Diversity and complexity of microbial communities from a chlor-alkali tailings dump

Revegetation of the tailings dumps produced by various industrial activities is necessary to prevent dust storms and erosion and represents a great challenge for ecological restoration. Little is known about the microbial colonisation and community structure of revegetated tailings following site exploitation. Here, we report the sequencing of 16S rRNA and internal transcribed spacer (ITS) fungal RNA gene amplicons from chlor-alkali residue and from an adjacent undisturbed soil to define the composition and assembly of the rhizosphere microbial communities. After quality filtering, a total of 72,373 and 89,929 bacterial sequences and 122,618 and 111,209 fungal sequences remained for community analysis from undisturbed soil and tailings dump samples, respectively. These reads were affiliated with 45 bacterial and 9 fungal phyla and 113 bacterial and 35 fungal classes. We observed a clear dominance of Gammaproteobacteria at our study site (24% of total sequences), especially of the Pseudomonas genera (72% of Gammaproteobacteria sequences), together with the dominance of a few fungal taxa, such as Hebeloma and Geopora. However, we also noticed that the core microbiome comprised 64.4% and 62.4% of the bacterial and fungal genera, respectively, despite marked differences in soil physico-chemical properties. A heatmap of correlations between soil parameters and taxa confirmed that approximately 50% of the 33 dominant taxa colonised both types of soil. We further demonstrated that the global bacterial-fungal network topology of the dump approximated that of the undisturbed soil. Our approach illuminates the importance of studying more than just a single component of the microbial community and represents a step forward in uncovering the microbial ecology of disturbed environments beyond what is generally found in conventional studies. Our study also provides novel global community proxies that have led us to conclude that environmental filtering is more likely to occur through the activity of tree roots rather than as a result of specific soil characteristics and could be an important force in the assembly of at least some microbial communities.     

Soil microbial biomass, dehydrogenase activity, bacterial community structure in response to long-term fertilizer management

This study describes the effects of balanced versus nutrient-deficiency fertilization on soil microbial biomass, activity, and bacterial community structure in a long-term (16 years) field experiment. Long-term fertilization greatly increased soil microbial biomass C and dehydrogenase activity, except that the P-deficiency fertilization had no significant effect. Organic manure had a significantly greater (P<0.05) impact on the biomass C and the activity, compared with mineral fertilizers. Microbial metabolic activity (dehydrogenase activity per microbial biomass C) was significantly higher (P<0.05) under balanced fertilization than under nutrient-deficiency fertilization. General bacterial community structure was analyzed by PCR-denaturing gradient gel electrophoresis (DGGE) targeting eubacterial 16S rRNA gene. Mineral fertilization did not affect the DGGE banding pattern, while specific DGGE band was observed in organic manure-fertilized soils. Phylogenetic analysis showed that the change of bacterial community in organic manure-fertilized soil might not be because of the direct influence of the bacteria in the compost, but because of the promoting effect of the compost on the growth of an indigenous Bacillus sp. in the soil. We emphasize the importance of balanced-fertilization, as well as the role of P, in maintaining soil organic matter, and promoting the biomass and activity of microorganisms.     

Increased N availability in grassland soils modifies their microbial communities and decreases the abundance of arbuscular mycorrhizal fungi

Two complementary studies were performed to examine (1) the effect of 18 years of nitrogen (N) fertilization, and (2) the effects of N fertilization during one growing season on soil microbial community composition and soil resource availability in a grassland ecosystem. N was added at three different rates: 0, 5.44, and 27.2 g N m⁻² y⁻¹. In both studies, Schizachyrium scoparium was the dominant plant species before N treatments were applied. Soil microbial communities from each experiment were characterized using fatty acid methyl ester (FAME) analysis. Discriminant analysis of the FAMEs separated the three N fertilizer treatments in both experiments, indicating shifts in the composition of the microbial communities. In general, plots that received N fertilizer at low or high application rates for 18 years showed increased proportions of bacterial FAMEs and decreased fungal FAMEs. In particular, control plots contained a significantly higher proportion of fungal FAMEs C18:1(cis9) and C18:2(cis9,12) and of the arbuscular mycorrhizal fungal (AMF) FAME, C16:1(cis11), than both of the N addition treatment plots. A significant negative effect of N fertilization on the AMF FAME, C16:1(cis11), was measured in the short-term experiment. Our results indicate that high rates of anthropogenic N deposition can lead to significant changes in the composition of soil microbial communities over short periods and can even disrupt the relationship between AMF and plants.     

Plant growth promoting bacteria increases biomass,effective constituent,and modifies rhizosphere bacterial communities of Panax ginseng

Purpose: The main aim of this study was to introduce and explore plant growth-promoting bacteria (PGPB) indigenous to ginseng, and to evaluate their ability to improve production and quality, and effect on rhizosphere niche in ginseng.

Materials and methods: Endophytic bacteria were isolated from root, stem, and leaf of ginseng from different sites and genotype in China and Korea, screened based on their beneficial properties as PGPB. Nine bacterial isolates were selected according to their plant growth properties including soluble phosphate and potassium, ammonia, auxin and siderophore producing, ACC deaminase, and antagonistic pathogen as well. Changes in ginseng after PGPB inoculation were evaluated with respect to the non-inoculated control.

Results and Conclusions: The PGPB isolates were identified as genera Bacillus, Lysinibacillus, Rhizobium, Stenotrophomonas, Erwinia, Ochrobactrum, Enterobacter and Pantoea based on 16S rRNA sequences. Inoculation of G209 and G119 increased not only plant height, root length, fresh weight, and dry weight, but also root activity and the amount of ginsenosides significantly. In particular, using the Illumina Miseq platform, the native bacterial community of rhizospheric soil maintained high community diversity and increased abundance of specific bacteria. Therefore, they may be play a crucial role in sustainable ginseng cultivating in farmland.     

Environmental conditions rather than microbial inoculum composition determine the bacterial composition,microbial biomass and enzymatic activity of reconstructed soil microbial communities

The composition of microbial communities and the level of enzymatic activity in the soil are both important indicators of soil quality, but the mechanisms by which a soil bacterial community is generated and maintained are not yet fully understood. Two soil samples were collected from the same location, but each had been subjected to a different long-term fertilization treatment and was characterized by different microbial diversity, biomass and physicochemical properties. These samples were γ-sterilized and swap inoculated. Non-sterilized soil samples along with sterilized and inoculated soil samples were incubated for eight months before their nutrient content, microbial biomass, enzymatic activity and bacterial composition were analyzed. Total phosphorus, and potassium concentrations along with the overall organic matter content of the non-sterilized soil were all equal to those of the same soil that had been sterilized and self/swap inoculated. Additionally, the microbial biomass carbon concentration was not affected by the specific inoculum and varied only by soil type. The activities of catalase, invertase, urease, protease, acid phosphatase and phytase were smaller in the sterilized soils that had been inoculated with organisms from chemical fertilizer amended soil (NPK) when compared to sterilized soil inoculated with organisms from manure and chemical fertilizer amended soil (NPKM) and non-sterilized soil samples. Bacterial 16S rRNA examined by 454-pyrosequencing revealed that the composition of bacterial community reconstructed by immigrant microbial inoculum in the soil was mainly influenced by its physicochemical properties, although the microbial inoculum contained different abundances of bacterial taxa. For example, the pH of the soil was the dominant factor in reconstructing a new bacterial community. Taken together, these results demonstrated that both soil microbial composition and functionality were primarily determined by soil properties rather than the microbial inoculum, which contributed to our understanding of how soil microbial communities are generated and maintained.     

Sewage sludge processing determines its impact on soil microbial community structure and function

Composting and thermal drying are amongst the most commonly used post-digestion processes for allowing sanitation and biological stabilization of sewage sludge from municipal treatment plants, and making it suitable as soil conditioner for use in agriculture. To assess the impact of sludge-derived materials on soil microbial properties, fresh (LAF), composted (LAC) and thermally dried (LAT) sludge fractions, each resulting from a different post-treatment process of a same aerobically digested sewage sludge, were added at 1% (w/w) application rate on two contrasting (a loam and a loamy sand) soils and incubated under laboratory conditions for 28 days. Soil respiration, microbial ATP content, hydrolytic activities and arginine ammonification rate were monitored throughout the incubation period. Results showed that soil biochemical variables, including the metabolic quotient (qCO₂), were markedly stimulated after sludge application, and the magnitude of this stimulatory effect was dependent on sludge type (precisely LAT > LAF > LAC), but not on soil type. This effect was related to the content of stable organic matter, which was lower in LAT. Genetic fingerprinting by PCR–DGGE revealed that compositional shifts of soil bacterial and, at greater extent, actinobacterial communities were responsive to the amendment with a differing sludge fraction. The observed time-dependent changes in the DGGE profiles of amended soils reflected the microbial turnover dependent on the sludge nutrient input, whereas no indications of adverse effects of sludge-borne contaminants were noted. Our findings indicate that composting rather thermal drying can represent a more appropriate post-digestion process to make sewage sludge suitable for use as soil conditioner in agriculture.     

喀斯特生态系统中乔木和灌木林根际土壤微生物生物量及其多样性的比较

我国喀斯特区域面积分布较广,而喀斯特生态系统的退化已成为当前西南地区面临的严重的生态问题。本研究选取贵州中部两种不同植被类型的生态系统—乔木林和灌木林,以乔木林中的白栎、园果化香和灌木林中的火棘、竹叶椒等主要优势树种为对象,研究不同的植物树种对根际土壤微生物生物量及其细菌群落结构的影响。结果显示:乔木林系统中根际土壤微生物生物量碳、氮显著性高于灌木林,植物的根际效应在乔木林中表现更为显著;同时乔木林中的优势树种通过根系分泌物的作用显著提高根际土壤细菌多样性指数,而灌木林中优势树种的根际土壤微生物量及多样性均未表现出明显的根际效应。因此,植被的演替通过改变土壤微生物的特性影响植物-微生物-土壤之间的物质和能量循环,进一步影响喀斯特生态系统的稳定和健康功能。     

Bacterial communities in soybean rhizosphere in response to soil type, soybean genotype, and their growth stage

Three experiments were conducted in this study in order to investigate the impacts of soil type, soybean genotype, and the reproductive growth stage on bacterial communities in the soybean rhizosphere. Communities were evaluated by principal component analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns and sequencing of partial 16S rDNA polymerase chain reaction (PCR) amplicons. A pot experiment analyzing three soybean genotypes grown in two different types of soil (Black soil and Dark Brown soil) indicated that soil type was the major factor in influencing the bacterial communities in the soybean rhizosphere, with a more significant effect observed in the Black soil samples than in the Dark Brown soil samples. A field experiment was conducted in Dark Brown soil using three soybean genotypes, and the results gleaned from both pot and field experiments indicated that bacterial communities in the soybean rhizosphere changed with growth stages, and higher number of DGGE bands observed in early reproductive growth stages, while surprisingly, a significant impact of genotype on the bacterial communities was not observed in these experiments. However, a plate culture experiment targeting the culturable bacterial communities detected a remarkable difference in the community structures of the rhizosphere between the two soybean genotypes, suggesting that a small portion of the total bacteria was influenced by genotype. Sequence analysis of DGGE bands indicated that bacterial phyla of Proteobacteria, Actinobacteria, Bacteroidetes, Nitrospirae, Firmicutes, Verrucomicrobia and Acidobacteria commonly inhabit the soybean rhizosphere.     

Phosphomonoesterase production and persistence and composition of bacterial communities during plant material decomposition in soils with different pH values

The aim of this work was to study the synthesis and persistence of acid and alkaline phosphomonoesterases in three soils with different pH values amended with ryegrass residues. The organic input increased soil respiration, as estimated by CO₂-C evolution in all soils. The ATP content of the three soils showed a 3-7-fold increase between 7 and 10 d in the different soils since the amendment. The dsDNA content of the three amended soils also peaked between 7 and 10 d, increasing by 2.5-3.5 times in the different soils. The bacterial species richness increased in the amended as compared to the control soils during the early stages (7-10 d) of organic matter decomposition, as indicated by the decreasing values of the Sørensen similarity index between the treatments in this period. Soil amendment increased the alkaline phosphomonoesterase activity by 6, 8 and 15 times in the Vallombrosa acidic, Romola neutral and Vicarello alkaline soil, respectively, whereas the acid phosphomonoesterase activity showed a 6-, 2- and 10-fold increase in the Vallobrosa acidic, Romola neutral and Vicarello alkaline soil, respectively. Phosphatase activities peaked between 4 and 10 d, depending on the activity and the soil considered, but activity of alkaline phosphomonoesterase was higher in alkaline soils and persisted longer than the acid phosphomonoesterase activities; the opposite occurred in the acid soil. During a 180 d decomposition period, both acid and alkaline phosphomonoesterase activities were related to dsDNA and ATP contents in all soils. Peaks of phosphmonoesterase activity coincided with the changes in the composition of the bacterial microflora, as detected by 16S-rDNA analysis, although no relationship between bacterial community composition and persistence of the phosphomonoesterase activities could be shown. It was concluded that acid and alkaline phosphomonoesterases are produced in greater amounts during plant residue decomposition, and that in acid soils acid phosphomonoesterase activity predominates and in neutral and alkaline soils alkaline phosphomonoesterase activity predominates. However, the persistence of newly produced enzymes is determinated by other factors such as soil texture, organic matter content and formation of soil colloid-enzyme complexes.     

Soil carbon quality and nitrogen fertilization structure bacterial communities with predictable responses of major bacterial phyla

Agricultural practices affect the soil ecosystem in multiple ways and the soil microbial communities represent an integrated and dynamic measure of soil status. Our aim was to test whether the soil bacterial community and the relative abundance of major bacterial phyla responded predictably to long-term organic amendments representing different carbon qualities (peat and straw) in combination with nitrogen fertilization levels and if certain bacterial groups were indicative of specific treatments. We hypothesized that the long-term treatments had created distinctly different ecological niches for soil bacteria, suitable for either fast-growing copiotrophic bacteria, or slow-growing oligotrophic bacteria. Based on terminal-restriction fragment length polymorphism of the 16S rRNA genes from the total soil bacterial community and taxa-specific quantitative real-time PCR of seven different groups, all treatments significantly affected the community structure, but nitrogen fertilization was the most important driver for changes in the relative abundances of the studied taxa. According to an indicator species analysis, the changes were largely explained by the decline in the relative abundances of Acidobacteria, Gemmatimonadetes and Verrucomicrobia with nitrogen fertilization. Conditions more favourable for copiotrophic life strategies were indicated in these plots by the decreased metabolic quotient, i.e. the ratio between basal respiration rate and soil biomass. Apart from the Alphaproteobacteria that were significantly associated with peat, no taxa were indicative of organic amendment in general. However, several significant indicators of both peat and straw were identified among the terminal restriction fragments suggesting that changes induced by the organic amendments were mainly manifested at a lower taxonomical level. Our findings strengthen the proposition that certain higher bacterial taxa adapt in an ecologically coherent way in response to changes induced by fertilization.     

Rapid effects of plant species diversity and identity on soil microbial communities in experimental grassland ecosystems

Changes in plant community structure, including the loss of plant diversity may affect soil microbial communities. To test this hypothesis, plant diversity and composition were experimentally varied in grassland plots cultivated with monocultures or mixtures of 2, 3 or 4 species. We tested the effects of monocultures versus mixtures and of plant species composition on culturable soil bacterial activity, number of substrates used and catabolic diversity, microbial biomass N, microbial respiration, and root biomass. These properties were all measured 10 months after seeding the experiment. Soil bacterial activity, number of substrates used and catabolic diversity were measured in the different plant communities using BIOLOG GN and GP microplates, which are redox-based tests measuring capacity of soil culturable bacteria to use a variety of organic substrates. Microbial biomass N, microbial respiration, and root biomass were insensitive to plant diversity. Culturable soil microbial activity, substrates used and diversity declined with declining plant diversity. Their activity, number of substrates used and diversity were significantly higher in plots with 3 and 4 plant species than in monocultures and in plots with 2 species. There was also an effect of plant species composition. Culturable soil microbial activity and diversity was higher in the four-species plant community than in any of the plant monocultures suggesting that the effect of plant diversity could not be explained by the presence of a particular plant species. Our results showed that changes in plant diversity and composition in grassland ecosystems lead to a rapid response of bacterial activity and diversity.     

黑土农田施加AM菌剂对大豆根际菌群结构的影响

为揭示在黑土农田条件下施加丛枝菌根(AM)菌剂对作物根际微生物群落的影响,试验以大豆为研究对象,田间播种时分别施加根内球囊霉(Glomus intraradices,GI)和摩西球囊霉(Glomus mosseae,GM)两种AM菌剂,以单施化肥处理(F)和不施加AM菌剂及化肥处理(CK)作为对照,采用传统与现代分子生物学手段,研究大豆根际土壤中菌群结构及根系内AM真菌多样性。结果表明:GI、GM处理的大豆菌根侵染率最高达到78.3%和86.6%;GI、GM、F处理的大豆根际土壤中可培养细菌、真菌和放线菌三大菌群的数量与CK处理相比显著提高(p0.05)。分离大豆结荚期根际土壤中AM真菌孢子,共获得Acaulospora属真菌3种,Glomus属真菌7种,孢子密度均较低,G.intraradices和G.mosseae均为各自处理的优势种群。对大豆结荚期根系和根际土壤PCR-DGGE图谱条带的丰度及优势条带测序分析,结果表明根际土壤中的AM真菌菌群数明显高于根系中AM真菌的菌群数量,GI处理的大豆根际土壤中AM真菌丰度值最大,GM处理大豆根系里的AM真菌丰度值最大,F处理的根际土壤中总AM真菌的数量最少;施加AM菌剂处理的大豆根系及根际土壤中的优势菌群分别为外源施加的两种AM真菌。     

The influence of tillage on the structure of rhizosphere and root-associated arbuscular mycorrhizal fungal communities

Soil environmental factors affect the structure of arbuscular mycorrhizal (AM) fungal communities present in soil. However, it is not understood to which degree management practices such as tillage lead to dissimilarities between intra- and extraradical AM fungal communities. This study aims to assess the influence of two different soil management practices (conventional tillage and no-till) on the diversity of AMF communities, both in rhizosphere soil and inside corn roots. We hypothesized that under no-till, roots are colonized as they grow through the undisturbed fungal mycelia left from the previous crop whereas under conventional tillage they are colonized by those propagules that survived disturbance and can re-establish in their new relocated and mixed environment. We predicted that the degree of similarity of AM fungal communities inside versus outside the roots would be greater under no-till than under tillage. Using terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis we observed a different AM fungal community present in roots under no-till than under conventional tillage. Moreover, the communities present in the rhizosphere soil were different than in the roots of the corn plants. These results suggest that soil management does alter the diversity of AM fungal communities associated with corn roots and that plants influence the structure of the AMF community colonizing their roots. Sequencing results indicated that the majority of AMF species found in this agricultural soil was Glomus spp. However, further work is required to determine the extent to which AM fungal genotypic alterations by soil management influences competitive relationships.     

Influence of introduced potential biocontrol agents on maize seedling growth and bacterial community structure in the rhizosphere

Two species of Pseudomonas chromosomally tagged with gfp, which had shown antagonistic activity against the tomato pathogen Ralstonia solanacearum in a previous study, were assessed for their impact in the rhizosphere of maize. Plant growth characteristics, numbers of indigenous heterotrophic bacteria, changes in the bacterial community structure according to the r/K strategy concept, and shifts in MIDI-FAME profiles of culturable bacterial fractions as well as total rhizosphere microbial communities were determined in relation to seed and soil treatment with the exogenous pseudomonads. The maize rhizosphere proved to be a suitable habitat for the introduced P. chlororaphis IDV1 and P. putida RA2, which showed good survival after introduction. However, both inoculants showed a small growth-reducing effect towards maize, which might have been caused by the high densities of inoculants used (i.e. competition for nutrients and action of metabolites produced) and/or changes in microbial community structure (both culturable bacterial fraction and the total microflora). Probably, an altered balance among the indigenous maize rhizosphere populations occurred. Thus, the culturable bacteria, as well as the total microflora in the rhizosphere, changed in response to the introduced pseudomonads, and their development was dependent on the growth stage of the plant. The FAME analyses showed that these microbial communities comprised different populations, and were separated according to, first, the method used (direct versus cultivation-based), second, sampling time, and, finally, inoculation level.     

Functional resilience of soil microbial communities depends on both soil structure and microbial community composition

The effects of soil structure and microbial community composition on microbial resistance and resilience to stress were found to be interrelated in a series of experiments. The initial ability of Pseudomonas fluorescens to decompose added plant residues immediately after a copper or heat stress (resistance) depended significantly on which of 26 sterile soils it was inoculated into. Subsequent studies showed that both the resistance and subsequent recovery in the ability of P. fluorescens to decompose added plant residues over 28 days after stress (resilience) varied significantly between a sandy and a clay-loam soil. Sterile, sandy and clay-loam soil was then inoculated with a complex microbial community extracted from either of the soils. The resulting microbial community structure depended on soil type rather than the source of inoculum, whilst the resistance and resilience of decomposition was similarly governed by the soil and not the inoculum source. Resilience of the clay-loam soil to heat stress did not depend on the water content of the soil at the time of stress, although the physical condition of the soil when decomposition was measured did affect the outcome. We propose that soil functional resilience is governed by the physico-chemical structure of the soil through its effect on microbial community composition and microbial physiology.     

长期施肥处理对玉米生长期潮土微生物生物量和活度的影响

以1989年建立的中国科学院封丘农田生态系统国家试验站的长期定位试验为平台,研究经18a连续不同施肥处理后玉米季土壤微生物生物量碳氮和微生物活度的动态变化及其与土壤有机碳之间的相互关系,并探讨施肥措施对土壤微生物及其活性的影响。施肥处理包括:(1)有机肥(OM);(2)1/2化肥和1/2有机肥(1/2OM+1/2NPK);(3)氮磷钾肥(NPK);(4)氮磷肥(NP);(5)磷钾肥(PK);(6)氮钾肥(NK);(7)不施肥,即对照(CK)7个处理。结果表明,微生物生物量碳氮和微生物活度在玉米生长期内均有明显的时间变异性,其中微生物生物量碳与微生物活度的动态变化比较一致,其间的极显著相关关系表明潮土微生物生物量碳的变化可以在很大程度上代表土壤微生物活度的变化。施肥制度显著影响微生物生物量碳氮和微生物活度的变化,总体趋势为OM1/2OM+1/2NPKNPKNPPKNKCK,表明OM有利于保持土壤的生物化学环境及促进土壤的生物学活性;与OM处理相比,化学肥料的长期施用有降低土壤微生物生物量和微生物活度的趋势,尤其是缺素处理的表现更为明显,其中以缺磷处理的表现最为严重。土壤微生物生物量碳氮、微生物活度与土壤有机碳变化均呈极显著正相关。     

Exploring soil microbial communities and soil organic matter: Variability and interactions in arable soils under minimum tillage practice

This study describes an integrated approach (1) to monitor the quantity and quality of water extractable organic matter (WEOM) and size, structure and function of microbial communities in space (depth) and time, and (2) to explore the relationships among the measured properties. The study site was an arable field in Southern Germany under integrated farming management including reduced tillage. Samples of this Eutric Cambisol soil were taken in July 2001, October 2001, April 2002 and July 2002 and separated into three depths according to the soil profile (0–10 cm, 10–28 cm and 28–40 cm). For each sample, the quantity and quality (humification index, HIX) of water extractable organic matter (WEOM) were measured concomitantly with soil enzyme activities (alkaline phosphatase, β-glucosidase, protease) and microbial community size (C_mic). Furthermore, microbial community structure was characterised based on the fingerprints of nucleic acids (DNA) as well as phospholipid fatty acids (PLFA). We observed strong influences of sampling date and depth on the measured parameters, with depth accounting for more of the observed variability than date. Increasing depth resulted in decreases in all parameters, while seasonal effects differed among variants. Principal component (PC) analysis revealed that both DNA and PLFA fingerprints differentiated among microbial communities from different depths, and to a smaller extent, sampling dates. The majority of the 10 PLFAs contributing most to PC 1 were specific for anaerobes. Enzyme activities were strongly related to C_mic, which was depending on water extractable organic carbon and nitrogen (WEOC and WEON) but not to HIX. HIX and WEOM interact with the microbial community, illustrated by (1) the correlation with the number of PLFA peaks (community richness), and (2) the correlations with community PC analysis scores.     

外源腐解微生物的物种组合对土壤微生物群落结构及代谢活性的影响

本文采用饲料类芽孢杆菌(Paenibacillus pabuli,P)、深红紫链霉菌(Streptomyces violaceorubidus,S)和黄绿木霉(Trichoderma aureoviride,T),组合构建了3种单菌剂(P、S和T)、3种两菌种复合菌剂(PT、PS和ST)及1种3菌种复合菌剂(PST),并将之添加到红壤中,监测各菌剂添加后土壤总磷脂脂肪酸(PLFAs)量、特征微生物PLFAs百分含量、土壤呼吸速率及总代谢熵的变化,旨在探明外源腐解微生物的物种组合对土壤微生物群落结构和代谢活性的影响,进而为优化有机物分解菌剂种群配置提供参考。结果显示,添加单菌剂的P、S和T处理及添加两菌种复合菌剂的PT和PS处理,土壤微生物生物量显著增加,增幅17.2%~121.6%(P0.05)。添加外源腐解微生物后,各处理的土壤微生物群落的细菌百分含量基本稳定在79.6%~83.1%,真菌百分含量显著增加8.8%~50.6%;而放线菌百分含量除P和ST处理外,其他处理显著降低9.4%~69.8%。PLFAs数据的主成分分析表明,各外源菌剂处理与CK处理间的群落结构变异由小到大依次为:接种单菌剂的P、S和T处理,接种两菌种复合菌剂的PT、PS和ST处理,接种3菌种复合菌剂的PST处理。添加单菌剂的P、T处理以及添加两菌种复合菌剂的ST处理,在短期内影响了土壤微生物的对数生长,使土壤呼吸速率的峰值分别提高48.7%、53.7%和78.7%;且外源腐解微生物组合的物种数量越多,土壤微生物进入潜伏期所需的时间越长。从外源腐解微生物对土壤肥力的长期影响来看,两菌种复合菌剂ST的添加使土壤微生物代谢活性提高28.9%,因此该处理的土壤碳矿化量增加11.1%;添加单菌剂的S处理使土壤微生物代谢活性显著降低32.4%,因此该处理的土壤碳矿化量仅降低7.3%;而添加两菌种复合菌剂的PS处理和3菌种复合菌剂的PST处理,在保持代谢活性不变的情况下,其土壤碳矿化量也降低5.8%~8.7%,其原因有待进一步研究。综上所述,外源腐解微生物的添加会改变土壤微生物的群落结构及其生长轨迹,且随外源腐解微生物组合的物种数量增多这一干扰程度越大,而土壤微生物代谢活性与外源腐解微生物组合的物种数量无显著相关性。     

Impact of artificial root exudates on the bacterial community structure in bulk soil and maize rhizosphere

Bacterial densities, metabolic signatures and genetic structures were evaluated to measure the impact of soil enrichment of soluble organic carbon on the bacterial community structures. The exudates chosen were detected in natural maize exudates (glucose, fructose, saccharose, citric acid, lactic acid, succinic acid, alanine, serine and glutamic acid) and were used at a rate of 100 μg C g⁻¹ day⁻¹ for 14 days. Moreover two synthetic solutions with distinct carbon/nitrogen ratios (20.5 and 40.1), obtained by varying carboxylic and amino acids concentrations, were compared in order to evaluate the potential role of organic N availability. The in vitro experiment consisted of applying exudate solutions to bulk soil. In the case of the control, only distilled water was added. Both solutions significantly increased bacterial densities and modified the oxidation pattern of Biolog® GN2 plates with no effect of the C/N ratio on these two parameters. Genetic structure, measured by means of ribosomal intergenic spacer analysis (RISA), was also consistently modified by the organic amendments. N availability levels led to distinct genetic structures. In a second experiment, one of the previous exudate solutions (C/N 20.5) was applied to 15-day-old maize plants to determine the structural influence of exudates on the rhizosphere microbial community (in situ experiment). Bacterial densities were significantly increased, but to a lesser extent than had been found in the in vitro experiment. Metabolic potentials and RISA profiles were also significantly modified by the organic enrichment.     

Effects of soil frost on growth, composition and respiration of the soil microbial decomposer community

Most climate change scenarios predict that the variability of weather conditions will increase in coming decades. Hence, the frequency and intensity of freeze-thaw cycles in high-latitude regions are likely to increase, with concomitant effect on soil carbon biogeochemistry and associated microbial processes. To address this issue we sampled riparian soil from a Swedish boreal forest and applied treatments with variations in four factors related to soil freezing (temperature, treatment duration, soil water content and frequency of freeze-thaw cycles), at three levels in a laboratory experiment, using a Central Composite Face-centred (CCF) experimental design. We then measured bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, basal respiration, soil microbial phospholipid fatty acid (PLFA) composition, and concentration of dissolved organic carbon (DOC). Fungal growth was higher in soil exposed to freeze-thawing perturbations and freezing temperatures of −6 °C and −12 °C, than under more constant conditions (steady 0 °C). The opposite pattern was found for bacteria, resulting in an increasing fungal-to-bacterial growth ratio following more intensive winter conditions. Soil respiration increased with water content, decreased with treatment duration and appeared to mainly be driven by treatment-induced changes in the DOC concentration. There was a clear shift in the PLFA composition at 0 °C, compared with the two lower temperatures, with PLFA markers associated with fungi as well as a number of unsaturated PLFAs being relatively more common at 0 °C. Shifts in the PLFA pattern were consistent with those expected for phenotypic plasticity of the cell membrane to low temperatures. There were small declines in PLFA concentrations after freeze-thawing and with longer durations. However, the number of freeze-thaw events had no effect on the microbiological variables. The findings suggest that the higher frequency of freeze-thaw events predicted to follow the global warming will likely have a limited impact on soil microorganisms.