Efficient afforestation programs are crucial to limit soil degradation in various arid and semi-arid ecosystems. However, the success of these programs is dependant to the plant type selected for revegetation and the methods used for seedling production. Exotic fast-growing trees have been largely planted but their use is currently controversial because of their potential negative ecological impacts. Whereas the positive impact of arbuscular mycorrhizal (AM) fungal inoculation in nursery was demonstrated, few studies focused on the monitoring of mycorrhizal inoculation in arid and semi-arid ecosystems. In addition, the majority of studies are based on single-species inocula with non native AM fungal strains. The current study aims at evaluating the efficiency of mycorrhizal inoculation of the emblematic Mediterranean carob tree (Ceratonia siliqua) in a Moroccan degraded site, through an ecological engineering strategy based on the use of a complex native AM community (naturally associated to carob trees). Results demonstrate the high potential of this approach by improving sustainably the growth and nutrient status of carob trees in a 3-year-old plantation and also by inducing a positive soil microbial environment for nutrient cycling and environmental stress resistance. (Résumé d'auteur)

Efficient afforestation programs are crucial to limit soil degradation in various arid and semi-arid ecosystems. However, the success of these programs is dependant to the plant type selected for revegetation and the methods used for seedling production. Exotic fast-growing trees have been largely planted but their use is currently controversial because of their potential negative ecological impacts. Whereas the positive impact of arbuscular mycorrhizal (AM) fungal inoculation in nursery was demonstrated, few studies focused on the monitoring of mycorrhizal inoculation in arid and semi-arid ecosystems. In addition, the majority of studies are based on single-species inocula with non native AM fungal strains. The current study aims at evaluating the efficiency of mycorrhizal inoculation of the emblematic Mediterranean carob tree (Ceratonia siliqua) in a Moroccan degraded site, through an ecological engineering strategy based on the use of a complex native AM community (naturally associated to carob trees). Results demonstrate the high potential of this approach by improving sustainably the growth and nutrient status of carob trees in a 3-year-old plantation and also by inducing a positive soil microbial environment for nutrient cycling and environmental stress resistance.

The influence of plant diversity on slope stabilitywas investigated at early phases of succession in a mixed forest in Sichuan, China. The first phase comprised big node bamboo (Phyllostachys nidularia Munro) only. In the second phase, bamboo co-existed with deciduous tree species and in the third phase, deciduous species existed alone. Root density at different depths and root tensile strength were determined for each species. The factor of safety (FOS) was calculated for slopes with and without vegetation for each succession phase. For phase 2, FOS was determined for different species mixtures and positions. In phase 3, simulations were performed with a single tree at the top, middle or toe of the slope. Due to its shallow root system, bamboo contributed little to slope stability. In simulations with the tree at the top or middle of the slope, FOS decreased because tree weight added a surcharge to the slope. FOS increased with the tree at the bottom of the slope. Different mixtures of species along the slope had no influence on FOS. Differences in root tensile strength between species played a small role in FOS calculations, and tree size and density were the most important factors affecting slope stability, excluding hydrological factors. (Résumé d'auteur)

Special Issue: Vegetation and Slope Stability | International audience | The influence of plant diversity on slope stability was investigated at early phases of succession in a mixed forest in Sichuan, China. The first phase comprised big node bamboo (Phyllostachys nidularia Munro) only. In the second phase, bamboo co-existed with deciduous tree species and in the third phase, deciduous species existed alone. Root density at different depths and root tensile strength were determined for each species. The factor of safety (FOS) was calculated for slopes with and without vegetation for each succession phase. For phase 2, FOS was determined for different species mixtures and positions. In phase 3, simulations were performed with a single tree at the top, middle or toe of the slope. Due to its shallow root system, bamboo contributed little to slope stability. In simulations with the tree at the top or middle of the slope, FOS decreased because tree weight added a surcharge to the slope. FOS increased with the tree at the bottom of the slope. Different mixtures of species along the slope had no influence on FOS. Differences in root tensile strength between species played a small role in FOS calculations, and tree size and density were the most important factors affecting slope stability, excluding hydrological factors

The discharge of raw faecal sludge directly into the environment is a common practice that threatens environmental and public health in low-income countries. Planted drying beds are a promising and low-cost option for treatment of faecal sludge and the production of fodder plants, but current research shows the leachate quality does not meet guidelines for discharge. This paper investigates the use of Vertical Flow Constructed Wetlands (VFCWs) planted with Echinochloa pyramidalis for polishing of leachate from faecal sludge drying beds. At a pilot-scale, three hydraulic loads (50, 100 and 150 mm/d) were applied with single loadings once a week on VFCWs (corresponding to 31 ± 9, 63 ± 19, 94 ± 28 g/m2/month of BOD5; 21 ± 6, 42 ± 12, 63 ± 19 TKN; and 2 ± 1; 4 ± 4; 6 ± 6 PO4–P, respectively, for hydraulic loading rates of 50, 100 and 150 mm/d). Infiltration flow rate, plant growth, rhizospheric bacteria, and leachate characteristics were monitored. VFCWs were effective in reducing on average more than 80% of the pollutants monitored (COD, BOD5, NH4–N, TKN, PO4–P, and faecal bioindicators), which met all National Cameroon and WHO guidelines for safe reuse in agriculture, except for total nitrogen and faecal indicators. Results confirmed a correlation between plant density and rhizospheric bacteria growth with increasing hydraulic load. These are important results, demonstrating that VFCWs can operate efficiently at multiple hydraulic loadings, and are hence adaptable to different sized treatment schemes. It also illustrates that if plant production for fodder is a goal, increased loading rates are preferable as they achieve overall treatment goals and result in greater plant production.

The discharge of raw faecal sludge directly into the environment is a common practice that threatens environmental and public health in low-income countries. Planted drying beds are a promising and low-cost option for treatment of faecal sludge and the production of fodder plants, but current research shows the leachate quality does not meet guidelines for discharge. This paper investigates the use of Vertical Flow Constructed Wetlands (VFCWs) planted with Echinochloa pyramidalis for polishing of leachate from faecal sludge drying beds. At a pilot-scale, three hydraulic loads (50, 100 and 150mm/d) were applied with single loadings once a week on VFCWs (corresponding to 31±9, 63±19, 94±28g/m2/month of BOD5; 21±6, 42±12, 63±19 TKN; and 2±1; 4±4; 6±6 PO4–P, respectively, for hydraulic loading rates of 50, 100 and 150mm/d). Infiltration flow rate, plant growth, rhizospheric bacteria, and leachate characteristics were monitored. VFCWs were effective in reducing on average more than 80% of the pollutants monitored (COD, BOD5, NH4–N, TKN, PO4–P, and faecal bioindicators), which met all National Cameroon and WHO guidelines for safe reuse in agriculture, except for total nitrogen and faecal indicators. Results confirmed a correlation between plant density and rhizospheric bacteria growth with increasing hydraulic load. These are important results, demonstrating that VFCWs can operate efficiently at multiple hydraulic loadings, and are hence adaptable to different sized treatment schemes. It also illustrates that if plant production for fodder is a goal, increased loading rates are preferable as they achieve overall treatment goals and result in greater plant production.

The finite element (FE) method has been used in recent years to simulate overturning processes in trees and to better comprehend plant anchorage mechanics. We aimed at understanding the fundamental mechanisms of root-soil reinforcement by simulating direct shear of rooted and non-rooted soil. Two- (2D) and three-dimensional (3D) FE simulations of direct shear box tests were carried out using readily available software for routine strength assessment of the root-soil composite. Both rooted and non-rooted blocks of soil were modelled using a simplified model of root distribution and root material properties representative of real roots. Linear elastic behaviour was assumed for roots and the soil was modelled as an ideally plastic medium. FE analysis showed that direct shear tests were dependent on the material properties specified for both the soil and roots. 2D and 3D simulations of direct shear of non-rooted soil produced similar results and any differences between 2D and 3D simulations could be explained with regard to the spatial complexity of roots used in the root distribution model. The application of FE methods was verified through direct shear tests on soil with analogue roots and the results compared to in situ tests on rooted soil in field conditions. (Résumé d'auteur)

International audience | The finite element (FE) method has been used in recent years to simulate overturning processes in trees and to better comprehend plant anchorage mechanics. We aimed at understanding the fundamental mechanisms of root-soil reinforcement by simulating direct shear of rooted and non-rooted soil. Two- (2D) and three-dimensional (3D) FE simulations of direct shear box tests were carried out using readily available software for routine strength assessment of the root-soil composite. Both rooted and non-rooted blocks of soil were modelled using a simplified model of root distribution and root material properties representative of real roots. Linear elastic behaviour was assumed for roots and the soil was modelled as an ideally plastic medium. FE analysis showed that direct shear tests were dependent on the material properties specified for both the soil and roots. 2D and 3D simulations of direct shear of non-rooted soil produced similar results and any differences between 2D and 3D simulations could be explained with regard to the spatial complexity of roots used in the root distribution model. The application of FE methods was verified through direct shear tests on soil with analogue roots and the results compared to in situ tests on rooted soil in field conditions

The finite element (FE) method has been used in recent years to simulate overturning processes in trees and to better comprehend plant anchorage mechanics. We aimed at understanding the fundamental mechanisms of root–soil reinforcement by simulating direct shear of rooted and non-rooted soil. Two- (2D) and three-dimensional (3D) FE simulations of direct shear box tests were carried out using readily available software for routine strength assessment of the root–soil composite. Both rooted and non-rooted blocks of soil were modelled using a simplified model of root distribution and root material properties representative of real roots. Linear elastic behaviour was assumed for roots and the soil was modelled as an ideally plastic medium. FE analysis showed that direct shear tests were dependent on the material properties specified for both the soil and roots. 2D and 3D simulations of direct shear of non-rooted soil produced similar results and any differences between 2D and 3D simulations could be explained with regard to the spatial complexity of roots used in the root distribution model. The application of FE methods was verified through direct shear tests on soil with analogue roots and the results compared to in situ tests on rooted soil in field conditions.

Vegetation is increasingly used to protect artificial and natural slopes against shallow landslides. Mechanically, plant roots reinforce soil along a slope by providing cohesion (cr). cr is usually estimated using either of two models: a Wu and Waldron's Model (WWM) or a Fiber Bundle Model (FBM). The WWM assumes that all fine and medium roots break simultaneously during shearing, whereas the FBM assumes progressive breakage of these roots. Both models are based on measurements of root density (RD), root tensile strength (Tr) and root orientation (Rf). RD is highly variable and influences cr significantly more than the other variables. We investigated RD in a mixed forest stand dominated by Fagus sylvatica and Abies alba growing at an altitude of 1400 m and a mixed stand of Abies alba and Picea abies located at 1700 m. We assumed that our sites were composed of different plant functional groups, i.e. (1) only trees and shrubs were present and (2) trees, shrubs and herbaceous plants coexisted within the same site. Results showed that RD was significantly influenced by soil depth, tree spatial density and species composition. cr was then estimated by the WWM and three different FBMs; each FBM differed in the manner that load was apportioned to the roots (as a function of root cross-sectional area (CSA), root diameter or number of intact roots). Results showed that cr values differed significantly depending on the model used: cr (FBM, root number) < cr (FBM, root diameter) < cr (FBM, root CSA) < cr (WWM). Through a meta-analysis of literature data relating to changes in Tr with root diameter, we found that compared with other factors, plant functional group had a limited effect on the estimation of cr. The use of a generic equation for Tr is therefore justified when studying the stability of temperate forested slopes with mixed species. (Résumé d'auteur)

Vegetation is increasingly used to protect artificial and natural slopes against shallow landslides. Mechanically, plant roots reinforce soil along a slope by providing cohesion (cᵣ). cᵣ is usually estimated using either of two models: a Wu and Waldron's Model (WWM) or a Fiber Bundle Model (FBM). The WWM assumes that all fine and medium roots break simultaneously during shearing, whereas the FBM assumes progressive breakage of these roots. Both models are based on measurements of root density (RD), root tensile strength (Tᵣ) and root orientation (Rf). RD is highly variable and influences cᵣ significantly more than the other variables. We investigated RD in a mixed forest stand dominated by Fagus sylvatica and Abies alba growing at an altitude of 1400m and a mixed stand of Abies alba and Picea abies located at 1700m. We assumed that our sites were composed of different plant functional groups, i.e. (1) only trees and shrubs were present and (2) trees, shrubs and herbaceous plants coexisted within the same site. Results showed that RD was significantly influenced by soil depth, tree spatial density and species composition. cᵣ was then estimated by the WWM and three different FBMs; each FBM differed in the manner that load was apportioned to the roots (as a function of root cross-sectional area (CSA), root diameter or number of intact roots). Results showed that cᵣ values differed significantly depending on the model used: cᵣ (FBM, root number)<cᵣ (FBM, root diameter)<cᵣ (FBM, root CSA)<cᵣ (WWM). Through a meta-analysis of literature data relating to changes in Tᵣ with root diameter, we found that compared with other factors, plant functional group had a limited effect on the estimation of cᵣ. The use of a generic equation for Tᵣ is therefore justified when studying the stability of temperate forested slopes with mixed species.

International audience | Vegetation is increasingly used to protect artificial and natural slopes against shallow landslides. Mechanically, plant roots reinforce soil along a slope by providing cohesion(cr). cr is usually estimated using either of two models : a Wu and Waldron's Model(WWM)or a Fiber Bundle Model(FBM). The WWM assumes that all fine and medium roots break simultaneously during shearing, whereas the FBM assumes progressive breakage of these roots. Both models are based on measurements of root density(RD),root tensile strength (Tr) and root orientation(Rf). RD is highly variable and influences cr significantly more than the other variables. We investigated RD in a mixed forest stand dominated by Fagus sylvatica and Abies alba growing at an altitude of 1400 m and a mixed stand of Abies alba and Picea abies located at 1700m. We assumed that our sites were composed of different plant functional groups,i.e.(1)only trees and shrubs were present and(2)trees, shrubs and herbaceous plants coexisted within the same site. Results showed that RD was significantly influenced by soil depth, tree spatial density and species composition. cr was then estimated by the WWM and three different FBMs ; each FBM differed in the manner that load was apportioned to the roots (as a function of root cross-sectional area(CSA), root diameter or number of intact roots). Results showed that cr values differed significantly depending on the model used: cr (FBM, root number) < cr (FBM, rootdiameter) < cr (FBM, rootCSA)< cr (WWM). Through a meta-analysis of literature data relating to changes in Tr with root diameter, we found that compared with other factors, plant functional group had a limited effect on the estimation of cr. The use of a generic equation for Tr is therefore justified when studying the stability of temperate forested slopes with mixed species.

Vegetation is increasingly used to protect artificial and natural slopes against shallow landslides. Mechanically, plant roots reinforce soil along a slope by providing cohesion (cr). cr is usually estimated using either of two models: a Wu and Waldron's Model (WWM) or a Fiber Bundle Model (FBM). The WWM assumes that all fine and medium roots break simultaneously during shearing, whereas the FBM assumes progressive breakage of these roots. Both models are based on measurements of root density (RD), root tensile strength (Tr) and root orientation (Rf). RD is highly variable and influences cr significantly more than the other variables. We investigated RD in a mixed forest stand dominated by Fagus sylvatica and Abies alba growing at an altitude of 1400 m and a mixed stand of Abies alba and Picea abies located at 1700 m. We assumed that our sites were composed of different plant functional groups, i.e. (1) only trees and shrubs were present and (2) trees, shrubs and herbaceous plants coexisted within the same site. Results showed that RD was significantly influenced by soil depth, tree spatial density and species composition. cr was then estimated by the WWM and three different FBMs; each FBM differed in the manner that load was apportioned to the roots (as a function of root cross-sectional area (CSA), root diameter or number of intact roots). Results showed that cr values differed significantly depending on the model used: cr (FBM, root number) < cr (FBM, root diameter) < cr (FBM, root CSA) < cr (WWM). Through a meta-analysis of literature data relating to changes in Tr with root diameter, we found that compared with other factors, plant functional group had a limited effect on the estimation of cr. The use of a generic equation for Tr is therefore justified when studying the stability of temperate forested slopes with mixed species. (Résumé d'auteur)

The main objective was to evaluate the impact of nurse plant species commonly found in Mediterranean areas (Lavandula dentata and Thymus satureoides) on microbial soil functions, on the native inoculum potential of AM fungi involved in the rock phosphate weathering and to measure the potential benefits to the growth of Atlas Cypress (Cupressus atlantica G.), an endemic Cupressacea of Morocco. Soils collected from an old C. atlantica forest and pre-cultivated with each of the target plant species (L. dentata and T. satureoides). After 5 months of cultivation, they were uprooted and the treated substrate was amended or not with Khouribga Rock Phosphate (KRP). Then pots were filled with the soil mixtures and planted with one pre-germinated seed of C. atlantica. The results show that pre-cultivation step with native mycotrophic plant species improves the mycorrhizal soil infectivity, modifies soil microbial functionalities and increases the impact of rock phosphate amendment on the C. atlantica growth. This low cost cultivation practice by improving forest plant development and cultural soil quality constitutes a promising ecological engineering tool to improve the performances of ecosystem restoration. (Résumé d'auteur)

The main objective was to evaluate the impact of nurse plant species commonly found in Mediterranean areas (Lavandula dentata and Thymus satureoides) on microbial soil functions, on the native inoculum potential of AM fungi involved in the rock phosphate weathering and to measure the potential benefits to the growth of Atlas Cypress (Cupressus atlantica G.), an endemic Cupressacea of Morocco. Soils collected from an old C. atlantica forest and pre-cultivated with each of the target plant species (L. dentata and T. satureoides). After 5months of cultivation, they were uprooted and the treated substrate was amended or not with Khouribga Rock Phosphate (KRP). Then pots were filled with the soil mixtures and planted with one pre-germinated seed of C. atlantica. The results show that pre-cultivation step with native mycotrophic plant species improves the mycorrhizal soil infectivity, modifies soil microbial functionalities and increases the impact of rock phosphate amendment on the C. atlantica growth. This low cost cultivation practice by improving forest plant development and cultural soil quality constitutes a promising ecological engineering tool to improve the performances of ecosystem restoration.

The analysis of the influence of the roots on the shear resistance of the soil requires identifying the effect of the different root-soil interaction processes depending on soil and roots properties. For that purpose, a numerical model of direct shear tests of non-rooted and rooted granular soils based on the Discrete Element Method was developed. The soil is modeled as an assembly of locally interacting spheres and the roots are modeled as deformable cylinders in the soil matrix. The model allows accounting for the root tensile loading until breakage, the root bending loading, the root-soil adhesive links until adhesion breakage, the root slippage associated with a frictional resistance at the root-soil interface. The study focuses on identifying the different root-soil interaction mechanisms depending on the soil type. Both frictional and cohesive granular soil types were used in the simulations. The effects of the roots mechanical properties - tensile, bending modulus and root-soil interfacial friction angle - and of the root number were also analyzed for the different soil types. The results first show that the effect of the roots strongly depends on the shear strain for any soil type. For frictional soils, an increasing shear strain induces progressively a pure tensile loading of the roots until slippage of the root-soil interface. For cohesive granular soils, the pure tensile loading of the roots is followed by a progressive breakage of the adhesive root-soil links and by a complete slippage of the roots. The results show that the influence of the root number is significant if the prevailing processes are root tensile loading combined with slippage whereas it is less important if root loading is combined with progressive breakage of the adhesive links for the root configurations explored. Finally, the results show that the shear strain range associated with the different processes strongly depends on the relative rigidities of the roots and soil matrix. The mo

The analysis of the influence of the roots on the shear resistance of the soil requires identifying the effect of the different root–soil interaction processes depending on soil and roots properties. For that purpose, a numerical model of direct shear tests of non-rooted and rooted granular soils based on the Discrete Element Method was developed. The soil is modeled as an assembly of locally interacting spheres and the roots are modeled as deformable cylinders in the soil matrix. The model allows accounting for the root tensile loading until breakage, the root bending loading, the root–soil adhesive links until adhesion breakage, the root slippage associated with a frictional resistance at the root–soil interface.The study focuses on identifying the different root–soil interaction mechanisms depending on the soil type. Both frictional and cohesive granular soil types were used in the simulations. The effects of the roots mechanical properties – tensile, bending modulus and root–soil interfacial friction angle – and of the root number were also analyzed for the different soil types.The results first show that the effect of the roots strongly depends on the shear strain for any soil type. For frictional soils, an increasing shear strain induces progressively a pure tensile loading of the roots until slippage of the root–soil interface. For cohesive granular soils, the pure tensile loading of the roots is followed by a progressive breakage of the adhesive root–soil links and by a complete slippage of the roots. The results show that the influence of the root number is significant if the prevailing processes are root tensile loading combined with slippage whereas it is less important if root loading is combined with progressive breakage of the adhesive links for the root configurations explored. Finally, the results show that the shear strain range associated with the different processes strongly depends on the relative rigidities of the roots and soil matrix.The model developed was shown of great interest to analyze the shear resistance of the rooted soil assemblies depending on the shear strain. Such an approach could therefore be used to test the different assumptions done in the analytical models. Developing analytical models of slope stability based on the calculation of the shear resistance of rooted soil depending not only on soil and root properties but also on shear strain intensity also constitutes a perspective for the use of the model developed.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN | International audience | The analysis of the influence of the roots on the shear resistance of the soil requires identifying the effect of the different root-soil interaction processes depending on soil and roots properties. For that purpose, a numerical model of direct shear tests of non-rooted and rooted granular soils based on the Discrete Element Method was developed. The soil is modeled as an assembly of locally interacting spheres and the roots are modeled as deformable cylinders in the soil matrix. The model allows accounting for the root tensile loading until breakage, the root bending loading, the root-soil adhesive links until adhesion breakage, the root slippage associated with a frictional resistance at the root-soil interface. The study focuses on identifying the different root-soil interaction mechanisms depending on the soil type. Both frictional and cohesive granular soil types were used in the simulations. The effects of the roots mechanical properties - tensile, bending modulus and root-soil interfacial friction angle - and of the root number were also analyzed for the different soil types. The results first show that the effect of the roots strongly depends on the shear strain for any soil type. For frictional soils, an increasing shear strain induces progressively a pure tensile loading of the roots until slippage of the root-soil interface. For cohesive granular soils, the pure tensile loading of the roots is followed by a progressive breakage of the adhesive root-soil links and by a complete slippage of the roots. The results show that the influence of the root number is significant if the prevailing processes are root tensile loading combined with slippage whereas it is less important if root loading is combined with progressive breakage of the adhesive links for the root configurations explored. Finally, the results show that the shear strain range associated with the different processes strongly depends on the relative rigidities of the roots and soil matrix. The model developed was shown of great interest to analyze the shear resistance of the rooted soil assemblies depending on the shear strain. Such an approach could therefore be used to test the different assumptions done in the analytical models. Developing analytical models of slope stability based on the calculation of the shear resistance of rooted soil depending not only on soil and root properties but also on shear strain intensity also constitutes a perspective for the use of the model developed.