Rhizosphere bacterial community composition in natural stands of Carex arenaria (sand sedge) is determined by bulk soil community composition

The relative importance of specific plant properties versus soil characteristics in shaping the bacterial community structure of the rhizosphere is a topic of considerable debate. Here, we report the results of a study on the bacterial composition of the rhizosphere of the wild plant Carex arenaria (sand sedge) growing at 10 natural sites in The Netherlands. The soil properties of the sandy soils at these sites were highly disparate, most notably in pH, chloride and organic matter content. Rhizosphere and bulk soil bacterial communities were examined by culture-independent means, namely, 16S rDNA-directed PCR-DGGE profiling. Large differences were observed between the bacterial communities of the different sites for both bulk and rhizosphere soil. Cluster analysis of bacterial profiles revealed that the rhizosphere community of each site was generally more closely related to the bulk soil community of that site rather than to rhizosphere communities of other sites. Hence, bacterial community structure within the rhizosphere of C. arenaria appeared to be determined to a large extent by the bulk soil community composition. This conclusion was supported by a reciprocal planting experiment, where C. arenaria shoots of different sites yielded highly similar rhizosphere communities when planted in the same soil.     

Soil feedback effects to the foredune grass Ammophila arenaria by endoparasitic root-feeding nematodes and whole soil communities

In coastal foredunes, the grass Ammophila arenaria develops a soil community that contributes to die-back and replacement by later successional plant species. Root-feeding nematodes and pathogenic soil microorganisms are involved in this negative feedback. Regular burial by wind-blown beach sand results in vigorous growth of A. arenaria, probably because of enabling a temporary escape from negative soil feedback. Here, we examine the role of root-feeding nematodes as compared to the whole soil community in causing negative feedback to A. arenaria. We performed a 3-year sand burial experiment in the field and every year we determined the feedback of different soil communities to plant growth in growth chamber bioassays.In the field, we established A. arenaria in tubes with beach sand, added three endoparasitic root-feeding nematode species (Meloidogyne maritima, Heterodera arenaria and Pratylenchus penetrans) or root zone soil to the plants, and created series of ceased and continued sand burial. During three subsequent years, plant biomass was measured and numbers of nematodes were counted. Every year, bioassays were performed with the field soils and biomass of seed-grown A. arenaria plants was measured to determine the strength of feedback of the established soil communities to the plant.In the field, addition of root zone soil had a negative effect on biomass of buried plants. In the bioassays, addition of root zone soil also reduced the biomass of newly planted seedlings, however, only in the case when the field plants had not been buried with beach sand. Addition of the three endoparasitic root-feeding nematodes did not influence plant biomass in the field and in the bioassays. Our results strongly suggest that the negative feedback to A. arenaria is not due to the combination of the three endoparasitic nematodes, but to other components in the soil community, or their interactions with the nematodes.     

Seasonal variation in trehalose contents of roots and nodules of leguminous trees in a tropical deciduous forest in Mexico

Seasonal variations of trehalose contents in roots and root-nodules of five legumes in a tropical deciduous forest in Jalisco, México, were determined. The tree species were: Lonchocarpus eriocarinalis and Erythrina lanata (sub-family Papilionoideae) and Piptadenia constricta, Albizia occidentalis and Lysiloma microphylum (sub-family Mimosoideae). Trehalose accumulation in nodules and roots varied seasonally and among species. For example, the Papilionoid-species retained nodules longer than those in the Mimosoideae (5 and 4 months, respectively), and accumulated the highest amounts of trehalose (average values of 178 vs. 2.88 mg g⁻¹ nodule (d. wt), respectively). Generally, maximum trehalose contents in nodules and roots were observed in November, at the beginning of the dry season.     

Rhizosphere fungal communities are influenced by Senecio jacobaea pyrrolizidine alkaloid content and composition

The contents and the compositions of the pyrrolizidine alkaloid (PA) complex of ragwort (Senecio jacobaea L.) were examined as potential drivers of fungal community structure in the rhizosphere. S. jacobaea plants within the coastal sand dune reserve of Meijendel (the Netherlands) were assayed for concentration and composition of PAs in roots. Rhizosphere soil was collected from pre-flowering plants, which differed up to 8-fold in PA production, and represented both jacobine and senecionine/seneciphylline chemotypes. Bulk soil samples from the same site were also collected for comparative examination. A culture-independent approach, involving direct DNA isolation, PCR of fungal 18S rRNA genes, and denaturing gradient gel electrophoresis (DGGE), was applied to compare the fungal communities of plants with different PA contents, as well as differences between bulk and rhizosphere samples. Cluster analysis of PCR-DGGE profiles revealed no clear evidence for PA-induced selection of specific fungal communities. However, canonical variance analysis showed that fungal communities associated with high-PA jacobine chemotypes could be discriminated from low PA samples and from the senecionine/seneciphylline chemotypes. The diversity of DGGE banding patterns, both in terms of band number and evenness, showed a trend toward lower diversity in the rhizosphere of high-PA plants as compared to low-PA plants and bulk soil. These results indicate that PA chemotypes of S. jacobaea differ in their influence on soil-borne fungal communities, with jacobine-containing plants exerting a greater selection in the rhizosphere than plants containing senecionine/seneciphylline.     

Belowground interactions between intercropped wheat and Brassicas in acidic and alkaline soils

Our previous studies showed that, under P-limiting conditions, growth and P uptake were lower in the wheat genotype Janz than in three Brassica genotypes when grown in monoculture. The present study was conducted to answer the question if P mobilised by the Brassicas is available to wheat; leading to improved growth of wheat when intercropped with Brassicas compared to monocropped wheat. To assess if the interactions between the crops depend on soil type, the wheat genotype Janz and three Brassica genotypes (two canolas and one mustard) were grown for 6 weeks in monoculture or wheat intercropped with each Brassica genotype in an acidic and an alkaline soil with low P availability (with two plants per pot). Wheat grew equally well in the two soils, but the Brassicas grew better in the acidic than in the alkaline soil. In the acidic soil, monocropped Brassicas had a 3 to 4 fold greater plant dry weight (dw) and P uptake than wheat; plant dw and P uptake in wheat were decreased or not affected by intercropping and increased in the Brassicas. In the alkaline soil, dw and P uptake of the Brassicas was twice as high as in wheat, with intercropping having no effect on these parameters. The contribution of wheat to the total shoot dw and P uptake per pot was 4-21% and 32-40% in acidic and alkaline soil, respectively. Mycorrhizal colonisation was low in all genotypes in the acidic soil (1-6%). In the alkaline soil, mycorrhizal colonisation of monocropped wheat was 62%, but only 43-47% in intercropped wheat. Intercropping decreased P availability in the rhizosphere of wheat in the acidic soil but had no effect on rhizosphere P availability in the alkaline soil. Intercropping had a variable effect on rhizosphere microbial community composition (assessed by fatty acid methylester analysis (FAME) and ribosomal intergenic spacer amplification (RISA)), ranging from intercropping having no effect on the rhizosphere communities to intercropping resulting in a new and similar rhizosphere community composition in both genotypes. The results of this study show that intercropping with Brassicas does not improve growth and P uptake of wheat; thus there is no indication that P mobilised by the Brassicas is available to wheat.     

Genetic variation in captive population of chinese alligator, Alligator sinensis, revealed by random amplified polymorphic DNA (RAPD)

Chinese alligator (Alligator sinensis) is an endemic species in China. The likely extinction of it in the wild has been recognised. To prevent this species becoming extinct, the Anhui Research Centre of Chinese Alligator Reproduction (ARCCAR) was established in Xuanzhou, Anhui Province in 1979, where has been established the largest captive population of Chinese alligator (XZSP) in the world. Another farm (CXSP) was established by villagers in Changxin, Zhejiang Province. The results of an investigation of the two captive subpopulation structures by genetic analysis are presented in this paper. We examined the genetic variation in the two captive subpopulations using RAPDs. Thirty-one random primers were selected among 199 random primers screened. A total of 193 reproducible RAPD fragments were scored among 43 individuals, of which 21 (10.88%) were polymorphic. The genetic distances between 43 individuals ranged from 0 to 0.0376 with average of 0.0104±0.0055 S.E. The genetic similarity in CXSP (0.9948±0.0029 S.E.) was higher than that in XZSP (0.9894±0.0055 S.E.). The founder effect is a possible explanation for very low genetic variation in CXSP. Analysis of the RAPD data showed that the mean phenotypic band frequencies of each polymorphic loci was 0.6656±0.3730 S.E. The lowest phenotypic band frequency (0.0233) was found in four of those polymorphic loci. There was no genetic difference between the two subpopulations (D_ij=0.0009). According to the dendrogram and the distribution of polymorphic fragments in two subpopulations, CXSP originated genetically from XZSP. This paper summarises a preliminary research on genetic structure in populations of Chinese alligator. Although there is higher genetic similary (0.9896±0.0055 S.E.) in captive population of A. sinensis, we did not determine whether or not loss of genetic variation had occurred in relation to a wild control population. The data of malformed offspring will be collected carefully, and wild samples be added to set up a control population in future study.     

Interactions between root-feeding nematodes depend on plant species identity

Root-feeding nematodes play an important role in structuring the composition of natural plant communities. Little is known about the role of intra- and interspecific interactions in determining the abundance of root-feeding nematodes in natural ecosystems. We examined interactions between two ectoparasitic root-feeding nematodes on two plant species: a good host plant for both nematode species and a good host for only one of the nematodes. We tested the hypothesis that root herbivore competitiveness depends on host suitability and related the experimental results to field data. In a greenhouse, we added different densities of the nematodes Tylenchorhynchus microphasmis and Tylenchorhynchus ventralis to Ammophila arenaria (the good host for both) and Carex arenaria (a good host for T. microphasmis only). Addition of T. ventralis did not significantly affect multiplication of T. microphasmis on both plant species. In contrast, on A. arenaria, T. ventralis experienced interspecific competition. However, on C. arenaria, T. microphasmis facilitated multiplication of T. ventralis. To explain this effect, we studied systemic plant-mediated effects in a split-root experiment. Nematode addition to one root compartment did not significantly influence nematode multiplication in the other root compartment, irrespective of nematode species identity. Therefore, the observed nematode interactions were not related to induced changes in the roots. In a two-choice experiment we tested whether host suitability was related to root attractiveness. Both nematode species were attracted to seedlings of A. arenaria, but not to C. arenaria. The low multiplication of T. ventralis on C. arenaria could be related to poor attraction to the roots. However, the poor attraction of T. microphasmis cannot be related to poor host suitability. Adding T. ventralis reduced shoot biomass of A. arenaria more than T. microphasmis did, whereas for C. arenaria the effect was the reverse. The interaction of the two nematodes on A. arenaria and C. arenaria shoot biomass was insignificant. However, the effect on root biomass of A. arenaria was interactive; adding T. ventralis to plants with high inoculation densities of T. microphasmis further decreased root biomass. Adding T. microphasmis further decreased root biomass of plants inoculated with low levels of T. ventralis. Depending on host plant identity, interactions between root-feeding nematodes may lead to competition or facilitation. Our results suggest that facilitation by T. microphasmis contributes to persistence of T. ventralis on C. arenaria. Thus, the population dynamics of root-feeding nematodes is influenced both by host plant identity and the presence of other root-feeding nematodes.     

Partitioning of molecular variation at local spatial scales in the vulnerable neotropical freshwater turtle, Hydromedusa maximiliani (Testudines, Chelidae): implications for the conservation of aquatic organisms in natural hierarchical systems

Hydromedusa maximiliani is a vulnerable freshwater turtle endemic to mountainous regions of the Atlantic rainforest in southeastern Brazil. Random amplified polymorphic DNA (RAPD) were surveyed with the purpose of assessing the genetic structure and determining the partitioning of molecular variation in H. maximiliani across the natural spatial hierarchical scale of its habitat. The goal of the study was to integrate ecological data with estimates of molecular genetics diversity to develop strategies for the conservation of this freshwater turtle. Specimens were sampled from rivers and streams across three drainages. Nine of the 80 primers used generated 27 scoreable bands of which 10 (37%) were polymorphic and produced 16 RAPD phenotypes. Significant heterogeneity was found in the distribution of RAPD molecular phenotypes across the three drainages. Analysis of molecular variance for molecular phenotypes showed that the heterogeneity had a spatial structure since a significant amount (22%) of the total variance was attributable to variation among rivers and streams. Since the genetic variation of this turtle seems to be structured according to the natural hierarchical system of rivers and streams within drainages, it is suggested that local populations should be considered as separate management units.     

Diversity and antagonistic potential of Pseudomonas spp. associated to the rhizosphere of maize grown in a subtropical organic farm

Pseudomonas spp. are one of the most important bacteria inhabiting the rhizosphere of diverse crop plants and have been frequently reported as biological control agents (BCAs). In this work, the diversity and antagonistic potential of Pseudomonas spp. in the rhizosphere of maize cultivars Nitroflint and Nitrodent grown at an organic farm in Brazil was studied by means of culture-dependent and -independent methods, respectively. Sampling of rhizosphere soil took place at three different stages of plant development: 20, 40 and 106 days after sowing. A PCR-DGGE strategy was used to generate specific Pseudomonas spp. fingerprints of 16S rRNA genes amplified from total community rhizosphere DNA. Shifts in the relative abundance of dominant populations (i.e. PCR-DGGE ribotypes) along plant development were detected. A few PCR-DGGE ribotypes were shown to display cultivar-dependent relative abundance. No significant differences in diversity measures of DGGE fingerprints were observed for different maize cultivars and sampling times. The characterisation and assessment of the antagonistic potential of a group of 142 fluorescent Pseudomonas isolated from the rhizosphere of both maize cultivars were carried out. Isolates were phenotypically and genotypically characterised and screened for in vitro antagonism towards three phytopathogenic fungi and the phytopathogenic bacterium Ralstonia solanacearum. Anti-fungal activity was displayed by 13 fluorescent isolates while 40 isolates were antagonistic towards R. solanacearum. High genotypic and phenotypic diversity was estimated for antagonistic fluorescent Pseudomonas spp. PCR-DGGE ribotypes displayed by antagonists matched dominant ribotypes of Pseudomonas DGGE fingerprints, suggesting that antagonists may belong to major Pseudomonas populations in the maize rhizosphere. Antagonists differing in their genotypic and phenotypic characteristics shared the same DGGE electrophoretic mobility, indicating that an enormous genotypic and functional diversity might be hidden behind one single DGGE band. Cloning and sequencing was performed for a DGGE double-band which had no corresponding PCR-DGGE ribotypes among the antagonists. Sequences derived from this band were affiliated to Pseudomonas stutzeri and P. alcaligenes 16S rRNA gene sequences. As used in this study, the combination of culture-dependent and -independent methods has proven to be a powerful tool to relate functional and structural diversity of Pseudomonas spp. in the rhizosphere.     

Influence of single trees on spatial and temporal patterns of belowground properties in native pine forest

Linkages between forest dynamics and ecosystem processes are poorly understood and this limits our ability to adequately estimate future changes in forest ecosystems due to human-induced global change. In particular at the single tree level, our understanding of temporal and spatial changes of belowground properties during forest succession is limited. Thus, our aim was to test whether we find a spatial and temporal gradient in nutrient availability and an associated shift in microbial community structure with increasing distance and age of single trees. We found that inorganic nitrogen was less available below the crown of single trees, while soluble organic carbon (DOC) was much more abundant, in particular in the inner zone of influence, i.e. close to the stem. The fungal:bacterial PLFA ratio was greater while microbial biomass carbon (MicC) was lower below the tree crown, indicating a strong influence of trees on spatial patterns of microbial biomass and community structure. Moreover, the positive correlation between MicC and total extractable N, and the negative correlation between fungal:bacterial biomass and δ¹⁵N, suggested that the microbial biomass was N limited below the tree crown and as a consequence nutrient cycling was presumably decelerated compared to open conditions. We also found a temporal pattern of increasing surface soil C and N content with increasing tree age (up to 250 years), underlining the significant role of single trees in creating spatial and temporal heterogeneity in forests.     

Pseudomonas corrugata: A suitable bacterial inoculant for maize grown under rainfed conditions of Himalayan region

Colonization and survival of the inoculated bacteria in rhizosphere of maize were investigated in field and pot experiments conducted for 3 consecutive years under rainfed conditions of Himalayan region. The effect of bacterial inoculations on growth and yield related parameters of maize were also evaluated. While three bacterial species, viz. Bacillus megaterium, Bacillus subtilis and Pseudomonas corrugata were tested in 1st year experiments, P. corrugata (based on the 1st year results) was chosen for inoculation in the subsequent experiments. All the three bacterial inoculants showed good rhizosphere competence giving high inoculum numbers (log₁₀ 11.13-11.34 cfu g⁻¹). The bacterial inoculations by B. megaterium, B. subtilis and P. corrugata resulted in an increment in grain yield of maize up to 122.4%, 135.2% and 194.3%, respectively, as compared to respective control. In 1st year, the antibiotic marked (Nal^r Rif^r) inoculant P. corrugata resulted in the highest increase in grain yield, statistically significant (P<0.05) as compared to control, B. megaterium and B. subtilis. In 2nd and 3rd year experiments, P. corrugata increased the grain yield up to 147.28% and 149.93%, respectively, as compared to control. The best performance and consistent trend of P. corrugata to increase plant yields was credited to its initial isolation from rhizosphere of maize growing under temperate conditions. The overall beneficial effects of bacterial inoculations on maize were contributed to (1) the colonization and survival of the introduced bacteria, and (2) stimulation of the indigenous microflora in the rhizosphere. Based on the comprehensive results obtained in this study, P. corrugata may be recommended as suitable bioinoculant for maize fields of temperate climate grown under rainfed conditions.     

Subtle changes in rhizosphere microbial community structure in response to increased boron and sodium chloride concentrations

Two of the major constraints to grain production in large areas of South-East Australia and cropping soils worldwide are high levels of subsoil boron (B) and excessive salinity (NaCl). Although the effect of these constraints is often studied in plants, the effect on microbially mediated plant-beneficial processes is unclear. To that end, we investigated the impact of B and NaCl on soil microbial community structure (MCS) in the wheat rhizosphere using BIOLOG ecoplates and terminal restriction fragment length polymorphism (T-RFLP). In addition, the effects of B and NaCl on the nitrogen (N) cycle processes of N fixation and ammonia oxidation were assessed by the construction of clone libraries of diazotrophic (nifH) and ammonia oxidising (amoA) rhizobacteria. Analysis of BIOLOG plates using non-metric multidimensional scaling (MDS) revealed addition of both B and NaCl significantly changed MCS, the latter of which was also significant through the analysis of T-RFLP data. Utilisation of several chemical groups of BIOLOG substrates significantly changed in NaCl-amended soil; both B and NaCl affected utilisation of several individual substrates indicative of plant stress including serine and malic acid. A significant decrease in diversity and species richness was observed in high B rhizosphere soil. The community structure of ammonia-oxidising bacteria (AOB), all of which clustered with Nitrosospira-like sequences, did not significantly change in response to addition of B or NaCl, but addition of the latter resulted in a significant increase of diazotroph clones within the α-proteobacteria similar to Azospirillum sp. It appeared that the addition of B and NaCl to soil changed rhizosphere MCS indirectly through increased soil moisture and subtle changes in root exudate patterns, the addition of the latter producing a more distinct change through increased osmotic pressure, leading to a greater increase in rhizodeposition of nutrients, especially carbohydrates. The implications for the current study are that B and NaCl are more likely to affect rhizosphere MCS indirectly through root exudate quantity and/or quality than directly through microbial toxicity, and that plant health is a major determinant in rhizosphere MCS and normal N cycling.     

Brassica genotypes differ in growth, phosphorus uptake and rhizosphere properties under P-limiting conditions

Compared to other crops, Brassicas are generally considered to grow well in soils with low P availability, however, little is known about genotypic differences within Brassicas in this respect. To assess the role of rhizosphere properties in growth and P uptake by Brassicas, three Brassica genotypes (mustard, Brassica juncea cv Chinese greens and canola, Brassica napus cvs Drum and Outback) were grown in an acidic soil with low P availability at two treatments of added P: 25 and 100 mg P kg⁻¹ as FePO₄ (P25 and P100). The plants were harvested at the 6-leaf stage, at flowering and at maturity. Shoot and root dry weight (dry weight) and root length increased with time and were lower in P25 than in P100. In P25, shoot dry weight was lowest in Outback and highest in Chinese greens. In the P100 treatment, Chinese greens had a higher shoot dry weight than the two canola cultivars. Chinese greens had a lower root dry weight and root length at flowering and maturity than the canola genotypes in both P treatments. Irrespective of P treatment, shoot P concentration was lower in Chinese greens than in the two canola genotypes. Specific P uptake (μg P m⁻¹ root length) decreased with time. In P25, Chinese greens had the lowest specific P uptake at the 6-leaf stage but it was higher than in the two canola genotypes at flowering and maturity. In P100, Outback had the lowest specific P uptake. Available P in the rhizosphere (resin P) decreased over time with the greatest decrease from the 6-leaf stage to flowering. In P25, resin P in the rhizosphere was greatest in Chinese greens at the 6-leaf stage and flowering and smallest in Outback at flowering. Microbial P and acid phosphatase activity changed little over time, were not affected by P treatment and there were only small differences between the genotypes. The rhizosphere microbial community composition [assessed by fatty acid methyl ester (FAME) analysis] of Outback and Chinese greens differed from that of the other two genotypes at the 6-leaf stage and flowering, respectively. At maturity, all three genotypes had distinct microbial communities. Plant traits such as production of high biomass at low shoot P concentrations as well as the capacity to maintain high P availability in the rhizosphere by P mobilisation can explain the observed differences in plant growth and P uptake among the Brassica genotypes.     

Microbial communities in forest floors under four tree species in coastal British Columbia

We compare forest floor microbial communities in pure plots of four tree species (Thuja plicata, Tsuga heterophylla, Pseudotsuga menziesii, and Picea sitchensis) replicated at three sites on Vancouver Island. Microbial communities were characterised through community level physiological profiles (CLPP), and profiling of phospholipid fatty acids (PLFA).Microbial communities from cedar forest floors had higher potential C utilisation than the other species. The F layer of the forest floor under cedar contained significantly higher bacterial biomass (PLFA) than the F layer under the other three tree species. There were differences in microbial communities among the three sites: Upper Klanawa had the highest bacterial biomass and potential C utilisation; this site also had the highest N availability in the forest floors. Forest floor H layers under hemlock and Douglas-fir contained greater biomass of Gram positive, Gram negative bacteria and actinomycetes than F layers based on PLFA, and H layers under spruce contained greater biomass of Gram negative bacteria than F layers. There were no significant differences in bacterial biomass between forest floor layers under cedar. Fungal biomass displayed opposite trends to bacteria and actinomycetes, being lowest in cedar forest floors, and highest in the F layer and at the site with lowest N availability. There were also differences in community composition among species and sites, with cedar forest floors having a much lower fungal:bacterial ratio than spruce, hemlock and Douglas-fir. The least fertile Sarita Lake site had a much greater fungal:bacterial ratio than the more fertile San Juan and Upper Klanawa sites. Forest floor layer had the greatest effect on microbial community structure and potential function, followed by site, and tree species. The similarity in trends among measures of N availability and microbial communities is further evidence that these techniques provide information on microbial communities that is relevant to N cycling processes in the forest floor.     

Temperature sensitivity of soil respiration is affected by prevailing climatic conditions and soil organic carbon content: A trans-China based case study

Understanding the spatial variation of temperature sensitivity (i.e. Q₁₀) of soil respiration (R_s) and its controlling factors, is critical to improve the precision of carbon budget estimations at regional scales. In this study, data from 2-3 continuous years of R_s measurements over 15 ecosystems of ChinaFLUX were summarized to analyze the response of R_s to soil temperature. Moreover, we improved our dataset by collecting previously published Q₁₀ values from 34 ecosystems in China. The ecosystems studied were located in the main climatic zones of China, spanning from alpine via temperate to tropical. Spatial variations of Q₁₀ and its controlling factors were analyzed. The results showed that soil temperature at a 5 cm depth satisfactorily explained the seasonal variations in R_s of the 15 ChinaFLUX ecosystems (R² varying from 0.37 to 0.83). Based on the overall data, the Q₁₀ values of R_s in China ranged from 1.28 to 4.75. The spatial variations in Q₁₀ were primarily determined by soil temperature during measurement periods, soil organic carbon (SOC) content, and ecosystem type. Ecosystems in colder regions and with higher SOC content had relatively higher Q₁₀ values. Moreover, ecosystems of different vegetation types showed different Q₁₀ values. A temperature- and SOC-dependent function for Q₁₀ is suggested, which could be a valuable reference for improving the regional-scale models of R_s and ecosystem carbon cycles.     

Pre-symbiotic and symbiotic interactions between Glomus intraradices and two Paenibacillus species isolated from AM propagules. In vitro and in vivo assays with soybean (AG043RG) as plant host

Two indole-producing Paenibacillus species, known to be associated with propagules of arbuscular mycorrhizal (AM) fungi, were examined for their mycorrhization helper bacteria activity at pre-symbiotic and symbiotic stages of the AM association. The effects were tested under in vitro and in vivo conditions using an axenically propagated strain of the AM fungus Glomus intraradices and Glycine max (soybean) as the plant host. The rates of spore germination and re-growth of intraradical mycelium were not affected by inoculation with Paenibacillus strains in spite of the variation of indole production measured in the bacterial supernatants. However, a significant promotion in pre-symbiotic mycelium development occurred after inoculation of both bacteria under in vitro conditions. The Paenibacillus rhizosphaerae strain TGX5E significantly increased the extraradical mycelium network, the rates of sporulation, and root colonization in the in vitro symbiotic association. These results were also observed in the rhizosphere of soybean plants grown under greenhouse conditions, when P. rhizosphaerae was co-inoculated with G. intraradices. However, soybean dry biomass production was not associated with the increased development and infectivity values of G. intraradices. Paenibacillus favisporus strain TG1R2 caused suppression of the parameters evaluated for G. intraradices during in vitro symbiotic stages, but not under in vivo conditions. The extraradical mycelium network produced and the colonization of soybean roots by G. intraradices were promoted compared to the control treatments. In addition, dual inoculation had a promoting effect on soybean biomass production. In summary, species of Paenibacillus associated with AM fungus structures in the soil, may have a promoting effect on short term pre-symbiotic mycelium development, and little impact on AM propagule germination. These findings could explain the associations found between some bacterial strains and AM fungus propagules.     

Functional diversity of microbial communities in the mixed boreal plain forest of central Canada

This study was designed to examine whether or not specific tree species (Picea glauca, Picea mariana, Pinus banksiana, Populus tremuloides), their post-fire stand age, or their position in a successional pathway had any significant effect on the functional diversity of associated soil microbial communities in a typical mixed boreal forest ecosystem (Duck Mountain Provincial Forest, Manitoba, Canada). Multivariate analyses designed to identify significant biotic and/or abiotic variables associated with patterns of organic substrate utilization (assessed using the BIOLOG™ System) revealed the overall similarity in substrate utilization by the soil microbial communities. The five clusters identified differed mainly by their substrate-utilization value rather than by specific substrate utilization. Variability in community functional diversity was not strongly associated to tree species or post-fire stand age; however, redundancy analysis indicated a stronger association between substrate utilization and successional pathway and soil pH. For example, microbial communities associated with the relatively high pH soils of the P. tremuloides-P. glauca successional pathway, exhibited a greater degree of substrate utilization than those associated with the P. banksiana-P. mariana successional pathway and more acidic soils. Differences in functional diversity specific to tree species were not observed and this may have reflected the mixed nature of the forest stands and of their heterogeneous forest floor. In a densely treed, mixed boreal forest ecosystem, great overlap in tree and understory species occur making it difficult to assign a definitive microbial community to any particular tree species. The presence of P. tremuloides in all stand types and post fire stand ages has probably contributed to the large amount of overlap in utilization profiles among soil samples.     

Non-symbiotic bacterial diazotrophs in crop-farming systems: can their potential for plant growth promotion be better exploited?

Biological N₂ fixation (BNF) by associative diazotrophic bacteria is a spontaneous process where soil N is limited and adequate C sources are available. Yet the ability of these bacteria to contribute to yields in crops is only partly a result of BNF. A range of diazotrophic plant growth-promoting rhizobacteria participate in interactions with C₃ and C₄ crop plants (e.g. rice, wheat, maize, sugarcane and cotton), significantly increasing their vegetative growth and grain yield. We review the potential of these bacteria to contribute to yield increases in a range of field crops and outline possible strategies to obtain such yield increases more reliably. The mechanisms involved have a significant plant growth-promoting potential, retaining more soil organic-N and other nutrients in the plant-soil system, thus reducing the need for fertiliser N and P. Economic and environmental benefits can include increased income from high yields, reduced fertiliser costs and reduced emission of the greenhouse gas, N₂O (with more than 300 times the global warming effect of CO₂), as well as reduced leaching of NO₃⁻-N to ground water. Obtaining maximum benefits on farms from diazotrophic, plant growth promoting biofertilisers will require a systematic strategy designed to fully utilise all these beneficial factors, allowing crop yields to be maintained or even increased while fertiliser applications are reduced.     

Dominant effects of litter substrate quality on the difference between leaf and root decomposition process above- and belowground

Initial decomposition rates, changes in organic chemical components (acid-insoluble fraction, holocellulose, polyphenols, soluble carbohydrates) and nutrient dynamics (K, Mg, Ca, P, N) were examined for fine roots and leaves of Japanese cypress (Chamaecyparis obtusa). Litterbag experiments designed to evaluate the relative effects of litter type and position of litter supply in the soil were carried out, considering that root and leaf litter typically occupy different locations and have different substrate qualities. Litterbags of roots and leaves were placed at two positions (on the soil surface and in the humus layer), and collected every 3 months over one year. The mass loss rate and N release were slower during root decomposition in the humus layer than during leaf decomposition on the soil surface. These differences between root and leaf decomposition were mainly caused by the litter type, and the effect of the position on decomposition was relatively small. Root litter was less influenced by position related effects, such as differences in humidity, than leaf litter, and this recalcitrant trait to environmental effects may be responsible for the slower mass loss rate and N release in root decomposition. The results of the present study suggest that fine roots are persistent in the soil and serve an important role in N retention in forest ecosystems because of their litter substrate quality.