How does the Fungal Endophyte Neotyphodium coenophialum Affect Tall Fescue (Festuca arundinacea) Rhizodeposition and Soil Microorganisms?

The goal of our study was to investigate the impact of fungal endophytes in tall fescue (Festuca arundinacea) on rhizodeposition and in turn, the soil microbial community. Sand-based, aseptic microlysimeter units were constructed for the collection of rhizodeposit solutions for chemical analyses from the roots of endophyte-free (E−) and endophyte-infected (E+) tall fescue plants. E+ plants were infected with Neotyphodium coenophialum, the most common endophyte found in tall fescue. Rhizodeposit solutions collected over nine weeks from E+ grass contained more organic carbon and carbohydrates than E−. These solutions were allowed to percolate through columns of plant-free soils to assess the response of the soil microbial communities. Soils to which solutions from E+ grass were applied had significantly higher respiration rates than those receiving solutions from E− grass, suggesting that microbial activity was stimulated by changes in the rhizodeposits. Culture-based assays of the soil microbial community (plate counts and community-level physiological profiling) suggest that the basic structure of the microbial community was not affected by application of rhizodeposit solutions from E+ plants as compared to E−. Our results indicate that the presence of a fungal endophyte may enhance rhizodeposition by tall fescue and could consequently influence microbial mineralization processes in the soil. In grasslands where nutrients may be limiting, hosting a fungal endophyte has the potential to enhance plant nutrient supply indirectly via a stimulatory effect on the soil microbial biomass. Megan M. Van Hecke and Amy M. Treonis - Both authors contributed equally to this work.     

Microbiomic Comparison of the Intestine of the Earthworm Eisenia fetida Fed Ergovaline

Tall fescue toxicosis and ergot alkaloids cost U.S. livestock producers approximately one billion dollars in annual livestock production loss annually. Ergovaline (EV) is the tall fescue alkaloid primarily responsible for clinical disease in livestock. Since native ruminal microorganisms have not been attributed to the detoxification of EV, finding detoxifying microbes from other environments is desirable. One possible source for potential microorganisms that can degrade EV is the anaerobic gut of the earthworm, Eisenia fetida. This study describes a comparative microbial analysis of earthworm digestive tracts receiving 10,000 ppb EV (E+ treatment) when compared with a control treatment with no detectable amounts of EV (E− treatment). An HPLC assay determined a 25% loss of EV from the E+ treatment was microbial in nature. A community microbiomic approach of constructing 16S-rRNA gene clone libraries was used to compare the microbes affected by the two treatments. RDPII tools such as Classifier and Libcompare were used in the analysis of 16S sequences. DOTUR analysis was used to examine the richness and diversity of the two microbial populations in these experiments. The results indicate there are few significant differences in the microbial community structure between the two microbiomes.     

Degradation of Polycyclic Aromatic Hydrocarbons in the Rhizosphere of Festuca arundinacea and Associated Microbial Community Changes

A phytoremediation growth chamber study was conducted to evaluate the contribution of soil microbial diversity to the contaminant degradation. Target contaminant removal from soil was assessed by monitoring concentrations of polycyclic aromatic hydrocarbons (PAHs), along with changes in the bacterial community structure over a time period of 10 months in the presence of tall fescue (Festuca arundinacea). Enhanced degradation of PAHs was observed in rhizosphere soil, with a maximum reduction in pyrene at a rate 36% higher than that noted for the unvegetated control. The dissipation of < 4-ring PAHs, 4-ring PAHs, and > 4-ring PAHs in unvegetated soil was 70%, 54%, and 49% respectively, whereas a higher dissipation rate was observed in tall fescue treated soil of 78%, 68%, and 61% at the end of the study. Microbial enumeration results showed greater total bacterial numbers and PAH-degrading bacteria in rhizosphere soil when compared to unvegetated soil. The results from the terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that there was a shift in the rhizosphere bacterial community during the phytoremediation process.     

Soil organic carbon fractions differ in two contrasting tall fescue systems

The value of tall fescue (Festuca arundinacea Schreb.) for C sequestration in addition to forage production and soil conservation is of current interest. However, studies relating to the impacts of endophyte infected (E+) and endophyte free (E?) tall fescue on soil organic matter fractions are few. This study examined how E+ and E? growth affected soil C fractions 4 years after establishment. The study site was at the University of Kentucky research farm, Lexington, Kentucky, USA. From soil cores in replicated fields we measured total C, particulate organic matter C (POM C), microbial biomass C (MBC), mineralizable C (Min C), C in aggregates, and aggregate distribution at depths of 0 to 15 and 15 to 30 cm. Significant effects between E+ and E- fescue were sometimes observed for MBC, Min C, C in micro-aggregates, and aggregate distribution, but not for total C, and POM C. At 0?C15 cm MBC (E+ 26% greater than E?), Min C (E+ 43% lower than E?), C associated with micro-aggregates (E+ 15% lower than E?), and micro-aggregates (46% more micro-aggregates in E+ than E?), were affected by endophyte infection, confirming hypotheses that early changes in soil properties were reflected in labile C fractions and soil structure. Endophyte infection status in tall fescue has quantifiable effects on C sequestration and soil structure, achievable in a relatively short period that can be used to monitor conservation efforts and the consequences of pasture renovation strategies.     

Low allelopathic potential of an invasive forage grass on native grassland plants: a cause for encouragement?

Tall fescue (Festuca arundinacea Schreb.), a highly competitive European grass that invades US grasslands, is reportedly allelopathic to many agronomic plants, but its ability to inhibit the germination or growth of native grassland plants is unknown. In three factorial glasshouse experiments, we tested the potential allelopathic effects of endophyte-infected (E+) and uninfected (E−) tall fescue on native grasses and forbs from Midwestern tallgrass prairies. Relative to a water control, at least one extract made from ground seed, or ground whole plant tissue of E+ or E− tall fescue reduced the germination of 10 of 11 species in petri dishes. In addition, the emergence of two native grasses in potting soil was lower when sown with E+ and E− tall fescue seedlings than when sown with seeds of conspecifics or tall fescue. However, when seeds of 13 prairie species were sown in sterilized, field-collected soil and given water or one of the four tall fescue extracts daily, seedling emergence was lower in one extract relative to water for only one species, and subsequent height growth did not differ among treatments for any species. We conclude that if tall fescue is allelopathic, its inhibitory effects on the germination and seedling growth of native prairie plants are limited, irrespective of endophyte infection. On the other hand, the apparent inability of these plants to detect tall fescue in field soil could hinder prairie restoration efforts if germination near this strong competitor confers fitness consequences. We propose that lack of chemical recognition may be common among resident and recently introduced non-indigenous plants because of temporally limited ecological interactions, and offer a view that challenges the existing allelopathy paradigm. Lastly, we suggest that tall fescue removal will have immediate benefits to the establishment of native grassland plants.     

Influence of phosphorus on the growth and ergot alkaloid content of Neotyphodium coenophialum-infected tall fescue (Festuca arundinacea Schreb.)

Tall fescue (Festuca arundinacea Schreb.) plants infected by the fungal endophyte Neotyphodium coenophialum (Morgan-Jones & Gams) (Glenn et al., 1996) often perform better than noninfected plants, especially in marginal resource environments. There is a lack of information about endophyte related effects on the rhizosphere of grasses. In a greenhouse experiment, four endophyte-infected (E+) tall fescue clones (DN2, DN4, DN7, DN11) and their endophyte-free (E–) forms were grown in limed (pH 6.3) Porter soil (low fertility, acidic, high aluminum and low phosphorus content, coarse-loamy mixed mesic Umbric Dystrochrept) at three soil P levels (17, 50, and 96 mg P kg^-1 soil) for five months. Excluding the genotype effect, endophyte infection significantly increased cumulative herbage DM yield by 8% at 17 mg P kg^-1 soil but reduced cumulative herbage DM yield by 12% at 96 mg P kg^-1 soil. With increased P availability in the soil, shoot and root DM, and root/shoot ratio in E+ plants were significantly less when compared to E– plants. Endophyte infection increased specific root length at 17 and 50 mg P kg^-1soil. At soil P level of 17 mg P kg^-1soil, E+ plants had significantly higher P concentrations both in roots and shoots. Similar relationships were found for Mg and Ca. E+ plants had significantly higher Zn, Fe, and Al concentration in roots, and lower Mn and Al concentration in shoots when compared to E– plants. Ergot alkaloid concentration and content in shoot of E+ plants increased with increasing P availability in the soil from 17 to 50 mg P kg^-1 but declined again at 96 mg P kg^-1 soil. Ergot alkaloid accumulation in roots increased linearly with P availability in the soil. Results suggest that endophyte infection affects uptake of phosphorus and other mineral nutrients and may benefit tall fescue grown on P-deficient soils. Phosphorus seems also to be involved in ergot alkaloid accumulation in endophyte-infected tall fescue.     

Temporal and spatial patterns of soil water following wildfire-induced changes in plant communities in the Great Basin in Nevada,USA

Infection of tall fescue (Festuca arundinacea Schreb.) with its endemicNeotyphodium coenophialum-endophyte (Morgan-Jones and Gams) Glenn, Bacon and Hanlin appears to reduce copper (Cu) concentrations in forage and serum of grazing animals, contributing to a range of immune-related disorders. A greenhouse experiment was conducted to identify effects of novel endophyte strains on Cu acquisition by tall fescue (Festuca arundinacea Schreb.) varieties Grasslands Flecha and Jesup infected with a novel, non ergot producing endophyte strain AR542, and two perennial ryegrass (Lolium perenne L.) varieties Aries and Quartet infected with a novel, non lolitrem B producing strain AR1, and their noninfected (E−) forms. Individual endophyte/grass associations were cultivated in nutrient solutions at 1.0 (P+) and 0.0 mM (P−) phosphorus concentrations. The Cu²⁺-binding activity of extracellular root exudates, and concentrations of Cu and other heavy metals in roots and shoots were measured. Extracellular root exudates of AR542-infected vs. E− tall fescue had higher Cu²⁺-binding activity only in P− nutrient solution as shown by lower concentration of free Cu²⁺ (0.096 vs. 0.188 mmol Cu²⁺ g⁻¹ root DM, respectively). The Cu²⁺-binding activity by root exudates of perennial ryegrass was not affected by endophyte infection, but was higher (i.e., lower concentration of free Cu²⁺) in P− vs. P+ nutrient solution (0.068 vs. 0.114 mmol Cu²⁺ g⁻¹ root DM). In this hydroponic experiment, Cu concentrations in shoots of both grasses were not a function of Cu²⁺-binding activity and endophyte effects on heavy metal concentrations in shoots and roots were specific for each variety. The Cu²⁺-binding activity of extracellular root exudates may affect Cu accumulation by field-grown, endophyte-infected tall fescue under P-limiting growth conditions and warrants verification by more specific methods.     

Rhizobacteria (Pseudomonas sp. SB) assist phytoremediation of oily-sludge-contaminated soil by tall fescue (Testuca arundinacea L.)

Background and aims

The objectives of this study were to examine the effect of direct inoculation of seeds with the rhizobacteria Pseudomonas sp. SB on the growth of tall fescue and phytodegradation efficiency in an oily-sludge-contaminated soil.

Methods

SB isolated from rhizosphere soil of tall fescue was evaluated for their plant-growth-promoting characters and ability to produce biosurfactant. A pot experiment was conducted to study the effect of inoculation of SB on phytoremediation.

Results

SB reduced the surface tension of culture media and produced indole acetic acid, siderophores, and 1-aminocyclopropane-1-carboxylate deaminase. Inoculation of SB increased shoot and root dry weights of tall fescue in oily-sludge-contaminated soil by 28 % and 19 %, respectively. Over 120 days, the content of total petroleum hydrocarbon in soil decreased by 33.9 %, 68.0 %, and 84.5 %, and of polycyclic aromatic hydrocarbons (PAHs) by 32.9 %, 40.9 %, and 46.2 %, respectively, in the no-plant control, tall fescue, and tall fescue + SB treatments. Inoculation of SB also increased the activity and biodiversity of soil microbial communities in the planted treatments.

Conclusions

SB could produce biosurfactant and exhibited a number of characters of plant-growth-promoting rhizobacteria. Inoculation of SB to tall fescue led to more effective remediation of oily-sludge-contaminated soils.     

Encroachment of Endophyte-infected on Endophyte-free Tall Fescue

Persistence of endophyte-free (E-) tall fescue (Festuca arundinaceaSchreb.)is erratic. Little information exists as to how fast endophyte(Neotyphodium coenophialum)-infected (E+) tall fescue mightencroach on E- tall fescue and whether specific conditions mightinfluence the speed of encroachment. Plots of E+ and E- tallfescue genotypes 7 and 17 were established using a modifiedNelder's design to compare performance of the E+ forms of theplants in pure and mixed communities at different populationdensities. The plots were planted at the USDA Southern PiedmontConservation Research Laboratory in Watkinsville, Georgia, andthe University of Georgia Plant Sciences Farm in Bogart, Georgia.Plants were grown over a 5 year period and dry matter yieldmonitored 1, 3, and 5 years after establishment. Relative crowdingcoefficients were calculated for each to establish trends ofencroachment of the E+ on the E- plants in the mixed communities.Generally, dry matter yields of E+ tall fescue were greaterthan E- tall fescue regardless of whether they were grown inpure or mixed communities. As time progressed, the differencein dry matter yield between E+ and E- tall fescue grown in mixedcommunities was greater than that of the pure communities. Relativecrowding coefficients increased as time progressed. Relativecrowding coefficients at the Watkinsville location were greaterafter 5 years than those at the Plant Sciences Farm. Therefore,site specific conditions exist which affect the competitivenessof E- tall fescue and degree of encroachment by E+ tall fescue.Research is needed to identify which biotic, abiotic and managementvariables exacerbate encroachment of E+ tall fescue to betterdefine the conditions which best suit E- tall fescue.Copyright1998 Annals of Botany Company tall fescue, endophyte,Neotyphodium coenophialum,Festuca arundinacea, competition, population density     

Effects of above-ground plant species composition and diversity on the diversity of soil-borne microorganisms

A coupling of above-ground plant diversity and below-ground microbial diversity has been implied in studies dedicated to assessing the role of macrophyte diversity on the stability, resilience, and functioning of ecosystems. Indeed, above-ground plant communities have long been assumed to drive below-ground microbial diversity, but to date very little is known as to how plant species composition and diversity influence the community composition of micro-organisms in the soil. We examined this relationship in fields subjected to different above-ground biodiversity treatments and in field experiments designed to examine the influence of plant species on soil-borne microbial communities. Culture-independent strategies were applied to examine the role of wild or native plant species composition on bacterial diversity and community structure in bulk soil and in the rhizosphere. In comparing the influence of Cynoglossum officinale (hound's tongue) and Cirsium vulgare (spear thistle) on soil-borne bacterial communities, detectable differences in microbial community structure were confined to the rhizosphere. The colonisation of the rhizosphere of both plants was highly reproducible, and maintained throughout the growing season. In a separate experiment, effects of plant diversity on bacterial community profiles were also only observed for the rhizosphere. Rhizosphere soil from experimental plots with lower macrophyte diversity showed lower diversity, and bacterial diversity was generally lower in the rhizosphere than in bulk soil. These results demonstrate that the level of coupling between above-ground macrophyte communities and below-ground microbial communities is related to the tightness of the interactions involved. Although plant species composition and community structure appear to have little discernible effect on microbial communities inhabiting bulk soil, clear and reproducible changes in microbial community structure and diversity are observed in the rhizosphere. This revised version was published online in August 2006 with corrections to the Cover Date.     

Soil and plant effects on microbial community structure

We investigated the effects of two different plant species (corn and soybean) and three different soil types on microbial community structure in the rhizosphere. Our working hypothesis was that the rhizosphere effect would be strongest on fast-growing aerobic heterotrophs, while there would be little or no rhizosphere effect on oligotrophic and other slow-growing microorganisms. Culturable bacteria and fungi had larger population densities in the rhizosphere than in bulk soil. Communities were characterized by soil fatty acid analysis and by substrate utilization assays for bacteria and fungi. Fatty acid analysis revealed a very strong soil effect but little plant effect on the microbial community, indicating that the overall microbial community structure was not affected by the rhizosphere. There was a strong rhizosphere effect detected by the substrate utilization assay for fast-growing aerobic heterotrophic bacterial community structure, with soil controls and rhizosphere samples clearly distinguished from each other. There was a much weaker rhizosphere effect on fungal communities than on bacterial communities as measured by the substrate utilization assays. At this coarse level of community analysis, the rhizosphere microbial community was impacted most by soil effects, and the rhizosphere only affected a small portion of the total bacteria.     

Soil and cultivar type shape the bacterial community in the potato rhizosphere

The rhizospheres of five different potato cultivars (including a genetically modified cultivar) obtained from a loamy sand soil and two from a sandy peat soil, next to corresponding bulk soils, were studied with respect to their community structures and potential function. For the former analyses, we performed bacterial 16S ribosomal RNA gene-based PCR denaturing gradient gel electrophoresis (PCR-DGGE) on the basis of soil DNA; for the latter, we extracted microbial communities and subjected these to analyses in phenotype arrays (PM1, PM2, and PM4, Biolog), with a focus on the use of different carbon, sulfur and phosphorus sources. In addition, we performed bacterial PCR-DGGE on selected wells to assess the structures of these substrate-responsive communities. Effects of soil type, the rhizosphere, and cultivar on the microbial community structures were clearly observed. Soil type was the most determinative parameter shaping the functional communities, whereas the rhizosphere and cultivar type also exerted an influence. However, no genetically modified plant effect was observed. The effects were imminent based on general community analysis and also single-compound analysis. Utilization of some of the carbon and sulfur sources was specific per cultivar, and different microbial communities were found as defined by cultivar. Thus, both soil and cultivar type shaped the potato root-associated bacterial communities that were responsive to some of the substrates in phenotype arrays.     

Comparison of Rhizosphere Bacterial Communities in Arabidopsis thaliana Mutants for Systemic Acquired Resistance

Systemic acquired resistance (SAR) is an inducible systemic plant defense against a broad spectrum of plant pathogens, with the potential to secrete antimicrobial compounds into the soil. However, its impact on rhizosphere bacteria is not known. In this study, we examined fingerprints of bacterial communities in the rhizosphere of the model plant Arabidopsis thaliana to determine the effect of SAR on bacterial community structure and diversity. We compared Arabidopsis mutants that are constitutive and non-inducible for SAR and verified SAR activation by measuring pathogenesis-related protein activity via a β-glucoronidase (GUS) reporter construct driven by the β-1-3 glucanase promoter. We used terminal restriction fragment length polymorphism (T-RFLP) analysis of MspI- and HaeIII-digested 16S rDNA to estimate bacterial rhizosphere community diversity, with Lactobacillus sp. added as internal controls. T-RFLP analysis showed a clear rhizosphere effect on community structure, and diversity analysis of both rhizosphere and bulk soil operational taxonomic units (as defined by terminal restriction fragments) using richness, Shannon–Weiner, and Simpson’s diversity indices and evenness confirmed that the presence of Arabidopsis roots significantly altered bacterial communities. This effect of altered soil microbial community structure by plants was also seen upon multivariate cluster analysis of the terminal restriction fragments. We also found visible differences in the rhizosphere community fingerprints of different Arabidopsis SAR mutants; however, there was no clear decrease of rhizosphere diversity because of constitutive SAR expression. Our study suggests that SAR can alter rhizosphere bacterial communities, opening the door to further understanding and application of inducible plant defense as a driving force in structuring soil bacterial assemblages.     

Contribution of root to soil respiration and carbon balance in disturbed and undisturbed grassland communities, northeast China

Changes in the composition of plant species induced by grassland degradation may alter soil respiration rates and decrease carbon sequestration; however, few studies in this area have been conducted. We used net primary productivity (NPP), microbial biomass carbon (MBC), and soil organic carbon (SOC) to examine the changes in soil respiration and carbon balance in two Chinese temperate grassland communities dominated by Leymus chinensis (undisturbed community; Community 1) and Puccinellia tenuiflora (degraded community; Community 2), respectively. Soil respiration varied from 2.5 to 11.9 g CO₂ m⁻² d⁻¹ and from 1.5 to 9.3 g CO₂ m⁻² d⁻¹, and the contribution of root respiration to total soil respiration from 38% to 76% and from 25% to 72% in Communities 1 and 2, respectively. During the growing season (May–September), soil respiration, shoot biomass, live root biomass, MBC and SOC in Community 2 decreased by 28%, 39%, 45%, 55% and 29%, respectively, compared to those in Community 1. The considerably lower net ecosystem productivity in Community 2 than in Community 1 (104.56 vs. 224.73 g C m⁻² yr⁻¹) suggests that the degradation has significantly decreased carbon sequestration of the ecosystems.     

杉木人工林土壤微生物群落结构特征

采用氯仿熏蒸法、稀释平板法和磷脂脂肪酸(phospholipid fatty acid,PLFA)方法,分析了常绿阔叶林转变成杉木人工林后土壤微生物种群数量和群落结构的变化特征.结果表明:常绿阔叶林转变为杉木人工林后,林地土壤的微生物生物量碳、可培养细菌和放线菌数降低.杉木人工林地总PLFAs、细菌PLFAs、真菌PLFAs比常绿阔叶林分别降低了49.4％、52.4％和46.6％,革兰氏阳性和阴性细菌PLFAs远低于常绿阔叶林.杉木人工林根际土壤微生物生物量碳、可培养细菌和放线菌数显著高于杉木人工林林地土壤,根际土壤中总PLFAs、细菌PLFAs、革兰氏阳性和阴性细菌PLFAs的含量也高于林地土壤,但真菌PLFAs和细菌PLFAs之比却低于林地土壤.对土壤微生物群落结构进行主成分分析发现,第1主成分和第2主成分共解释了土壤微生物群落结构变异的78.2％.表明常绿阔叶林与杉木人工林土壤的微生物群落结构间存在差异.     

Bacterial community evaluation during establishment of tall fescue (Festuca arundinacea) in soil contaminated with pyrene

A greenhouse study was conducted to monitor microbial community dynamics related to contaminant concentration and plant growth during the phytoremediation of pyrene. Soil microbial communities in both bulk and rhizosphere soils were assessed using a polymerase chain reaction--denaturing gradient gel electrophoresis (PCR-DGGE). However, no correlation between pyrene concentration and bacterial community shifts was observed. Even though plants significantly enhanced pyrene degradation (undetectable after 91 d for planted treatments compared with 82 mg/kg for unplanted treatments after 147 d with initial concentration of 758 mg/kg), biodegradation may have been accomplished by the pre-existing microbial community. It is also possible that the method was unable to pick up subtle community shifts, considering that the carbon source from pyrene was only 3.7% of the existing soil organic matter. This research suggests that plants significantly enhance degradation and mineralization of pyrene in soil, although there was no conclusive evidence supporting any specific bacterial activity responsible for this enhancement.     

Endophytic fungi (Neotyphodium coenophialum) affect the growth and mineral uptake, transport and efficiency ratios in tall fescue (Festuca arundinacea)

Neotyphodium coenophialuminteracts mutualistically with its host grasses. Tall fescue (Festuca arundinacea Schreb.) plants infected by the fungal endophyte,Neotyphodium coenophialum(Morgan-Jones and Gams) Glenn, Bacon and Hanlin, often perform better than non-infected plants, especially in limited resource environments. However, there is a scarcity of information about endophyte-grass ecotypes interaction in Andisols of temperate regions. Clones of three tall fescue ecotypes (Fukaura, Koiwai and Showa) either infected with N. coenophialum (E+) or noninfected (E–) were grown in Andisols (Black Andisol: naturally low content of phosphorus, high in other nutrients; Red Andisol: naturally high content of phosphorus, low in other nutrients) for 133 days in a controlled environment. Cumulative shoot dry weight, daily regrowth rates (tiller number, plant height and shoot dry matter) after clippings and nutrient uptake, transport and efficiency ratios were measured. In Black Andisol, E+ plants had significantly higher cumulative shoot dry weight as well as daily regrowth rates than E– plants, while in Red Andisol the reverse was true. Among the ecotypes studied, Showa had the highest shoot growth. Significantly higher phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) uptake as well as transport were identified in E+ vs. E– plants grown in Black Andisol. With few exceptions, values for nutrient efficiency ratios were not significantly different between E+ and E– plants grown in both soils. Significant three-way interaction (endophyte × ecotype × soil) for cumulative shoot dry weight and regrowth rate revealed that the ecotype specific regrowth responses to endophyte infection were depended on soil nutrient conditions. Vegetative growth and nutrient acquisition in tall fescue varied with ecotype and were modified by abiotic (soil fertility status) as well as biotic (endophyte infection) factors.     

内生真菌感染对禾草宿主生境土壤特性和微生物群落的影响

以羊草(Leymus chinensis)-内生真菌共生体为研究对象, 分别在野外样地和室内盆栽两种实验条件下研究了内生真菌感染对土壤特性和微生物群落结构的影响。结果显示：在处理时间较长并伴随有枯落物分解的羊草样地中, 内生真菌感染促进了土壤氮(N)的积累, 提高了30天培养时间内土壤初始碳(C)矿化速率和前3天土壤矿化量和土壤矿化总量; 而在处理时间较短且没有地上枯落物分解的盆栽羊草中, 内生真菌感染对土壤的C、N含量及C矿化均无显著影响。无论是野外样地还是室内盆栽实验, 内生真菌感染均未引起土壤微生物磷脂脂肪酸种类的变化, 但内生真菌感染均有提高土壤微生物生物量的趋势, 内生真菌显著增加了盆栽羊草土壤中细菌、革兰氏阴性细菌、真菌磷脂脂肪酸含量和磷脂脂肪酸总量, 增加了羊草样地土壤中革兰氏阳性细菌和放线菌的磷脂脂肪酸含量。总体看来, 内生真菌感染能够改变土壤N积累和C矿化率, 并且改变土壤中微生物群落的结构, 这有助于进一步认识内生真菌与羊草之间的共生关系及其在生态系统C、N循环中所起的作用。     

Use of 13C labeling to assess carbon partitioning in transgenic and nontransgenic (parental) rice and their rhizosphere soil microbial communities

Photosynthetic assimilation of CO₂ is a primary source of carbon in soil and root exudates and can influence the community dynamics of rhizosphere organisms. Thus, if carbon partitioning is affected in transgenic crops, rhizosphere microbial communities may also be affected. In this study, the temporal effects of gene transformation on carbon partitioning in rice and rhizosphere microbial communities were investigated under greenhouse conditions using the ¹³C pulse-chase labeling method and phospholipid fatty acid (PLFA) analysis. The ¹³C contents in leaves of transgenic (Bt) and nontransgenic (Ck) rice were significantly different at the seedling, booting and heading stages. There were no detectable differences in ¹³C distribution in rice roots and rhizosphere microorganisms at any point during rice development. Although a significantly lower amount of Gram-positive bacterial PLFAs and a higher amount of Gram-negative bacterial PLFAs were observed in Bt rice rhizosphere as compared with Ck at all plant development stages, there were no significant differences in the amount of individual ¹³C-PLFA between Bt and Ck rhizospheres at any growing stage. These findings indicate that the insertion of cry1Ab and marker genes into rice had no persistent or adverse effect on the photosynthate distribution in rice or the microbial community composition in its rhizosphere.     

Impact of elevated CO2 on the metabolic diversity of microbial communities in N-limited grass swards

The impact of elevated atmospheric CO2 on qualitative and qua ntitative changes in rhizosphere carbon flow will have important consequences fo r nutrient cycling and storage in soil, through the effect on the activity, biom ass size and composition of soil microbial communities. We hypothesized that mic robial communities from the rhizosphere of Danthonia richardsonii, a n ative C3 Australian grass, growing at ambient and twice ambient CO2 a nd varying rates of low N application (20, 60, 180 kg N ha-1) will be different as a consequence of qualitative and quantitative change in rhizosphere carbon flow. We used the Biolog^TM system to construct sole carbon source utilisation profiles of these communities from the rhizosphere of D. richardsonii. Biolog^TM GN and MT plates, the latter to which more ecologically relevant root exudate carbon sources were added, were used to characterise the communities. Microbial communities from the rhizosphere of D. richardsonii grown for four years at twice ambient CO2 had significantly greater utilisation of all carbon sources except those with a low C:N ratio (neutral and acidic amino acids, amides, N-heterocycles, long chain aliphatic acids) than communities from plants grown at ambient CO2. This indicates a change in microbial community composition suggesting that under elevated CO2 compounds with a higher C:N ratio were exuded. Enumeration of microorganisms, using plate counts, indicated that there was a preferential stimulation of fungal growth at elevated CO2 and confirmed that bacterial metabolic activity (C utilisation rates), not population size (counts), were stimulated by additional C flow at elevated CO2. Nitrogen was an additional rate-limiting factor for microbial growth in soil and had a significant impact on the microbial response to elevated CO2. Microbial populations were higher in the rhizosphere of plants receiving the highest N application, but the communities receiving the lowest N application were most active. These results have important implications for carbon turnover and storage in soils where changes in soil microbial community structure and stimulation of the activity of microorganisms which prefer to grow on rhizodeposits may lead to a decrease in the composition of organic matter and result in an accumulation of soil carbon.