Abstract

The objectives of this study were to determine whether the invasive plant Amaranthus viridis influenced soil microbial and chemical properties and to assess the consequences of these modifications on native plant growth. The experiment was conducted in Senegal at two sites: one invaded by A. viridis and the other covered by other plant species. Soil nutrient contents as well as microbial community density, diversity and functions were measured. Additionally, five sahelian Acacia species were grown in (1) soil disinfected or not collected from both sites, (2) uninvaded soil exposed to an A. viridis plant aqueous extract and (3) soil collected from invaded and uninvaded sites and inoculated or not with the arbuscular mycorrhizal (AM) fungus Glomus intraradices. The results showed that the invasion of A. viridis increased soil nutrient availability, bacterial abundance and microbial activities. In contrast, AM fungi and rhizobial development and the growth of Acacia species were severely reduced in A. viridis-invaded soil. Amaranthus viridis aqueous extract also exhibited an inhibitory effect on rhizobial growth, indicating an antibacterial activity of this plant extract. However, the inoculation of G. intraradices was highly beneficial to the growth and nodulation of Acacia species. These results highlight the role of AM symbiosis in the processes involved in plant coexistence and in ecosystem management programs that target preservation of native plant diversity.

Introduction

Varied and considerable impacts on native flora and fauna can result from exotic plant species that invade natural ecosystems and public lands (Adair & Groves, 1998). Exotic plants may become invasive in their introduction to a geographical area through a number of biological processes such as higher performance in their introduced ranges (Thébaud & Simberloff, 2001), release from their native specialized antagonists (Mitchell & Power, 2003), direct chemical interference (allelopathic effect) with native plant species (Callaway & Ridenour, 2004) and resistance variability of native plant communities to invasion (Levine & D'Antonio, 1999). These disturbances of the native plant ecosystem resulting from exotic plant invasion modify succession, dominance, community structure and composition and vegetation dynamics (del Moral & Muller, 1970). It has also been suggested that exotic plants could disrupt mutualistic associations within native microbial communities (Richardson et al., 2000; Callaway & Ridenour, 2004; Stinson et al., 2006). Terrestrial ecosystems' functioning and stability are mainly determined by plant-specific richness and composition, and accumulating evidences indicated that plant species dynamics are strongly interlinked with soil microbiota (Grayston & Campbell, 1996; Hooper & Vitousek, 1997; Hart et al., 2003; Kisa et al., 2007). Therefore, one of the main success ways of exotic invasive plant species might result from these exogenous organisms-mediated modifications in the structure and activities of soil microbial communities (Kourtev et al., 2003; Wolfe & Klironomos, 2005; Batten et al., 2006; Mummey & Rillig, 2006; Stinson et al., 2006).

Among soil microbial communities, arbuscular mycorrhizal (AM) fungi form an important component of the sustainable soil–plant system (Bruno et al., 2003; Johansson et al., 2004; Finlay, 2008). AM symbiosis influences plant development (Schreiner et al., 2003; Duponnois et al., 2005) as well as plant community diversity (van der Heijden et al., 1998b; O'Connor et al., 2002) and could affect relationships between plants (van der Heijden et al., 2003). Another important group of mutualists are the nitrogen-fixing bacterial associates of legumes called rhizobia (Sprent, 2001). Rhizobia induce root nodules and fix atmospheric nitrogen into ammonium that is transferred to their leguminous hosts. This rhizobial symbiosis is important because nitrogen is one of the main nutrients that limits plant productivity in natural ecosystems, especially under tropical environmental conditions. More recently, it has been demonstrated that symbiotic interactions between legumes and rhizobia contribute to plant productivity, plant community structure and acquisition of limiting resources in legume-rich grassland communities (van der Heijden et al., 2006).

Amaranthus viridis L. (Amaranthaceae) is an annual weed native from Central America. This plant is now largely widespread in all warm parts of the world including regions between 30°S and 30°N (Le Bourgeois & Merlier, 1995). Amaranthaceae are characterized by their nitrophily and the C4 photosynthesis pathway. These physiological traits are advantageous in man-made habitats, under conditions of fertilization and/or irrigation (Maillet & Lopez-Garcia, 2000). Furthermore, A. viridis and other species of Amaranthus are also characterized by an extended period of germination, rapid growth and high rates of seed production (Bensch et al., 2003) that might facilitate plant invasiveness (Rejmanek & Richardson, 1996). In Pakistan, where A. viridis is considered as an invasive plant species, Hussain et al. (2003) observed that aqueous extracts from shoots, rain leachate, litter and root exudation significantly reduced the germination and seedling growth of pearl millet (Pennisetum americanum), wheat (Triticum aestivum) and corn (Zea mays). Moreover, they also found that the shoot extract of this exotic plant retarded the development of meristematic cells of test species underlying the allelopathic potential of A. viridis against crops. Of the 50 species of the genus Amaranthus, nine of them, including A. viridis, have already been considered invasive and noxious in the United States and Canada (USDA Plant Database). In Senegal, A. viridis was reported in agrosystems, and increasingly invaded fallow lands, areas of pasture and domestic waste deposit areas. Its growth is positively correlated to soil fertility (e.g. organic matter and nitrogen contents) (Le Bourgeois & Merlier, 1995). Amaranthus viridis, by invading large areas, compromised native plant (grass-, shrub- and woodland) growth. The mechanisms underlying A. viridis' capacity to enter and proliferate within sahelian native plant communities have not yet been investigated. Among native threatened plants, sahelian Acacia species represent keystone trees in sub-Saharan arid and semi-arid areas. Indeed, they are particularly adapted to these regions' harsh climatic conditions and some of them prevent wind and rain erosion, control sand dunes, are sources of wood and provide fodder for browsing livestock. These legume trees have high economic value, and their rhizobial and mycorrhizal symbiosis improves soil fertility (Dreyfus & Dommergues, 1981; Duponnois et al., 2001).

The purpose of this study was to examine the potential for soil-mediated effects resulting from A. viridis establishment, with an emphasis on effects mediated via symbiotic soil microorganisms such as AM fungi and rhizobia, to affect the growth of several sahelian Acacia species [Acacia raddiana thorns, Acacia senegal (L.) Willd. Syn., Acacia seyal Del., Acacia nilotica Willd. ex Del. and Faidherbia albida (Del.) A. Chev], the most representative Acacia species in West Africa. We hypothesized that soil microbiota will respond to A. viridis by changing in both structure and global activities. We studied the modifications on the growth of Acacia species and their mycorrhizal and rhizobial status. We further hypothesized that an enhancement of AM propagule density (through controlled AM fungal inoculation of Acacia species) would minimize the effect of this invasive plant species.

Materials and methods

Experimental ecosystem area and soil sampling

The experimental area was located in the IRD experimental station of Bel Air (14°43′N, 17°26′W) (Dakar, Senegal). The climate is semi-arid with a long dry season (6–8 months), a mean annual temperature of 24 °C and a mean annual rainfall of 300 mm (Gassama-Dia et al., 2003). The plant cover was sparse and degraded. The vegetation is composed of grasses (Alysicarpus ovalifolius L., Fabaceae; Boerhavia diffusa L., Nyctaginaceae; Commelina forskalaei Vahl, Commelinaceae, Eragrostis tremula L., Poaceae, etc.) and tree species (Acacia spp., Mimosaceae; Leucaena spp., Mimosaceae; Balanites aegypticum, Zygophyllaceae), and A. viridis was highly abundant. The average soil physicochemical characteristics were as follows: pH (H2O) 8.2, clay 5.2%, fine silt 1.8%, coarse silt 2.0%, fine sand 66.5%, coarse sand 24.5%, carbon 0.36%, total nitrogen 0.04% and soluble phosphorus 39.6 mg kg−1. A total of 10 sampling (1 × 1 m quadrat) plots were distributed in the studied area according to the presence or not of A. viridis plants (five quadrats with a history of A. viridis and five quadrats without a history of A. viridis, but covered with grasses). Then the soil (0–15-cm depth) of each quadrat was collected, mixed and sieved (mesh size, <2 mm) to remove plant root materials and kept at 4 °C before further analysis. All soil samples were characterized by measuring the total carbon, total nitrogen, total phosphorous and soluble phosphorous contents (Olsen et al., 1954; Aubert, 1978) in the Laboratoire des Moyens Analytiques (LAMA) [Certifié International Standard for Organization 9001, version 2000, Dakar, Senegal; US Imago (Unité de Service Instrumentations, Moyens Analytiques, Observatoires en Géophysique et Océanographie), Institut de Recherche pour le Développement (http://www.lama.ird.sn)].

Measurement of soil microbial global activities

The total microbial activity in soil samples was measured using the fluorescein diacetate [3′, 6′,-diacetylfluorescein (FDA)] hydrolysis assay according to the method of Alef (1998) in which the fluorescein released was assayed colorimetrically at 490 nm, after 1 h of soil incubation. The total microbial activity was expressed as milligrams of product corrected for background fluorescence per hour and per gram of soil. Dehydrogenase activity was measured by readings at A490 nm following the method of Prin et al. (1989) and Schinner et al. (1996) with iodo nitrotetrazolium (INT) as an artificial electron acceptor to form INT-formazan (INTF).

Assessment of soil microbial structure

Bacterial community 16S rRNA gene copy number quantification

Real-time PCR assays were used to quantify the 16S rRNA gene copy number in soil samples according to Cébron et al. (2008). Total DNA from 0.5 g of soil samples were extracted using a bead-beating protocol (Cébron et al., 2008). Samples were mixed with glass beads, 800 μL of extraction buffer [100 mM Tris, 100 mM EDTA, 100 mM NaCl, 1% (w/v) polyvinylpolypyrrolidone, 2% (w/v) sodium dodecyl sulfate, pH 8.0] and 40 μL of 6% CTAB in 5 mM CaCl2. After bead beating on a horizontal grinder Retsch (Roucaire Instruments Scientifiques, France), DNA was extracted using phenol/chloroform/isoamyl alcohol (25/24/1) and washed twice with chloroform/isoamyl alcohol (24/1). DNA precipitation was carried out using isopropanol and then resuspended in 100 μL Tris-HCl buffer (10 mM, pH 8) and stored at −20 °C.

A real-time PCR experiment was conducted in an iCycler iQ (Bio-Rad) associated with iCycler optical system interface software (version 2.3; Bio-Rad). Real-time PCR were performed in 25-μL reaction volumes containing 1 × iQ SYBR Green Supermix (Bio-Rad), 0.4 μM of each primers (968f and 1401r), 0.9 μg μL−1 bovine serum albumin (BSA), 0.5 μL dimethyl sulfoxide (DMSO), 0.1 μL of T4 bacteriophage gene 32 product (QBiogene) and 1 μL of template DNA or distilled water (negative control). Ten times dilution series of a plasmid standard with a known concentration from 108 to 101 target gene copies per microliter were used for a quantification calibration curve. The following temperature profiles were used for rRNA gene amplifications: step one, heating to 95 °C (5 min), followed by 50 cycles of four steps of 30 s of denaturation at 95 °C, 30 s at the primer-specific annealing temperature (56 °C) and 30 s of elongation at 72 °C, and the SYBR Green I signal intensities were measured during a 10-s step at 80 °C. The final step consisted of 7 min at 72 °C. Then a melting curve analysis was performed as a final step by measuring the SYBR Green I signal intensities during a 0.5 °C temperature increment every 10 s from 51 to 95 °C.

Temporal thermal gradient gel electrophoresis (TTGE) analysis

The eubacterial primer set, 968f-GC [with the GC clamp described by Muyzer et al. (1993)] and 1401r, was used to amplify a 475-bp fragment of the 16S rRNA gene (Felske et al., 1998; Heuer et al., 1999). For each amplification reaction, 1 μL of crude DNA extract was added to 49 μL of PCR mix consisting of 50 mM buffer, 3 mM MgCl2, 0.2 mM dNTPs, 3 mg mL−1 of BSA, 1.5 μL of DMSO, 2 μM of (each) primer and 5 U of Taq polymerase (FastStart; Roche Diagnostic). DNA was amplified in an iCycler (Bio-Rad) with the following amplification program: 94 °C for 5 min (one cycle); 94 °C for 40 s, 56 °C for 30 s and 72 °C for 40 s (38 cycles); and 72 °C for 5 min (one cycle). PCR products, stained with ethidium bromide, were visualized under UV light after electrophoresis in a 1% (w/v) agarose gel to verify the size and quantity of amplified PCR products.

16S rRNA gene fingerprints of the bacterial community present in soil samples were performed by TTGE using the Dcode universal mutation detection system (Bio-Rad Laboratories) under the conditions modified from Corgié (2004). This molecular method has already been proven to give sufficient and reproducible information about modifications in the soil microbial community structure (Muyzer & Smalla, 1998) and has been used in various environmental samples (Halos et al., 2006). The polyacrylamide gels [6% (w/v) acrylamide, 0.21% (w/v) bisacrylamide, 7 M urea, 1.25 × Tris-acetate-EDTA and 0.2% (v/v) glycerol] were allowed to polymerize for 1 h. DNA samples (10 μL) were separated by electrophoresis in 1.25 × Tris-acetate-EDTA at a constant voltage (100 V), with a temperature gradient from 57 to 67 °C (temperature increment of 2 °C h−1). After electrophoresis, gels were stained with ethidium bromide and enumerated under UV light.

Quantification of band intensity was performed using Gel Doc (Biorad) coupled to quantity one software. For each profile, the intensity of the bands was summed and the relative intensity of each band was calculated in order to assess the abundance of the species in the initial sample (Marschner & Baumann, 2003).

Assessment of AM fungus community structure

AM hyphal length was measured on membrane filters according to Jakobsen et al. (1992). The total hyphal length was estimated using the Gridline intersect method (Hanssen et al., 1974). The AM fungi hyphae were distinguished from hyphae of other soil fungi following the morphological criteria described by Nicolson (1959). AM fungal spores were also extracted from soil samples by wet sieving and decanting, followed by sucrose centrifugation (Gerdemann & Nicolson, 1963). After centrifugation, the supernatant was poured through a 50-mm sieve and rinsed with tap water. Spores were counted under a stereomicroscope and grouped according to morphological characteristics. The relative abundance of each fungal type was calculated per 100 g of dry soil. Spore size and color were assessed in water under a stereomicroscope (Olympus SZ H10 research stereomicroscope) whereas spore wall structures and other attributes were observed on permanent slides prepared according to Azcon-Aguilar et al. (2003) under a microscope connected to a computer with a digital image analysis software. Morphotype classification to the genus level, and when possible to the species, was mainly based on morphological features such as color, size, wall structure and hyphal attachment (Morton & Benny, 1990; INVAM, 1997). The relative abundance of each fungal species in each treatment was calculated.

Greenhouse experiments

Assessment of A. viridis mycorrhizal dependence

The AM fungus Glomus intraradices, Schenk and Smith (DAOM 181602, Ottawa Agricultural Herbarium) was propagated on maize (Z. mays L.) for 12 weeks under greenhouse conditions in calcined clay (Plenchette et al., 1996). Maize plants were uprooted, gently washed and colonized roots were hand cut into 1–3-mm-long pieces, bearing around 250 vesicles cm−1, each considered as one propagule (Plenchette, 2000). To obtain a logarithmic scale of inoculum density (0, 3, 30 and 100 propagules per 100 g soil), AM root pieces were counted under a dissecting microscope and, for each inoculum rate, the number was adjusted to 100 root pieces per 100 g of soil with nonmycorrhizal maize roots, prepared as above. Root pieces were then thoroughly mixed with a disinfected sandy soil (120 °C, 60 min) whose physicochemical characteristics were as follows: pH (H2O) 5.3, clay (%) 3.6, fine silt (%) 0.0, coarse silt (%) 0.8, fine sand (%) 55.5, coarse sand (%) 39.4, carbon (%) 0.17, nitrogen (%) 0.02, total phosphorous (mg kg−1) 39 and Olsen phosphorous (mg kg−1) 4.8.

Seeds of A. viridis collected from the experimental area were surface-sterilized with 1% NaOCl for 15 min and rinsed with demineralized water. They were pre germinated for 2 days in Petri dishes on a humid filter paper at 25 ° C in the dark. The germinating seeds were used when rootlets were 1–2 cm long. For each inoculum density, plastic pots (5.5 cm diameter; 6 cm high) were filled with 100 g of soil containing the required number of AM propagules, and one pre germinated seed of A. viridis was planted per pot. Pots were arranged in a randomized complete block design with six replicates per treatment. They were placed under greenhouse conditions (30 ° C day, 20 ° C night, 10-h photoperiod) and watered daily with deionized water without adding nutrients. After 3 months of culture, the plants were harvested, and the oven-dried weight (1 week at 65 ° C) of the shoot was recorded. The root systems were gently washed, cleared and stained according to the method of Phillips & Hayman (1970). About 50 1-cm root pieces were observed per plant under a microscope (magnification, × 250). The extent of mycorrhizal colonization was expressed as (the number of mycorrhizal root pieces)/(total number of observed root pieces) × 100. The remaining roots were oven dried (1 week at 65 °C) and weighed.

Effect of soil origins on Acacia seedling growth

Seeds of Sahelian Acacia species, A. raddiana, A. senegal, A. seyal, A. nilotica and F. albida were surface sterilized with 95% concentrated sulfuric acid for 15 min (A. senegal), 30 min (A. seyal, F. albida), 60 min (A. raddiana) and 120 min (A. nilotica). They were pre germinated for 2 days in Petri dishes on a humid filter paper at 25 ° C in the dark. Soils from the same origin were pooled in the lab to obtain two types of soil. One part of the soil collected from A. viridis -invaded area or -uninvaded area was disinfected by autoclaving (120 ° C, 60 min) and the other part was not (undisinfected) to perform four treatments: (1) soil with a history of A. viridis, (2) disinfected soil with a history of A. viridis, (3) soil without a history of A. viridis and (4) disinfected soil without a history of A. viridis. For each soil treatment and each origin, plastic pots (4-cm diameter, 20-cm height) were filled with 250 g of soil and one pre germinated seed of the tested Acacia species was planted per pot. Pots were arranged in a randomized complete block design with six replicates per treatment under the same greenhouse conditions as described previously. After 5 months of culture, the plants were harvested and the oven-dried weight (1 week at 65 ° C) of the shoots was measured. Their entire root systems were washed under tap water. On each plant, the extent of AM colonization was assessed (Phillips & Hayman, 1970) and the total dry weight of root nodule per plant (1 week at 65 °C) was determined. Then the roots were oven dried (1 week at 65 °C) and weighed.

Effect of A. viridis plant extract on Acacia seedling growth

Amaranthus viridis plant extract was prepared by grinding 10 g fresh weight of the whole plant material in 100 mL of deionized water in a Waring Blender and filtering through Whatman no. 1 paper with a Buchner funnel. A 100-mL aqueous extract (or 100-mL distilled water for the control treatment) was added to plastic pots filled with 250 g of soil without a history of A. viridis. After 1 week of soil exposure to the extract, one germinating seed of each Acacia species was planted per pot with six replicates per treatment. Pots were arranged in a randomized complete block design under the same greenhouse conditions as before. After 8 months of culture, the plants were harvested, shoot and root biomasses were determined and the total dry weight of root nodule per plant (1 week at 65 ° C) and percent mycorrhizal colonization of roots were assayed.

Effect of G. intraradices inoculation on Acacia seedling growth planted in soil invaded by A. viridis or uninvaded by A. viridis

One germinating seed of each Acacia species was planted per pot filled with 250 g of the following soil treatments: (1) soil invaded by A. viridis, (2) soil invaded by A. viridis and inoculated with G. intraradices, (3) soil uninvaded by A. viridis and (4) soil uninvaded by A. viridis and inoculated with G. intraradices. The inoculum of G. intraradices was produced as described previously. The AM inoculation consisted of adding 1 g of fresh maize root (mycorrhizal, or not, for the control without fungus) to a hole (1 cm × 5 cm) made in each pot. The experimental design and the environmental conditions of the culture in the greenhouse were the same as described before. After an 8-month culture, plants were harvested, dried at 65 ° C for 1 week, weighed to determine biomass and the root nodulation was calculated. Root AM colonization was determined according to Phillips & Hayman (1970).

Plant-trapping tests

Pregerminated seeds (1–2-cm-long rootlets) of Acacia species were transferred under aseptic conditions into Gibson tubes (Gibson, 1963) containing a sterile Jensen nitrogen-free medium (Vincent, 1970). The tubes were placed in a growth chamber under controlled conditions (12 h day length at 28 °C, 40 000 lx, 75% relative humidity and 20 °C at nights). After 1 week of growth, 1 mL of the stirred soil solution was added to each tube. A soil suspension was obtained by adding 10 g of each soil sample to 90 mL of sterile saline buffer pH 7 (NaCl 0.15 M, KH2PO4 0.002 M, Na2HPO4 0.004 M) for 1 h. Soil treatments were as follows: (1) soil invaded by A. viridis, (2) soil uninvaded by A. viridis and (3) soil uninvaded by A. viridis and moistened by the extract of whole plants of A. viridis at the same rate as described before. Four replicates were made for each soil. Uninoculated plants were used as controls. Plants were observed for nodule formation for 6 weeks after inoculation, and fresh nodules were collected, numbered and oven dried.

Test of antibacterial activity

The aqueous extract of whole plants of A. viridis was tested for antibacterial activity by the diffusion technique on solid media against a seeded culture of selected strains of nitrogen-fixing bacteria (Table 1) (Vincent & Vincent, 1944). Sterilized filter paper disks (diameter 13 mm) were saturated with 0.1 mL of an A. viridis aqueous extract. One hundred milliliters of a 48-h liquid culture of rhizobia was used in seeding the Petri dishes. After inoculation, dishes were usually allowed to dry for a few hours before the test disks were added. The test plates were routinely examined and the zones of inhibition were recorded after 1 week of incubation at 25 °C in the dark.

1

Effect of fresh extract of Amaranthus viridis whole plant on nitrogen-fixing bacteria

Rhizobial strains Genus Host plant Country of collection Effect of fresh extract of A. viridis whole plant 
ORS 1080 Rhizobium A. raddiana Sénégal +* 
ORS 1081 Rhizobium A. raddiana Sénégal + 
ORS 1082 Rhizobium A. raddiana Sénégal + 
ORS 1083 Rhizobium A. raddiana Sénégal + 
ORS 1084 Rhizobium A. raddiana Sénégal + 
ORS 1140 Rhizobium F. albida Sénégal + 
ORS 1141 Rhizobium F. albida Sénégal + 
ORS 1142 Rhizobium F. albida Sénégal + 
ORS 1143 Rhizobium F. albida Sénégal + 
ORS 1146 Rhizobium F. albida Sénégal + 
ORS 1302 Rhizobium A. seyal Burkina Faso + 
ORS 1317 Rhizobium A. seyal Mauritanie + 
ORS 1320 Rhizobium A. seyal Mauritanie + 
ORS 3453 Mesorhizobium A. seyal Sénégal + 
ORS 3454 Mesorhizobium A. seyal Sénégal + 
ORS 3160 Rhizobium A. nilotica Sénégal + 
ORS 3161 Rhizobium A. nilotica Sénégal + 
ORS 3162 Rhizobium A. nilotica Sénégal + 
ORS 3163 Rhizobium A. nilotica Sénégal + 
ORS 3164 Rhizobium A. nilotica Sénégal + 
ORS 3416 Mesorhizobium A. senegal Sénégal + 
ORS 3417 Mesorhizobium A. senegal Sénégal + 
ORS 3418 Mesorhizobium A. senegal Sénégal + 
ORS 3419 Mesorhizobium A. senegal Sénégal + 
ORS 3420 Mesorhizobium A. senegal Sénégal + 
Rhizobial strains Genus Host plant Country of collection Effect of fresh extract of A. viridis whole plant 
ORS 1080 Rhizobium A. raddiana Sénégal +* 
ORS 1081 Rhizobium A. raddiana Sénégal + 
ORS 1082 Rhizobium A. raddiana Sénégal + 
ORS 1083 Rhizobium A. raddiana Sénégal + 
ORS 1084 Rhizobium A. raddiana Sénégal + 
ORS 1140 Rhizobium F. albida Sénégal + 
ORS 1141 Rhizobium F. albida Sénégal + 
ORS 1142 Rhizobium F. albida Sénégal + 
ORS 1143 Rhizobium F. albida Sénégal + 
ORS 1146 Rhizobium F. albida Sénégal + 
ORS 1302 Rhizobium A. seyal Burkina Faso + 
ORS 1317 Rhizobium A. seyal Mauritanie + 
ORS 1320 Rhizobium A. seyal Mauritanie + 
ORS 3453 Mesorhizobium A. seyal Sénégal + 
ORS 3454 Mesorhizobium A. seyal Sénégal + 
ORS 3160 Rhizobium A. nilotica Sénégal + 
ORS 3161 Rhizobium A. nilotica Sénégal + 
ORS 3162 Rhizobium A. nilotica Sénégal + 
ORS 3163 Rhizobium A. nilotica Sénégal + 
ORS 3164 Rhizobium A. nilotica Sénégal + 
ORS 3416 Mesorhizobium A. senegal Sénégal + 
ORS 3417 Mesorhizobium A. senegal Sénégal + 
ORS 3418 Mesorhizobium A. senegal Sénégal + 
ORS 3419 Mesorhizobium A. senegal Sénégal + 
ORS 3420 Mesorhizobium A. senegal Sénégal + 
*

The zone of bacterial growth inhibition around the filter paper disks moistened with the fresh extract of Amaranthus viridis whole plant.

Statistical analysis

All data were subjected to a one-way anova and the mean values were compared using Newman–Keul's test (P<0.05). For percentage mycorrhizal infection, data were transformed by Arc sin√x. Community similarities between TTGE profiles, based on the relative band intensity, were analyzed by principal component analysis (Wikström et al., 1999) with ade-4 software (http://pbil.univ-lyon1.fr/ADE-4/) (Thioulouse et al., 1997).

Results

Chemical and microbial soil characteristics

Soil nitrogen, carbon, total phosphorus and soluble phosphorus contents were significantly higher in the soil collected from the A. viridis-invaded area than in the soil sampled from the uninvaded area (Table 2). The same positive effects of A. viridis were observed on the total microbial activity (FDA) and dehydrogenase activity (Table 2).

2

Chemical characteristics and enzymatic activities of soils invaded or not by Amaranthus viridis plants

 Uninvaded by A. viridis Invaded by A. viridis 
Total nitrogen (%) 0.11 (0.003)* a† 0.20 (0.006) b 
Total carbon (%) 1.21 (0.037) a 2.46 (0.067) b 
Total phosphorus (mg kg−1625.3 (5.24) a 1675.7 (322.48) b 
Soluble phosphorus (mg kg−1107.6 (19.57) a 211.6 (5.03) b 
16S rRNA gene copy number per gram of soil (× 1074.26 a 33.73 b 
Total microbial activity (mg of hydrolyzed fluorescein diacetate h−1 g−1 of soil) 8.94 (0.56) a 18.04 (1.01) b 
Dehydrogenase activity (mg INTF day−1 g−1 of soil) 5.03 (0.55) a 29.44 (1.96) b 
 Uninvaded by A. viridis Invaded by A. viridis 
Total nitrogen (%) 0.11 (0.003)* a† 0.20 (0.006) b 
Total carbon (%) 1.21 (0.037) a 2.46 (0.067) b 
Total phosphorus (mg kg−1625.3 (5.24) a 1675.7 (322.48) b 
Soluble phosphorus (mg kg−1107.6 (19.57) a 211.6 (5.03) b 
16S rRNA gene copy number per gram of soil (× 1074.26 a 33.73 b 
Total microbial activity (mg of hydrolyzed fluorescein diacetate h−1 g−1 of soil) 8.94 (0.56) a 18.04 (1.01) b 
Dehydrogenase activity (mg INTF day−1 g−1 of soil) 5.03 (0.55) a 29.44 (1.96) b 
*

SE.

Data in the same line followed by the same letter are not significantly different according to the Newman–Keul test (P<0.05).

The total bacterial community structures from both soils were assessed by TTGE analysis of the 16S rRNA gene (Fig. 1). The TTGE gel displayed numerous bands (mean number, 14) of various intensities that resulted from differences between the 16S rRNA gene sequences of different bacterial species. The two soils shared most of the TTGE bands, suggesting that a common bacterial population colonized these soils. However, one bacterial species seemed to be more represented in soils invaded by A. viridis. Ordination plots generated by principal component analysis of band intensity data (representation of the first two axes, about 75% of the total inertia) clearly separated the soils on the basis of their origin (Fig. 2). The four replicates of soil collected from invaded sites presented a very close group in comparison with soils sampled from uninvaded sites, indicative of a strong disturbance in recipient soil bacterial populations and, therefore, their homogenization. Modifications occurring in the bacterial community structure were accompanied by shifts in the community density in soil samples. The data from the quantification of 16S rRNA gene copy number indicated significant stimulation of bacterial population in the soil invaded by A. viridis. The 16S rRNA gene copy number averaged 33.7 × 107 in soil collected from invaded sites and about 4.3 × 107 in uninvaded soil (Table 2).

1

16S rRNA gene-TTGE patterns of the total bacterial communities from the soils invaded or not by Amaranthus viridis plants.

1

16S rRNA gene-TTGE patterns of the total bacterial communities from the soils invaded or not by Amaranthus viridis plants.

2

Principal component analysis of the band intensity data of soils invaded or not with Amaranthus viridis.

2

Principal component analysis of the band intensity data of soils invaded or not with Amaranthus viridis.

The total number of AM spores as well as the length of external hyphae found in the uninvaded soil were significantly higher than that recorded in the invaded soil (Table 3). Two genera (Glomus and Scutellospora) and four species (Glomus constrictum, G. intraradices, Scutellospora armeniaca and Scutellospora claroideum) were present in both soil origins, but their abundance was significantly higher in the uninvaded soil (Table 3).

3

Influence of Amaranthus viridis on the structure of AM fungal communities and hyphal length

 Uninvaded by A. viridis Invaded by A. viridis 
AM fungal species (number of AM spores per 100 g of soil) 
Glomus constrictum 290.2 (23.4)* b† 104.2 (6.1) a 
Glomus intraradices 16.1 (3.1) b 5.2 (1.1) a 
Scutellospora armeniaca 31.5 (2.6) b 13.7 (2.1) a 
Scutellospora claroideum 12.7 (1.9) b 6.7 (1.7) a 
Total number of AM spores per 100 g of soil 350.3 (26.2) b 129.7 (8.4) a 
Hyphal length (m g−1 of soil) 3.21 (0.32) b 0.83 (0.08) a 
 Uninvaded by A. viridis Invaded by A. viridis 
AM fungal species (number of AM spores per 100 g of soil) 
Glomus constrictum 290.2 (23.4)* b† 104.2 (6.1) a 
Glomus intraradices 16.1 (3.1) b 5.2 (1.1) a 
Scutellospora armeniaca 31.5 (2.6) b 13.7 (2.1) a 
Scutellospora claroideum 12.7 (1.9) b 6.7 (1.7) a 
Total number of AM spores per 100 g of soil 350.3 (26.2) b 129.7 (8.4) a 
Hyphal length (m g−1 of soil) 3.21 (0.32) b 0.83 (0.08) a 
*

SE.

Data in the same line followed by the same letter are not significantly different according to the Newman–Keul test (P<0.05).

Mycorrhizae (vesicles) were observed on A. viridis root systems only at the highest inoculum rates (30 and 100 AM propagules per 100 g of soil) (Table 4). Plant growth was negatively linked with the rates of AM inoculation (r2=0.34, P=0.0028).

4

Mycorrhizal dependency assessment of Amaranthus viridis plants on a disinfected soil inoculated with the AM fungus Glomus intraradices

 Number of mycorrhizal root fragments per 100 g of soil 
30 100 
Shoot biomass (mg dry weight) 322.9 b 274.3 a 206.1 a 141.5 a 
Root biomass (mg dry weight) 270.2 b 171.2 ab 129.7 ab 119.8 a 
Mycorrhizal colonization (%) 0 a 0 a 4.3 b 7.5 c 
 Number of mycorrhizal root fragments per 100 g of soil 
30 100 
Shoot biomass (mg dry weight) 322.9 b 274.3 a 206.1 a 141.5 a 
Root biomass (mg dry weight) 270.2 b 171.2 ab 129.7 ab 119.8 a 
Mycorrhizal colonization (%) 0 a 0 a 4.3 b 7.5 c 

Data in the same line followed by the same letter are not significantly different according to the Newman–Keul test (P<0.05).

Growth response of Acacia species in soils collected from uninvaded and invaded soils

The shoot and root growth, root nodule biomass and AM colonization of all the tested Acacia species were significantly decreased when plants were cultivated in the soil invaded by A. viridis than in the uninvaded soil (Table 5). In both sterilized soils invaded by A. viridis and uninvaded soils, no significant difference in the shoot and root biomasses of A. raddiana, A. senegal and F. albida was observed while these biomasses were significantly lower for other Acacia species after culturing in invaded soil (Table 5). Moreover, reductions in growth of A. raddiana, A. senegal and F. albida plants when cultured in the unsterilized soil, but invaded by A. viridis, were similar to those observed when seedlings were grown in sterilized soil from both sites invaded or not by A. viridis (Table 5).

5

Growth, mycorrhizal colonization and nodule biomass of Acacia species in Amaranthus viridis-invaded and -uninvaded soils sterilized or not

 Uninvaded by A. viridis Invaded by A. viridis 
Sterilized Unsterilized Sterilized Unsterilized 
A. raddiana 
Shoot biomass (mg dry weight) 128.3 b 165.1 c 111.7 ab 100.0 a 
Root biomass (mg dry weight) 88.3 ab 96.7 b 51.7 a 44.7 a 
Mycorrhizal colonization (%) – 20.8 b – 9.7 a 
Nodule biomass (mg dry weight) – 5.5 b – 0.0 a 
A. senegal 
Shoot biomass (mg dry weight) 205.0 a 230.1 b 201.7 a 206.7 a 
Root biomass (mg dry weight) 210.1 a 223.3 a 195.0 a 181.7 a 
Mycorrhizal colonization (%) – 75.5 b – 33.1 a 
Nodule biomass (mg dry weight) – 8.8 b – 2.3 a 
F. albida 
Shoot biomass (mg dry weight) 391.7 a 485.1 b 306.7 a 385.1 a 
Root biomass (mg dry weight) 500.1 ab 580.0 b 461.8 a 461.9 a 
Mycorrhizal colonization (%) – 83.7 b – 29.8 a 
Nodule biomass (mg dry weight) – 17.5 b – 6.9 a 
A. seyal 
Shoot biomass (mg dry weight) 1083.3 c 1850.1 d 816.7 a 1366.7 b 
Root biomass (mg dry weight) 1636.7 b 2066.7 c 1333.3 a 1633.3 b 
Mycorrhizal colonization (%) – 38.7 b – 11.8 a 
Nodule biomass (mg dry weight) – 116.6 b – 72.5 a 
A. nilotica 
Shoot biomass (mg dry weight) 1953.3 b 2541.7 c 1513.3 a 2201.7 b 
Root biomass (mg dry weight) 760.1 b 1006.7 c 611.7 a 930.1 c 
Mycorrhizal colonization (%) – 51.8 b – 28.3 a 
Nodule biomass (mg dry weight) – 112.8 b – 69.7 a 
 Uninvaded by A. viridis Invaded by A. viridis 
Sterilized Unsterilized Sterilized Unsterilized 
A. raddiana 
Shoot biomass (mg dry weight) 128.3 b 165.1 c 111.7 ab 100.0 a 
Root biomass (mg dry weight) 88.3 ab 96.7 b 51.7 a 44.7 a 
Mycorrhizal colonization (%) – 20.8 b – 9.7 a 
Nodule biomass (mg dry weight) – 5.5 b – 0.0 a 
A. senegal 
Shoot biomass (mg dry weight) 205.0 a 230.1 b 201.7 a 206.7 a 
Root biomass (mg dry weight) 210.1 a 223.3 a 195.0 a 181.7 a 
Mycorrhizal colonization (%) – 75.5 b – 33.1 a 
Nodule biomass (mg dry weight) – 8.8 b – 2.3 a 
F. albida 
Shoot biomass (mg dry weight) 391.7 a 485.1 b 306.7 a 385.1 a 
Root biomass (mg dry weight) 500.1 ab 580.0 b 461.8 a 461.9 a 
Mycorrhizal colonization (%) – 83.7 b – 29.8 a 
Nodule biomass (mg dry weight) – 17.5 b – 6.9 a 
A. seyal 
Shoot biomass (mg dry weight) 1083.3 c 1850.1 d 816.7 a 1366.7 b 
Root biomass (mg dry weight) 1636.7 b 2066.7 c 1333.3 a 1633.3 b 
Mycorrhizal colonization (%) – 38.7 b – 11.8 a 
Nodule biomass (mg dry weight) – 116.6 b – 72.5 a 
A. nilotica 
Shoot biomass (mg dry weight) 1953.3 b 2541.7 c 1513.3 a 2201.7 b 
Root biomass (mg dry weight) 760.1 b 1006.7 c 611.7 a 930.1 c 
Mycorrhizal colonization (%) – 51.8 b – 28.3 a 
Nodule biomass (mg dry weight) – 112.8 b – 69.7 a 

Data in the same line followed by the same letter are not significantly different according to the Newman-Keul test (P<0.05).

Extracts of whole plants of A. viridis significantly decreased the shoot growth of A. senegal, F. albida, A. seyal and A. nilotica and the root growth of A. raddiana and A. nilotica (Table 6). For all the Acacia species, mycorrhizal colonization and root nodule biomass were significantly lower in the soil inoculated with A. viridis aqueous extract (Table 6).

6

Growth, mycorrhizal colonization and nodule biomass of Acacia species planted in Amaranthus viridis-uninvaded soil inoculated or not (control) with fresh extract of A. viridis whole plant

 Without A. viridis plant extract With A. viridis plant extract 
A. raddiana 
Shoot biomass (mg dry weight) 1474.0 a 1232.1 a 
Root biomass (mg dry weight) 758.2 b 586.1 a 
Mycorrhizal colonization (%) 67.2 b 30.6 a 
Nodule biomass (mg dry weight) 40.0 b 29.2 a 
A. senegal 
Shoot biomass (mg dry weight) 1010.2 b 764.3 a 
Root biomass (mg dry weight) 942.1 a 824.2 a 
Mycorrhizal colonization (%) 91.6 b 31.4 a 
Nodule biomass (mg dry weight) 40.1 b 14.9 a 
F. albida 
Shoot biomass (mg dry weight) 1450.2 b 990.3 a 
Root biomass (mg dry weight) 1250.0 a 1007.1 a 
Mycorrhizal colonization (%) 92.4 b 32.1 a 
Nodule biomass (mg dry weight) 52.6 b 21.8 a 
A. seyal 
Shoot biomass (mg dry weight) 2640.1 b 1724.3 a 
Root biomass (mg dry weight) 2680.2 a 2206.1 a 
Mycorrhizal colonization (%) 69.8 b 27.6 a 
Nodule biomass (mg dry weight) 137.9 b 46.5 a 
A. nilotica 
Shoot biomass (mg dry weight) 3674.1 b 3122.0 a 
Root biomass (mg dry weight) 882.0 b 738.2 a 
Mycorrhizal colonization (%) 57.4 b 19.8 a 
Nodule biomass (mg dry weight) 130.4 b 87.9 a 
 Without A. viridis plant extract With A. viridis plant extract 
A. raddiana 
Shoot biomass (mg dry weight) 1474.0 a 1232.1 a 
Root biomass (mg dry weight) 758.2 b 586.1 a 
Mycorrhizal colonization (%) 67.2 b 30.6 a 
Nodule biomass (mg dry weight) 40.0 b 29.2 a 
A. senegal 
Shoot biomass (mg dry weight) 1010.2 b 764.3 a 
Root biomass (mg dry weight) 942.1 a 824.2 a 
Mycorrhizal colonization (%) 91.6 b 31.4 a 
Nodule biomass (mg dry weight) 40.1 b 14.9 a 
F. albida 
Shoot biomass (mg dry weight) 1450.2 b 990.3 a 
Root biomass (mg dry weight) 1250.0 a 1007.1 a 
Mycorrhizal colonization (%) 92.4 b 32.1 a 
Nodule biomass (mg dry weight) 52.6 b 21.8 a 
A. seyal 
Shoot biomass (mg dry weight) 2640.1 b 1724.3 a 
Root biomass (mg dry weight) 2680.2 a 2206.1 a 
Mycorrhizal colonization (%) 69.8 b 27.6 a 
Nodule biomass (mg dry weight) 137.9 b 46.5 a 
A. nilotica 
Shoot biomass (mg dry weight) 3674.1 b 3122.0 a 
Root biomass (mg dry weight) 882.0 b 738.2 a 
Mycorrhizal colonization (%) 57.4 b 19.8 a 
Nodule biomass (mg dry weight) 130.4 b 87.9 a 

Data in the same line followed by the same letter are not significantly different according to the Newman-Keul test (P<0.05).

For both soils invaded by A. viridis and uninvaded by A. viridis, G. intraradices inoculation had significantly improved shoot and root growth of all Acacia species (Table 7), with higher growth stimulation in the soil invaded by A. viridis (Table 7). AM inoculation had also improved root nodule biomass in both soils (Table 7).

7

Growth, mycorrhizal colonization and nodule biomass of Acacia species planted in Amaranthus viridis-invaded or -uninvaded soils and inoculated (with Gi) or not with Glomus intraradices (without Gi)

 Uninvaded by A. viridis Invaded by A. viridis 
With Gi Without Gi With Gi Without Gi 
A. raddiana 
Shoot biomass (mg dry weight) 1255.1 c 988.3 b 1431.7 c 720.2 a 
Root biomass (mg dry weight) 1130.2 c 976.7 b 1190.0 c 670.1 a 
Mycorrhizal colonization (%) 73.3 d 8.2 b 56.1 c 2.1 a 
Nodule biomass (mg dry weight) 40.5 c 24.5 b 35.6 c 13.9 a 
A. senegal 
Shoot biomass (mg dry weight) 1123.3 d 638.3 b 863.3 c 481.7 a 
Root biomass (mg dry weight) 1205.0 c 943.3 b 1163.3 c 685.1 a 
Mycorrhizal colonization (%) 49.2 d 17.8 b 28.2 c 4.3 a 
Nodule biomass (mg dry weight) 68.1 d 14.7 b 29.7 c 0.0 a 
F. albida 
Shoot biomass (mg dry weight) 1293.3 c 828.3 b 1073.3 c 570.0 a 
Root biomass (mg dry weight) 1346.7 c 1140.0 b 1518.3 c 898.3 a 
Mycorrhizal colonization (%) 51.2 c 15.8 b 53.3 c 5.3 a 
Nodule biomass (mg dry weight) 28.6 c 18.4 b 17.7 ab 12.5 a 
A. seyal 
Shoot biomass (mg dry weight) 2583.3 c 1891.7 b 2268.3 c 1457.7 a 
Root biomass (mg dry weight) 3233.3 c 2515.0 b 2710.0 c 1498.3 a 
Mycorrhizal colonization (%) 74.2 b 14.2 a 85.1 c 10.8 a 
Nodule biomass (mg dry weight) 92.7 d 13.5 b 21.4 c 0.0 a 
A. nilotica 
Shoot biomass (mg dry weight) 3320.0 b 2728.3 a 2945.1 b 2535.1 a 
Root biomass (mg dry weight) 738.3 a 671.7 a 760.1 a 686.7 a 
Mycorrhizal colonization (%) 36.7 b 3.2 a 25.3 b 1.8 a 
Nodule biomass (mg dry weight) 29.1 c 10.1 b 18.6 bc 2.2 a 
 Uninvaded by A. viridis Invaded by A. viridis 
With Gi Without Gi With Gi Without Gi 
A. raddiana 
Shoot biomass (mg dry weight) 1255.1 c 988.3 b 1431.7 c 720.2 a 
Root biomass (mg dry weight) 1130.2 c 976.7 b 1190.0 c 670.1 a 
Mycorrhizal colonization (%) 73.3 d 8.2 b 56.1 c 2.1 a 
Nodule biomass (mg dry weight) 40.5 c 24.5 b 35.6 c 13.9 a 
A. senegal 
Shoot biomass (mg dry weight) 1123.3 d 638.3 b 863.3 c 481.7 a 
Root biomass (mg dry weight) 1205.0 c 943.3 b 1163.3 c 685.1 a 
Mycorrhizal colonization (%) 49.2 d 17.8 b 28.2 c 4.3 a 
Nodule biomass (mg dry weight) 68.1 d 14.7 b 29.7 c 0.0 a 
F. albida 
Shoot biomass (mg dry weight) 1293.3 c 828.3 b 1073.3 c 570.0 a 
Root biomass (mg dry weight) 1346.7 c 1140.0 b 1518.3 c 898.3 a 
Mycorrhizal colonization (%) 51.2 c 15.8 b 53.3 c 5.3 a 
Nodule biomass (mg dry weight) 28.6 c 18.4 b 17.7 ab 12.5 a 
A. seyal 
Shoot biomass (mg dry weight) 2583.3 c 1891.7 b 2268.3 c 1457.7 a 
Root biomass (mg dry weight) 3233.3 c 2515.0 b 2710.0 c 1498.3 a 
Mycorrhizal colonization (%) 74.2 b 14.2 a 85.1 c 10.8 a 
Nodule biomass (mg dry weight) 92.7 d 13.5 b 21.4 c 0.0 a 
A. nilotica 
Shoot biomass (mg dry weight) 3320.0 b 2728.3 a 2945.1 b 2535.1 a 
Root biomass (mg dry weight) 738.3 a 671.7 a 760.1 a 686.7 a 
Mycorrhizal colonization (%) 36.7 b 3.2 a 25.3 b 1.8 a 
Nodule biomass (mg dry weight) 29.1 c 10.1 b 18.6 bc 2.2 a 

Data in the same line followed by the same letter are not significantly different according to the Newman–Keul test (P<0.05).

Nodulation assessment and antibacterial activity of A. viridis aqueous extract

The number of nodule and their total biomass per plant recorded on each Acacia species were significantly lower in the invaded soil than in the uninvaded soil (Table 8). This negative effect was found when the A. viridis aqueous extract was mixed with the uninvaded soil (Table 8). In addition, the A. viridis aqueous extract exhibited an inhibitory effect against all the rhizobial strains tested (Table 1).

8

Nodulation of Acacia species in vitro conditions with soil suspensions from Amaranthus viridis-invaded or -uninvaded soils and from the uninvaded soil mixed with fresh extract of A. viridis whole plant

Acacia species Soil suspension origins 
A. viridis- uninvaded soil A. viridis- invaded soil A. viridis- uninvaded soil +A. viridis extract 
A. raddiana 
Number of nodule per plant 7.7 b 0.0 a 1.7 a 
Total biomass of nodule per plant (mg dry weight) 5.4 b 0.0 a 0.3 a 
A. senegal 
Number of nodule per plant 5.7 b 3.7 ab 1.2 a 
Total biomass of nodule per plant 4.1 b 1.6 ab 0.4 a 
F. albida 
Number of nodule per plant 50.5 c 21.6 b 1.6 a 
Total biomass of nodule per plant 20.8 b 9.8 a 8.2 a 
A. seyal 
Number of nodule per plant 11.0 b 0.6 a 4.2 ab 
Total biomass of nodule per plant 18.2 b 9.8 a 8.2 a 
A. nilotica 
Number of nodule per plant 2.4 a 0.0 a 0.4 a 
Total biomass of nodule per plant 3.3 b 0.0 a 0.1 a 
Acacia species Soil suspension origins 
A. viridis- uninvaded soil A. viridis- invaded soil A. viridis- uninvaded soil +A. viridis extract 
A. raddiana 
Number of nodule per plant 7.7 b 0.0 a 1.7 a 
Total biomass of nodule per plant (mg dry weight) 5.4 b 0.0 a 0.3 a 
A. senegal 
Number of nodule per plant 5.7 b 3.7 ab 1.2 a 
Total biomass of nodule per plant 4.1 b 1.6 ab 0.4 a 
F. albida 
Number of nodule per plant 50.5 c 21.6 b 1.6 a 
Total biomass of nodule per plant 20.8 b 9.8 a 8.2 a 
A. seyal 
Number of nodule per plant 11.0 b 0.6 a 4.2 ab 
Total biomass of nodule per plant 18.2 b 9.8 a 8.2 a 
A. nilotica 
Number of nodule per plant 2.4 a 0.0 a 0.4 a 
Total biomass of nodule per plant 3.3 b 0.0 a 0.1 a 

Data in the same line followed by the same letter are not significantly different according to the Newman-Keul test (P<0.05).

Discussion

Our results clearly show that the weed plant species, A. viridis, which is usually considered as a nonmycorrhizal plant, exerts a positive effect on soil nutrient content by increasing carbon, nitrogen and phosphorous concentrations and microbial activities (FDA and dehydrogenase) whereas it negatively affects the growth of the Acacia species by altering the development of rhizobia and AM communities. Moreover, AM inoculation was beneficial to the growth of Acacia species irrespective of whether the natural population of AM fungi in the soil was impoverished or not.

In our study, we observed differences between the average soil physicochemical characteristics in the IRD experimental station and those measured in the quadrant sampled for the experimental survey (with and without A. viridis). These differences might be due to the fact that in the study, soils have been sampled only in patches colonized by vegetation that is able to strongly mediate nutrient cycling and availability. By contrast, the determination of the global physicochemical characteristics of the site was assayed on soil samples representative of the site, including bulk soil patches on which nutrient cycling might be fairly reduced.

Importantly, it has been reported in many studies that plant invasions were associated with elevated or fluctuating resource levels (Davis et al., 2000; Daehler, 2003; Ehrenfeld, 2003). Invasive species grew better than native species at a higher nutrient concentration, but not at lower nutrient availability, where native vegetation is more competitive (Daehler, 2003). Studies have demonstrated that high nutrient-demanding invasive species can generate their own nutrient-rich sites, thus possibly promoting their own invasion (Vitousek et al., 1987; Ehrenfeld et al., 2001). Exotic plant species can cause an increase in soil pH as well as organic carbon content and nitrification rates, providing more available nitrate (Callaway, 1995; Kourtev et al., 1998, Kourtev et al., 1999, Kourtev et al., 2003; Ehrenfeld et al., 2001) and also phosphorus content (Ehrenfeld, 2003). The changes in the amounts and availability of nutrients may be attributed to concurrent changes in plant biomass or litter nutrient concentration with invasion (Ehrenfeld, 2003; Kao-Kniffin & Balser, 2008). Data of the present study well support this invasive plant strategy.

In addition, our results indicate that the presence of the invasive plant A. viridis has induced severe disturbances in soil microbial communities, the primary mediators of soil nutrient cycling. It has been well established that the structure and functional diversity of soil bacterial communities were mainly dependent on the aboveground plant composition (Grayston et al., 2001). Our data are in accordance with these previous studies because a higher global fertility is observed under A. viridis. It also seems that the invasive plant exerts a selective effect on some components of these bacterial communities, which may vary in abundance, and thus altering links between native aboveground and belowground communities by ways including the timing, quality, quantity and spatial structure of plant-derived soil inputs (Wolfe & Klironomos, 2005). A shift in the composition or the abundance of particular members of the microbial community has also been suggested to alter nutrient pools and fluxes (Balser & Firestone, 2005).

From the present study, we found a higher bacterial abundance and bacterial activity in the soil collected under A. viridis, but, in contrast, a decrease in AM spore abundance and hyphal length. This invasive plant effect has already been reported by Kourtev et al. (2002), who found, in soil collected from the invasive Japanese barbery, Barberis thunbergii, an overall decrease in fungal abundance and a conversion to a microbial community dominated by bacteria. Moreover, some authors have found that increased soil fertility shifted the microbial community structure, with a noticeable decrease in fungi and increase in bacteria (Pennanen et al., 1999; Bradley et al., 2006).

Because it belongs to the presumed nonmycorrhizal family Amaranthaceae (Tester et al., 1987), A. viridis has usually been considered as a nonmycorrhizal plant species. However, it has been recently reported that, under natural conditions, AM associations were found in A. viridis (Muthukumar et al., 2006). Our results corroborate these observations, but at a higher G. intraradices inoculum rate. In addition, the presence of AM structures in the roots of A. viridis was linked to a depressive effect on the plant growth. Reports of negative plant growth responses are common (Bougher et al., 1990). Mycorrhizal associations are detrimental (parasitic) to plants when the net costs are higher than net benefits (Johnson et al., 1997). Costs of mycorrhizae are traditionally expressed in terms of carbon (photosynthate) allocated to the fungus (Fitter, 1991). It has been hypothesized that AM fungal parasitism could result from (1) developmental factors, (2) environmental factors and (3) genotypic factors (Johnson et al., 1997). From the present study, the parasitic effect of G. intraradices inoculation could result from the first and the third hypothesis, but not from the ‘environmental factors’ hypothesis because the soil used in this experiment had a low fertility (in particular, low phosphorous and nitrogen contents). For instance, it has been reported that AM symbiosis can depress seedling growth at the first stages of seedling development (Koide, 1985). Because A. viridis plant growth was depressed by AM fungal inoculation and had a low affinity for AM establishment, it could be a good competitor in areas with low densities of AM fungi (Grime et al., 1987; Hartnett et al., 1993). This reduced dependence on mycorrhizal fungi can be viewed as part of a life-history strategy that is successful in disturbed environments (Francis & Read, 1995).

Our results also indicated that Acacia species nodulation was significantly reduced when these plants were grown in A. viridis-invaded soil. This negative effect of the invasive plant on the symbiotic microorganisms (AM fungi and rhizobia) could result from abiotic factors. Indeed, higher phosphorus and nitrogen contents in soil are known to inhibit the development of the fungal and rhizobial symbionts (Vincent, 1970; Smith & Read, 2008). However, A. viridis could also interfere with AM and rhizobia colonization of Acacia species root systems and slow their growth in invaded soil. These inhibitions were similar to those recorded for most of the Acacia species in sterilized soil from both A. viridis-invaded and A. viridis-free sites. This result strongly suggested that the invasive plant reduced Acacia growth through a microbially mediated effect and that this depletion did not only result from soil differences or direct allelopathy against plants. It has been demonstrated recently that an invasive plant, Alliaria petiolata (garlic mustard), suppressed native plant growth by disrupting AM symbiotic relationships through root exudation of antifungal compounds (Stinson et al., 2006). These authors showed that garlic mustard inhibited AM formation in native tree species, more particularly by reducing the germination rates of native AM spores (Stinson et al., 2006). In an earlier study, Vaughn & Berhow (1999) also observed that phytochemicals could have direct effects on plant growth through allelopathy as well as indirect effects via disruption of AM fungi. In our experimental work, the suppressive effect of A. viridis on Acacia species nodulation and root AM establishment was also found after the addition of an A. viridis plant aqueous extract to uninvaded soil. In addition, this plant extract exhibited a strong antibiotic effect against all the rhizobial strains tested (rhizobia isolated from the root systems of the targeted Acacia species). Hence, these results clearly demonstrated that A. viridis disrupts the formation of AM associations and nodulation, probably through phytochemical inhibition. Inhibition effects on nitrogen-fixing bacteria have been observed previously in pioneer plants such as Amaranthus retroflexus and suggested that this mechanism may be important in competition and succession among plants (Rice, 1964).

Hence, abundant soil mutualists' declines may initiate a positive feedback to maintain exotic plants in their introduced area (Vogelsang et al., 2004). With the successful establishment of exotic species, re-establishment of soil mutualists may be slowed, thereby impeding native plant species that exhibit high dependencies on these microorganisms. Therefore, in systems where native plants have strong mutualistic relationships with soil symbionts such as rhizobia or AM fungi, disturbances that disrupt these microbial symbiotic relationships could facilitate the establishment of exotic species having a low dependence on these microorganisms for their growth and survival.

The inoculation of G. intraradices was highly beneficial to the nodulation as well as the growth of Acacia species in both soil origins. Promoting the effect of AM inoculation on plant nodulation could probably result from close synergistic interactions between mycorrhizal fungi and rhizobia (Cornet & Diem, 1982; Founoune et al., 2002). In this regard, it has been demonstrated that mycorrhizal infection helps nodule formation and functioning under stress conditions (Azcon et al., 1988; André, 2005). Moreover, our result is in accordance with other studies, from which it has been established that this fungal isolate was very effective on the growth of plant species (Duponnois & Plenchette, 2003; Villenave et al., 2003; Duponnois et al., 2005). Importantly, it is also well established that AM fungi could affect plant community structure by enhancing the growth of the stronger mycorrhizal plant species (Gange et al., 1993; van der Heijden et al., 1998a). In the case of this study on A. viridis-invaded soil, our results provide evidence that an increase in soil AM propagule density could mediate the invasive plant antagonistic effect on Acacia species. It has been observed that microorganisms can act as allelochemical mediators, inactivating or metabolizing toxic compounds. Previous findings suggested that AM fungi, associated with their mycorrhizosphere microbial communities, could protect seedlings from allelopathy and other phytochemical compounds (Pellissier & Souto, 1999; Blum et al., 2000; Renne et al., 2004).

In conclusion, our results confirm the high importance of biological mechanism by which an invasive plant can alter native communities by disrupting the development of AM fungi and rhizobia. They also highlight the key role of AM fungi in mediating plant coexistence and the necessity to manage AM community in soil (i.e. controlled mycorrhization of Acacia tree species) to improve successful re-establishment of native species. However, further research must be undertaken to identify allelopathic processes (e.g. compounds produced by the invasive plant) and therefore to better understand the mechanisms of soil mutualistic community degradation.

References

Adair
RJ
Groves
RH
(
1998
)
Impact of Environmental Weeds on Biodiversity: A Review and Development of Methodology
 .
Environment Australia
,
Canberra
.
Alef
K
(
1998
)
Estimation of the hydrolysis of fluorescein diacetate
.
Methods in Applied Soil Microbiology and Biochemistry
  (
Alef
K
Nannipieri
P
eds), pp.
232
233
.
Academic Press
,
London
.
André
S
Galiana
A
Le Roux
C
Prin
Y
Neyra
M
Duponnois
R
(
2005
)
Ectomycorrhizal symbiosis enhanced the efficiency of two Bradyrhizobium inoculated on Acacia holosericea plant growth
.
Mycorrhiza
 
15
:
357
364
.
Aubert
G
(
1978
)
Méthodes d'Analyse des sols
 .
CRDP
,
Marseille
.
Azcon
R
El-Atrach
F
Barea
JM
(
1988
)
Influence of mycorrhiza vs. soluble phosphate on growth and N2 fixation (15N) in alfalfa under different levels of water potential
.
Biol Fertil Soils
 
7
:
28
31
.
Azcon-Aguilar
C
Palenzuela
J
Roldan
A
Bautista
S
Vallejo
R
Barea
JM
(
2003
)
Analysis of the mycorrhizal potential in the rhizosphere of representative plant species from desertification-threatened Mediterranean shrublands
.
Appl Soil Ecol
 
14
:
165
175
.
Balser
TC
Firestone
MK
(
2005
)
Linking microbial community composition and soil processes in a California annual grassland and mixed-conifer forest
.
Biogeochemistry
 
73
:
395
415
.
Batten
KM
Scow
KM
Davies
KF
Harrison
SP
(
2006
)
Two invasive plants alter soil microbial community composition in serpentine grasslands
.
Biol Invasions
 
8
:
217
230
.
Bensch
CN
Horak
MJ
Peterson
D
(
2003
)
Interference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean
.
Weed Sci
 
51
:
37
43
.
Blum
U
Statman
KL
Flint
LJ
Shaefer
SR
(
2000
)
Induction and/or selection of phenolic acid-utilizing bulk-soil and rhizospheric bacteria and their influence on phenolic acid phytotoxicity
.
J Chem Ecol
 
26
:
2059
2078
.
Bougher
NL
Grove
TS
Malajczuk
N
(
1990
)
Growth and phosphorus of karri (Eucalyptusdiversicolor F. Muell.) seedlings inoculated with ectomycorrhizal fungi in relation to phosphorous supply
.
New Phytol
 
114
:
77
85
.
Bradley
K
Drijber
RA
Knops
J
(
2006
)
Increased N availability in grassland soils modifies their microbial communities and decreases the abundance of arbuscular mycorrhizal fungi
.
Soil Biol Biochem
 
38
:
1583
1595
.
Bruno
JF
Stachowicz
JJ
Bertness
MD
(
2003
)
Inclusion of facilitation into ecological theory
.
Trends Ecol Evol
 
18
:
119
125
.
Callaway
RM
(
1995
)
Positive interactions among plants
.
Bot Rev
 
61
:
306
349
.
Callaway
RM
Ridenour
WM
(
2004
)
Novel weapons: invasive success and the evolution of increased competitive ability
.
Front Ecol Environ
 
2
:
436
443
.
Cébron
A
Norini
MP
Beguiristain
T
Leyval
C
(
2008
)
Real-Time PCR quantification of PAH-ring hydroxylating dioxygenase (PAH-RHDα) genes from Gram positive and Gram negative bacteria in soil and sediment samples
.
J Microbiol Meth
 
73
:
148
159
.
Corgié
SC
Beguiristain
T
Leyval
C
(
2004
)
Spatial distribution of bacterial communities and phenanthrene degradation in the rhizosphere of Lolium perenne L
.
Appl Environ Microb
 
70
:
3552
3557
.
Cornet
F
Diem
HG
(
1982
)
Etude comparative de l'efficacité des souches de Rhizobium d'Acacia isolées de sols du Sénégal et effet de la double symbiose Rhizobium-Glomusmosseae sur la croissance de Acacia holosericea et A. raddiana
.
Bois Forêt Trop
 
189
:
3
15
.
Daehler
C
(
2003
)
Performance comparisons of co-occurring native and alien invasive plants: implications for conservation and restoration
.
Annu Rev Ecol Evol S
 
34
:
183
211
.
Davis
MA
Grime
PJ
Thompson
K
(
2000
)
Fluctuating resources in plant community: a general theory of invisibility
.
J Ecol
 
72
:
319
330
.
Del Moral
R
Muller
CH
(
1970
)
The allelopathic effects of Eucalyptus camaldulensis
.
Am Midl Nat
 
83
:
254
282
.
Dreyfus
BL
Dommergues
YR
(
1981
)
Nodulation of Acacia species by fast- and slow-growing tropical strains of Rhizobium
.
Appl Environ Microb
 
41
:
97
99
.
Duponnois
R
Plenchette
C
(
2003
)
A mycorrhiza helper bacterium (MHB) enhances ectomycorrhizal and endomycorrhizal symbiosis of Australian Acacia species
.
Mycorrhiza
 
13
:
85
91
.
Duponnois
R
Plenchette
C
AM
(
2001
)
Growth stimulation of seventeen fallow leguminous plants inoculated with G. aggregatum in Senegal
.
Eur J Soil Biol
 
37
:
181
186
.
Duponnois
R
Colombet
A
Hien
V
Thioulouse
J
(
2005
)
The mycorrhizal fungus Glomus intraradices and rock phosphate amendment influence plant growth and microbial activity in the rhizosphere of Acacia holosericea
.
Soil Biol Biochem
 
37
:
1460
1468
.
Ehrenfeld
J
Kourtev
P
Huang
W
(
2001
)
Changes in soil functions following invasions of exotic understory plants in deciduous forests
.
Ecol Appl
 
11
:
1287
1300
.
Ehrenfeld
JG
(
2003
)
Effects of exotic plant invasions on soil nutrient cycling processes
.
Ecosystems
 
6
:
503
523
.
Felske
A
Akkermans
ADL
De Vos
WM
(
1998
)
Quantification of 16S rRNAs in complex bacterial communities by multiple competitive reverse transcription-PCR in temperature gradient gel electrophoresis fingerprints
.
Appl Environ Microb
 
64
:
4581
4587
.
Finlay
RD
(
2008
)
Ecological aspects of mycorrhizal symbiosis: with special emphasis on the functional diversity of interactions involving the extraradical mycelium
.
J Exp Bot
 
59
:
1115
1126
.
Fitter
AH
(
1991
)
Costs and benefits of mycorrhizas: implications for functioning under natural conditions
.
Experientia
 
47
:
350
355
.
Founoune
H
Duponnois
R
AM
El Bouami
F
(
2002
)
Influence of the dual arbuscular endomycorrhizal/ectomycorrhizal symbiosis on the growth of Acacia holosericea (A. Cunn ex G. Don) in glasshouse conditions
.
Ann For Sci
 
59
:
93
98
.
Francis
R
Read
DJ
(
1995
)
Mutualism and antagonism in the mycorrhizal symbiosis, with special reference to impacts on plant community structure
.
Can J Bot
 
73
:
1301
1309
.
Gange
AC
Brown
VK
Sinclair
GS
(
1993
)
VA mycorrhizal fungi: a determinant of plant community structure in early succession
.
Funct Ecol
 
7
:
616
622
.
Gassama-Dia
YK
Sané
D
N'Doye
M
(
2003
)
Reproductive biology of Faidherbia albida (Del.) A. Chev
.
Silva Fenn
 
37
:
429
436
.
Gerdemann
JW
Nicolson
TH
(
1963
)
Spores of mycorrhizal endogone species extracted from soil by wet sieving and decanting
.
Trans Br Mycol Soc
 
46
:
235
.
Gibson
AH
(
1963
)
Physical environment and symbiotic nitrogen fixation. I. The effect of temperature on recently nodulated Trifolium subterraneum L. plant
.
Aust J Biol Sci
 
16
:
28
42
.
Grayston
SJ
Campbell
CD
(
1996
)
Functional biodiversity of microbial communities in the rhizosphere of hybrid larch (Larix eurolepis) and Sitka spruce (Picea sitchensis)
.
Tree Physiol
 
16
:
1031
1038
.
Grayston
SJ
Griffith
GS
Mawdsley
JL
Campbell
CD
Bardgett
RD
(
2001
)
Accounting for variability in soil microbial communities of temperate upland grassland ecosystems
.
Soil Biol Biochem
 
33
:
533
551
.
Grime
JP
Mackey
JML
Hillier
SH
Read
DJ
(
1987
)
Floristic diversity in a model system using experimental microcosms
.
Nature
 
328
:
420
422
.
Halos
L
Mavris
M
Vourc'h
G
Maillard
R
Barnouin
J
Boulouis
HJ
Vayssier-Taussat
M
(
2006
)
Broad-range PCR-TTGE for the first-line detection of bacterial pathogen DNA in ticks
.
Vet Res
 
37
:
245
253
.
Hanssen
JF
Thingstad
TF
Goksoyr
J
(
1974
)
Evaluation of hyphal lengths and fungal biomass in soil by a membrane filter technique
.
Oikos
 
25
:
102
107
.
Hart
MM
Reader
RJ
Klironomos
JN
(
2003
)
Plant coexistence mediated by arbuscular mycorrhizal fungi
.
Trends Ecol Evol
 
18
:
418
423
.
Hartnett
DC
Heterick
BAD
Wilson
GWT
Gibson
DJ
(
1993
)
Mycorrhizal influence of intra- and interspecific neighbour interactions among co-occurring prairie grasses
.
J Ecol
 
81
:
785
795
.
Heuer
H
Hartung
K
Wieland
G
Kramer
I
Smalla
K
(
1999
)
Polynucleotide probes that target a hypervariable region of 16S rRNA genes to identify bacterial isolates corresponding to bands of community fingerprints
.
Appl Environ Microb
 
65
:
1045
1049
.
Hooper
DU
Vitousek
PM
(
1997
)
The effects of plant composition and diversity on ecosystem processes
.
Science
 
277
:
1302
1305
.
Hussain
F
Gilani
SS
Fatima
I
Durrani
MJ
(
2003
)
Some autecological studies on Amaranthus viridis L
.
Pak J Weed Sci Res
 
9
:
117
124
.
INVAM
(
1997
)
International Culture Collection of (Vesicular) Arbuscular Mycorrhizae
 . Available at http://www.invam.caf.wvu.edu/
Jakobsen
I
Abbott
LK
Robson
AD
(
1992
)
External hyphae of vesicular–arbuscular mycorrhizal fungi associated with Trifolium subterraneum. I. Spread of hyphae and phosphorus inflow into roots
.
New Phytol
 
120
:
371
380
.
Johansson
JF
Paul
LR
Finlay
RD
(
2004
)
Microbial interactions in the mycorrhizosphere and their significance for sustainable agriculture
.
FEMS Microbiol Ecol
 
48
:
1
13
.
Johnson
NC
Graham
JH
Smith
FA
(
1997
)
Functioning of mycorrhizal associations along the mutualism–parasitism continuum
.
New Phytol
 
135
:
575
585
.
Kao-Kniffin
J
Balser
TC
(
2008
)
Soil fertility and the impact of exotic invasion on microbial communities in Hawaiian forests
.
Microb Ecol
 
56
:
55
63
.
Kisa
M
Sanon
A
Thioulouse
J
et al
(
2007
)
Arbuscular mycorrhizal symbiosis counterbalance the negative influence of the exotic tree species Eucalyptus camaldulensis on the structure and functioning of soil microbial communities in a sahelian soil
.
FEMS Microbiol Ecol
 
62
:
32
44
.
Koide
R
(
1985
)
The nature of growth depressions in sunflower caused by vesicular–arbuscular mycorrhizal infection
.
New Phytol
 
99
:
449
462
.
Kourtev
P
Ehrenfeld
J
Huang
W
(
1998
)
Effects of exotic plant species on soil properties in hardwood forests of New Jersey
.
Water Air Soil Poll
 
105
:
493
501
.
Kourtev
P
Huang
W
Ehrenfeld
J
(
1999
)
Differences in earthworm densities and nitrogen dynamics in soils under exotic and native plant species
.
Biol Invasions
 
1
:
237
245
.
Kourtev
PS
Ehrenfeld
JG
Häggblom
M
(
2002
)
Exotic plant species alter the microbial community structure and function in the soil
.
Ecology
 
83
:
3152
3166
.
Kourtev
PS
Ehrenfeld
JG
Häggblom
M
(
2003
)
Experimental analysis of the effect of exotic and native plant species on the structure and function of soil microbial communities
.
Soil Biol Biochem
 
35
:
895
905
.
Le Bourgeois
T
Merlier
H
(
1995
)
Adventrop. Les adventices d'Afrique soudano-sahélienne
 .
CIRAD-CA
,
Montpellier, France
.
Levine
JM
D'Antonio
CM
(
1999
)
Elton revisited: a review of evidence linking diversity and invasibility
.
Oikos
 
87
:
15
26
.
Maillet
J
Lopez-Garcia
C
(
2000
)
What criteria are relevant for predicting the invasive capacity of a new agricultural weed? The case of invasive American species in France
.
Weed Res
 
40
:
11
26
.
Marschner
P
Baumann
K
(
2003
)
Changes in bacterial community structure induced by mycorrhizal colonisation in spilt-root maize
.
Plant Soil
 
251
:
279
289
.
Mitchell
CE
Power
AG
(
2003
)
Release of invasive plants from viral and fungal pathogens
.
Nature
 
421
:
625
627
.
Morton
JB
Benny
GL
(
1990
)
Revised classification of arbuscular mycorrhizal fungi (Zygomycetes): a new order, Glomales, two new suborders, Glomineae and Gigasporineae, and two new families, Acaulosporaceae and Gigasporaceae, with an emendation of Glomaceae
.
Mycotaxon
 
37
:
471
491
.
Mummey
DL
Rillig
MC
(
2006
)
The invasive plant species Centaurea maculosa alters arbuscular mycorrhizal fungal communities in the field
.
Plant Soil
 
288
:
81
90
.
Muthukumar
T
Senthilkumar
M
Rajangam
M
Udaiyan
K
(
2006
)
Arbuscular mycorrhizal morphology and dark septate fungal associations in medicinal and aromatic plants of Western Ghats, Southern India
.
Mycorrhiza
 
17
:
11
24
.
Muyzer
G
Smalla
K
(
1998
)
Application of denaturing gradient gel electrophoresis (DGGE) and temperature gradient gel electrophoresis (TGGE) in microbial ecology
.
Antonie van Leeuwenhoek
 
73
:
127
141
.
Muyzer
G
De Waal
EC
Uitterlinden
AG
(
1993
)
Profiling of complex microbial populations by denaturing gradient gel electrophoresis analysis of polymerase chain reaction-amplified genes coding for 16S rDNA
.
Appl Environ Microb
 
59
:
695
700
.
Nicolson
TH
(
1959
)
Mycorrhiza in the Gramineae. I. Vesicular–arbuscular endophytes, with special reference to the external phase
.
Trans Brit Mycol Soc
 
42
:
421
438
.
O'Connor
PJ
Smith
SE
Smith
FA
(
2002
)
Arbuscular mycorrhizas influence plant diversity and community structure in a semiarid herbland
.
New Phytol
 
154
:
209
218
.
Olsen
SR
Cole
CV
Watanabe
FS
Dean
LA
(
1954
)
Estimation of available phosphorus in soils by extraction with sodium bicarbonate
.
U.S. Department of Agriculture Circular
 , Vol.
939
.
U.S. Department of Agriculture
,
Washington, DC
, p.
9
.
Pellissier
F
Souto
X
(
1999
)
Allelopathy in Northern temperate and boreal semi-natural woodland
.
Crit Rev Plant Sci
 
18
:
637
652
.
Pennanen
T
Liski
J
Bååth
E
Kitunen
V
Uotila
J
Westman
CJ
Fritze
H
(
1999
)
Structure of the microbial communities in coniferous forest soils in relation to site fertility and stand development stage
.
Microb Ecol
 
38
:
168
179
.
Phillips
JM
Hayman
DS
(
1970
)
Improved procedures for clearing roots and staining parasitic and vesicular–arbuscular mycorrhizal fungi for rapid assessment of infection
.
Trans Br Mycol Soc
 
55
:
158
161
.
Plenchette
C
(
2000
)
Receptiveness of some tropical soils from banana fields in Martinique to the arbuscular fungus Glomus intraradices
.
Appl Soil Ecol
 
15
:
253
260
.
Plenchette
C
Declerck
S
Diop
T
Strullu
DG
(
1996
)
Infectivity of monoaxenic cultures of the AM fungus Glomus versiforme associated with Ri-TDNA transformed root
.
Appl Microbiol Biot
 
46
:
545
548
.
Prin
Y
Neyra
M
Ducousso
M
Dommergues
YR
(
1989
)
Viabilité d'un inoculum déterminée par l'activité réductrice de l'INT
.
Agron Trop
 
44
:
13
19
.
Rejmanek
M
Richardson
DM
(
1996
)
What attributes make some plant species more invasive?
Ecol
 
77
:
1655
1661
.
Renne
IJ
Rios
BG
Fehmi
JS
Tracy
BF
(
2004
)
Low allelopathic potential of an invasive forage grass on native grassland plant: a cause for encouragements?
Basic Appl Ecol
 
5
:
261
269
.
Rice
EL
(
1964
)
Inhibition of nitrogen-fixing and nitrifying bacteria by seed plants
.
Ecology
 
45
:
824
837
.
Richardson
DM
Allsopp
N
D'Antonio
CM
Milton
SJ
Rejmanek
M
(
2000
)
Plant invasion – the role of mutualisms
.
Biol Rev
 
75
:
65
93
.
Schinner
F
Ohlinger
R
Kandeler
E
Margesin
R
(
1996
)
Methods in Soil Biology
 .
Springer-Verlag
,
Berlin
,
426
pp.
Schreiner
RP
Mihara
KL
Mc Daniel
H
Benthlenfalvay
GJ
(
2003
)
Mycorrhizal fungi influence plant and soil functions and interactions
.
Plant Soil
 
188
:
199
209
.
Smith
SE
Read
DJ
(
2008
)
Mycorrhizal Symbiosis
 ,
3rd
edn.
Academic Press
,
London
.
Sprent
JI
(
2001
)
Nodulation in Legumes
 .
Royal Botanical Gardens
,
Kew
.
Stinson
KA
Campbell
SA
Powell
JR
Wolfe
BE
Callaway
RM
Thelen
GC
Hallett
SG
Prati
D
Klironomos
JN
(
2006
)
Invasive plant suppresses the growth of native tree seedling by disrupting belowground mutualisms
.
PLoS Biol
 
4
:
727
731
.
Tester
M
Smith
SE
Smith
FA
(
1987
)
The phenomenon of non-mycorrhizal plants
.
Can J Bot
 
65
:
419
431
.
Thébaud
C
Simberloff
D
(
2001
)
Are plants really larger in their introduced ranges?
Am Nat
 
157
:
231
236
.
Thioulouse
J
Chessel
D
Dolédec
S
Olivier
JM
(
1997
)
ADE-4: a multivariate analysis and graphical display software
.
Stat Comput
 
7
:
75
83
.
Van Der Heijden
MGA
Bollet
T
Wiemken
A
Sanders
IR
(
1998a
)
Different arbuscular mycorrhizal fungal species are potential determinants of plant community structure
.
Ecology
 
79
:
2082
2091
.
Van Der Heijden
MGA
Klironomos
JN
Ursic
M
Moutoglis
P
Streitwolf-Engel
R
Boller
T
Wiemken
A
Sanders
IR
(
1998b
)
Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity
.
Nature
 
396
:
72
75
.
Van Der Heijden
MGA
Wiemken
A
Sanders
IR
(
2003
)
Different arbuscular mycorrhizal fungi alter coexistence and resource distribution between co-occuring plant
.
New Phytol
 
157
:
569
578
.
Van Der Heijden
MGA
Bakker
R
Verwaal
J
Scheublin
TR
Rutten
M
Van Logtestijn
R
Staehelin
C
(
2006
)
Symbiotic bacteria as a determinant of plant community structure and plant productivity in dune grassland
.
FEMS Microbiol Ecol
 
56
:
178
187
.
Vaughn
SF
Berhow
MA
(
1999
)
Allelochemicals isolated from tissues of the invasive weed garlic mustard (Alliaria petiolata)
.
J Chem Ecol
 
25
:
2495
2504
.
Villenave
C
Leye
K
Chotte
JL
Duponnois
R
(
2003
)
Nematofauna associated with the mycorrhizosphere and hyphosphere of Glomus intraradices and exotic or native leguminous plant species mycorrhizal formation in West Africa
.
Biol Fert Soils
 
38
:
161
169
.
Vincent
JG
Vincent
HW
(
1944
)
Filter paper disc modification of the Oxford cup penicillin determination
.
P Soc Exp Biol Med
 
55
:
162
164
.
Vincent
JM
(
1970
)
A Manual for Practical Study of the Root Nodule Bacteria
 .
Blackwell Scientific Publications
,
Oxford
.
Vitousek
PM
Walker
LR
Whiteaker
LD
Mueller-Dombois
D
Matson
PA
(
1987
)
Biological invasion by Myrica faya alters ecosystem development in Hawaii
.
Science
 
238
:
802
804
.
Vogelsang
KM
Bever
JD
Griswold
M
Schulz
PA
(
2004
)
The use of mycorrhizal fungi in erosion control applications
.
Final report for Caltrans
 ,
California Department of Transportation Contract No. 65 A0070
,
Sacramento, CA
.
Wikström
P
Andersson
AC
Forsman
M
(
1999
)
Biomonitoring complex microbial communities using random amplified polymorphic DNA and principal component analysis
.
FEMS Microbiol Lett
 
28
:
131
139
.
Wolfe
BE
Klironomos
JN
(
2005
)
Breaking new ground: soil communities and exotic plant invasion
.
BioScience
 
55
:
477
487
.

Author notes

Editor: Philippe Lemanceau