Diversity and characterization of <Emphasis Type="Italic">Azotobacter</Emphasis> isolates obtained from rice rhizosphere soils in Taiwan

Azotobacter species, free-living nitrogen-fixing bacteria, have been used as biofertilizers to improve the productivity of non-leguminous crops, including rice, due to their various plant growth-promoting traits. The purposes of this study were to characterize Azotobacter species isolated from rice rhizospheres in Taiwan and to determine the relationship between the species diversity of Azotobacter and soil properties. A total of 98 Azotobacter isolates were isolated from 27 paddy fields, and 16S rRNA gene sequences were used to identify Azotobacter species. The characteristics of these Azotobacter strains were analyzed including carbon source utilization and plant growth-promoting traits such as nitrogen fixation activity, indole acetic acid production, phosphate-solubilizing ability, and siderophore secretion. Of the 98 strains isolated in this study, 12 were selected to evaluate their effects on rice growth. Four species of Azotobacter were identified within these 98 strains, including A. beijerinckii, A. chroococcum, A. tropicalis, and A. vinelandii. Of these four species, A. chroococcum was predominant (51.0%) but A. beijerinckii had the highest level of nucleotide diversity. Strains within individual Azotobacter species showed diverse profiles in carbon source utilization. In addition, the species diversity of Azotobacter was significantly related to soil pH, Mn, and Zn. Members of the same Azotobacter species showed diverse plant growth-promoting traits, suggesting that the 98 strains isolated in this study may not equally effective in promoting rice growth. Of the 12 strains evaluated, A. beijerinckii CHB 461, A. chroococcum CHB 846, and A. chroococcum CHB 869 may be used to develop biofertilizers for rice cultivation because they significantly promoted rice growth. This study contributes to the selection of suitable Azotobacter strains for developing biofertilizer formulations and soil management strategies of Azotobacter for paddy fields.     

Effect of plant–soil feedbacks on the growth and competition of <Emphasis Type="Italic">Lactuca</Emphasis> species

Plant species generate specific soil communities that feedback on plant growth and competition. These feedbacks have been implicated in plant community composition and dispersion. We used Lactuca sativa and its wild progenitor Lactuca serriola to test the hypotheses that separate Lactuca species generate unique soil communities and that these soil communities differentially influence host, and neighboring, plant growth and competition. We grew each Lactuca in competition with the other, in sterile and non-sterile soils. We then examined the growth of each Lactuca species in sterile, non-sterile, and preconditioned soil. Finally, we used TRFLP techniques to explore whether the two Lactuca species generate significantly different bacterial communities in their rhizosphere soils. L. sativa proved to be the stronger competitor of the two species. However, sterilization increased the competitive effect of L. serriola background competitors. The growth experiment showed a significant effect on plant species, soil treatment, and the interaction of the two. Preconditioning soil caused reduced growth in both Lactuca species. Only L. serriola showed significantly increased growth in sterile soils. Our TRFLP analysis showed that the L. sativa soil community was significantly less diverse and that soil preconditioning had the largest impact on the community composition. These results show that Lactuca serriola’s rhizosphere communities generate a stronger negative feedback for plant growth than do the communities associated with L. sativa. Our study suggests that selection for plants that are able to grow in dense monoculture may have released Lactuca from species-specific negative soil feedbacks. This has important implications for both agriculture and the evolution of invasive plant species.     

Interactions of the manganese hyperaccumulator <Emphasis Type="Italic">Phytolacca americana</Emphasis> L. with soil pH and phosphate

Hyperaccumulators are plants that store exceptionally high concentrations of heavy metals or metalloids in their leaves. Phytolacca americana is one of the few species known to hyperaccumulate manganese (Mn); however, it is a common weedy species and has no specific association with high-Mn soils. Neither the mechanism by which P. americana hyperaccumulates Mn nor the ecological significance of this trait are well understood. It has recently been suggested that P. americana secretes acids into the rhizosphere as a means of acquiring phosphate, which might coincidentally increase Mn uptake. To determine whether P. americana acidifies the surrounding soil, plants were grown in rhizoboxes providing access to living roots. A thin layer of agar containing bromocresol green pH indicator dye was placed on the roots to observe color changes indicating acidification. Comparative studies showed that P. americana acidifies the rhizosphere significantly more than the non-accumulating plant Acalypha rhomboidea. A second experiment studied whether adjustment of soil pH and phosphate affect foliar Mn concentrations of P. americana. Concentrations of Mn in leaves were highest when plants were grown in acidified soils but were significantly lower in soils that were alkaline and/or enriched with phosphate. These results suggest that Mn hyperaccumulation may be a side effect of rhizosphere acidification as a phosphorus-acquisition mechanism, rather than an adaptation in its own right. The findings provide fundamental information about hyperaccumulator physiology and evolution, and may be relevant to attempts to utilize P. americana for phytoremediation.     

Rhizosphere processes induce changes in dissimilatory iron reduction in a tidal marsh soil: a rhizobox study

Background and aims

Although the role of microbial iron respiration in tidal marshes has been recognized for decades, the effect of rhizosphere processes on dissimilatory ferric iron reduction (FeR) is poorly known. Herein, we examined the FeR surrounding the root zone of three tidal marsh plants.

Methods

Using in situ rhizoboxes, we accurately separated rhizobox soil as one rhizosphere zone, and three bulk soil zones. Dissimilatory and sulfidic-mediated FeR were quantified by accumulation of non-sulfidic Fe(II) and Fe sulfides over time, respectively.

Results

The rates of dissimilatory FeR attained 42.5 μmol Fe g^?1 d^?1 in the rhizosphere, and logarithmically declined by up to 19.1 μmol Fe g^?1 d^?1 in the outer bulk soil. The rates of sulfidic-mediated FeR were less than 2 μmol Fe g^?1 d^?1 among all zones. Poorly crystalline Fe(III), DOC and DON, porewater Fe²⁺, and SO₄^2? were all enriched in the rhizosphere, whereas non-sulfidic Fe(II) and Fe sulfides gradually accumulated away from the roots. Iron reducers (Geobacter, Bacillus, Shewanella, and Clostridium) had higher populations in the rhizosphere than in the bulk soil. Higher rates of dissimilatory FeR were observed in the Phragmites australis and Spartina alterniflora rhizoboxes than in the Cyperus malaccensis rhizoboxes.

Conclusions

The radial change pattern of dissimilatory FeR rates were determined by allocation of poorly crystalline Fe(III) and dissolved organic carbon. The interspecies difference of rhizosphere dissimilatory FeR was associated with the root porosity and aerenchyma of the tidal marsh plants.

     

Impact of soil salinity on the microbial structure of halophyte rhizosphere microbiome

The rhizosphere microbiome plays a significant role in the life of plants in promoting plant survival under adverse conditions. However, limited information is available about microbial diversity in saline environments. In the current study, we compared the composition of the rhizosphere microbiomes of the halophytes Urochloa, Kochia, Salsola, and Atriplex living in moderate and high salinity environments (Khewra salt mines; Pakistan) with that of the non-halophyte Triticum. Soil microbiomes analysis using pyrosequencing of 16S rRNA gene indicated that Actinobacteria were dominant in saline soil samples whereas Proteobacteria predominated in non-saline soil samples. Firmicutes, Acidobacteria, Bacteriodetes and Thaumarchaeota were predominant phyla in saline and non-saline soils, whereas Cyanobacteria, Verrucomicrobia, Gemmatimonadetes and the unclassified WPS-2 were less abundant. Sequences from Euryarchaeota, Ignavibacteriae, and Nanohaloarchaeota were identified only from the rhizosphere of halophytes. Dominant halophilic bacteria and archaea identified in this study included Agrococcus, Armatimonadetes gp4, Halalkalicoccus, Haloferula and Halobacterium. Our analysis showed that increases in soil salinity correlated with significant differences in the alpha and beta diversity of the microbial communities across saline and non-saline soil samples. Having a complete inventory of the soil bacteria from different saline environments in Pakistan will help in the discovery of potential inoculants for crops growing on salt-affected land.     

Elucidating the interactions and phytotoxicity of zinc oxide nanoparticles with agriculturally beneficial bacteria and selected crop plants

There is a growing interest in the use of bioinoculants to assist mineral fertilizers in improving crop production and yield. Azotobacter and Pseudomonas are two agriculturally relevant strains of bacteria which have been established as efficient bioinoculants. An experiment involving addition of graded concentrations of zinc oxide (ZnO) nanoparticles was undertaken using log phase cultures of Azotobacter and Pseudomonas. Growth kinetics revealed a clear trend of gradual decrease with Pseudomonas; however, Azotobacter exhibited a twofold enhancement in growth with increase in the concentration of ZnO concentration. Scanning electron microscopy (SEM), supported by energy-dispersive X-ray (EDX) analyses, illustrated the significant effect of ZnO nanoparticles on Azotobacter by the enhancement in the abundance of globular biofilm-like structures and the intracellular presence of ZnO, with the increase in its concentration. It can be surmised that extracellular mucilage production in Azotobacter may be providing a barrier to the nanoparticles. Further experiments with Azotobacter by inoculation of wheat and tomato seeds with ZnO nanoparticles alone or bacteria grown on ZnO-infused growth medium revealed interesting results. Vigour index of wheat seeds reduced by 40–50% in the presence of different concentrations of ZnO nanoparticles alone, which was alleviated by 15–20%, when ZnO and Azotobacter were present together. However, a drastic 50–60% decrease in vigour indices of tomato seeds was recorded, irrespective of Azotobacter inoculation.     

Phosphorus intensity determines short-term P uptake by pigeon pea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> L.) grown in soils with differing P buffering capacity

Phosphorus (P) uptake by plant roots depends on P intensity (I) and P quantity (Q) in the soil. The relative importance of Q and I on P uptake is unknown for soils with large P sorption capacities because of difficulties in determining trace levels of P in the soil solution. We applied a new isotope based method to detect low P concentrations (<20 μg P l⁻¹). The Q factor was determined by assessment of the isotopically exchangeable P in the soil (E-value) and the I factor was determined by measurement of the P concentration in the pore water. A pot trial was set up using four soils with similar labile P quantities but contrasting P buffering capacities. Soils were amended with KH₂PO₄ at various rates and pigeon pea (Cajanus cajan L.) was grown for 25 days. The P intensity ranged between 0.0008 and 50 mg P l⁻¹ and the P quantity ranged between 10 and 500 mg P kg⁻¹. Shoot dry matter (DM) yield and P uptake significantly increased with increasing P application rates in all soils. Shoot DM yield and P uptake, relative to the maximal yield or P uptake, were better correlated with the P concentration in the pore water (R ² = 0.83–0.90) than with the E-value (R ²=0.40–0.53). The observed P uptakes were strongly correlated to values simulated using a mechanistic rhizosphere model (NST 3.0). A sensitivity analysis reveals that the effect of P intensity on the short-term P uptake by pigeon pea exceeded the effect of P quantity both at low and high P levels. However, DM yield and P uptake at a given P intensity consistently increased with increasing P buffering capacity (PBC). The experimental data showed that the intensity yielding 80% of the maximal P uptake was 4 times larger in the soil with the smallest PBC compared to the soil with the largest PBC. This study confirms that short-term P uptake by legumes is principally controlled by the P intensity in the soil, but is to a large extent also affected by the PBC of the soil. Section Editor: N. J. Barrow     

Effect of <Emphasis Type="Italic">Pinus ponderosa</Emphasis> afforestation on soilborne <Emphasis Type="Italic">Frankia</Emphasis> and saprophytic Actinobacteria in Northwest Patagonia,Argentina

Large areas in the extra-Andean region in the forest - steppe ecotone in “Northwestern Argentinean Patagonia” have been replaced by plantations of the exotic conifer Pinus ponderosa which modify soils physical and chemical factors and alter the biodiversity. Considering that in the region occur saprophytic soilborne actinobacteria that play important role as the fixation of atmospheric nitrogen (N₂) in symbiosis with native plant species and the production of bioactive molecules in plants rhizosphere, we aimed to study the effect of the plantation on the abundance of the N₂ fixer Frankia and on the genus diversity of cultivable rhizospheric actinobacteria. The study was performed with soils of six paired sites with pine plantations and natural neighbor areas (including steppes or shrublands). Abundance of infective Frankia was estimated by evaluating the nodulation capacity of soils, through a plant bioassay using Ochetophila trinervis as trap plant. Isolation trials for saprophytic actinobacteria were performed by applying chemotactic and successive soils dilutions methods. We concluded that P. ponderosa afforestation affect soil actinobacteria. This was mainly evidenced by a decrease in the Frankia nodulation capacity in O. trinervis, which was related to plantation age, to lower soil carbon and nitrogen content, higher available phosphorus, and to a slight decrease in soils pH. Pine plantation influence on the cultivable saprophytic actinobacteria was less clear. The study highlights the importance of soils as source of Frankia and rhizospheric actinobacteria in relation to disturbance caused by pine plantation in natural environments with native actinorhizal plant species.     

Bacterial community structure associated with the rhizosphere soils and roots of <Emphasis Type="Italic">Stellera chamaejasme</Emphasis> L. along a Tibetan elevation gradient

The effect of altitude on the composition and diversity of microbial communities have attracted highly attention recently but is still poorly understood. We used 16S rRNA gene clone library analyses to characterize the bacterial communities from the rhizosphere and roots of Stellera chamaejasme in the Tibetan Plateau. Our results revealed that Actinobacteria and Proteobacteria were dominant bacteria in this medicinal plant in the rhizosphere and root communities. The Shannon diversity index showed that the bacterial diversity of rhizosphere follows a small saddle pattern, while the roots possesses of a hump-backed trend. Significant differences in the composition of bacterial communities between rhizosphere and roots were detected based on multiple comparisons analysis. The community of Actinobacteria was found to be significantly negative correlated with soil available P (p?<?0.01), while the phylum of Proteobacteria showed a positive relationship with available P (p?<?0.05). Moreover, redundancy analysis indicated that soil phosphorus, pH, latitude, elevation and potassium positively correlated with bacterial communities associated with rhizosphere soils. Taken together, we provide evidence that bacterial communities associated with S. chamaejasme exhibited some certain elevational pattern, and bacterial communities of rhizosphere soil were regulated by environmental characteristics along elevational gradients in this alpine ecosystem.     

Carbon Inputs from <Emphasis Type="Italic">Miscanthus</Emphasis> Displace Older Soil Organic Carbon Without Inducing Priming

The carbon (C) dynamics of a bioenergy system are key to correctly defining its viability as a sustainable alternative to conventional fossil fuel energy sources. Recent studies have quantified the greenhouse gas mitigation potential of these bioenergy crops, often concluding that C sequestration in soils plays a primary role in offsetting emissions through energy generation. Miscanthus is a particularly promising bioenergy crop and research has shown that soil C stocks can increase by more than 2 t C ha^?1 yr^?1. In this study, we use a stable isotope (¹³C) technique to trace the inputs and outputs from soils below a commercial Miscanthus plantation in Lincolnshire, UK, over the first 7 years of growth after conversion from a conventional arable crop. Results suggest that an unchanging total topsoil (0–30 cm) C stock is caused by Miscanthus additions displacing older soil organic matter. Further, using a comparison between bare soil plots (no new Miscanthus inputs) and undisturbed Miscanthus controls, soil respiration was seen to be unaffected through priming by fresh inputs or rhizosphere. The temperature sensitivity of old soil C was also seen to be very similar with and without the presence of live root biomass. Total soil respiration from control plots was dominated by Miscanthus-derived emissions with autotrophic respiration alone accounting for ～50 % of CO₂. Although total soil C stocks did not change significantly over time, the Miscanthus-derived soil C accumulated at a rate of 860 kg C ha^?1 yr^?1 over the top 30 cm. Ultimately, the results from this study indicate that soil C stocks below Miscanthus plantations do not necessarily increase during the first 7 years.     

Distribution and accumulation of trace elements in rhizosphere and non-rhizosphere soils on a karst plateau after vegetation restoration

Aims

The restoration of vegetation in the rocky desertified areas of karst plateaus is a major problem for present-day ecological studies. The aim of this study was to determine the effects of vegetation restoration on the distribution and accumulation of trace elements in rhizosphere and non-rhizosphere soils.

Methods

Four representative areas containing the plants Coriaria nepalensis Wall., Pinus armandii Franch., Elaeagnus pungens Thunb., and Cotoneaster hissaricus Pojark. were selected within a vegetation restoration area in the Karst Plateau of Caohai County, Guizhou Province, China. Soils were sampled using a grid method to measure the total contents of the trace elements iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in rhizosphere and non-rhizosphere soils.

Results

The representative area containing Elaeagnus displayed the greatest amount of accumulation in the rhizosphere of both total and available trace elements, except for total Zn. Representative areas of the rhizosphere with other types of vegetation showed accumulation of only some of the trace elements studied. All types of vegetation were associated with the bioenrichment of available trace elements in both rhizosphere and non-rhizosphere soils, except for available Cu in areas associated with Cotoneaster.

Conclusions

Representative areas containing Pinus displayed the greatest degree of bioenrichment for both total and available trace elements in both rhizosphere and non-rhizosphere soils.

     

Illumina-based analysis of the rhizosphere microbial communities associated with healthy and wilted Lanzhou lily (<Emphasis Type="Italic">Lilium davidii</Emphasis> var. <Emphasis Type="Italic">unicolor</Emphasis>) plants grown in the field

Lanzhou lily (Liliumdavidii var. unicolor) is the best edible lily as well as a traditional medicinal plant in China. The microbes associated with plant roots play crucial roles in plant growth and health. However, little is known about the differences of rhizosphere microbes between healthy and wilted Lanzhou lily (Lilium davidii var. unicolor) plants. The objective of this study was to compare the rhizosphere microbial community and functional diversity of healthy and wilted plants, and to identify potential biocontrol agents with significant effect. Paired end Illumina Mi-Seq sequencing of 16S rRNA and ITS gene amplicons was employed to study the bacterial and fungal communities in the rhizosphere soil of Lanzhou lily plants. BIOLOG technology was adopted to investigate the microbial functional diversity. Our results indicated that there were major differences in the rhizosphere microbial composition and functional diversity of wilted samples compared with healthy samples. Healthy Lanzhou lily plants exhibited lower rhizosphere-associated bacterial diversity than diseased plants, whereas fungi exhibited the opposite trend. The dominant phyla in both the healthy and wilted samples were Proteobacteria and Ascomycota, i.e., 34.45 and 64.01 %, respectively. The microbial functional diversity was suppressed in wilted soil samples. Besides Fusarium, the higher relative abundances of Rhizoctonia, Verticillium, Penicillium, and Ilyonectria (Neonectria) in the wilted samples suggest they may pathogenetic root rot fungi. The high relative abundances of Bacillus in Firmicutes in healthy samples may have significant roles as biological control agents against soilborne pathogens. This is the first study to find evidence of major differences between the microbial communities in the rhizospheric soil of healthy and wilted Lanzhou lily, which may be linked to the health status of plants.     

Plant growth promotion and suppression of <Emphasis Type="Italic">Phytophthora drechsleri</Emphasis> damping-off in cucumber by cellulase-producing <Emphasis Type="Italic">Streptomyces</Emphasis>

Phytophthora drechsleri damping-off is one of the most important diseases of cucumber (Cucumis sativus). Salinity is a serious problem for crop production and affects diversity and activity of soil microorganisms. Application of salt-tolerant biocontrol agents may be beneficial in order to protect plants against pathogenic fungi in saline soils. In this study, a total of 717 Streptomyces isolates were isolated from the rhizosphere of cucumber, out of which two isolates showed more than 70% inhibitory effect against P. drechsleri and had cellulase activity in the presence and absence of NaCl. In a greenhouse experiment, two Streptomyces isolates with the highest antagonistic activity, strains C 201 and C 801, reduced seedling damping-off of cucumber caused by P. drechsleri by 77 and 80%, respectively, in artificially infested soils. Strain C 201 increased dry weight of seedlings up to 21% in greenhouse experiments. Phylogenetic analyses of 16S rRNA gene sequence reveals that strains C 201 and C 801 are closely related to S. rimosus and S. monomycini respectively. Increased activity of polyphenol oxidase (PPO) and peroxidase (POX) enzymes in Streptomyces-treated plants proved the biocontrol-induced systemic resistance (ISR) in cucumber plants against P. drechsleri.     

Temperature and soil microorganisms interact to affect <Emphasis Type="Italic">Dodonaea viscosa</Emphasis> growth on mountainsides

There is considerable interest in understanding the drivers of plant growth in the context of climate change. Soil microorganisms play an important role in affecting plant growth and functional traits. However, the role of interaction between soil microbes and temperature in affecting plant growth and functional traits remains unclear. The objective of this research was to investigate the effects of soil microbes, temperature, and their interaction on the growth and functional traits of Dodonaea viscosa in a mountain in Yuanmou county, southwest China. The experiment was conducted in climate chambers with a factorial design of three soil microbial communities (inoculated rhizosphere microbes from high elevation, inoculated rhizosphere microbes from low elevation, and autoclaved control) and two temperature conditions (colder and warmer). D. viscosa planted in inoculated rhizosphere microbes from both high and low elevations produced more total biomass with a lower root–shoot allometric exponent, and accumulated significantly more N and P nutrients than those in an autoclaved control, with no significant differences between the two microbial inoculations. Thus, rhizosphere soil microorganisms had positive effects on D. viscosa growth. However, the effect of the microbes on plant growth strongly depended on temperature. Warming had a positive effect on D. viscosa growth in inoculated rhizosphere microbe treatments, while the positive effect disappeared in the autoclaved control treatment. Our results indicate that temperature and soil microorganisms interact to affect D. viscosa growth. As the climate changes in the future in the studied region, the growth of D. viscosa may be greatly affected both directly and indirectly through the temperature–soil microbe interaction.     

Community Structure of Rhizomicrobiomes in Four Medicinal Herbs and Its Implication on Growth Management

Medicinal plants are the basic materials of traditional Chinese medicine. Soil characteristics and microbial contribution play important roles in the growth and product quality of medicinal plants, but the link between them in the rhizosphere of medicinal plants has been overlooked. Accordingly, Mentha haplocalyx, Perilla frutescens, Glycyrrhiza uralensis, and Astragalus membranaceus, four plants used in traditional Chinese medicines, were investigated in this study in order to elucidate bacterial and arbuscular mycorrhizal fungal (AMF) diversity in the rhizosphere and its possible association with soil quality. DGGE-based 16S rRNA and 18S rRNA gene sequencing results indicated that the diversity of both bacteria and AMF in Glycyrrhiza uralensis and Astragalus membranaceus was significantly higher than those in Mentha haplocalyx and Perilla frutescens, suggesting that medicinal plants have different preferences even under the same conditions. In addition, enzymatic activities and nutrition were enhanced in the rhizospheric soil of Mentha haplocalyx and Perilla frutescens, and the correlation among AMF diversity, soil enzymatic activities and nutrition was confirmed using RDA analysis. These results suggest the potential to grow medicinal plants with a reasonable rotation or intercrop in order to maintain long-term continuous soil development.     

Biofilm of <Emphasis Type="Italic">Aureobasidium pullulans</Emphasis> var. <Emphasis Type="Italic">pullulans</Emphasis> on winter wheat kernels and its effect on other microorganisms

Winter wheat, grown under greenhouse conditions, was protected four times with a cell suspension of Aureobasidium pullulans var. pullulans during the growing season. After harvest, the distribution and survival rates of the studied biocontrol agent were analyzed under a scanning electron microscope. The abundance of filamentous fungi, yeasts, pseudomonads and Azotobacter bacteria was determined by inoculation onto selective agar media. A. pullulans produced mostly unicellular chlamydospores on the surface and in the brush of kernels. Multicellular blastospore conglomerates secreted extracellular polymeric substances (EPS), and their biofilms were found in the brush and crease of kernels. The application of a cell suspension of A. pullulans with the density of 10⁴ CFU to winter wheat spikes, repeated four times, inhibited the growth of pseudomonads, Azotobacter bacteria and filamentous fungi.     

Hunting for cultivable <Emphasis Type="Italic">Micromonospora</Emphasis> strains in soils of the Atacama Desert

Innovative procedures were used to selectively isolate small numbers of Micromonospora strains from extreme hyper-arid and high altitude Atacama Desert soils. Micromonosporae were recognised on isolation plates by their ability to produce filamentous microcolonies that were strongly attached to the agar. Most of the isolates formed characteristic orange colonies that lacked aerial hyphae and turned black on spore formation, whereas those from the high altitude soil were dry, blue-green and covered by white aerial hyphae. The isolates were assigned to seven multi- and eleven single-membered groups based on BOX-PCR profiles. Representatives of the groups were assigned to either multi-membered clades that also contained marker strains or formed distinct phyletic lines in the Micromonospora 16S rRNA gene tree; many of the isolates were considered to be putatively novel species of Micromonospora. Most of the isolates from the high altitude soils showed activity against wild type strains of Bacillus subtilis and Pseudomonas fluorescens while those from the rhizosphere of Parastrephia quadrangulares and from the Lomas Bayas hyper-arid soil showed resistance to UV radiation.     

Distribution and stress tolerance of <Emphasis Type="Italic">Fimbristylis dichotoma</Emphasis> subsp. <Emphasis Type="Italic">podocarpa</Emphasis> (Cyperaceae) growing in highly acidic solfatara fields

Solfatara fields, areas surrounding fumaroles (volcanic vents) near the hot springs or volcanoes, are characterized by severe environmental conditions such as low soil pH and high aluminum contents. Fimbristylis dichotoma subsp. podocarpa is an endangered plant distributed in solfatara fields of Kyushu, western Japan. This species is known to form stands in sites closed to fumaroles where another solfatara plant Miscanthus sinensis do not colonize. We conducted field survey and culture experiments to test the hypothesis that F. dichotoma has higher tolerance to low pH and high aluminum conditions than M. sinensis, which corresponds to the distribution pattern of the two species. In the study site of Myoban Hot Spring, Oita Prefecture, rhizosphere soils of F. dichotoma showed lower pH and higher aluminum contents than those of M. sinensis. The culture experiment showed that germination of F. dichotoma was not inhibited even at pH 2, whereas significant decline in germination (%) was observed in M. sinensis. However, because of the low germination (%) of F. dichotoma, the values of the two species were similar at pH 2. In the pot culture, seedling growth of M. sinensis was suppressed at the aluminum concentrations tested (400 mg AlCl₃ L^?1). Conversely, no significant decline in the seedling growth was observed in F. dichotoma at the same aluminum levels. Considering soil conditions in the field, we concluded that that the difference in the tolerance to aluminum between the two species played a significant role in determining their distribution pattern.     

Crop rotation associating a legume and the nickel hyperaccumulator <Emphasis Type="Italic">Alyssum murale</Emphasis> improves the structure and biofunctioning of an ultramafic soil

Nickel (Ni) agromining aims to phytoextract heavy metals using hyperaccumulators whilst at the same time rehabilitating ultramafic soils. After removing the bioavailable metal, ultramafic soils are improved in terms of their agronomic properties with the aim of future agricultural uses. The low fertility of ultramafic soils can be compensated by integrating legumes already used in traditional agro-systems because of their importance in soil nitrogen enrichment. However, few studies have evaluated the potential profits of legumes on Ni agromining and their potential benefits on soil biological fertility. Here, we characterized the effect of a crop rotation with two plants, a legume (Vicia sativa) and a hyperaccumulator (Alyssum murale), on the phytoextraction efficiency and on soil structure and biofunctioning. A pot experiment was set up in controlled conditions to grow A. murale and four treatments were tested: rotation with V. sativa (Ro), fertilized mono-culture (FMo), non-fertilized mono-culture (NFMo) and bare soil without plants (BS). No significant difference was found between the Ro and NFMo treatments for the dry biomass yield. However, the Ro treatment showed the highest Ni concentrations ([Ni]) in A. murale shoots compared to FMo and NFMo treatments. The Ro treatment plants had more than twice as many leaves [Ni] compared to FMo. Soil physico-chemical analyses showed that the Ro treatment was better structured and showed the highest presence of bacterial micro-aggregates, as well as less non-aggregated particles. Legumes integration in Ni-agromining systems could be a pioneering strategy to reduce chemical inputs and to improve soil biofunctioning and thus fertility.     

Seasonal differences in arbuscular mycorrhizal fungal communities in two woody species dominating semiarid caatinga forests

Tropical dry forests are strongly affected by seasonality, but its effects on belowground communities are poorly studied. Thus, the objective of this study was to reveal the effect of the season (dry versus wet) on the mycorrhizal status of roots and their potential colonization, and to determine the composition and abundance of spore-based communities of arbuscular mycorrhizal fungi (AMF) in rhizospheric soil of two dominant woody species in caatinga communities (tropical dry forest of the Brazilian Northeast). Soil and root samples were taken four times in each season (dry and wet). In the cases of the number of glomerospores and the number of infective propagules of AMF, there were significant differences between the hosts, with greater values observed in the rhizosphere of Commiphora leptophloeos than Mimosa tenuiflora. Mycorrhizal colonization and the number of infective propagules of AMF differed also between the seasons, being higher in the dry than the wet season. In total, fourteen AMF species were found in the rhizosphere of C. leptophloeos and twelve species were associated with M. tenuiflora. There was a predominance of the fungal genus Acaulospora, with seven species, followed by Gigaspora and Glomus. The species studied and the seasons differ in the composition and structure of the AMF community in the rhizosphere of the plants. The ecological significance of those differences needs to be examined further.