Trapping of phosphate solubilizing bacteria on hyphae of the arbuscular mycorrhizal fungus Rhizophagus irregularis DAOM 197198

A simple method is described for trapping phosphate solubilizing bacteria (PSB) strongly attached to the hyphae of the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis (Ri). Bacteria were isolated from the hyphosphere of mycorrhizal leek plants growing on Turface previously inoculated with soil suspensions, obtained from the mycorrhizosphere of mycorrhizal plants growing in agricultural settings or maple forests in Quebec, Canada. Among the best PSB strongly attached to the hyphae of Ri, 26 isolates belonged to Burkholderia spp. and one was identified as Rhizobium miluonense. Four hyphobacteria exhibiting high potential of inorganic and organic P mobilization were further compared with four equivalent mycorrhizobacteria directly isolated from mycorrhizospheric soils sampled. In general, hyphobacteria were superior in mobilizing P from hydroxyapatite and from a low reactivity igneous phosphate rock from Quebec. Release of gluconic acid or the product of its oxidation 2-ketogluconic acid, are the main mechanisms involved in P solubilization. In a two compartments Petri plate system, Ri extraradical hyphal exudates, supported PSB growth and activity. In the absence of PSB Ri showed a negligible P solubilization activity. In the presence of PSB a substantial increase in P mobilization was observed, and the superiority of hyphobacterial activity was also observed under this system. Our results suggest that in developing a bioinoculant based on selected PSB, their interaction with AMF hyphae should not be overlooked.     

Phosphate-solubilization activity of bacterial strains in soil and their effect on soybean growth under greenhouse conditions

The efficiency of 13 phosphate-solubilizing bacteria (PSB; four Burkholderia sp., five Enterobacter sp., and four Bradyrhizobium sp.) was assessed in a soil plate assay by evaluating soil phosphorus (P) availability. A commercial argentine strain, Pseudomonas fluorescens, was used for comparing solubilizing activity. Burkholderia sp. PER2F, Enterobacter sp. PER3G, and Bradyrhizobium sp. PER2H strains solubilized the largest quantities of P in the soil plate assay after 60 days as compared with the other strains, including the commercial one. The effect of PSB inoculation on growth and nutrient uptake of soybean plants was also studied under greenhouse conditions. Plants inoculated with Burkholderia sp. PER2F had the highest aerial height and showed an appropriate N/P ratio. However, none of the PSB increased P uptake by plants. This suggests that PSB inoculation does not necessarily improve P nutrition in soybean, nor was there any relationship between P availability in the soil plate assay and P content in the soybean shoot in the greenhouse. We concluded that the selection of efficient PSB strains as possible inoculation tools for P-deficient soils should focus on the integral interpretation of soil assays, greenhouse experiments, and field trials.     

Isolation and characterization of phosphate-solubilizing bacteria from walnut and their effect on growth and phosphorus mobilization

The objectives of this work were to isolate and characterize walnut phosphate-solubilizing bacteria (PSB) and to evaluate the effect of inoculation with the selected PSB stains to walnut seedlings fertilized with or without insoluble phosphate. Thirty-four PSB strains were isolated and identified under the genera Pseudomonas, Stenotrophomonas, Bacillus, Cupriavidus, Agrobacterium, Acinetobacter, Arthrobacter, Pantoea, and Rhodococcus through a comparison of the 16S ribosomal DNA sequences. All isolated PSB strains could solubilize tricalcium phosphate (TCP) in solid and liquid media. Phosphate-solubilizing activity of these strains was associated with a drop in the pH of medium. A significantly negative linear correlation was found between culture pH and phosphorus (P) solubilized from inorganic phosphate. Three isolates Pseudomonas chlororaphis (W24), Bacillus cereus (W9), and Pseudomonas fluorescens (W12) were selected for shade house assays because of their higher phosphate-solubilizing abilities. Under shade house conditions, application of W24 or W12 remarkably improved plant height, shoot and root dry weight, and P and nitrogen (N) uptake of walnut seedlings. These increases were higher on combined inoculation of PSB with TCP addition. The most pronounced beneficial effect on growth of walnut plants was observed in the co-inoculation of the three PSB strains with TCP addition. In comparison, the isolate of W9 failed to increase available soil P, nutrient levels in plants, or to promote plant growth, suggesting that more insoluble phosphate compounds than tricalcium phosphate should be used as substrates to assess the phosphate-solubilizing ability of PSB under greenhouse conditions. The present results indicated that strains P. chlororaphis or P. fluorescens could be considered for the formulation of new inoculants of walnut, even of more woody plants.     

Effect of Arbuscular Mycorrhizal Fungi and Phosphate-Solubilizing Bacteria Consortia Associated with Phospho-Compost on Phosphorus Solubilization and Growth of Tomato Seedlings (Solanum lycopersicum L.)

ABSTRACT

The exploitation of phosphate mines generates an important quantity of phosphate sludge that remains accumulated and not valorized. In this context, composting with organic matter and rhizospheric microorganisms offers an interesting alternative and that is more sustainable for agriculture. This work aims to investigate the synergetic effect of arbuscular mycorrhizal fungi (AMF), phosphate-solubilizing bacteria (PSB) and phospho-compost (PC), produced from phosphate-laundered sludge and organic wastes, and their combination on plant growth, phosphorus solubilization and phosphatase activities (alkaline and acid). Inoculated mycorrhizae and bacteria strains used in this study were selected from plant rhizosphere grown on phosphate-laundered sludge. Significant (p < .05) increases in plant growth was observed when inoculated with both consortia and PC (PC+ PSB+ AMF) similar to those recorded in plants amended with chemical fertilizer. Tripartite inoculated tomato had a significantly (p < .05) higher shoot height; shoot and root dry weight, root colonization and available P content, than the control. Co-inoculation with PC and AMF greatly increased alkaline phosphatase activity and the rate of mycorrhizal intensity. We conclude that PC and endophytic AMF and PSB consortia contribute to a tripartite inoculation in tomato seedlings and are coordinately involved in plant growth and phosphorus solubilization. These results open up promising prospects for using formulate phospho-compost enriched with phosphorus-solubilizing microorganisms (PSM) in crop cultivation as biofertilizers to solve problems of phosphate-laundered sludge accumulation.     

解磷细菌、磷酸岩和化肥对玉米和小麦轮作及其经济收入的影响

A two-year field study was conducted to test the effects of two phosphate-solubilizing bacteria (PSB), Pantoea cypripedii (PSB-3) and Pseudomonas plecoglossicida (PSB-5), inoculated singly or together with rock phosphate (RP) fertilization on maize and wheat cropping cycle by comparing with chemical P fertilizer (diammonium phosphate, DAP), mainly in the crop yield, soil fertility and economic returns. Inoculation of PSB together with RP fertilization increased the crop growth in terms of shoot height, shoot and root dry biomass, grain yield and total P uptake in both maize and wheat crops compared to the other treatments. Soil fertility in the context of available P, enzyme activities and PSB population in both maize and wheat crops was significantly improved with PSB inoculation together with RP fertilization compared to DAP treatment. The combined use of PSB inoculation and RP fertilization was more economical due to minimal cost and maximum returns. These results suggested that PSB inoculation along with RP fertilization would be an appropriate substitute for chemical phosphate fertilizer application in sustainable agriculture systems.     

分室磷添加下菌根对滇池流域红壤间作玉米生长及磷素利用的影响

随着全球范围内磷矿资源短缺问题的日益严重,间作或菌根技术强化作物对土壤磷(P)的利用及增产增收的效应受到越来越多的关注。通过三室隔网盆栽模拟试验研究了分室磷处理[不添加磷(P0)、添加有机磷(OP50)、添加无机磷(IOP50)]和根室不接种(NM)、根室接种丛枝菌根真菌Glomus mosseae(GM)对与大豆间作的玉米的生长及磷素利用的影响。研究结果表明:所有复合处理中,以间作?GM?IOP50组合处理下的玉米根系最短和地上部生物量最高;OP50处理下,间作玉米的菌根侵染率显著高于单作处理。间作条件下,无论分室磷添加与否,接种GM处理的玉米地上部生物量明显高于NM处理;接种GM处理的玉米根系生物量和株高均显著高于NM处理,且根系生物量以间作?GM?OP50组合处理下最高。接种GM条件下,P0、IOP50、OP50处理下的间作植株生物量较单作处理分别提高45.98%、111.33%、33.56%。单作条件下,无论分室磷添加与否,接种GM处理的玉米地上部磷含量均显著高于NM处理;无论何种种植模式及分室磷添加与否,接种GM处理的植物根系磷含量均显著高于NM处理。无论磷添加与否,间作?GM组合条件下的玉米地上部磷吸收量均显著较高,其中IOP50处理下的地上部磷吸收量显著高于OP50处理。间作?GM组合条件下,IOP50处理玉米根系的磷吸收效率均显著高于OP50处理。可见,接种GM、分室磷添加和间作各自在一定程度上促进了玉米的生长。综合菌根侵染、生物量及磷含量与吸收量、磷吸收效率等指标,所有复合处理中以间作?GM?IOP50组合对玉米地上部的促生作用最好,玉米磷素吸收最多,可望有效强化滇池流域红壤坡耕地磷素的利用。     

Effects of interactions between arbuscular mycorrhizal fungi and bacteria on the growth of Lotus corniculatus L.: From the perspective of regulating rhizosphere fungal community

Arbuscular mycorrhizal fungi (AMF) provide essential nutrients to crops and are affected by fertilizers. Phosphate-solubilizing bacteria (PSB), nitrogen-fixing bacteria (NFB), and AMF have mutually beneficial relationships with plants, but the effects of their interactions on plant growth by regulating rhizosphere fungal community have not been sufficiently studied. In this study, a greenhouse pot experiment was conducted to investigate the interactions between AMF and bacteria (PSB and NFB) on the growth of Lotus corniculatus L. Specifically, the role of rhizosphere fungal community in the growth of Lotus corniculatus L. was explored using Illumina MiSeq high-throughput sequencing. The results showed that combined inoculation of AMF with PSB and NFB increased plant biomass, plant height, and fungal colonization rate. The richness, complexity, and stability of rhizosphere fungal community also increased after combined inoculation of AMF with PSB and/or NFB, particularly with PSB. In addition, combined inoculation of AMF with PSB and NFB enriched the abundance of beneficial microorganisms, with Chaetomium and Humicola showing the greatest alterations. The structural equation model showed that the interactions of AMF with PSB and NFB promoted plant growth by affecting fungal network structure and soil enzyme activities involved in carbon, nitrogen, and phosphorus cycling. These findings provide evidence for the effects of interactions of AMF with PSB and NFB on rhizosphere fungal community and plant growth.     

Isolation and characterization of phosphate-solubilizing bacteria from betel nut (Areca catechu) and their effects on plant growth and phosphorus mobilization in tropical soils

Phosphate-solubilizing bacteria (PSB) were isolated and characterized from the rhizosphere and bulk soils of Areca catechu plants. A long history of phosphate fertilizer use has elicited a direct effect on the incidence of soil PSB. Their abundance and ability to solubilize insoluble phosphate were significantly greater (P?<?0.0001) in soils with low available phosphorus (P) content than in other soil types. Three efficient PSB strains, namely, ASL12, ASG34, and ADH302, were identified as Acinetobacter pittii, Escherichia coli, and Enterobacter cloacae by characterizing 16S rRNA sequences and biochemical characteristics; they produced gluconic acid at concentrations of 7862.4, 4306.5, and 2663.8 mg L^?1, respectively. The highest amount of solubilized P was determined in Pikovskaya (PVK) medium for the bacterial strain ASL12. The secretion of gluconic acid was related to the available P of rhizosphere soils and P solubilization. Under shaded conditions, the application of these three strains significantly improved plant height, shoot and root dry weight, and nutrient uptake of A. catechu seedlings. A further increase in P solubilization was observed by co-inoculating the three strains and also applying tricalcium phosphate (TCP) or aluminum phosphate (AP). A significant (P?<?0.05) correlation was also observed between P-solubilization activity and A. catechu plant growth in pot experiments. Thus, the three strains can be potentially applied as inoculants in tropical and aluminum-rich soils.     

Plant Growth Promoting Traits of Indigenous Phosphate Solubilizing Pseudomonas aeruginosa Isolates from Chilli (Capsicumannuum L.) Rhizosphere

Phosphorus (P) is the second key nutrient for plants and it affects several attributes of plant growth. Identification of a potent phosphate solubilizing microorganism capable of transforming the insoluble P into soluble and plant-accessible forms is considered as the best eco-friendly option for providing inexpensive P to plants. Hence, this study was focused to assess the growth enhancement traits of the phosphate solubilizing bacteria (PSB) isolated from chili rhizosphere. Twelve PSB were isolated by enrichment culture technique and its P solubilization efficiency was checked using Vanadomolybdate phosphoric yellow color method. Among them, two potent strains PS2 and PS3, identified as Pseudomonas aeruginosa KR270346 and KR270347 based on biochemical and molecular characterization, were selected for further study. The Pseudomonas aeruginosa isolates interestingly showed the presence of various potential plant growth-promoting properties including indole acetic acid and siderophore production. The growth enhancement effect of Pseudomonas aeruginosa isolates on chilli showed promising results, and the growth parameters were found to be statistically signi?cant when compared to control. The results demonstrated an eloquent impact on various aspects, namely microbial count and PSB population, phosphatase and dehydrogenase activity, available phosphorous in the soil, plant nutrient uptake, and yield parameters. Inoculation of these two isolates together with the addition of rock phosphate increased comparable amount of available P and these treatments were statistically at par throughout the growth period. The results con?rmed the growth-promoting potential of the isolates to develop as biofertilizers either alone or as components of integrated nutrient management systems.     

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.     

Assessment of the arbuscular mycorrhizal fungal community in roots and rhizosphere soils of Bt corn and their non-Bt isolines

Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) in combination with sequencing of amplified partial 18S ribosomal DNA was used to assess the effects of transgenic Bt corn 5422Bt1 (event Bt 11) and 5422CBCL (event MON810) on the community structure of a non-target microorganism, namely the arbuscular mycorrhizal fungi (AMF) Glomus in corn roots and rhizosphere soils, relative to their non-Bt isolines 5422 (conventional parent) and 5422wx (conventional hybrid). AMF colonization in roots of different corn genotypes was also assessed using microscopic visualization. No adverse effect was detected on the indigenous AMF colonization of the roots of Bt hybrids 5422Bt1 and 5422CBCL. Two-way indicator species analysis (TWINSPAN^®) and detrended correspondence analysis (DCA) of the DGGE data from corn roots presented differences between Bt and non-Bt corn isolines (5422Bt1 vs. 5422wx, and 5422CBCL vs. 5422wx). However, differences were also recognized between the two non-Bt corn cultivars (5422 vs. 5422wx), and between the two Bt corn lines (5422Bt1 vs. 5422CBCL) in roots. Our results suggest that corn genotypes may have a greater influence on the AMF community structure of plant roots and rhizosphere soils than other factors, such as the age of the growing plants.     

Distribution of arbuscular mycorrhizal fungi spores in soils of smallholder agroforestry and monocultural coffee systems in southwestern Ethiopia

Arbuscular mycorrhizal fungi (AMF) are associated with the root system of coffee (Coffea arabica L.) plants, but their distribution in smallholder agroforestry and monocultural coffee systems is not well known. This study investigates the spatial distribution of AMF spores in a field study in southwestern Ethiopia. Soil samples from different depths (0–50 cm) were collected under the tree canopies of Acacia abyssinica, Albizia gummifera, Ficus sur, Ficus vasta and randomly selected unshaded coffee plants at different sampling points (canopy base, radius, edge and outside canopy). Significantly higher AMF spore densities were recorded at canopy bases and at 0–30 cm soil depth. Spore populations were found to belong to five genera: Acaulospora, Entrophospora, Glomus, Gigaspora and Scutellospora, with Glomus and Acaulospora dominating. Sampling points, sites and depths, shade tree species and shade tree/coffee plant age affected AMF spore density. Agroforestry practices including the use of leguminous shade trees effectively maintained AMF numbers in soils even at depth compared with unshaded coffee plants (monocultures).     

Association between Burkholderia species and arbuscular mycorrhizal fungus spores in soil

Burkholderia pseudomallei, the bacterial cause of the potentially fatal infection known as melioidosis, has a facultative intracellular lifestyle. The intracellular presence of B. pseudomallei in various eukaryotes including arbuscular mycorrhizal fungus (AMF) spores can be demonstrated in vitro. AMF spores were isolated from soils in a melioidosis-endemic area. B. pseudomallei and other Burkholderia spp. DNA was detected in these AMF spore samples, confirming an AMF spore-Burkholderia spp. association in soils which did not yield Burkholderia spp. by culture. This association may explain the environmental persistence, difficulty of recovery and dispersal of Burkholderia spp. in specific environments.     

Defoliation effects on Plantago lanceolata resource allocation and soil decomposers in relation to AM symbiosis and fertilization

Plants can mediate interactions between aboveground herbivores and belowground decomposers as both groups depend on plant-provided organic carbon. Most vascular plants also form symbiosis with arbuscular mycorrhizal fungi (AMF), which compete for plant carbon too. Our aim was to reveal how defoliation (trimming of plant leaves twice to 6 cm above the soil surface) and mycorrhizal infection (inoculation of the fungus Glomus claroideum BEG31), in nutrient poor and fertilized conditions, affect plant growth and resource allocation. We also tested how these effects can influence the abundance of microbial-feeding animals and nitrogen availability in the soil. We established a 12-wk microcosm study of Plantago lanceolata plants growing in autoclaved soil, into which we constructed a simplified microfood-web including saprotrophic bacteria and fungi and their nematode feeders. We found that fertilization, defoliation and inoculation of the mycorrhizal fungus all decreased P. lanceolata root growth and that fertilization increased leaf production. Plant inflorescence growth was decreased by defoliation and increased by fertilization and AMF inoculation. These results suggest a negative influence of the treatments on P. lanceolata belowground biomass allocation. Of the soil organisms, AMF root colonization decreased with fertilization and increased with defoliation. Fertilization decreased numbers of bacterial-feeding nematodes, probably because fertilized plants produced less root mass. On the other hand, bacterial feeders were more abundant when associated with defoliated than non-defoliated plants despite defoliated plants having less root mass. The AMF inoculation per se increased the abundance of fungal feeders, but the reduced and increased root AM colonization rates of fertilized and defoliated plants, respectively, were not reflected in the numbers of fungal feeders. We found no evidence of plant-mediated effects of the AM fungus on bacterial feeders, and against our prediction, soil inorganic nitrogen concentrations were not positively associated with the concomitant abundances of microbial-feeding animals. Altogether, our results suggest that (1) while defoliation, fertilization and AMF inoculation all affect plant resource allocation, (2) they do not greatly interact with each other. Moreover, it appears that (3) while changes in plant resource allocation due to fertilization and defoliation can influence numbers of bacterial feeders in the soil, (4) these effects may not significantly alter mineral N concentrations in the soil.     

Response of the pigeonpea-Rhizobium symbiosis to salinity stress: variation among Rhizobium strains in symbiotic ability

Summary There were significant differences among pigeonpea [Cajanus cajan (L.) Millsp] Rhizobium sp. strains (IC 3506, IC 3484, IC 3195, and IC 3087) in their ability to nodulate and fix N₂ under saline conditions. Pigeonpea plants inoculated with IC 3087 and IC 3506 were less affected in growth by salinity levels of 6 and 8 dS m^-1 than plants inoculated with the other strains. For IC 3506, IC 3484, and IC 3195, there was a decrease in the number of nodules with increasing salinity, while the average nodule dry weight and the specific nitrogenase activity remained unaffected. However, in IC 3087, the number of nodules increased slightly with increasing salinity. Leaf-P concentrations increased with salinity in the inoculated plants irrespective of the Rhizobium sp. strain, and leaf-N concentrations decreased with increasing salinity in IC 3484 and IC 3195 only. Shoot-Na and-Cl levels were further increased in these salt-sensitive strains only at 8 dS m^-1. Therefore there may be scope for selecting pigeonpea Rhizobium sp. symbioses better adapted to saline conditions. The Rhizobium sp. strains best able to form effective symbioses at high salinity levels are not necessarily derived from saline soils.Submitted as JA No. 919 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

适应玉米的溶磷细菌筛选及其对玉米生长的影响

从石灰性土壤中分离获得4株高效溶磷细菌X5、X6、Z4和Z8,研究其生物学特征,探索其单独及复合的溶磷促生潜能。研究发现菌株X5、X6、Z4和Z8均可以利用玉米根系分泌物作碳源生长。菌株X6和Z4均能产生吲哚乙酸(IAA)和铁载体,菌株Z8可产生IAA不产生铁载体,菌株X5可产生铁载体不产生IAA。盆栽试验结果表明,接种单一溶磷菌及4株菌复合处理均可促进玉米生长,但复合菌群的溶磷促生效果显著高于单一菌株。通过16S r RNA基因序列分析研究菌株的分类地位,初步鉴定X5、X6、Z4、Z8分别为荧光假单孢菌(Pseudomonas fluorescens)、草假单胞菌(Pseudomonas poae)、巨大芽孢杆菌(Bacillus megaterium)和枯草芽孢杆菌(Bacillus subtilis)。     

Phosphate Solubilizing Ability and Phylogenetic Diversity of Bacteria from P-Rich Soils Around Dianchi Lake Drainage Area of China

The phylogenetic diversity of phosphate solubilizing bacteria(PSB)distributed in P-rich soils in the Dianchi Lake drainage area of China was characterized,and the tricalcium phosphate(TCP)solubilizing activities of isolated PSB were determined.Among 1 328 bacteria isolated from 100 P-rich soil samples,377 isolates(28.39%of the total)that exhibited TCP solubilization activity were taken as PSB.These PSB showed different abilities to solubilize TCP,with the concentrations of solubilized P in bacterial cultures varying from 33.48 to 69.63 mg L^-1.A total of 123 PSB isolates,with relatively high TCP solubilization activity(>54.00 mg L^-1),were submitted for restriction fragment length polymorphism(RFLP)analysis,which revealed 32 unique RFLP patterns.Based on these patterns,62 representative isolates,one to three from each RFLP pattern,were seffected for 16S rRNA sequencing.Phylogenetic analysis placed the 123 PSB into three bacterial phyla,namely Proteobacteria,Actinobacteria and Firmicutes.Members of Proteobacteria were the dominant PSB,where 107 isolates represented by 26 RFLP patterns were associated with the genera of Burkholderia,Pseudomonas,Acinetobacter,Enterobacter,Pantoea,Serratia,Klebsiella,Leclercia,Raoultella and Cedecea.Firmicutes were the subdominant group,in which 13 isolates were affiliated with the genera of Bacillus and Brevibacterium.The remaining 3 isolates were identified as three species of the genus Arthrobacter.This research extends the knowledge on PSB in P-rich soils and broadens the spectrum of PSB for the development of environmentally friendly biophosphate fertilizers.     

Nitrogen and phosphorus fertilization leads to soil arbuscular mycorrhizal fungal diversity changes and rainfed crop yield increase on the Loess Plateau of China: A 37-year study

More than 80% of plants form mutualistic symbiotic relationships with arbuscular mycorrhizal fungi (AMF), and the application of fertilizers, such as nitrogen (N) and phosphorus (P) fertilizers, is a common agricultural management practice to improve crop yield and quality. However, the potential effects of long-term N and P fertilization on the AMF community in the rainfed agricultural system of the Loess Plateau of China are still not well understood. In this study, a long-term field experiment was conducted based on orthogonal design, with three N levels (0, 90, and 180 kg ha^-1 year^-1) and three P levels (0, 90, and 180 kg ha^-1 year^-1) for wheat fertilization. Changes in AMF community and correlations between AMF community composition, soil environmental factors, and wheat yield component traits were analyzed using traditional biochemical methods and high-throughput sequencing technology. The results showed that long-term N and P addition had a significant effect on the AMF community structure and composition. Nitrogen application alone significantly reduced the richness and diversity of AMF community, whereas the combined application of N and P significantly increased the richness and diversity of AMF community. The AMF community was driven mainly by soil available P, total P, and pH. There was a significant positive correlation between Glomus abundance and wheat yield and a significant negative correlation between Paraglomus abundance and wheat yield. Long-term N and P addition directly increased crop yield and affected yield indirectly by influencing soil chemical properties and the AMF community. Combined application of N and P both at 90 kg ha^-1 year^-1 could improve the ecological and physiological functions of the AMF community and benefit the sustainable development of rainfed agriculture.     

Nitrogenase activity and nitrogen-fixing bacteria associated with the roots ofAtriplex spp. growing in saline sodic soils of Pakistan

Summary TwoAtriplex spp. growing in low-fertility saline sodic soils were assayed for root-associated nitrogenase activity. The excised washed and unwashed root of the two species.A. lentiformis andA. amnicola, showed high root-associated nitrogenase activity. Acetylene-reducing activity seemed to be directly influenced by moisture. The highest number of diazotrophs, enumerated using a most probable number technique was observed on the root surface. Most of the isolated diazotrophs were identified asEnterobacter agglomerans. Root-associated nitrogenase activity inAtriplex spp. may explain the high protein and biomass content of these plants growing in low-fertility saline sodic soils.     

Differential interaction between two Glomus intraradices strains and a phosphate solubilizing bacterium in maize rhizosphere

Arbuscular mycorrhizal (AM) fungi and phosphate solubilizing bacteria (PSB) have a positive effect on plant productivity primarily through increasing phosphate availability. In order to study the interaction between AM fungi and PSB, we used Bacillus megaterium, a PSB isolated from the sterilized surface of AM germinated spores, and two strains of the AM fungus Glomus intraradices with different mycelial architecture. A greenhouse experiment was designed with maize as host plant with the addition of tribasic calcium phosphate. We tested the hypothesis that PSB, intimately linked with AM fungi, could interact differentially with the two AM strains. We concluded that inoculation with the PSB positively affected maize mycorrhization. Insoluble phosphate alone did not influence the AM extraradical mycelium (ERM) length and maize mycorrhization when bacteria were not inoculated. The results provide evidence that the adverse effect on infectivity for some AM strains might be caused by solubilized phosphorus release to the rhizosphere by PSB. Differences related to the mycelium architecture of each AM strain were observed: the density of PSB in rhizosphere soil was significantly higher only with the GA8 strain coinciding with the highest values of maize biomass. The density of bacteria associated with GA8 mycelium could be the result of the transfer of photosynthates through the rhizosphere; this close contact would favor the persistence of the intimate relationship between PSB and AM hyphae. In the bacteria-free treatments, soil adherence was not significantly altered. Although the highest development of ERM occurred with GA5, plants inoculated with GA8 showed the highest values for soil adherence. This may be due to the AM mycelium which modifies bacterial persistence in the rhizosphere and consequently soil adherence. Our results show that for potential applications, some characteristics of the AM strains are key in the selection of the AM fungi–PSB combinations. These include the tolerance to soluble phosphorus, the rate of root colonization, and ERM development that favors the persistence of bacteria in rhizosphere soil.