Effects of biochar on the root morphological and mechanical properties of Lespedeza bicolor Turcz. on slopes in high-altitude areas

Received: 23-Mar-2026, Manuscript No. mp-26-186737; Accepted: 16-Apr-2026, Pre QC No. mp-26-186737 (PQ); Editor assigned: 25-Mar-2026, Pre QC No. mp-26-186737 (PQ); Reviewed: 08-Apr-2026, QC No. mp-26-186737; Revised: 15-Apr-2026, Manuscript No. mp-26-186737 (R); Published: 23-Apr-2026, DOI: 10.5281/zenodo.19689284

Introduction

In recent years, the continuous expansion of transportation, energy, and other infrastructure projects into high-altitude areas has boosted socioeconomic development, yet has also exerted certain adverse impacts on the ecological environment. High-altitude regions are characterized by infertile soil, low soil fertility, and fragile ecosystems. As a result, disturbances caused by engineering activities tend to induce severe soil erosion and slope exposure, which not only undermine the stability of the ecosystem but also pose potential risks to the safety and durability of infrastructure (Sun, et al. 2023). Accordingly, slope restoration has become an urgent issue to be addressed.

Vegetation restoration is an important measure for slope ecological restoration. The presence of roots not only increases soil particle friction and cohesion (Dupuy, et al. 2005), thereby enhancing the anchoring effect of plant roots, but also mitigates soil erosion induced by water flow (De Baets and Poesen 2010). Consequently, these root-induced effects improve the shear resistance of slopes and minimize deformation caused by slope instability (Eab, et al. 2015). However, the full exertion of these mechanical effects by roots largely depends on the physical structure and fertility of the slope soil. The particularity of high-altitude regions severely restricts the effect of vegetation restoration (Wang, et al. 2024). High-altitude areas in China exhibit distinct characteristics including drastic fluctuations in hydrothermal conditions, inferior soil quality, and slow nutrient cycling. These factors greatly limit vegetation growth and can hardly meet the demands of slope vegetation restoration (Barberis, et al. 2023). Therefore, the development of efficient restoration materials capable of improving soil properties in high-altitude areas and promoting plant root growth has become a breakthrough to enhance slope remediation.

Biochar is a cost-effective soil amendment that has been widely used for the management and restoration of degraded soils (El-Naggar, et al. 2018). Owing to its unique porous structure and physicochemical properties, biochar not only enhances soil nutrient retention and water-holding capacity but also influences plant root morphology and mechanical characteristics (Coomes and Miltner 2017, Xiang et al. 2017). Numerous studies have so far focused on the application of biochar in soils and low-altitude agricultural systems (Oram, et al. 2014, Isimikalu, et al. 2023, Murtaza, et al. 2023), and the results have confirmed its significant effects on improving crop yield and ameliorating degraded soils (Major, et al. 2010, Sadaf, et al. 2017, Murtaza, et al. 2021, Bezzalla, et al. 2026). In recent years, biochar has been widely used in slope ecological restoration due to its significant advantages in improving soil structure and enhancing soil nutrient and water retention capacities, which provides a new strategy for strengthening the soil-fixing ability of slope vegetation (Sharma, 2024). Nevertheless, research on the mechanical properties of plant roots under engineering disturbance in high-altitude regions remains relatively limited. Particularly in ecologically fragile alpine areas, there is still a lack of empirical studies to evaluate the regulatory mechanism of biochar on the mechanical properties of plant roots and its relationship with the synergistic effects of soil water and fertilizer. Therefore, exploring the effects of biochar on the mechanical properties of plant roots can not only provide a theoretical basis for the synergistic regulation of water and fertilizer in high-altitude regions, but also be of great practical significance for slope vegetation restoration.

The Yalong River Basin belongs to the plateau alpine gorge region (Li, et al. 2025). Its fragile ecosystem and infertile soil make vegetation restoration extremely difficult. Lespedeza bicolor Turcz. is an adaptable leguminous shrub. Aiming to address the difficulty of natural vegetation restoration in the Yalong river basin, six treatments with different biochar application rates were set up in Yajiang county, Garze Tibetan autonomous prefecture, Sichuan province. We systematically determine and analyze the morphological traits, chemical constituents, and mechanical properties of L. bicolor roots. The following hypotheses were proposed: (1) Biochar application would significantly affect the root morphological and mechanical properties of L. bicolor; (2) There is a threshold for biochar application rate. Excessive biochar may destroy the soil physicochemical properties, disturb the balance between soil nutrient supply and plant uptake, and thus inhibit plant growth.

Materials and Methods

Study site

The study site is located in Yajiang County, Garze Tibetan autonomous prefecture, Sichuan province, southwestern China. It lies in the middle reaches of the Yalong river basin, within the Hengduan Mountains region of the Qinghai-Tibet Plateau. Yajiang County belongs to the subhumid climate zone of the Qinghai-Tibet Plateau, characterized by a continental monsoon alpine climate. The annual average temperature is 11.1°C, with monthly average temperatures ranging from 1.4°C to 18.0°C, and the annual average sunshine duration reaches 2319 hours-characterized by abundant sunshine, intense solar radiation, and a significant diurnal temperature variation. The annual average precipitation is 783.1 mm, and the annual average relative humidity is 54.4%; the winter and spring seasons are cold and arid, with no distinct summer period. Hydropower development in the Yalong river basin has resulted in extensive bare slopes, which are highly susceptible to geological hazards such as landslides. Consequently, ecological restoration of these slopes is of urgent necessity. In this study, one representative slope was selected as the research object, and its location and fundamental parameters are illustrated in Fig. 1.

Figure 1: Effect of different biochar application rates on plant growth status. (a) Plant height; (b) Basal diameter; (c) Number per unit area. Data are presented as the means ± standard deviations. Control (CK), I, II, III, IV, and V represent biochar application rates of 0, 2.5, 5, 7.5, 10, and 12.5 kg·m^-2, respectively. Lowercase letters indicate statistically significant differences among different biochar content treatments at the 0.05 significance level.

Experimental design

Experimental materials: The biochar material was provided by Liao Ning Golden Future Agriculture Technology Co., Ltd. It was produced via high-temperature carbonization of rice straw at 500°C under anaerobic conditions. The basic physicochemical properties of the biochar were as follows: Specific surface area of 8.85 m²·g^-1, average pore diameter of 16.25 nm, pH 9.32, ash content of 12.6%, total nitrogen content of 0.6%, available phosphorus content of 20.89 g·kg^-1, and available potassium content of 2.51 g·kg^-1.

Plant materials: Lespedeza bicolor, provided by Yunnan Jinye Ecological Construction Group Co., Ltd.

Experimental setup: Given the shallow topsoil on the experimental slope, slope vegetation restoration was carried out using the foreign soil covering method, with a covering thickness of 12 cm (the basic physicochemical properties of the foreign soil are presented in Tab. 1). The foreign soil was mixed with L. bicolor seeds at a sowing density of 15 g·m^-2. The experiment was established in early May 2016, with conventional water management applied during the entire test period. Upon completion of the experiment in May 2024, plant growth conditions were investigated and the relevant indicators were measured.

Table 1. Basic physical and chemical properties of foreign soil (when biochar is not added).

A Randomized Complete Block Design (RCBD) was adopted, comprising six biochar application treatments: Control (CK, 0 kg·m^-2), Treatment I (I, 2.5 kg·m^-2), Treatment II (II, 5 kg·m^-2), Treatment III (III, 7.5 kg·m^-2), Treatment IV (IV, 10 kg·m^-2), and Treatment V (V, 12.5 kg·m^-2). Each treatment was assigned to a 5 m × 5 m plot, with three replicates per treatment, resulting in a total of 18 standard plots. To avoid mutual interference, a spacing of 3 m was maintained between adjacent plots.

Measurement and calculation of indicators

Measurement of plant growth status: The number of L. bicolor individuals in each quadrat was counted. Three plants were randomly selected from each quadrat to measure plant height and basal diameter. Plant height was measured as the straight-line distance from the rhizome node to the plant apex using a measuring tape with an accuracy of 1 mm. The mean value of repeated measurements was taken as the plant height for each individual. Basal diameter was determined at the soil surface using a Vernier caliper with an accuracy of 0.01 mm. Measurements were conducted in two mutually perpendicular directions, and the average of the two values was recorded as the basal diameter for each individual. Data on plant height and basal diameter of the selected individual plants within each treatment group were statistically analyzed separately. The mean values within each group were calculated and used as the representative values of plant height and basal diameter of L. bicolor under different biochar application treatments.

Measurement of root morphology: After selecting the L. bicolor individuals for excavation, surface debris including fallen leaves, gravel, and loose topsoil around the plant base was carefully removed. The position where the rhizome contacts the soil surface was marked, and the aboveground part of the plant was moderately pruned to lower its center of gravity and reduce tensile damage to the root system caused by external forces during excavation. During excavation, soil removal started from the surface layer and proceeded layer by layer from top to bottom and from outside to inside. Gentle digging and stripping were adopted to avoid root breakage caused by forced pulling. Upon completion of excavation, the root systems were observed. All plants exhibited well-developed root systems with distinct taproots, primary lateral roots, and secondary lateral roots. Subsequently, the number, length (distance from the growth point to the root tip), and average diameter of first-order and second-order lateral roots for each plant were determined. The average diameter was measured with a vernier caliper at 10 cm from the root attachment point, as the mean of the maximum and minimum diameters in two perpendicular directions. The length and diameter of the taproots were also measured, and fibrous roots on the lateral roots were collected. Based on the measured data, the mean values of taproot length and diameter, as well as the mean values of the number, length, and diameter of first-order lateral roots and second-order lateral roots for each individual plant were calculated. After root excavation and measurement, impurities adhering to the root surfaces were carefully removed. The different root types were classified, packaged, and stored in a refrigerator at 4°C for low-temperature preservation, and all subsequent tests were completed within 7 days.

Measurement of root tensile strength: For each plant root sample, the tensile strength of the taproot, first-order lateral roots, and second-order lateral roots was determined separately. Three replicates were randomly selected for both first-order lateral roots and second-order lateral roots. All test root segments were prepared according to a uniform standard: the segment length was fixed at 10 cm, with only one segment taken per root; segments were cut starting at 5 cm from the root growth point. Prepared root segments were immediately subjected to tensile resistance testing using an electronic universal testing machine (KD III, Shenzhen Kaiqiangli Testing Instrument Co., Ltd.).

Root tensile strength was calculated from the measured data, using the following formula:

Where Pg is the tensile strength of the root (MPa), Fmax is the maximum tensile force (N), and D is the diameter of the root segment (mm).

Determination of root biomass and chemical constituents: The plant roots (including those used for tensile strength testing) were oven-dried at 80°C for 48 h and then weighed to determine the root dry weight per plant. The contents of cellulose, hemicellulose, and lignin in the taproots, first-order lateral roots, and second-order lateral roots were determined via the nitric acid-ethanol method (Chen, et al. 2018), the hydrochloric acid hydrolysis-DNS method (Xiong, et al. 2005), and the 72% concentrated sulfuric acid method (Zhang, et al. 2014), respectively.

Collection and analysis of soil samples: Soil samples were collected using a cutting ring. During sampling, impurities such as stones, crop roots, and debris were removed from the samples. After sampling, the samples were sealed immediately and transported to the laboratory for analysis. The soil samples were air-dried naturally, ground, and then passed through a 2 mm nylon sieve. Soil saturated water content and field capacity were measured via the cutting ring method (Ma, et al. 2024). The soil pH value was measured by the potentiometric method with a soil-to-water ratio of 1:2.5; available nitrogen was determined via the alkali-hydrolysis diffusion method (Li, et al. 2021); available phosphorus was determined via the 0.5 mol.L^-1 NaHCO₃ extraction-molybdenum antimony anticolorimetric method (Scrimgeour, 2008); and available potassium was measured via the NH₄OAc extraction-flame photometry method (Zanati, et al. 1973).

Statistical analysis

Data analysis was performed using SPSS 26.0. According to different sample sizes, the Kolmogorov-Smirnov test was used to test the normality of plant root traits (n>50), and the Shapiro-Wilk test was used to test the normality of soil properties (n<50). The homogeneity of variance was assessed via the Levene test. One-way Analysis Of Variance (ANOVA) was conducted for data satisfying normality and homogeneity of variance. The Tukey test was used for post hoc multiple comparisons among different biochar application treatments, and the LSD test was used for post hoc multiple comparisons among different root types. For data that violated the homogeneity of variance assumption, the Welch test was applied, followed by the Games-Howell test for post hoc multiple comparisons. The graphs were generated using Origin 2024.

Results

Plant growth status

As shown in Fig. 2, plant height (P=0.000) and basal diameter (P=0.002) increased first and then decreased with increasing biochar application rate, reaching the maximum under Treatment III. Moreover, Treatment III was significantly higher than Treatments IV and V. In addition, plant height under Treatment III was significantly higher than that under Control (CK) and Treatment I, while there were no significant differences in basal diameter among Control (CK), Treatments I, II and III. The quantity per unit area increased with increasing biochar application rate (P=0.000). Compared with Control (CK), no significant increases were observed in Treatments I, II and III, while significant differences were found in Treatments IV and V.

Figure 2: Effect of different biochar application rates on plant growth status. (a) Plant height; (b) Basal diameter; (c) Number per unit area. Data are presented as the means ± standard deviations. Control (CK), I, II, III, IV, and V represent biochar application rates of 0, 2.5, 5, 7.5, 10, and 12.5 kg·m^-2, respectively. Lowercase letters indicate statistically significant differences among different biochar content treatments at the 0.05 significance level.

Soil physical and chemical properties

Different biochar application treatments caused significant differences in soil saturated water content (P=0.000) and field capacity (P=0.000). As shown in Fig. 3, compared with the control, Treatments I to V increased soil saturated water content and field capacity during the growth period of L. bicolor to varying degrees, with an overall increasing trend.

Figure 3: Effects of different biochar application rates on soil physical properties: (a) Saturated water content; (b) Field capacity. Data are presented as the means ± standard deviations. Control (CK), I, II, III, IV, and V represent biochar application rates of 0, 2.5, 5, 7.5, 10, and 12.5 kg·m^-2, respectively. Lowercase letters indicate statistically significant differences among different biochar content treatments at the 0.05 significance level.

Different biochar application treatments also resulted in significant differences in soil available nitrogen (P=0.000), available phosphorus (P=0.000), available potassium (P=0.000), and soil pH (P=0.000). Fig. 4 shows that soil pH increased significantly and continuously with increasing biochar application rate, indicating that biochar addition significantly increased soil pH. Available nitrogen exhibited a trend of increasing first and then decreasing with increasing biochar application rate, reaching the maximum under Treatment III. However, compared with Control (CK), available nitrogen contents in Treatments I, II and III showed no significant increase, while those in Treatments IV and V decreased significantly. Available phosphorus increased significantly with increasing biochar application rate, peaking under Treatment III. Compared with Control (CK), no significant increase was observed in Treatment I, while significant increases were found in Treatments II, III, IV and V, but there were no significant differences between Treatment III and Treatments II, IV and V. Biochar application significantly increased available potassium content. Compared with CK, significant differences were detected in Treatments II, III, IV and V, but no significant differences were observed between Treatment III and Treatments IV and V.

Figure 4: Effects of different biochar application rates on soil chemical properties. Data are presented as the mean ± standard deviation. Control (CK), I, II, III, IV, and V represent biochar application rates of 0, 2.5, 5, 7.5, 10, and 12.5 kg·m^-2, respectively. Lowercase letters indicate statistically significant differences among different biochar content treatments at the 0.05 significance level.

Root morphological characteristics

According to the ANOVA results, among the root morphological traits investigated, different biochar application rates significantly affected Taproot Length (TRL) (P=0.005) and Taproot Diameter (TRD) (P=0.001). Significant differences were observed in the length of first-order lateral roots (P=0.000), as well as in the length (P=0.000) and number (P = 0.000) of second-order lateral roots across the different biochar application rates. In contrast, the diameter (P=0.996) and number (P=0.990) of first-order lateral roots, as well as the diameter of second-order lateral roots (P=0.807), were not significantly affected by the biochar application rate. Taproot length, taproot diameter, first-order lateral root length, and second-order lateral root length and number were significantly greater in Treatments II and III than in Treatments IV and V.

Different biochar application rates significantly affected the root biomass (P=0.001), total root length (P=0.001), and specific root length (P=0.025) of L. bicolor. The root biomass in Treatments II and III was significantly greater than that in Treatments IV and V. The total root length in Treatment âÂ…¢ were significantly greater than those in the other treatments. Additionally, the SRL values were similar among the Control (CK) and Treatments I, II , and IV (Fig. 5).

Figure 5: Effects of different biochar application rates on root parameters. MR represents the Taproot, LR1 represents the first-order Lateral Root, and LR2 represents the second-order Lateral Root. (a) Length and (b) diameter of MR, LR1, and LR2. (c) The numbers of LR1 and LR2. (d) Root biomass. (e) SRL, specific root length. (f) TRL, total root length. Data are presented as the mean ± standard deviation. Lowercase letters indicate statistically significant differences among different biochar content treatments at the 0.05 significance level.

Root chemical composition

Under different biochar application rates, no significant differences were observed in the cellulose content (P=0.985), hemicellulose content (P=0.979), and lignin content (P=0.650) of the taproots. With respect to first-order lateral roots, the biochar application rate significantly affected the cellulose content (P=0.000), whereas the hemicellulose content (P=0.895) and lignin content (P=0.064) were not significantly influenced. In contrast, for second-order lateral roots, both the cellulose content (P=0.000) and lignin content (P=0.000) were significantly affected by the biochar application rate, whereas the hemicellulose content (P=0.922) was not significantly affected (Fig. 6).

Figure 6: Effects of different biochar application rates on root chemical properties. MR represents the taproot; LR1 represents the first-order lateral root; LR2 represents the second-order lateral root. (a) Cellulose content, (b) hemicellulose content, and (c) lignin content of MR, LR1 and LR2. Data are presented as the mean ± standard deviation. Different uppercase letters indicate significant differences in the chemical composition of the same type of root under different biochar contents (P<0.05). Different lowercase letters indicate significant differences in the chemical composition of different types of roots under the same biochar application treatment. (P<0.05).

There were also differences in the chemical composition among different root types. Under the same treatment, the cellulose content in taproots was lower than that in first-order and second-order lateral roots, whereas the lignin content was higher than that in first-order and second-order lateral roots. No significant differences were observed in hemicellulose content among different root types under the same biochar application rate.

Root tensile strength

Different biochar application rates did not significantly affect the tensile strength of the taproots (P=0.980). In contrast, significant differences in tensile strength were observed between first-order lateral roots (P=0.001) and second-order lateral roots (P=0.001) across the different treatments. Compared with that in the other treatments, the root tensile strength in Treatment III was significantly greater. Additionally, the tensile strength of the taproots was lower than that of the first-order lateral roots and second-order lateral roots (Fig. 7).

Figure 7: Effects of different biochar application rates on tensile strength. Data are presented as the mean ± standard deviation. Different uppercase letters above the bars indicate significant differences (P<0.05) in tensile strength of the same root type under different biochar application treatments. Different lowercase letters indicate significant differences (P<0.05) in tensile strength among different root types under the same biochar application treatment.

Relationships between root tensile strength and root chemical composition

The quantitative relationships between root tensile strength and root chemical composition in each treatment are shown in Fig. 8. Root tensile strength exhibited an increasing trend with increasing cellulose content, and a decreasing trend with increasing lignin content. In other words, root tensile strength was positively correlated with cellulose content and negatively correlated with lignin content. However, no obvious trend was observed between root tensile strength and hemicellulose content (Tab. 2).

Figure 8: Relationships between root tensile strength and cell wall chemical components of L. bicolor. (a) Relationship with cellulose content, (b) relationship with hemicellulose content, (c) relationship with lignin content.

Table 2. Regression equations between root tensile strength and root chemical compositions of L. bicolor.

Discussion

Effects of biochar application rate on the growth of L. bicolor

Biochar application rate had a significant dose effect on plant growth. Appropriate biochar application could create favorable conditions for plant growth by optimizing the soil microenvironment, whereas excessive addition would disrupt the ecological balance and thus inhibit plant growth. In the present experiment, Treatment III achieved a better balance between soil nutrient supply and plant uptake.

Compared with Control (CK), under the low biochar application rate groups (Treatments I, II, and III), biochar could improve soil health and enhance the plant’s absorption capacity for nitrogen, phosphorus, and potassium, thereby providing favorable conditions for plant growth (Singh, et al. 2015). Conversely, excessive biochar application can temporarily reduce the availability of soil mineral nutrients (Tammeorg, et al. 2014) and disrupt soil microbial biomass and activity (Ameloot, et al. 2014). Such detrimental effects ultimately impede plant growth, leading to significant reductions in both plant height and basal diameter.

Under the influence of the aforementioned regulatory mechanisms, both plant height and basal diameter in the present study exhibited a trend of increasing initially and then decreasing with escalating biochar application rate. Consistent with this observation, Yang, et al. 2024 also confirmed a similar pattern in flue-cured tobacco, where plant height first increased and subsequently decreased as biochar application rate rose, which aligns well with our experimental results. Similarly, Cai, et al. 2021 reported that biochar can significantly promote plant growth and biomass accumulation at an appropriate application rate, whereas an inhibitory effect is induced when the application rate exceeds a critical threshold.

In our experiment, the number of L. bicolor plants per unit area increased continuously with increasing biochar application rate. The main reason was the sustained improvement of soil conditions by biochar. The soil at the experimental site was infertile, and biochar application alleviated adverse environmental conditions such as soil acid-base imbalance and salt stress, thereby creating a more favorable microenvironment for seed germination and seedling survival of L. bicolor (Zulfiqar, et al. 2022, Su, et al. 2024, Antonangelo, et al. 2025). As an inert carbonaceous material, biochar did not promote plant growth through direct nutrient supply at high application rates. Instead, it increased population quantity by reducing the seedling missing rate and seedling mortality. Consequently, the number of L. bicolor plants per unit area continued to rise with increasing biochar application rate.

Notably, the individual growth of L. bicolor showed a threshold effect with increasing biochar application rate, while its population quantity continued to increase. The soil amelioration effect of biochar preferentially supported seed germination and seedling survival, rather than rapid individual growth (Marsh, et al. 2023). When the application rate exceeded the threshold, negative effects such as soil nutrient immobilization only inhibited individual growth, without offsetting the increase in population quantity (Laghari, et al. 2015).

Effects of biochar application rate on root morphology

As a critical organ for water and nutrient uptake, plant roots exhibit strong environmental adaptability (Montagnoli, 2022). Root biomass, as an important indicator of plant growth, is significantly influenced by external environmental conditions during its formation and accumulation (Helmisaari, et al. 2007). Due to its unique microporous structure (Atkinson, et al. 2010, Igalavithana, et al. 2017), biochar exerts multiple beneficial effects in soil amelioration: it can not only effectively increase soil organic matter (Pan, et al. 2009) but also possess excellent water and nutrient retention capacities (Laird, et al. 2010, Ahmad, et al. 2014), thereby significantly improving soil fertility (Mohamed, et al. 2016).

The results of this study indicate that suitable biochar application rates (Treatments I, II, and III) significantly promoted root biomass accumulation (Zou, et al. 2021). On the one hand, biochar addition significantly increases soil organic carbon content (Novak, et al. 2009, Dong, et al. 2018), enhances soil water-holding capacity and water regulation function, and improves soil physical structure by increasing porosity and aeration (Busscher, et al. 2010), thus creating a favorable environment for root growth. On the other hand, biochar can increase soil enzyme activity and the abundance of soil microbial communities, in turn increasing root activity.

In this study, root length, specific root length, and taproot diameter all showed a trend of increasing first and then decreasing, reaching the peak under Treatment III. As a porous material, biochar reduces soil bulk density and increases soil porosity when applied to soil (Jiang, et al. 2025). This modification positively affects root development, resulting in longer root systems. At the optimal application rate, biochar creates a more favorable growth environment for roots by improving soil physical structure, increasing water and nutrient retention capacity, and regulating the pH of acidic soils (Lee, et al. 2025). However, when the application rate exceeds this threshold, the strong adsorption capacity of biochar may cause excessive immobilization of soil nutrients, particularly nitrogen, thereby restricting nutrient acquisition by plants (Zheng, et al. 2013, Prapagdee, et al. 2014). In addition, excessive biochar application may lead to soil loosening, which impairs the contact between roots and soil particles and has complex effects on the structure and function of soil microbial communities, indirectly interfering with the normal development of roots. These adaptive changes in root morphology may help maximize surface area and nutrient absorption, thereby potentially increasing root growth rates.

Biochar application optimizes root morphology and thereby exerts a soil reinforcement effect. Plants under an appropriate biochar application rate present superior root morphological characteristics, such as greater total root length (Chang, et al. 2021), which provides critical support for the improved soil erosion resistance in this study. In terms of soil reinforcement mechanisms, longer total root length can not only expand the root-soil contact area (Blume-Werry, et al. 2019) and strengthen the entanglement and fixation between soil particles (Li, et al. 2020), but also further improve soil structure and enhance soil aggregate stability through root exudates (Lu, et al. 2020, Xiao, et al. 2024). An increase in total root length also indicates a more developed lateral root system. Well-developed lateral roots enhance slope stability and reduce the risk of shallow landslides via reinforcement and anchorage effects (Gray and Barker, 2004, Schwarz, et al. 2010). These results are consistent with previous studies: soil erosion resistance is directly determined by root biomass and morphological characteristics, and is positively correlated with root biomass, taproot length and root diameter. Yang, et al. 2021 further confirmed that root pull-out resistance, as an important indicator of soil erosion resistance, is closely related to key root morphological indices including root diameter, root length and root surface area. For L. bicolor, Zhou, et al. 2023 also clearly demonstrated that root biomass and morphological characteristics play a crucial role in strengthening soil anti-scourability. Accordingly, appropriate biochar amendment optimizes root morphology, and combined with the inherent root advantages of Lespedeza, ultimately leads to a significant improvement in soil erosion resistance.

Effects of biochar application rate on root tensile strength and chemical composition

The ability of plants to resist soil and water loss depends on root tensile properties. In the present study, the tensile strength of taproots showed no significant change, while that of lateral roots increased with root order. The overall tensile strength of the root system initially increased and then decreased with increasing biochar application rate (Fig. 7). Root presence influences the apparent cohesion of soil, and its reinforcement effect plays an important role in the overall shear strength of soil (Ali and Osman 2008, Pallewattha, et al. 2019). Moreover, the improvement in soil cohesion and shear strength is closely related to root tensile strength (Ali and Osman, 2008, Lin, et al. 2024). Therefore, the application of an appropriate amount of biochar can improve the tensile properties of plant roots, leading to a more prominent effect in root-reinforced soil for soil and water conservation.

Root tensile properties serve as a pivotal indicator for assessing soil and water conservation capacity as well as slope and bank stabilization performance. Our findings demonstrate that root tensile strength is intimately associated with root chemical composition. As the primary structural component of plant cell walls (Haigler, et al. 2001), cellulose polymerizes into microfibrils that form the structural framework of root cell walls (Anderson, et al. 2010), thereby governing the mechanical characteristics of plant organs (Bidhendi and Geitmann, 2016). Notably, cellulose microfibrils exhibit remarkable resistance to tensile failure; consequently, elevated cellulose content can substantially enhance root tensile strength. In contrast to cellulose, hemicellulose features a branched structure and functions as a filling and cross-linking component in plant cell walls, improving cell wall flexibility. Lignin represents another critical structural constituent of plant cell walls (Khan, et al. 2024). It interacts with cellulose through hemicellulose-mediated bridges, reinforcing cell wall integrity and augmenting root tensile resistance (Ye, et al. 2017). However, excessive lignin compromises cell wall elasticity while enhancing rigidity, rendering roots more susceptible to brittle fracture under tensile stress (Qi, et al. 2024). This phenomenon was corroborated in Yan, et al. 2022 study on the roots of Dolichos lablab L., a leguminous herbaceous species. Consistent with prior research (Genet, et al. 2005, Hales, et al. 2009), the current study reveals a significant positive correlation between tensile strength and cellulose content, alongside a significant negative correlation between tensile strength and lignin content (Fig. 8). Furthermore, Figs. 6 and 7 provide direct visual evidence that roots with higher tensile strength typically correspond to higher cellulose levels and lower lignin contents.

This study further analyzed the regulatory effects of biochar application rates on root chemical constituents. As illustrated in Fig. 6, for second-order lateral roots, an optimal biochar application rate significantly increased the root cellulose content while reducing the lignin content, which was consistent with the variation trend of the root tensile strength. This suggests that biochar may modulate the synthesis and deposition of root cell wall components (Ali, et al. 2022, Xu, et al. 2022) by improving the rhizosphere microenvironment and regulating carbon metabolic pathways (Kolton, et al. 2017, Jin, et al. 2024), thereby enhancing root mechanical properties from the perspective of chemical composition and providing support for improved root erosion resistance.

Conclusion

These results indicate that biochar plays a crucial role in regulating the soil-reinforcement capacity of the roots of L. bicolor. The addition of an optimal amount of biochar increases root biomass and lateral root cellulose content, decreases lignin content, promotes root development, and thereby significantly increases root tensile strength. In contrast, excessive biochar application has negative effects on root growth. Root tensile strength is positively correlated with the cellulose content and negatively correlated with the lignin content. Based on the findings of this study, a biochar application rate of 7.5 kg·m-2 is deemed suitable for improving the soil-reinforcement capacity of L. bicolor roots in practical applications. Future research should further investigate the long-term impacts of biochar across a broader range of application rates and plant species to provide a theoretical basis for ecological restoration projects.

Acknowledgments

Competing Interests

The authors declare that they have no conflict of interest.

Data Availability

The data are available from the authors upon reasonable request.

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