Optimization of plant growth-promoting bacteria-assisted phytostabilization of mine tailings

Recent studies have indicated that plant growth-promoting bacteria (PGPB) can improve revegetation of arid mine tailings as measured by increased biomass production. The goals of the present study were first to evaluate how mode of application of known PGPB affects plant growth, and second to evaluate the effect of this inoculation on rhizosphere microbial community structure. PGPB application strategies investigated include preliminary surface sterilization of seeds (a common practice in phytoremediation trials) followed by a comparison of two application methods; immersion and alginate encapsulation. Results with two native desert plant species, Atriplex lentiformis and Buchloe dactyloides, suggest that seed surface sterilization prior to inoculation is not necessary to achieve beneficial effects of introduced PGPB. Both PGPB application techniques generally enhanced plant growth although results were both plant and PGPB specific. These results demonstrate that alginate encapsulation, which allows for long-term storage and easier application to seeds, is an effective way to inoculate PGPB. In addition, the influence of PGPB application on B. dactyloides rhizosphere community structure was evaluated using PCR-DGGE (denaturing gradient gel electrophoresis) analysis of bacterial DNA extracted from rhizosphere samples collected 75 d following planting. A comparative analysis of DGGE profiles was performed using canonical correspondence analysis (CCA). DGGE-CCA showed that rhizosphere community profiles from PGPB-inoculated treatments are significantly different from both uninoculated tailings rhizosphere profiles and profiles from the compost used to amend the tailings. Further, community profiles from B. dactyloides inoculated with the best performing PGPB (Arthro Mix) were significantly different from two other PGPB tested. These results suggest that introduced PGPB have the potential to influence the development of the rhizosphere community structure found in plants grown in mine tailings.     

Analysis of bacterial communities in soil by PCR-DGGE targeting protease genes

We studied the effects of the application of organic (OM) and inorganic fertilizer (CF) on soil protease activity and proteolytic bacterial communities in rhizosphere and bulk soil on an experimental lettuce field in Hokkaido, Japan. The protease activity always was higher in soils of the OM than with the CF treatment, and also higher in the rhizosphere than in the bulk soil. We analyzed proteolytic bacterial communities by denaturing gradient gel electrophoresis (DGGE) of the alkaline metalloprotease (apr) and neutral metalloprotease (npr) genes. Most apr forms detected were closely related to apr of Pseudomonas fluorescens, and all npr variants closely resembled the gene of Bacillus megaterium. These results were consistent with findings from tests using cultured bacterial communities, indicating a high specificity of our PCR-DGGE for amplifying apr and npr genes. The community compositions of proteolytic bacteria were assessed by principal component analysis of the DGGE profiles. There were significant differences in the effects of CF and OM on the community compositions of apr- and npr-expressing bacteria, and the communities of the two types of bacteria played different roles in rhizosphere and bulk soil. We found significant correlations between the protease activity and the communities of the two types of bacteria. The results indicate that different proteolytic bacteria release different amounts or activities of protease, and that the composition of proteolytic bacterial communities may play a major role in determining overall soil protease activity.     

Bacillus amyloliquefaciens BNM122, a potential microbial biocontrol agent applied on soybean seeds,causes a minor impact on rhizosphere and soil microbial communities

The increase in soybean productivity has contributed to a greater use of agrochemicals, which cause major problems, such as soil and water pollution and reduction of biodiversity, and have a negative impact on non-target species. The development of microbial biocontrol agents for soybean diseases can help to reduce pesticide abuse. Bacillus amyloliquefaciens BNM122 is a potential microbial biocontrol agent able to control the damping-off caused by Rhizoctonia solani when inoculated in soybean seeds, both in a plant growth chamber and in a greenhouse. In this study, we report the effect of soybean seed treatments with strain BNM122 or with two fungicides (thiram and carbendazim) on the structure and function of the bacterial community that colonizes the soybean rhizosphere. Also, soybean root nodulation by Bradyrhizobium japonicum, mycorrhization by arbuscular mycorrhizal fungi and plant growth were evaluated. We used the r- and K-strategist concept to evaluate the ecophysiological structure of the culturable bacterial community, community-level physiological profiles (CLPP) in Biolog? EcoPlates to study bacterial functionality, and the patterns of 16S RNA genes amplified by PCR and separated by denaturing gradient gel electrophoresis (PCR–DGGE) to assess the genetic structure of the bacterial community. Neither the ecophysiological structure nor the physiological profiles of the soybean rhizosphere bacterial community showed important changes after seed inoculation with strain BNM122. On the contrary, seed treatment with fungicides increased the proportions of r-strategists and altered the metabolic profiles of the rhizosphere culturable bacterial community. The genetic structure of the rhizosphere bacterial community did not show perceptible changes between treated and non-treated seeds. Regarding the bacterial and fungal symbioses, seed treatments did not affect soybean nodulation, whereas soybean mycorrhization significantly decreased (P < 0.05) in plants obtained from seeds treated with strain BNM122 or with the fungicides. However, a higher negative effect was observed in plants which seeds were treated with the fungicides. Plant growth was not affected by seed treatments.It can be concluded that soybean seed treatment with B. amyloliquefaciens BNM122 had a lesser effect on soil microbial community than that with the fungicides, and that these differences may be attributed to the less environmental persistence and toxic effects of the strain, which deserve further studies in order to develop commercial formulations.     

Compositional shifts of bacterial groups in a solarized and amended soil as determined by denaturing gradient gel electrophoresis

Soil solarization is a widespread, nonchemical agricultural practice for disinfesting soils, which is often used in combination with organic amendment, and whose action represents an important factor impacting on soil bacterial communities structure and population dynamics. The present study was conducted to investigate whether and to which extent a 72-day plot-scale soil solarization treatment, either combined or not with organic amendment, could stimulate compositional changes in the genetic structure of indigenous soil bacterial communities. Soil solarization with transparent polyethylene film, in combination or not with farmyard manure addition, was carried out during a summer period on a clay loam agricultural soil located in Southern Italy. Soils from a four-treatment (NS, nonsolarized control soil; S, solarized soil; MA, manure-amended nonsolarized soil; MS, manure-amended and solarized soil) plot block were sampled after 0, 8, 16, 36 and 72 days. Compositional shifts in the genetic structure of indigenous soil bacterial communities were monitored by denaturing gradient gel electrophoresis (DGGE) fingerprinting of 16S rRNA gene fragments amplified from soil-extracted community DNA using primers specific for Bacteria, Actinomycetales, α- and β-Proteobacteria. Changes in soil temperature, pH, and electrical conductivity (EC_1:1) were also monitored from 0 to 72 days. Beneath the polyethylene film the average soil temperature at 8-cm depth reached 55 °C compared to 35 °C in nonsolarized soil. In general, without amendment both soil pH and EC_1:1 were not significantly affected by solarization, whereas in manured plots either variables were greatly increased (from 7.0 to 8.0 pH and from 271 to 3021 μS cm⁻¹ EC_1:1), and both showed long-lasting effects due to soil solar heating. The eubacterial DGGE profiles revealed that soil solarization was the main factor inducing strong time-dependent population shifts in the community structure either in unamended or amended soils. Conversely, the addition of organic amendment resulted in an altered bacterial community, which remained rather stable over time. A similar behaviour was also observed in the DGGE patterns of β-proteobacterial and actinomycete populations, and also, albeit to a lesser extent, in the DGGE profiles of α-Proteobacteria. An increased bacterial richness was evidenced by DGGE fingerprints in 16- and 36-day samplings, followed by a decrease appearing in 72-day samplings. This could be explained, other than by a direct thermal effect on soil microflora, by solarization-induced changes in the physico-chemical properties of soil microbial habitats or by other ecological factors (e.g. decreased competitiveness of dominating bacterial species, reduced grazing pressure of microfaunal predators, increased nutrient availability).     

Effect of Azospirillum brasilense inoculation on rhizobacterial communities analyzed by denaturing gradient gel electrophoresis and automated ribosomal intergenic spacer analysis

Nucleic acid-based techniques allow the exploration of microbial communities in the environments such as the rhizosphere. Azospirillumbrasilense, a plant growth promoting rhizobacterium (PGPR), causes morphological changes in the plant root system. These changes in root physiology may indirectly affect the microbial diversity of the rhizosphere. In this study, the changes in the rhizobacterial structure following A. brasilense inoculation of maize (Zea mays) plants was examined by PCR-denaturating gradient gel electrophoresis (DGGE) and automated ribosomal intergenic spacer analysis (ARISA), using two universal primers sets for the 16S rRNA gene, and an intergenic 16S-23S rDNA primer set, respectively. Similar results were obtained when using either ARISA or DGGE performed with these different primer sets, and analyzed by different statistical methods: no prominent effect of A. brasilense inoculation was observed on the bacterial communities of plant roots grown in two different soils and in different growth systems. In contrast, plant age caused significant shifts in the bacterial populations.     

Influence of plant species and phosphorus amendments on metal speciation and bioavailability in a smelter impacted soil: a case study of food-chain contamination

Purpose

The present research aimed to assess the influence of two phosphorous (P) amendments on metal speciation in rhizosphere soil and the soil–plant transfer of metals.

Materials and methods

Complementary experiments were performed: field experiments on a contaminated cultivated soil and laboratory experiments on an uncultivated contaminated soil to highlight the mechanisms involved in metal-phosphorous interactions. In laboratory experiment, P amendments were added at 120 mg P/kg of soluble KH₂PO₄ amendment and 9,000 mg P/kg of solid Ca₅(PO₄)₃OH amendment.

Results and discussion

Field-culture results showed the possible food-chain contamination due to Pb, Cd, Cu, and Zn phytoaccumulation by pea and mustard plants from a cultivated agricultural soil. Moreover, P-metal complexes were observed by microscopy in the rhizosphere soil. In laboratory experiments, the application of P amendments significantly increased Pb and Zn level in rhizosphere soil compared to control. Phosphate amendments significantly increased metal-P fraction and decreased “oxides” and “organic matter” fractions of Pb and Zn. Soluble-P amendment was more effective than solid P amendment in changing Pb and Zn speciation. The changes in metal speciation are higher in the rhizosphere soil of pea than tomato. Application of P amendments increased Pb and Zn TF root/soil but decreased TF shoot/root.

Conclusions

The effectiveness of in situ metal immobilization technique varies with the type and quantity of applied P amendment as well as plant and metal type.     

Analysis of bacterial communities on alkaline phosphatase genes in soil supplied with organic matter

Abstract

We studied the effects of the application of organic matter (OM) and chemical fertilizer (CF) on soil alkaline phosphatase (ALP) activity and ALP-harboring bacterial communities in the rhizosphere and bulk soil in an experimental lettuce field in Hokkaido, Japan. The ALP activity was higher in soils with OM than in soils with CF, and activity was higher in the rhizosphere for OM than in the bulk soil. Biomass P and available P in the soil were positively related to the ALP activity of the soil. As a result, the P concentration of lettuce was higher in OM soil than in CF soil. We analyzed the ALP-harboring bacterial communities using polymerase chain reaction based denaturing gradient gel electrophoresis (DGGE) on the ALP genes. Numerous ALP genes were detected in the DGGE profile, regardless of sampling time, fertilizer treatment or sampled soil area, which indicated a large diversity in ALP-harboring bacteria in the soil. Several ALP gene fragments were closely related to the ALP genes of Mesorhizobium loti and Pseudomonas fluorescens. The community structures of the ALP-harboring bacteria were assessed using principal component analysis of the DGGE profiles. Fertilizer treatment and sampled soil area significantly affected the community structures of ALP-harboring bacteria. As the DGGE bands contributing to the principal component were different from sampling time, it is suggested that the major bacteria harboring the ALP gene shifted. Furthermore, there was, in part, a significant correlation between ALP activity and the community structure of the ALP-harboring bacteria. These results raise the possibility that different ALP-harboring bacteria release different amounts and/or activity of ALP, and that the structure of ALP-harboring bacterial communities may play a major role in determining overall soil ALP activity.     

Responses of soil microbial communities to manure and biochar in wheat cultivation of a rice-wheat rotation agroecosystem in East China

Soil contamination in agroecosystems remains a global environmental problem. Biochar has been suggested as an organic amendment to alleviate soil pollution, sequester carbon(C), and improve soil fertility. However, information on how bacterial and fungal communities in acidic bulk and rhizosphere soils respond to swine manure and its biochar is still lacking. In this study, biochar and swine manure were applied at two rates of 1.5 and 3 t ha^-1 in a rice-wheat rotation field to assess ...     

Effect of calcium carbonate, sand, and organic matter levels on mortality of five species of Azospirillum in natural and artificial bulk soils

 Five bacterial strains, one from each of the five known species of the plant growth-promoting bacteria (PGPB) Azospirillum (A. brasilense, A. lipoferum, A. amazonense, A. halopraeference, and A. irakense) were inoculated into two natural, semiarid soils (terra rosa and loessial sandy) from Israel, and two artificial soils constructed to simulate the native soils. Within 60 days, the populations of all five Azospirillum species declined significantly in a linear fashion, in both the native soils and in the homologous artificial soils. Increased levels of CaCO₃, and fine and rough sand, had significant detrimental effects on the survival of the five Azospirillum species, whereas increased organic matter content improved survival. In contrast, when the bacterial strains were incubated in the rhizosphere of tomato seedlings grown in the artificial soils, manipulation of these soil variables had only a marginal effect on bacterial survival; all Azospirillum species survived well in the tomato rhizosphere under conditions that are otherwise detrimental. This study indicates that most cells of the strains of five known species of Azospirillum died out linearly over time in two semiarid soils, and that only the major soil components affected Azospirillum survival in soil. Because mortality was similar in native soils and in artificial homologous soils, artificial soils can be used to study the soil behavior of Azospirillum. Received: 9 April 1999     

不同细菌饲喂细菌线虫对土壤细菌数量、活性以及群落结构的影响

The effects of bacterial-feeding nematodes on bacterial number, activity, and community composition were studied through a microcosm experiment using sterilized soil inoculated with soil bacteria （soil suspension） and with bacteria and three species of bacterial-feeding nematodes （ Cephalobus persegnis, Protorhabditis filiformis, and Caenorhabditis elegans）. Catalyzed reporter deposition-fluorescence in situ hybridization, CO2 evolution, and denaturing gradient gel electrophoresis （DGGE） of PCR ampli- fied 16S rRNA gene fragments were used to investigate bacterial numbers, antivity, and community composition, respectively. Our results showed that bacterial numbers and activity significantly increased in the presence of bacterial-feeding nematodes, which indicated that bacterial-feeding nematodes had a significant positive effect on soil bacteria. The different nematode species had different effects on bacterial numbers and activity. C. persegnis and P. filiformis, isolated from native soil, increased the bacterial number and activity more than C. elegans. The DGGE analysis results showed that dominant bacterial species significantly differed among the treatments, which suggested that bacterial-feeding nematode species modified the bacterial community composition in soil. Further gene sequence analysis results showed that the dominant bacterial species in this study were gram-negative bacteria. Given the completely same conditions except nematode species, the varied selective feeding behavior of different nematode species was the most likely reason for the altered bacterial community composition. Overall, the alteration of bacterial numbers, activity and community composition resulting from the bacterial-feeding nematodes may ult!mately affect soil ecological functioning and processes.     

Shifts in soil bacterial communities associated with the potato rhizosphere in response to aromatic sulfonate amendments

Desulfonation is an important process in the sulfur cycle, through which organic sulfur compounds are mineralized, releasing S. Desulfonating bacteria are known to respond to inorganic S soil amendments. However, the extent to which these communities shift as a response to the addition of organic sulfur in the soil remains elusive. Here, we investigated how amendments of soil with inorganic or organic sulfur compounds influence the bacterial communities associated with potato, in a microcosm experiment. The soil was amended with two doses of linear alkylbenzene sulfonate (LAS), here used as a model aromatic sulfonate compound, or with sulfate. Degradation of LAS was observed already at the young plant stage, as in all treatments 10- to 50-fold reductions of the initial (background) LAS concentrations were noted. Quantitative PCR analyses showed no significant effects of treatment on the bacterial abundances, which tended to increase from the young plant to the flowering stages of plant development. The bacterial community structures, determined via PCR-DGGE, were strongly affected by the presence of plants. This rhizosphere effect became more apparent at the flowering stages. Both the bacterial and β-proteobacterial community structures were affected by the presence of LAS, but dose-related effects were not observed. LAS also caused significant changes in the community structures, as compared to those in inorganic sulfate amended soil. Sulfate did not influence the bacterial community structures and only affected the β-proteobacterial ones at the flowering stage. Surprisingly, the presence of LAS did not exert any significant effect on the abundance of the Variovorax asfA gene, although clone libraries revealed a dominance of Variovorax types in the rhizosphere, especially in the high-level LAS treatment. Our results suggest that rhizosphere communities are key players in LAS degradation in soils, and that desulfonator Variovorax spp. plays a minor role in the mineralization of aromatic sulfonates in soil cropped with potato.     

Detection of aluminium tolerance plasmids and microbial diversity in the rhizosphere of plants grown in acidic volcanic soil

The rhizosphere is considered as a hot-spot for gene exchange among bacteria in terrestrial ecosystems. Chilean volcanic soils are characterized by low pH and high concentrations of aluminium (Al) in the soil solution, thus Al tolerance could be important for the survival of microorganisms in these soils; loss of genes encoding for Al tolerance may affect competitiveness particularly in the rhizosphere where competition is strong. The occurrence of Al-tolerance plasmids was investigated in the rhizospheres of pasture and crop plants growing in acidic volcanic soils from southern Chile. Al tolerance plasmids were captured by biparental mating. Two types of Al tolerance plasmids could be distinguished, based on their endonuclease restriction pattern. One plasmid of each group (denoted as pRPA21 and pOPA21) was selected for further studies. The plasmids showed a high stability in presence and absence of Al. Additionally, microbial community composition in the rhizosphere soils was assessed by denaturing gradient gel electrophoresis (DGGE). Sequencing of DGGE bands revealed among others, members of the bacterial phylum Gemmatimonadetes and archaeal phylum Crenarchaeota. The present study shows that the rhizosphere of pasture and crop plants growing in Chilean volcanic soil harbors genetic mobile elements which could play a role in the adaptation of bacterial populations to environmental stressors, such as Al-toxicity.     

Influence of soil on the interactions between endomycorrhizae and Azospirillum in sorghum

Sorghum (S. bicolor L. Moench cv. Bok 8) plants were grown in soil or sand-perlite low in plant-available N and P. Plants were inoculated with a vesicular-arbuscular mycorrhizal (VAM) fungus, or a strain of Azospirillum brasilense or both endophytes together. Plants received a nutrient solution which did not contain N or P. Increases in plant dry weight, shoot-to-root ratios, and the N content of dually-infected plants could be accounted for by summing the VAM and Azospirillum effects. For sorghum inoculated with both endophytes, the presence of A. brasilense in the rhizosphere increased VAM colonization and biomass, while the N input due to Azospirillum decreased, possibly due to competition for carbohydrates.Comparisons between sorghum grown with or without VAM-fungal infection in four growth media showed that edaphic factors other than P availability determined the host response to VAM infection. The P-fixing capacity of the soil, rather than the amount of available (NaHCO₃-extractable) P, influenced the balance between mutualistic and parasitic VAM-fungal growth.     

Soil solarization with biodegradable materials and its impact on soil microbial communities

The application of soil solarization (SS), one of the most promising techniques for the control of soilborne pathogens, is seriously limited by the drawback regarding the disposal of the used plastic materials. A possible solution to this problem is the use of biodegradable plastics. The aim of this study was to make comparisons between the impact of SS performed with biodegradable materials and that of SS with plastic films and other pest management techniques (i.e. organic matter amendment, calcium cyanamide and Dazomet fungicide application) on crop productivity, soilborne disease incidence, weed suppression, and soil chemical (total N, NH₄-N, nitrate, available phosphorus, organic matter, hydrolysis of fluorescein diacetate) and microbial (cultivable Pseudomonas, DGGE fingerprinting of bacterial 16S- and fungal 28S rRNA gene fragments from total soil community DNA) parameters. We carried out field experiments in two types of soil with different textures (clay and sand) artificially inoculated with Fusarium oxysporum f.sp. lycopersici (vs. tomato) and Sclerotinia minor (vs. lettuce).The temperature of soils covered with solarizing materials was always higher than that of bare soils, but plastic cover was more effective and consistent in rising soil temperature compared to biodegradable materials. Plant growth promotion by SS was limited, especially compared to Dazomet and organic matter applications, and a positive effect was observed only for lettuce in the clay soil. Differently, both plastic and biodegradable solarizing materials were effective in reducing lettuce drop caused by S. minor. Weed development was significantly suppressed by Dazomet application and SS with plastic film, while control with biodegradable materials was limited. SS had a variable and limited effect on chemical and microbial parameters, with a general tendency to reduce richness of bacteria and fungi. Dazomet caused the most pronounced reduction of the microbial community diversity in both soil types and a significant stimulation of the fluorescent Pseudomonas group. Organic amendment significantly enhanced the organic matter content, the hydrolysis of fluorescein diacetate and the Pseudomonas population. Among all measured soil parameters, the size of the fluorescent Pseudomonas population emerged as the most important factor affecting crop productivity.The results of this experimentation show the potential of using biodegradable solarizing materials in place of plastic films, but also indicate the need for improving their properties to obtain performances comparable to those of other pest management techniques.     

Structure and function of the soil microbial community in a long-term fertilizer experiment

The effect of organic and inorganic fertiliser amendments is often studied shortly after addition of a single dose to the soil but less is known about the long-term effects of amendments. We conducted a study to determine the effects of long-term addition of organic and inorganic fertiliser amendments at low rates on soil chemical and biological properties. Surface soil samples were taken from an experimental field site near Cologne, Germany in summer 2000. At this site, five different treatments were established in 1969: mineral fertiliser (NPK), crop residues removed (mineral only); mineral fertiliser with crop residues; manure 5.2 t ha⁻¹ yr⁻¹; sewage sludge 7.6 t ha⁻¹ yr⁻¹ or straw 4.0 t ha⁻¹ yr⁻¹ with 10 kg N as CaCN₂ t straw⁻¹. The organic amendments increased the C_org content of the soil but had no significant effect on the dissolved organic C (DOC) content. The C/N ratio was highest in the straw treatment and lowest in the mineral only treatment. Of the enzymes studied, only protease activity was affected by the different amendments. It was highest after sewage amendment and lowest in the mineral only treatment. The ratios of Gram+ to Gram− bacteria and of bacteria to fungi, as determined by signature phospholipid fatty acids, were higher in the organic treatments than in the inorganic treatments. The community structure of bacteria and eukaryotic microorganisms was assessed by denaturing gradient gel electrophoresis (DGGE) and redundancy discriminate analyses of the DGGE banding patterns. While the bacterial community structure was affected by the treatments this was not the case for the eukaryotes. Bacterial and eukaryotic community structures were significantly affected by C_org content and C/N ratio.     

The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum)

Pea plants were grown in γ-irradiated soil in pots with and without addition of the AM fungus Glomus intraradices at sufficient N and limiting P. Depending on the growth phase of the plant presence of AM had negative or positive effect on rhizosphere activity. Before flowering during nutrient acquisition AM decreased rhizosphere respiration and number of protozoa but did not affect bacterial number suggesting top-down regulation of bacterial number by protozoan grazing. In contrast, during flowering and pod formation AM stimulated rhizosphere respiration and the negative effect on protozoa decreased. AM also affected the composition of the rhizosphere bacterial community as revealed from DNA analysis (DGGE). With or without mycorrhiza, rhizosphere respiration was P-limited on very young roots, not nutrient limited at more mature roots and C-limited at withering. This suggests changes in the rhizosphere community during plant growth also supported by changes in the bacteria (DGGE).     

Structural and functional diversity of bacterial community in soil treated with the herbicide napropamide estimated by the DGGE,CLPP and r/K-strategy approaches

Napropamide is one of the most commonly used herbicide in agricultural practice and can exhibit toxic effect to soil microorganisms. Therefore, the main objective of this study was to examine the genetic and functional diversity of microbial communities in soil treated with napropamide at field rate (FR, 2.25 mg kg⁻¹ of soil) and 10 times the FR (10 × FR, 22.5 mg kg⁻¹ of soil) by the denaturing gradient gel electrophoresis (DGGE) and the community level physiological profile (CLPP) methods. In addition, the r/K-strategy approach was used to evaluate the effect of this herbicide on the community structure of the culturable soil bacteria. DGGE patterns revealed that napropamide affected the structure of microbial community; however, the richness (S) and genetic diversity (H) values indicated that the FR dosage of napropamide experienced non-significant changes. In turn, the 10 × FR dosage of herbicide caused significant changes in the S and H values of dominant soil bacteria. DGGE profiles suggest an evolution of bacteria capable of degrading napropamide among indigenous microflora. Analysis of the CLPPs indicated that the catabolic activity of microbial community expressed as AWCD (average well-color development) was temporary positively affected after napropamide application and resulted in an increase of the substrate richness (S_R) as well as functional biodiversity (H) values. Analysis of the bacterial growth strategy revealed that napropamide affected the r- or K-type bacterial classes (ecotypes). In treated-soil samples K-strategists dominated the population, as indicated by the decreased ecophysiological (EP) index. Napropamide significantly affected the physiological state of culturable bacteria and caused a reduction in the rate of colony formation as well as a prolonged time of growth rate. Obtained results indicate that application of napropamide may poses a potential risk for soil functioning.     

Organic and synthetic fertility amendments influence soil microbial,physical and chemical properties on organic and conventional farms

Field experiments were conducted to examine the effects of organic and synthetic soil fertility amendments on soil microbial communities and soil physical and chemical properties at three organic and three conventional vegetable farms in Virginia and Maryland in 1996 and 1997. Two treatments, including either an alternative organic soil amendment (composted cotton-gin trash, composted yard waste, or cattle manure) or synthetic soil amendment (fertilizer) were applied to three replicated plots at each grower field location. Production history and time affected propagule densities of Trichoderma species which remained higher in soils from organic farms. Propagule densities of Trichoderma species, thermophilic microorganisms, and enteric bacteria were also detected in greater numbers in soils amended with alternative than synthetic amendments, whereas propagule densities of Phytophthora and Pythium species were lower in soils amended with alternative than synthetic fertility amendments. Concentrations of Ca, K, Mg, and Mn were higher in soils amended with alternative than synthetic fertility amendments. Canonical correlations and principle component analyses indicated significant correlation between these soil chemical factors and the biological communities. First-order canonical correlations were more negative in fields with a conventional history, and use of synthetic fertilizers, whereas canonical correlations were more positive in fields with a history of organic production and alternative soil amendments. In the first year, yields of corn or melon were not different in soil amended with either synthetic or organic amendments at four of six farms. In the second year, when all growers planted tomatoes, yields were higher on farms with a history of organic production, regardless of soil amendment type. Alternative fertility amendments, enhanced beneficial soil microorganisms reduced pathogen populations, increased soil organic matter, total carbon, and cation exchange capacity (CEC), and lowered bulk density thus improving soil quality.     

Microbial community composition and functioning in the rhizosphere of three Banksia species in native woodland in Western Australia

Annual plant species differ in their rhizosphere microbial community composition. However, rhizosphere communities are often investigated under controlled conditions, and it is unclear if perennial plants growing in the field also have rhizosphere communities that are specific to a particular plant species. The aim of our study was to determine the bacterial community composition of three species of Banksia (B. attenuata R. Brown, B. ilicifolia R. Brown and B. menziesii R. Brown) growing in close proximity in a native woodland in Western Australia and to relate community structure to function. All three species are small trees that produce cluster roots in the field following winter rains. Cluster roots and rhizosphere soil were sampled in early spring (August 2001) and again four weeks later (September 2001). Many new cluster roots were formed in the period between the August and the September sampling. Rhizosphere soil pH, percent soil moisture and C and N content did not differ significantly among species or sampling times. However, the bacterial community composition on the cluster roots and in the rhizosphere soil, studied by denaturing gradient gel electrophoresis (DGGE), differed among the three species, with cluster root age class (young or mature to senescing) and also between sampling times. These changes in community composition were accompanied by changes in the activity of some of the enzymes studied. The activities of β-glucosidase and protease increased over time. The three species differed in asparaginase activity, but not in the activity of acid and alkaline phosphatase in the rhizosphere. These results suggest a relationship between the changes in composition and function of bacterial communities.     

Using a mixture of microalgae,biochar, and organic manure to increase the capacity of soil to act as carbon sink

Purpose

Microalgae, biochar, or organic manure (OM) can be used as soil amendments to enhance soil organic carbon (OC) content. In the present study, a mixture of all three was used to test whether they could further improve soil OC content and the soil’s ability to retain and fix carbon.

Materials and methods

A laboratory incubation study was carried out to evaluate the efficacy of using microalgae, biochar, OM, or their mixture, as a soil amendment to improve OC in soil extract. Metabolic processes and soil microbial community structuring were analyzed to explore the mechanism by which the mixture increased the capacity of soil to act as a carbon sink.

Results and discussion

OC increased markedly (2.9 times its initial level) following the amendment of the soil with a treatment comprising microalgae, biochar, and the highest dose of OM. Microalgal metabolites were utilized by soil microorganisms as a carbon source. Biochar reduced the concentration of extracellular polysaccharides, whereas OM increased extracellular protein concentration. These metabolites affected the relative proportions of different groups of soil microorganisms, thereby increasing the proportion of Rhodobacter and Runella, which exerted a positive synergistic effect on soil OC and increased the soil’s capacity to fix carbon.

Conclusions

A mixture including microalga, biochar, and OM as a soil amendment improved the OC of soil extract, and its effect was greater than that of any of its components alone. The findings of this study can help in devising ways to increase the OC content and the CO₂-fixing capacity of the soil.