Selective interactions between arbuscular mycorrhizal fungi and<Emphasis Type="Italic"> Rhizobium leguminosarum</Emphasis> bv.<Emphasis Type="Italic"> viceae</Emphasis> enhance pea yield and nutrition

This study evaluated the response of pea (Pisum sativum cv. Trapper) to arbuscular mycorrhizal fungi (AMF) and Rhizobium leguminosarum bv. viceae strains varying in their effectiveness on pea. Plants were inoculated with the AMF species Glomus clarum NT4 or G. mosseae NT6 and/or ten Rhizobium strains, and grown for 90 days in soil containing indigenous AMF and rhizobia. The effectiveness of the Rhizobium strains on the growth (P <0.046; r =0.64) and N nutrition (P <0.04; r =0.65) of 6-week-old pea grown under gnotobiotic conditions was correlated with the growth and N nutrition of 90-day-old pea grown in natural soil for all strains except LX48. The growth and yield response of pea to co-inoculation with AMF and Rhizobium strains depended on the particular AMF-Rhizobium strain combination. In some cases, the yield and N nutrition of pea inoculated with a superior Rhizobium strain was significantly (P <0.05) enhanced by an apparently compatible AMF species compared to the Rhizobium treatment. On the other hand, an apparently incompatible AMF species significantly (P <0.05) reduced the performance of an effective Rhizobium strain. In general, treatments with effective Rhizobium strains or co-inoculation treatments with effective Rhizobium strains and a compatible AMF species produced the best results. Changes in total shoot dry matter production was significantly (P <0.05) correlated with the total shoot N (P <0.0001; r =0.95) and P content (P <0.0001; r =0.87), indicating that this response was mediated by enhanced N and P nutrition. Growth, yield and nutrition of pea were not related to AMF colonization of roots. Our results suggest that careful co-selection of AMF species and Rhizobium strains can enhance pea yield and nutrition.     

Functional diversity of soil microbial community,rock phosphate dissolution and growth of Acacia seyal as influenced by grass-, litter- and soil-feeding termite nest structure amendments

We tested termite mound materials belonging to different feeding groups: Cubitermes (soil-feeder), Trinervitermes (grass-feeder) and Macrotermes (litter-feeder), as natural microbial inoculum to promote plant growth and increase nutrient supplies from soil organic matter and inorganic amendments (rock phosphate), through their effects on soil microorganisms (functional diversity of soil microflora, arbuscular mycorrhizal fungi, rhizobia, fluorescent pseudomonads, actinomycetes and saprophytic fungi). Experiments were made in a pot experiment with Acacia seyal, a leguminous tree abundant in West Africa, with a sandy soil amended or not with rock phosphate. Results indicated a stimulation of plant growth with Cubitermes and Trinervitermes mound powder (plant height and shoot biomass), similar to what was obtained with rock phosphate alone. Leaf content in N was also increased in the termite treatments (except in Macrotermes soil), whereas mycorrhizal colonization was inhibited as compared to the control. The development of saprophytic fungi was significantly higher in the soils amended with rock phosphate and this effect was hypothesized to be related to the production of large quantities of oxalic acid by fungal populations. The fluorescent pseudomonad populations notably increased in the soils dually amended with mound powders and rock phosphate, and this could be due to the fact that some species of this bacterial group are able to dissolve rock phosphate. The organic and inorganic amendments decreased the soil catabolic evenness in all the mound powder treatments. Among the mound materials tested, Cubitermes mound powder had the most promising effect, especially on SIR response to oxalate. It is concluded that soils amended both with rock phosphate and Cubitermes mound soil could promote the development of microbial communities, which could help to metabolize this compound and consequently enhance plant growth.     

Pre-inoculation with arbuscular mycorrhizal fungi suppresses root knot nematode (<Emphasis Type="Italic">Meloidogyne incognita</Emphasis>) on cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis>)

A pot experiment was conducted to evaluate the influence of pre-inoculation of cucumber plants with each of the three arbuscular mycorrhizal (AM) fungi Glomus intraradices, Glomus mosseae, and Glomus versiforme on reproduction of the root knot nematode Meloidogyne incognita. All three AM fungi tested significantly reduced the root galling index, which is the percentage of total roots forming galls. Numbers of galls per root system were significantly reduced only in the G. intraradices + M. incognita treatment. The number of eggs per root system was significantly decreased by AM fungus inoculation, no significant difference among the three AM fungal isolates. AM inoculation substantially decreased the number of females, the number of eggs g⁻¹ root and of the number of eggs per egg mass. The number of egg masses g⁻¹ root was greatly reduced by inoculation with G. mosseae or G. versiforme. By considering plant growth, nutrient uptake, and the suppression of M. incognita together, G. mosseae and G. versiforme were more effective than G. intraradices.     

Influence of arbuscular mycorrhizal fungi and salinity on growth,biomass, and mineral nutrition of<Emphasis Type="Italic"> Acacia auriculiformis</Emphasis>

The effect of salinity on the efficacy of two arbuscular mycorrhizal fungi, Glomus fasciculatum and G. macrocarpum, alone and in combination was investigated on growth, development and nutrition of Acacia auriculiformis. Plants were grown under different salinity levels imposed by 0.3, 0.5 and 1.0 S m^-1 solutions of 1 M NaCl. Both mycorrhizal fungi protected the host plant against the detrimental effect of salinity. The extent of AM response on growth as well as root colonization varied with fungal species, and with the level of salinity. Maximum root colonization and spore production was observed with combined inoculation, which resulted in greater plant growth at all salinity levels. AM fungal inoculated plants showed significantly higher root and shoot weights. Greater nutrient acquisition, changes in root morphology, and electrical conductivity of soil in response to AM colonization was observed, and may be possible mechanisms to protect plants from salt stress.     

Comparative study of earthworm communities,microbial biomass,and plant nutrient availability under 1-year <Emphasis Type="Italic">Cajanus cajan</Emphasis> (L.) Millsp and <Emphasis Type="Italic">Lablab purpureus</Emphasis> (L.) Sweet cultivations versus natural regrowths in a guinea savanna zone

In tropical savannas where soils are generally sandy and nutrient poor, organic farming associated with enhanced soil biological activity may result in increased nutrient availability. Therefore, legumes have been introduced in the humid savanna zone of Côte d’Ivoire, owing to their ability to fix atmospheric N and to continually supply soil with great quantity of organic materials in relatively short time. The main objective of this study was to assess the influence of two legume (Cajanus cajan and Lablab purpureus) cultivations on earthworm communities and P and N availability. Trials were carried out under farmers' field conditions; C. cajan was planted on savanna soils (trial 1) while L. purpureus was established on new Chromolaena odorata-dominated fallow soils (trial 2). Native vegetations were considered as controls. Changes in soil properties (earthworm abundance and diversity, microbial biomass carbon (MBC), and plant available P and N) were assessed using the biosequential sampling. After 1 year, both the legume stands showed a significantly higher density of earthworms, compared with the respective controls. This trend was linked to an increase in the abundance of the detritivores Dichogaster baeri Sciacchitano 1952 and Dichogaster saliens Beddard 1893, and the polyhumic Stuhlmannia zielae Omodeo 1963. Equally, legume had beneficial impacts on the average number of earthworm species, the Shannon–Weaver index of diversity and MBC in savanna (trial 1). Available P and ammonium significantly increased under both legume cultivations and were significantly and concurrently linked to litter quality and earthworm activities as shown by multiple regressions. As a result, legumes could improve nutrient availability in the sandy soils of central Côte d’Ivoire by positively affecting soil biological activity and this could bring farmers to cultivate crops on savanna lands.     

<Emphasis Type="Italic">Trichoderma</Emphasis> improves the growth of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Bio-fertilizer application has been proposed as a strategy for enhancing soil fertility, regulating soil microflora composition, and improving crop yields, and it has been widely applied in the agricultural yields. However, the application of bio-fertilizer in grassland has been poorly studied. We conducted in situ and pot experiments to investigate the practical effects of different fertilization regimes on Leymus chinensis growth, with a focus on the potential microecological mechanisms underlying the responses of soil microbial composition. L. chinensis biomass was significantly (P?<?0.05) increased by treatment with 6000 kg ha^?1 of Trichoderma bio-fertilizer compared with other treatments. We found a positive (R² =?0.6274, P <?0.001) correlation between bacterial alpha diversity and L. chinensis biomass. Hierarchical cluster analysis and nonmetric multidimensional scaling (NMDS) revealed that soil bacterial and fungal community compositions were all separated according to the fertilization regime used. The relative abundance of the most beneficial genera in bio-fertilizer (BOF) (6000 kg ha^?1Trichoderma bio-fertilizer) was significantly higher than in organic fertilizer (OF) (6000 kg ha^?1 organic fertilizer) or in CK (non-amend fertilizer), there the potential pathogenic genera were reduced. There were significant negative (P?<?0.05) correlations between L. chinensis biomass and the relative abundance of several potential pathogenic genera. However, the relative abundance of most beneficial genera were significantly (P?<?0.05) positively correlated with L. chinensis biomass. Soil properties had different effects on these beneficial and on these pathogenic genera, further influencing L. chinensis biomass.     

Arbuscular mycorrhizal enhancement of iron concentration by <Emphasis Type="Italic">Poncirus trifoliata</Emphasis> L. Raf and <Emphasis Type="Italic">Citrus reticulata</Emphasis> Blanco grown on sand medium under different pH

The effect of the arbuscular mycorrhizal (AM) fungus (Glomus versiforme) on iron contents by two citrus rootstocks (trifoliate orange [Poncirus trifoliata L. Raf] and red tangerine [Citrus reticulata Blanco]) was studied in sand culture under different pH conditions. Seeds were sown in a mixed substrate (perlite/sand, 1:1 [v/v]) inoculated with or without mycorrhizal inoculum. The experiment was carried out at four pH levels by applying nutrient solution at pH 5.0, 6.0, 7.0, or 8.0 to P. trifoliata and pH 5.2, 6.2, 7.2, or 8.2 to C. reticulata. No AM colonization was found in uninoculated control (NM) and plants, and root colonization in AM plants was depressed under iron deficiency at high pH. Colonization by G. versiforme led to higher dry weights of shoots compared with NM treatments, suggesting that G. versiforme enhanced plant growth. Higher concentration of chlorophyll and active iron, lower ratios of P/Fe and 50(10P+K)/Fe were present in AM plants than NM treatments. Nevertheless, G. versiforme improved root Fe (III) chelate reductase activity of P. trifoliata and C. reticulata. The data indicate that plant uptake and translocation of iron were enhanced and AM fungi may be considered as a potential tool for bioremediation of citrus iron deficiency.     

Interactions between arbuscular mycorrhizal fungi and phosphate-solubilizing fungus (<Emphasis Type="Italic">Mortierella</Emphasis> sp.) and their effects on <Emphasis Type="Italic">Kostelelzkya virginica</Emphasis> growth and enzyme activities of rhizosphere and bulk soils at different salinities

This study investigated the interactions between two arbuscular mycorrhizal fungi (AMF) (Glomus aggregatum and Glomus mosseae) and a P-solubilizing fungus (Mortierella sp.), with respect to their effects on growth of Kostelelzkya virginica and urease, invertase, neutral phosphatase, alkaline phosphatase, and catalase activities of rhizosphere and bulk soils at different salinity levels (i.e., 0, 100, 200, and 300 mM NaCl). Percentage of AMF colonization, Mortierella sp. populations, pH, electrical conductivity, and available P concentration in soil were also determined. Combined inoculation of AMF and Mortierella sp. increased the percentage of AMF colonization and Mortierella sp. populations under salt stress (i.e., 100, 200, and 300 mM NaCl). The dual inoculation of Mortierella sp. with AMF (G. aggregatum or G. mosseae) had significant effects on shoot and root dry weights and available P concentrations, pH values, and electrical conductivities of rhizosphere and bulk soils under salt stress. The inoculation of Mortierella sp. significantly enhanced the positive effects of AMF on some enzyme activities (i.e., neutral phosphatase, alkaline phosphatase, and catalase in bulk soil; neutral phosphatase and urease in rhizosphere soil); on the contrary, it produced negative effects on urease activities in bulk soil and invertase activities in bulk and rhizosphere soils. The results indicated that the most effective co-inoculation was the dual inoculation with Mortierella sp. and G. mosseae, which may help in alleviating the deleterious effects of salt on plants growth and soil enzyme activities.     

A significant proportion of indigenous rhizobia from India associated with soybean (<Emphasis Type="Italic">Glycine max</Emphasis> L.) distinctly belong to <Emphasis Type="Italic">Bradyrhizobium</Emphasis> and <Emphasis Type="Italic">Ensifer</Emphasis> genera

The diversity among 269 rhizobia isolated from naturally occurring root nodules of soybean collected from two different agro-ecological regions of India, based on RFLP and sequences of the intergenic spacer (IGS) between the 16S and 23S rRNA genes, growth rate, and indole acetic acid production, revealed their significant, site-dependent genomic diversity. Among these bacteria, nine IGS genotypes were identified with two endonucleases. They were distributed into five divergent lineages by sequence analysis of each IGS representative strain, i.e., (1) comprising IGS genotypes I, II, III, and reference Bradyrhizobium yuanmingense; (2) with genotype IV and strains of unclassified bradyrhizobia genomic species; (3) including genotypes V, VI, and Bradyrhizobium liaoningense; (4) with IGS genotype VII and Bradyrhizobium elkanii strains; and (5) comprising IGS genotypes VIII, IX, and different Ensifer genus bacteria. Host-specificity test revealed that all rhizobia-nodulated soybean and cowpea and only part of them formed nodules on Arachis hypogeae and Cajanus cajan. The great diversity of soybean nodulators observed in this study emphasises that Indian soil is an important reservoir of nitrogen-fixing rhizobia.     

Influence of arbuscular mycorrhizal fungi,inoculum level and phosphorus placement on growth and phosphorus uptake of <Emphasis Type="Italic">Phyllanthus calycinus</Emphasis> under jarrah forest soil

Phosphorus uptake and biomass production may vary between arbuscular mycorrhizas (AM) species of contrasting life cycles and their modes of interaction with host plants. This research investigated differences among three AM fungi from different genera isolated from the jarrah forest with respect to their colonisation, biomass production and P uptake over time. The understorey plant Phyllanthus calycinus, has been shown to be extensively mycorrhizal. We examined differences in the capacity of fungi associated with this plant to access a P point source from increasing distances from the root. The methodology simulated “pockets” of P in forest soil. Phosphorus sources in root-exclusion mesh bags were inserted 2, 4 and 6 cm from plant roots restrained in separate mesh bags. Acaulospora laevis colonised plants to the greatest extent, producing higher plant biomass and P uptake compared to the other two fungi. Plant biomass and P uptake were enhanced where P sources were closest to the root, as expected, but there were differences among fungi in response to level of inoculum used for each fungus. The capacity of individual AM fungi to access a point source of P at different distances varied in a highly mycorrhizal native forest understorey plant in parallel with plant growth and P uptake.     

Characterization of rhizobia from <Emphasis Type="Italic">Sesbania</Emphasis> species native to seasonally wetland areas in Uruguay

Naturally growing Sesbania species with tolerance to unfavourable habitats are widely distributed in non-cultivated seasonally wetland areas in Uruguay. We investigated the relative abundance, diversity and symbiotic efficiency of Sesbania punicea and S. virgata rhizobia in three ecologically different undisturbed and water-logged sites in Uruguay. Numbers of native-soil rhizobia infective on S. punicea or S. virgata were low, with higher numbers associated with the presence of S. virgata. Plants of S. virgata inoculated with soil suspension showed aerial and nodule biomass greater than that obtained with S. punicea. The rhizobia nodulating Sesbania species in water-logged lands in different regions of Uruguay were diverse differing in growth rates, acid production, growth at 39°C and in LB medium, host range and symbiotic efficiency. Seventeen representative strains clustered into four groups on the basis of phenotypic characteristics, ARDRA and DNA fingerprinting (GTG₅-PCR). Partial sequence of 16S rRNA from eight of these strains classified them into at least two genera with four species: Azorhizobium doebereinerae, Rhizobium sp. related to R. etli and two different Rhizobium sp.-Agrobacterium. Our results confirm the presence of the specie Azorhizobium doebereinerae as microsymbionts of S. virgata in South America. No strain of Rhizobium etli has previously been reported as a microsymbiont of Sesbania, though R. etli like organisms have also been recovered from Dalea purpurea and Desmanthus illinoensis. Significant increases in dry matter production were obtained with S. virgata plants inoculated with selected rhizobial strains under growth chamber conditions.     

N<Subscript>2</Subscript> fixation by faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) in a gypsum-amended sodic soil

The response of faba bean to the application of four rates of gypsum (0, 2.5, 5.0, 10.0 t ha⁻¹) to a non-saline, alkaline sodic soil was measured in terms of grain yield, dry matter (DM) production, N accumulation and the proportional dependence of the legume on symbiotic N₂ fixation (P _atm). A yield-independent, time-integrated ¹⁵N-dilution model was used to estimate symbiotic dependence. A significant decrease in the exchangeable sodium percentage and significant increases in exchangeable Ca⁺⁺ and the Ca⁺⁺:Mg⁺⁺ ratio in the 0–10-cm soil layer were measured 30 months after application of 10 t ha⁻¹ gypsum. Despite low and erratic rainfall during crop growth, faba bean DM and N uptake responded positively to gypsum application. The symbiotic dependence of the legume at physiological maturity was little affected by sodicity (P _atm = 0.74 at zero gypsum and 0.81–0.82 at 2.5–10 t ha⁻¹ gypsum). The increase in fixed N due to gypsum application was mainly due to increases in legume DM and total N uptake. At 10 t ha⁻¹ of gypsum, faba bean fixed more than 200 kg N ha⁻¹ in above-ground biomass.     

Phytostabilization Ability of <Emphasis Type="Italic">Baccharis linearis</Emphasis> and Its Relation to Properties of a Tailings-Derived Technosol

Spontaneous colonization of mine tailing dams by plants is a potential tool for phytostabilization of such reservoirs. However, the physical and chemical properties of each mine tailings deposit determine the success of natural plant establishment. The plant Baccharis linearis is the main native nanophanerophyte species (evergreen sclerophyllous shrub) that naturally colonizes abandoned copper tailings dams in arid to semiarid north-central Chile. This study compare growth of B. linearis against the physical and chemical properties of a Technosol derived from copper mine tailings. Five sites inside the deposit were selected based on B. linearis vegetation density (VD), at two soil sampling depths under the canopy of adult individuals. Physical and chemical properties of tailings samples and nutrient concentrations in tailings and plants were each determined. Some morphological features of the plants (roots and aerial parts) were also quantified. There were significant differences in soil available water capacity (AW) and relative density (Rd) at different VD. Sites with low AW and high Rd had lower nutrient concentrations and higher Zn content in tailings, decreased infection by arbuscular mycorrhizal fungi, and increased fine root abundance and root hair length in individual plants. In contrast, higher AW, which was positively correlated with fine particles and organic matter content, had a positive effect on vegetation coverage, increased N and P contents in tailings, and increased N contents in leaf tissues, even when available N and P levels in tailings were low. Multiple constraints, such as low AW, N, P, and B contents and high Zn concentrations in the tailings restricted vegetation coverage, but no phenotypic differences were observed between individuals. Thus, in order to promote dense coverage by B. linearis, water retention in these tailings must be improved by increasing colloidal particles (organic and/or inorganic) contents, which have a positive effect on colonization by this species.     

Fungal degradation of chlorpyrifos by <Emphasis Type="Italic">Acremonium</Emphasis> sp. strain (GFRC-1) isolated from a laboratory-enriched red agricultural soil

The organophosphorus insecticide, chlorpyrifos, has been widely applied in agriculture; in veterinary, against household pests; and in subterranean termite control. Due to its slow rate of degradation in soil, it can persist for extended periods in soil with a significant threat to environment and public health. The mixed and pure fungi were isolated from three soils by enrichment technique. The enriched mixed fungal cultures were capable of biodegrading chlorpyrifos (300 mg L⁻¹) when cultivated in Czapek Dox medium. The identified pure fungal strain, Acremonium sp., utilized chlorpyrifos as a source of carbon and nitrogen. The highest chlorpyrifos degradation (83.9%) by Acremonium sp. strain GFRC-1 was found when cultivated in the nutrient medium with full nutrients. Desdiethyl chlorpyrifos was detected as a major biodegradation product of chlorpyrifos. The isolated fungal strain will be used for developing bioremediation strategy for chlorpyrifos-polluted soils.     

Comparison of nitrogen mineralisation patterns from root residues of<Emphasis Type="Italic"> Trifolium subterraneum</Emphasis> and<Emphasis Type="Italic"> Medicago sativa</Emphasis>

Crops grown in the first or second year after Medicago sativa L. (lucerne or alfalfa) in southern Australia have sometimes yielded less than crops grown after Trifolium subterraneum L. (subterranean clover), despite the greater annual accumulation of legume nitrogen (N) and enhanced N₂ fixation in lucerne-based pastures. To understand why, we studied the N mineralisation patterns of root residues collected from 16- or 20-week-old plants using incubation assays, in two separate experiments with contrasting soil types (a red loam and a grey clay loam). Fine roots of both species added to soil produced more mineral N than the control soil with no root residues added. In contrast, less mineral N accumulated in the presence of coarse roots than in the control soil. These patterns were not explained by differences in physical size or surface-area, but appeared to be more related to the carbon to nitrogen ratio (C:N) of fine and coarse roots. Fine roots of both species had a C:N of about 11, while the C:N of coarse roots ranged from 28 to 37. Subterranean clover had a high proportion of fine roots giving a weighted average C:N of 19 for the whole root system, and could be expected to result in a rapid net mineralisation of N. The root systems of lucerne on the other hand, consisted mainly of coarse roots giving an average C:N of 26, and would be likely to induce a transient net immobilisation of N and a delay in net mineralisation. The same general patterns of N mineralisation/immobilisation were observed in the second experiment. Tissue chemical analyses suggested that even where the total C:N of subterranean clover and lucerne residues were similar, the amounts of C and N in the soluble fraction of the residues and the C:N of that soluble fraction could largely account for the observed differences in N mineralisation/immobilisation.     

Effects of long-term mineral fertilization on microbial biomass,microbial activity,and the presence of <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">K</Emphasis>-strategists in soil

Long-term effects of mineral fertilization on microbial biomass C (MBC), basal respiration (R _B), substrate-induced respiration (R _S), β-glucosidase activity, and the r–K-growth strategy of soil microflora were investigated using a field trial on grassland established in 1969. The experimental plots were fertilized at three rates of mineral N (0, 80, and 160 kg ha⁻¹ year⁻¹) with 32 kg P ha⁻¹ year⁻¹ and 100 kg K ha⁻¹ year⁻¹. No fertilizer was applied on the control plots (C). The application of a mineral fertilizer led to lower values of the MBC and R _B, probably as a result of fast mineralization of available substrate after an input of the mineral fertilizer. The application of mineral N decreased the content of C extracted by 0.5 M K₂SO₄ (C _ex). A positive correlation was found between pH and the proportion of active microflora (R _S/MBC). The specific growth rate (μ) of soil heterotrophs was higher in the fertilized than in unfertilized soils, suggesting the stimulation of r-strategists, probably as the result of the presence of available P and rhizodepositions. The cessation of fertilization with 320 kg N ha⁻¹ year⁻¹ (NF) in 1989 also stimulated r-strategists compared to C soil, probably as the result of the higher content of available P in the NF soil than in the C soil.     

Root trait plasticity and plant nutrient acquisition in phosphorus limited soil

To overcome soil nutrient limitation, many plants have developed complex nutrient acquisition strategies including altering root morphology, root hair formation or colonization by arbuscular mycorrhizal fungi (AMF). The interactions of these strategies and their plasticity are, however, affected by soil nutrient status throughout plant growth. Such plasticity is decisive for plant phosphorus (P) acquisition in P‐limited soils. We investigated the P acquisition strategies and their plasticity of two maize genotypes characterized by the presence or absence of root hairs. We hypothesized that in the absence of root hairs plant growth is facilitated by traits with complementary functions, e.g., by higher root mycorrhizal colonization. This dependence on complementary traits will decrease in P fertilized soils. At early growth stages, root hairs are of little benefit for nutrient uptake. Regardless of the presence or absence of root hairs, plants produced average root biomass of 0.14 g per plant and exhibited 23% root mycorrhizal colonization. At later growth stages of maize, contrasting mechanisms with functional complementarity explained similar plant biomass production under P limitation: the presence of root hairs versus higher root mycorrhizal colonization (67%) favored by increased fine root diameter in absence of root hairs. P fertilization decreased the dependence of plant on specific root traits for nutrient acquisition. Through root trait plasticity, plants can minimize trade‐offs for developing and maintaining functional traits, while increasing the benefit in terms of nutrient acquisition and plant growth. The present study highlights the plasticity of functional root traits for efficient nutrient acquisition strategies in agricultural systems with low nutrient availability.     

Benefits of inoculation of the common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis>) crop with efficient and competitive<Emphasis Type="Italic"> Rhizobium tropici</Emphasis> strains

Cropping in low fertility soils, especially those poor in N, contributes greatly to the low common bean (Phaseolus vulgaris L.) yield, and therefore the benefits of biological nitrogen fixation must be intensively explored to increase yields at a low cost. Six field experiments were performed in oxisols of Paraná State, southern Brazil, with a high population of indigenous common bean rhizobia, estimated at a minimum of 10³ cells g^–1 soil. Despite the high population, inoculation allowed an increase in rhizobial population and in nodule occupancy, and further increases were obtained with reinoculation in the following seasons. Thus, considering the treatments inoculated with the most effective strains (H 12, H 20, PRF 81 and CIAT 899), nodule occupancy increased from an average of 28% in the first experiment to 56% after four inoculation procedures. The establishment of the selected strains increased nodulation, N₂ fixation rates (evaluated by total N and N-ureide) and on average for the six experiments the strains H 12 and H 20 showed increases of 437 and 465 kg ha^–1, respectively,in relation to the indigenous rhizobial population. A synergistic effect between low levels of N fertilizer and inoculation with superior strains was also observed, resulting in yield increases in two other experiments. The soil rhizobial population decreased 1 year after the last cropping, but remained high in the plots that had been inoculated. DGGE analysis of soil extracts showed that the massive inoculation apparently did not affect the composition of the bacterial community.     

Soil nematode community varies between rice cultivars but is not affected by transgenic Bt rice expressing <Emphasis Type="Italic">Cry1Ab</Emphasis> or <Emphasis Type="Italic">Cry1Ab</Emphasis>/<Emphasis Type="Italic">Cry1Ac</Emphasis>

Insecticidal crystal (Cry) proteins produced by transgenic Bacillus thuringiensis (Bt) rice that enter the soil via root exudation and plant residues may be harmful to non-target soil organisms. We conducted a 3-year field investigation to determine if soil nematode abundance and diversity were affected by exposure to two transgenic Bt rice cultivars, compared to their non-transgenic near isolines. Near isolines were Kemingdao (KMD-Bt) expressing the single Cry1Ab gene and its non-Bt near isoline Xiushui-11 (XSD), as well as Huahui-1 (HH-Bt) expressing the fused Cry1Ab/Cry1Ac gene and its non-Bt near isoline Minghui-63 (MH). Nematode variables including community composition, abundance, trophic groups, and most of the common genera differed significantly between the rice cultivars. However, these nematode variables were similar under transgenic Bt rice and its non-Bt near isoline, although higher Shannon’s index value and Pielou’s index value were found in soils planted with Bt rice than the non-Bt near isoline. During this 3-year field study, gene modification (single Cry1Ab gene and fused Cry1Ab/Cry1Ac gene) supports a more uniform distribution of nematode species but had no effect on soil nematode abundance and community composition. We conclude that continuous cultivation of KMD-Bt and HH-Bt rice varieties for 3 years is not detrimental to soil nematode communities under field conditions.     

Effects of an arbuscular mycorrhizal fungus and two plant-parasitic nematodes on<Emphasis Type="Italic"> Musa</Emphasis> genotypes differing in root morphology

In this study, the effect of an arbuscular mycorrhizal fungus (AMF) and two migratory endoparasitic nematodes on Musa plant growth, including the root system, were examined. In addition, the AMF-nematode interaction was studied. Seven Musa genotypes with different root systems were selected. Based on their relative mycorrhizal dependency, two genotypes (Calcutta 4 and Obino l'Ewai) were selected for AMF-nematode interaction studies. The experiments were performed under greenhouse conditions. Mycorrhization with Glomus mosseae resulted in a significantly better plant growth even in the presence of nematodes. The effect of AMF on the root system was genotype-dependent and seemed to be related to the relative mycorrhizal dependency of the genotype. The nematodes also affected the root system, decreasing branching. Nematode population densities were significantly reduced in the presence of AMF, except for Pratylenchus coffeae in Obino l'Ewai. In the root system, it appeared that the decreased branching caused by the nematodes was counterbalanced by the increased branching caused by the AMF.