus officinalis, Erigeron Canadensis, and Phragmites australis (main species and their importance values shown in the sample plots,

Table 3. Main Species and Important Value in Sample Plots.

No.	Sample Plots	Main Species and Important Value
1	Lujiaba	Cynodon dactylon (54.8%), Raphanus raphanistrum (14.7%), Erigeron canadensis (9.9%), Melilotus officinalis (8.7%)
2	Yuxingba	Cynodon dactylon (42.9%), Phragmites australis (16.6%), Duchesnea indica (10.0%), Melilotus officinalis (6.2%)
3	Zhangjiawan	Cynodon dactylon (48.8%), Phragmites australis (15.6), Raphanus raphanistrum (14.2%), Erigeron Canadensis (13.4%)
4	Zhongba	Erigeron Canadensis (25.9%), Cynodon dactylon (25.7%), Melilotus officinalis (17.3%), Setaria viridis (16.1%)
5	Zengjiagou	Cynodon dactylon (51.3%), Erigeron Canadensis (25.1%), Glycine soja (12.5%), Raphanus raphanistrum (3.4%)
6	Yuxi Town	Melilotus officinalis (32.7%), Cynodon dactylon (20.0%), Leonurus japonicas (11.2%), Raphanus raphanistrum (3.8%)
7	Yinglong Street	Cynodon dactylon (38.3%), Erigeron Canadensis (16.2%), Dysphania ambrosioides (14.5%), Artemisia selengensis (9.0%)
8	Hujiawan	Cynodon dactylon (53.5%), Erigeron canadensis (12.3%), Melilotus officinalis (9.6%), Leonurus japonicas (9.3%)

).

The R, D, H, and E of the eight plant communities are shown in Figure 7. The D of plant communities ranges from 0.497 to 0.731, with the highest D value found in Zhongba (0.731) and the lowest in Lujiaba (0.497). The H of communities ranges from 1.062 to 1.652, with the highest H value in Yuxingba (1.652) and the lowest in Zhangjiawan (1.062). The E of communities ranges from 0.485 to 0.708, with the highest E value in Yuxi Town (0.708) and the lowest in Yinglong Street (0.485). The R of communities ranges from 7 to 13, with the highest R value found in Yuxingba (13) and the lowest in Zhangjiawan and Lujiaba (7). The trends in Simpson dominance index, Shannon–Wiener diversity index, and Pielou evenness index tended to be similar.

From the perspective of land-use types surrounding the plots, R and H are more significantly influenced by the types of land use in the surrounding area, whereas the impact on D and E is comparatively less pronounced (Figure 8). For R, arable land is the highest, followed by forests and villages, with wasteland being the lowest. For H, arable land is again the highest, followed by forests and villages, with wasteland being the least abundant. There is not much difference in D and E among different land-use types.

3.2. Mechanical and Morphological Characteristics of Root

Selected five dominant plant species, including Cynodon dactylon, Setaria viridis, Melilotus officinalis, Erigeron canadensis, and Phragmites australis, were analyzed for their root morphological characteristics. Cynodon dactylon and Setaria viridis have shallow root systems, with a distribution depth of 0–10 cm, consisting mainly of horizontal and vertical roots. Conversely, the roots of Melilotus officinalis, Erigeron canadensis, and Phragmites australis grow deeper, with a distribution depth of 0–25 cm, 0–20 cm, and 0–20 cm, respectively. Erigeron canadensis and Phragmites australis both have strong taproots, with roots growing vertically or horizontally, while Melilotus officinalis lacks a clear taproot and has widely extending roots.

The root diameter distribution illustrated in Figure 9. Cynodon dactylon mainly consists of fine roots with a diameter of 0–1 mm; Setaria viridis has a taproot diameter between 1–2 mm and lateral root diameter of 0–1 mm; Melilotus officinalis exhibits a relatively wide root diameter distribution, ranging from 0–3 mm; Erigeron canadensis has a taproot diameter between 2–3 mm and lateral root diameter of 0–2 mm; Phragmites australis has a taproot diameter of 2–3 mm, with lateral roots distributed around the taproot in sections, with diameters ranging from 1–2 mm.

After selecting five dominant plant species, including Cynodon dactylon, Setaria viridis, Melilotus officinalis, Erigeron canadensis, and Phragmites australis, for tensile strength test, the results indicated that the root tensile strength of these five plant species decreases with the increase of root diameter, showing a negative power exponent relationship (

Table 4. Relationship Between Root Diameter and Tensile Strength.

Species	Function	R2	n
Cynodon dactylon	y = 20.38x−0.75	0.78	60
Setaria viridis	y = 44.73x−0.47	0.66	55
Melilotus officinalis	y = 49.61x−0.63	0.71	53
Erigeron canadensis	y = 23.00x−0.59	0.63	54
Phragmites australis	y = 23.18x−0.91	0.74	62

, Figure 10).

Due to the different root diameter ranges of different plants, there is a large difference in tensile strength. The roots diameter of Cynodon dactylon ranges from 0.05 to 1.47 mm, with a tensile strength range of 6.46–165.23 Mpa; the roots diameter of Setaria viridis ranges from 0.16 to 1.2 mm, with a tensile strength range of 32.69–101.64 Mpa; the roots diameter of Melilotus officinalis ranges from 0.22 to 2.95 mm, with a tensile strength range of 7.10–126.45 Mpa; the roots diameter of Erigeron canadensis ranges from 0.15 to 3.08 mm, with a tensile strength range of 9.94–72.25 Mpa; the roots diameter of Phragmites australis ranges from 0.26 to 2.34 mm, with a tensile strength range of 10.15–85.26 Mpa. Overall, when the diameter is less than 0.2 mm, the root tensile strength of Cynodon dactylon is the highest, and when the diameter is greater than 0.2 mm, the root tensile strength of Melilotus officinalis is the highest.

3.3. Hydraulic Effects of Roots on Scour

In this study, dominant shallow-rooted plants including Cynodon dactylon and Setaria viridis were selected for soil anti-scouribility tests, and a blank control (bare soil) was set up. The change of erosion depth over time is shown in Figure 11. In the experiment with bare soil samples, the scour hole showed a symmetrical shape, and the corresponding scour depth over time produced a relatively smooth curve. The scour depth gradually increased over time, and after 8 min, it stabilized at 95 mm. In the experiments with Cynodon dactylon and Setaria viridis, the scour hole had an irregular shape, and the corresponding scour depth over time produced a line graph with many inflection points. The scour depth gradually increased over time, and for the Cynodon dactylon samples, it stabilized after 9–12 min, ranging from 57–78 mm. For the Setaria viridis samples, the scour depth stabilized after 9–11 min, also ranging from 57–78 mm. Compared to the samples without roots, the stabilization time of the scour depth in samples with roots was delayed by 1–4 min, and the scour depth was reduced by 9–38 mm. The presence of plant roots significantly buffered the process of water flow scouring and reduced the scour depth.

The relationship between erosion mount and root biomass parameter is shown in Figure 12. The underground biomass length density of Cynodon dactylon ranges from 0.50 to 0.77 cm/cm³, and the underground biomass volume density ranges from 3.13 × 10⁻² to 8.30 × 10⁻² cm³/cm³. The underground biomass length density of Setaria viridis ranges from 0.23 to 0.32 cm/cm³, and the underground biomass volume density ranges from 1.32 × 10⁻⁴ to 1.66 × 10⁻² cm³/cm³. The length and volume density of underground biomass of Cynodon dactylon are much greater than those of Setaria viridis, with the length density being 2.17–2.41 times higher and the volume density being 5–234.84 times higher. The significant difference in length and volume density is due to the presence of a large number of underground stems in addition to the root system of Cynodon dactylon, and these stems have a larger diameter. To investigate the influence of root length and volume on soil anti-scouribility, the relationship between root length density, root volume density, and erosion amount was analyzed. The results indicated that erosion amount exhibit a nonlinear decreasing trend, with root length density and root volume density increasing.

3.4. Shear Characteristics of Root–Soil Composites

In this study, dominant deep-rooted plants, including Melilotus officinalis, Erigeron canadensis, and Phragmites australis, were selected for large-box direct shear tests, with a blank control (bare soil) set up for comparison. The relationship between shear stress and displacement is shown in Figure 13. During the shear process, the shear stress of the bare soil increased rapidly with shear displacement, reaching a peak value of 35.11 kPa at 2.67 cm, then gradually decreased to a stable value between 28.34 and 29.84 kPa. For the Melilotus officinalis root–soil composite, the shear stress also increased rapidly with shear displacement, but the growth rate slowed after reaching 2.40 cm of displacement. The peak shear stress was reached at 7.47 cm, with a value of 39.87 kPa. For the Erigeron canadensis root–soil composite, the shear stress increased rapidly with the increase of shear displacement, but the growth rate slowed down after reaching a displacement of 0.8 cm. The peak shear stress value was reached at 6.40 cm, measuring 45.55 kPa. Regarding the Phragmites australis root–soil composite, the shear stress also increased rapidly with shear displacement, reaching a peak value of 46.48 kPa at 2.93 cm, and then stabilized between 44.24 and 45.70 kPa. In comparison to bare soil, the presence of roots significantly enhanced the soil’s shear resistance. This delay in shear peak displacement ranged from 0.26 to 4.8 cm, and the shear peak value increased by 4.76 to 11.37 kPa. Among all the deep-rooted plants, Phragmites australis had the most substantial strengthening effect due to their strong taproot, thus playing a vital role in resisting shear failure.

The root parameters and energy consumption are shown in

Table 5. Root Parameters and Energy Consumption.

Species	Energy Consumption (J)	Root Biomass (cm3)	Root Area Ratio (%)
Bare soil	323.51	0	0
Melilotus officinalis	400.36	1.43	6.67 × 10−3
Erigeron canadensis	450.37	2.25	3.91 × 10−3
Phragmites australis	472.66	1.41	4.71 × 10−3

: 323.51 J for the bare soil, 400.36 J for Melilotus officinalis, 450.37 J for Erigeron canadensis, and 472.66 J for Phragmites australis. The presence of plant roots effectively enhances the shear strength of soil but also increases the energy consumption during the shear process by 23.76–46.11%. The root biomass of Melilotus officinalis was 1.43 cm³, with a root area ratio of 6.67 × 10⁻³; the root biomass of Erigeron canadensis was 2.25 cm³, with a root area ratio of 3.91 × 10⁻³; and the root biomass of Phragmites australis was 1.41 cm³, with a root area ratio of 4.71 × 10⁻³. Among them, Phragmites australis consumed the most energy, Erigeron canadensis had the largest root biomass, and Melilotus officinalis had the highest root tensile strength and root area ratio. The energy consumption was positively correlated with the peak shear value, whereas the correlation between root biomass, root area ratio, root tensile strength, and peak shear value was not significant. This indicates that the morphological characteristics of the roots also significantly impact soil shear resistance. The strong taproots and segmented fine root distribution of Phragmites australis enhance the connection with the soil on both sides of the shear plane, achieving a better reinforcement effect.

4. Discussion

Biodiversity directly impacts the stability and sustainability of ecosystems and is fundamental for human survival. It provides both a basic environment and abundant resources [29]. Human activities, such as land use and its changes, are the main factors affecting biodiversity [30,31]. The results of this study show that the H, D, and E of the Lujiaba, Zhangjiawan, and Yinglong Street plots are lower than those of other plots, primarily due to heavy human interference. Zhangjiawan has multiple temporary roads and accumulated construction waste, while Yinglong Street has livestock grazing, leading to a decline in biodiversity. The Lujiaba plot is adjacent to farmland with a relatively narrow vegetation buffer zone, affected by both sample size and human interference. Regarding surrounding land-use types, there are significant differences in R and H under different land-use types, whereas the differences in D and E are not significant.

The study found that the five types of root systems exhibit a negative power law relationship with both diameter and tensile strength. This aligns with the findings of Saifuddin [32] and Fan [33], indicating that under the same root area, the reinforcement effect of fine roots is better than that of coarse roots [34,35]. The variation in root tensile strength with diameter may be related to the composition or structure of the roots. Zhu et al. [36] suggested that the tensile strength characteristics of roots within different diameter ranges are related to the content of cellulose and lignin, while Baets [34] posited that the variation in root tensile strength may be associated with the root bark.

Plant root systems can improve soil texture, increase soil aggregates and organic matter, and enhance soil anti-scouribility through network interconnection, root–soil adhesion, and biochemical interactions [37,38]. The results of this study show that as root volume density and root length density increase, soil erosion decreases. The underground biomass of the Cynodon dactylon is much greater than that of the Setaria viridis and also exhibits better anti-scouribility. There is a significant positive correlation between root length density, root volume density, and soil anti-scouribility, which is consistent with the findings of others [27,39,40,41]. The root system, formed by interweaving and entanglement in the soil during plant growth, affects the mechanical properties of the soil [42], reduces soil splashing and erosion [43,44], and to some extent improves soil anti-scouribility.

Plant roots can significantly impact the mechanical properties of the soil through their density, branching, length, volume, and tilt direction [45,46]. The presence of root systems significantly enhances the soil’s shear strength, with Phragmites australis consuming the most energy during shearing. Although the root system of the Melilotus officinalis has the best tensile characteristics, it has a higher horizontal quantity. The vertical and inclined roots can better connect the soil above and below the shearing surface, and they have a better strengthening effect on the soil’s shear resistance compared to horizontal roots [23]. The root biomass of the Erigeron canadensis is the highest, but its tensile characteristics are the worst. The study results indicate that the reinforcement effect of root systems on soils is a combined result of both tensile characteristics and morphological features. Numerous studies have shown that the influence of root systems on soil shear strength mainly comes from the tensile strength of individual roots and the friction between roots and soil [47,48,49], which is consistent with the results of this study.

5. Conclusions

Research on the biodiversity and soil reinforcement effect of vegetation buffer zones shows that human disturbance is a significant factor affecting biodiversity. The type of surrounding land use has a greater impact on Patrick richness index and Shannon–Wiener diversity index than on Simpson diversity index and Pielou uniformity index. The presence of plant roots can effectively enhance soil anti-scouribility and shear strength. Cynodon dactylon and Setaria viridis can reduce soil erosion depth by 9–38 mm and erosion volume by 20.17–58.90%. Melilotus officinalis, Erigeron canadensis, and Phragmites australis can increase the soil’s shear peak value by 4.76–11.37 kPa and energy consumption by 23.76–46.11%. To achieve both the anti-scouribility and shear resistance effects of plant roots, it is recommended to pair shallow-rooted Cynodon dactylon with deep-rooted Phragmites australis to enhance riverbank soil and water conservation.