外来物种刺槐对土壤微生物功能多样性的影响

以延河流域不同植被区内人工刺槐(Robinia pseudoacacia)群落和乡土植物群落的土壤微生物为研究对象,利用Biolog微平板技术对土壤微生物功能多样性进行测定,分析人工引种刺槐在不同环境梯度(3个植被区)下对土壤微生物功能多样性的影响。结果表明:刺槐对不同环境梯度下的土壤微生物的影响明显不同。从草原区到森林区,刺槐林之间土壤微生物群落的平均颜色变化率(AWCD)和土壤微生物功能多样性指数均没有显著变化;但与乡土植物群落比较,草原区、森林草原区和森林区土壤AWCD分别表现为刺槐乡土植物、刺槐乡土植物、刺槐乡土植物;在草原区和森林草原区刺槐林土壤微生物群落的群落丰富度指数(S)、Shannon-Wiener指数(H)、Simpson指数(D)、Mc Intosh指数(U)均大于乡土植物,森林区刺槐林群落丰富度指数(S)、Shannon指数(H)、Simpson指数(D)均小于乡土植物;刺槐林和乡土植物群落下土壤微生物碳源利用存在差异,主要体现在对糖类、氨基酸类的利用上。PCA分析显示主成分1贡献较大的碳源有24种,在主成分分离中起主要贡献作用的是糖类、氨基酸类和羧酸类。土壤碳氮含量能影响土壤微生物功能多样性指数,土壤含水量和温湿度能够影响碳源的利用类型。刺槐对土壤微生物功能多样性的影响存在区域差异,在评价刺槐对土壤生态过程与功能的影响时必须要考虑这种空间差异性。     

Fertilization alters the abundance but not the diversity of soil fauna: A meta-analysis

Aim

Soil fauna, a functionally important group of soil organisms, are greatly affected by fertilization. However, it is still debated whether and how fertilization affects the soil faunal community. Here, we aimed to synthesize the global patterns of soil fauna communities in response to fertilization in terrestrial ecosystems.

Location

Global.

Time period

1997–2021.

Major taxa studied

Soil fauna.

Methods

We examined the effects of fertilization on the abundance, number of groups and Shannon diversity of soil fauna by synthesizing 1218 observations based on 39 published studies. We also explored the associations between fertilization-induced changes in the soil faunal community and changes in soil and microbial properties.

Results

Fertilization increased the abundance of soil fauna by 56.3%, without significantly affecting the number of groups and the Shannon index. The type of fertilizer affected the responses of soil faunal abundance, and the effects of fertilizer type were altered by climate zones, ecosystem types and soil depths. Both organic and organic–mineral fertilizer treatments significantly increased the abundance of soil fauna in most climate zones, ecosystem types and soil depths, whereas mineral fertilizer treatment had no such effect. Additionally, we found inconsistent responses of soil fauna to fertilization among different taxonomic groups, not only at the order level but also at the class level, providing evidence for the idiosyncratic nature of the effects of fertilization on soil fauna. Furthermore, our regression analysis showed that changes in food resources, including soil nutrients and microbes, were crucial controls for the response of soil faunal abundance to fertilization.

Main conclusions

Fertilization generally increased soil faunal abundance at the global scale by affecting food resources, and the effects of fertilization were dependent on the specific soil fauna and type of fertilizer. We suggest the use of organic or organic–mineral fertilizers, rather than mineral fertilizers, to increase the benefits on specific soil fauna.     

Effects of elevated atmospheric CO2 concentrations on soil microorganisms

Effects of elevated CO(2) on soil microorganisms are known to be mediated by various interactions with plants, for which such effects are relatively poorly documented. In this review, we summarize and synthesize results from studies assessing impacts of elevated CO(2) on soil ecosystems, focusing primarily on plants and a variety the of microbial processes. The processes considered include changes in microbial biomass of C and N, microbial number, respiration rates, organic matter decomposition, soil enzyme activities, microbial community composition, and functional groups of bacteria mediating trace gas emission such as methane and nitrous oxide. Elevated CO(2) in atmosphere may enhance certain microbial processes such as CH(4) emission from wetlands due to enhanced carbon supply from plants. However, responses of extracellular enzyme activities and microbial community structure are still controversy, because interferences with other factors such as the types of plants, nutrient availabilitial in soil, soil types, analysis methods, and types of CO(2) fumigation systems are not fully understood.     

Ecosystem properties and microbial community changes in primary succession on a glacier forefront

We studied microbial community composition in a primary successional chronosequence on the forefront of Lyman Glacier, Washington, United States. We sampled microbial communities in soil from nonvegetated areas and under the canopies of mycorrhizal and nonmycorrhizal plants from 20- to 80-year-old zones along the successional gradient. Three independent measures of microbial biomass were used: substrate-induced respiration (SIR), phospholipid fatty acid (PLFA) analysis, and direct microscopic counts. All methods indicated that biomass increased over successional time in the nonvegetated soil. PLFA analysis indicated that the microbial biomass was greater under the plant canopies than in the nonvegetated soils; the microbial community composition was clearly different between these two types of soils. Over the successional gradient, the microbial community shifted from bacterial-dominated to fungal-dominated. Microbial respiration increased while specific activity (respiration per unit biomass) decreased in nonvegetated soils over the successional gradient. We proposed and evaluated new parameters for estimating the C use efficiency of the soil microbial community: “Max” indicates the maximal respiration rate and “Acc” the total C released from the sample after a standard amount of substrate is added. These, as well as the corresponding specific activities (calculated as Max and Acc per unit biomass), decreased sharply over the successional gradient. Our study suggests that during the early stages of succession the microbial community cannot incorporate all the added substrate into its biomass, but rapidly increases its respiration. The later-stage microbial community cannot reach as high a rate of respiration per unit biomass but remains in an “energy-saving state,” accumulating C to its biomass. Received: 4 June 1998 / Accepted: 11 January 1999     

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.     

黄土丘陵区植被与地形特征对土壤和土壤微生物生物量生态化学计量特征的影响

研究黄土丘陵区植被与地形特征对土壤和土壤微生物生物量生态化学计量特征影响有助于深入理解黄土丘陵区不同植被带下土壤和土壤微生物相互作用及养分循环规律.选择黄土丘陵区延河流域3个植被区(森林区、森林草原区、草原区)和5种地形部位(阴/阳沟坡、阴/阳梁峁坡、峁顶)的土壤作为研究对象,利用生态化学计量学理论研究植被和地形对土壤和土壤微生物生物量生态化学计量特征的影响.结果表明: 土壤及土壤微生物生物量碳、氮、磷含量在不同地形之间的差别主要表现在沟坡位置和阴坡高于其他坡位和阳坡.植被类型的变化对两个土层(0～10、10～20 cm)土壤和土壤微生物生物量碳、氮、磷的影响均达到显著水平,坡向对表层(0～10 cm)土壤和土壤微生物生物量碳、氮、磷的影响强于坡位,而在10～20 cm土层,坡位对土壤和土壤微生物生物量碳、氮、磷影响更显著.植被类型显著影响土壤C∶N、C∶P、N∶P和土壤微生物生物量C∶N、C∶P,坡向和坡位仅影响土壤C∶P和N∶P,植被类型的变化是影响土壤C∶N的主要因素.同时,植被类型对土壤养分和微生物生物量碳、氮、磷含量及其生态化学计量特征的影响大于地形因子.标准化主轴分析结果表明,黄土丘陵区不同植被带土壤微生物具有内稳性,特别在草原带,土壤微生物生物量生态化学计量学特征具有更加严格的约束比例.在黄土丘陵区,土壤微生物生物量N∶P或许可以作为判断养分限制的另一个有力工具,若将土壤微生物生物量N∶P与植物叶片N∶P配合使用可能有助于我们更加精确地判断黄土丘陵区的土壤养分限制情况.     

Effects of a lignin peroxidase-expressing recombinant, Streptomyces lividans TK23.1, on biogeochemical cycling and the numbers and activities of microorganisms in soil.

A recombinant actinomycete, Streptomyces lividans TK23.1, expressing a pIJ702-encoded extracellular lignin peroxidase gene cloned from the chromosome of Streptomyces viridosporus T7A, was released into soil in flask- and microcosm-scale studies to determine its effects on humification and elemental cycling and on the numbers, types, and activities of microorganisms native to the soil. Strain TK23.1 had been shown previously to transiently increase the rate of organic carbon mineralization in soil via an effect that was recombinant specific and particularly significant in nonsterile soils already possessing an active microflora. The results of this study confirmed the previous findings and showed that additional effects were measurable upon release of the recombinant strain TK23.1 into unamended soil and into soil amended with lignocellulose. In addition to a transient enhancement of carbon mineralization, the recombinant affected soil pH, the rate of incorporation of carbon into soil humus fractions, nitrogen cycling, the relative populations of some microbial groups, and also certain soil enzyme activities. Whereas the survival or persistence in soil of the recombinant TK23.1 strain and that of its parent, TK23, were similar, the observed effects on microbial numbers, types, and activities were recombinant specific and did not occur when the parental strain was released into soil. All of the measured effects were transient, generally lasting for only a few days. While the effects were statistically significant, their ecological significance appears to be minimal. This is the first report showing that a recombinant actinomycete can affect the microbial ecology of soil in ways that can be readily monitored by using a battery of microbiological, enzymological, and chemical assays.     

Impacts of chronic nitrogen additions vary seasonally and by microbial functional group in tundra soils

Previous studies have shown that fertilization with nitrogen depresses overall microbial biomass and activity in soil. In the present study we broaden our understanding of this phenomenon by studying the seasonality of responses of specific microbial functional groups to chronic nitrogen additions in alpine tundra soils. We measured soil enzyme activities, mineralization kinetics for 8 substrates, biomass of 8 microbial functional groups, and changes in N and carbon pools in the soil. Our approach allowed us to compare the ability of the soil microbial biomass to utilize various substrates in addition to allowing us to estimate changes in biomass of microbial functional groups that are involved in carbon and nitrogen cycling. Overall microbial activity and biomass was reduced in fertilized plots, whereas pools of N in the soil and microbial biomass N were higher in fertilized plots. The negative effects of N were most prominent in the summer. Biomass of the dominant microbial functional groups recovered in fertilized soils during the winter and nitrogen storage in microbial biomass was higher in fertilized soils in the autumn and winter than in the summer. Microbial immobilization of N may therefore be a significant sink for added N during autumn and winter months when plants are not active. One large microbial group that did not recover in the winter in fertilized soils was phenol mineralizers, possibly indicating selection against microbes with enzyme systems for the breakdown of phenolic compounds and complex soil organic matter. Overall, this work is a step towards understanding how chronic N additions affect the structure and biogeochemical functioning of soil microbial communities.     

基于宏基因组技术分析全球不同温度带土壤微生物多样性

【背景】温度在塑造大尺度的土壤微生物群落方面发挥了重要作用,但目前针对全球不同温度带大尺度土壤微生物多样性方面的研究十分缺乏。【目的】明确不同温度带大尺度土壤微生物组成和功能的差异变化。【方法】从宏观的角度运用宏基因组技术对不同温度带土壤微生物群落的组成和功能进行分析。【结果】细菌的物种多样性随着温度带纬度的升高而增多,真菌的物种多样性在温带最多,在寒带最小且假丝酵母属(Candida)占绝对优势。3个温度带间除物种多样性存在差异外,微生物群落中物种丰度差异也较大,优势属和特殊属各有不同。其中值得注意的是,假单胞菌属(Pseudomonas)和芽孢杆菌属(Bacillus)的丰度在不同温度带间存在显著差异,且随着温度带纬度的升高而增多,而链霉菌属(Streptomyces)、地嗜皮菌属(Geodermatophilus)、红色杆菌属(Rubrobacter)和小单孢菌属(Micromonospora)的丰度随温度带纬度的升高而降低。在功能方面,发现与翻译后修饰、蛋白质周转、伴侣(posttranslational modification, protein turnover, chap...     

植被类型变化对土壤微生物碳利用效率的影响研究进展

植被类型变化强烈影响着土壤碳循环。土壤微生物碳利用效率（CUE）是微生物将从环境中获取的碳分配给自身生长的比例,是土壤碳循环的综合指标。研究植被类型变化对CUE的影响有助于从微生物视角理解该过程中的土壤碳动态,可以为评估植被类型变化对土壤质量及生态系统碳循环的影响提供基础,具有重要的理论及实际价值。通过系统查阅相关文献,综述了植被类型变化导致的CUE变化情况,以及该过程中影响CUE的因子与机制。目前,相关研究主要涉及以林地、草地和农业用地为起点或终点的植被变化类型。天然林（原生林、次生林）变化为人工林、林地变化为草地后CUE普遍下降,随终点植被的发展CUE可能恢复至起点水平。植被成熟度越高,发生转变时CUE变化越剧烈。植被类型变化以农业用地为起点或终点时,CUE变化方向的不确定性及幅度的变异性均增加。植被类型变化导致的CUE变化主要受到植被、土壤、微生物因子及其交互作用的驱动,指示CUE的指标、采样季节和土层也会一定程度上影响CUE的变化。今后相关研究应采用直接的CUE测定方法,拓宽研究气候区及植被变化类型,关注植被变化过程中CUE变化的土层差异及动态监测,深入对植被类型变化导致的生态环境因子变化与CUE的关系及作用机制的研究。     

Spatial patterns of microbial diversity and activity in an aged creosote-contaminated site

Restoration of polluted sites via in situ bioremediation relies heavily on the indigenous microbes and their activities. Spatial heterogeneity of microbial populations, contaminants and soil chemical parameters on such sites is a major hurdle in optimizing and implementing an appropriate bioremediation regime. We performed a grid-based sampling of an aged creosote-contaminated site followed by geostatistical modelling to illustrate the spatial patterns of microbial diversity and activity and to relate these patterns to the distribution of pollutants. Spatial distribution of bacterial groups unveiled patterns of niche differentiation regulated by patchy distribution of pollutants and an east-to-west pH gradient at the studied site. Proteobacteria clearly dominated in the hot spots of creosote pollution, whereas the abundance of Actinobacteria, TM7 and Planctomycetes was considerably reduced from the hot spots. The pH preferences of proteobacterial groups dominating in pollution could be recognized by examining the order and family-level responses. Acidobacterial classes came across as generalists in hydrocarbon pollution whose spatial distribution seemed to be regulated solely by the pH gradient. Although the community evenness decreased in the heavily polluted zones, basal respiration and fluorescein diacetate hydrolysis rates were higher, indicating the adaptation of specific indigenous microbial populations to hydrocarbon pollution. Combining the information from the kriged maps of microbial and soil chemistry data provided a comprehensive understanding of the long-term impacts of creosote pollution on the subsurface microbial communities. This study also highlighted the prospect of interpreting taxa-specific spatial patterns and applying them as indicators or proxies for monitoring polluted sites.     

猫儿山不同海拔植被带土壤微生物群落功能多样性

为研究中亚热带森林土壤微生物群落功能多样性特征及其随海拔梯度的变化,应用Biolog微平板技术,对猫儿山不同海拔植被带(常绿阔叶林(EBF)、落叶阔叶混交林(DBF)、针阔混交林(CBF))土壤微生物群落功能多样性差异进行了比较。结果表明,不同海拔植被带土壤微生物群落功能多样性差异显著。土壤平均颜色变化率(AWCD)随培养时间延长而逐渐增加,随着海拔升高,土壤AWCD值逐渐降低,大小顺序为EBFDBFCBF。土壤微生物群落Shannon指数和丰富度指数的总体趋势为EBF最高,DBF次之,CBF最低。不同海拔植被带土壤微生物群落均匀度指数之间差异不显著。不同海拔植被带土壤微生物对不同碳源的利用能力存在差异,其中EBF利用率最高,CBF利用率最低,氨基酸类、胺类和酯类碳源为各海拔植被带土壤微生物利用的主要碳源。主成分分析结果表明,主成分1和主成分2分别能解释变量方差的40.42%和15.97%,在主成分分离中起主要贡献作用的是酯类、胺类和氨基酸类碳源。土壤理化性质与土壤微生物群落功能多样性之间的相关性分析结果表明,微生物群落多样性的Shannon指数与全钾(TK)呈极显著正相关(P0.01),与含水量呈极显著负相关(P0.01),与总有机碳(TOC)、全氮(TN)、速效氮(AN)、有效P(AP)之间的相关性显著(P0.05)或极显著(P0.01),且为负相关。土壤TK含量和含水量可能是造成不同海拔土壤微生物群落功能多样性差异的主要原因。     

Microbial community composition controls carbon flux across litter types in early phase of litter decomposition

Leaf litter decomposition is a major carbon input to soil, making it a target for increasing soil carbon storage through microbiome engineering. We expand upon previous findings to show with multiple leaf litter types that microbial composition can drive variation in carbon flow from litter decomposition and specific microbial community features are associated with synonymous patterns of carbon flow among litter types. Although plant litter type selects for different decomposer communities, within a litter type, microbial composition drives variation in the quantity of dissolved organic carbon (DOC) measured at the end of the decomposition period. Bacterial richness was negatively correlated with DOC quantity, supporting our hypothesis that across multiple litter types there are common microbial traits linked to carbon flow patterns. Variation in DOC abundance (i.e. high versus low DOC) driven by microbial composition is tentatively due to differences in bacterial metabolism of labile compounds, rather than catabolism of non-labile substrates such as lignin. The temporal asynchrony of metabolic processes across litter types may be a substantial impediment to discovering more microbial features common to synonymous patterns of carbon flow among litters. Overall, our findings support the concept that carbon flow may be programmed by manipulating microbial community composition.     

贺兰山不同海拔典型植被带土壤微生物多样性

土壤微生物多样性在海拔梯度的分布格局研究近年来受到和植物动物一样的重视程度,但是干旱风沙区微生物多样性在海拔梯度上的多样性分布规律尚未揭示。本研究以处于干旱风沙区的贺兰山不同海拔的六个典型植被带（荒漠草原带、山地旱生灌丛带、温性针叶林带、针阔混交林带、寒温性针叶林带和亚高山草甸带）土壤为研究对象,利用Biolog微平板法和磷脂脂肪酸甲酯法(FAMEs)系统研究微生物多样性群落特征以及在不同植被带分布规律。结果表明：土壤微生物功能多样性随海拔增加发生变化,且微生物群落结构存在显著差异。Biolog分析显示土壤微生物群落代谢活性依次是：亚高山草甸>寒温性针叶林>针阔混交林>温性针叶林>山地旱生灌丛>荒漠草原,随海拔的升高土壤微生物群落物种丰富度指数（H）和均匀度指数（E）总体上均表现出增大的趋势,差异显著（P＜0.05）;FAMEs分析表明不同海拔的微生物区系发生了一定程度的变化,寒温性针叶林土壤微生物磷酸脂肪酸生物标记的数量和种类均最高,且细菌、真菌特征脂肪酸相对含量也最高;土壤微生物群落结构多样性次序是：寒温性针叶林带>针阔混交林带>温性针叶林带>亚高山草甸>山地旱生灌丛>荒漠草原。本研究结果表明贺兰山海拔梯度的微生物多样性分布规律不同于已有的植物多样性“中部膨胀”研究结果,这说明在高海拔地区有更多的适合该生境的微生物存在,这对维持干旱风沙区的生态系统功能稳定性具有重要意义。     

Co-occurring grass species differ in their associated microbial community composition in a temperate native grassland

Background and aims

Specific associations exist between plant species and the soil microbial community and these associations vary between habitat types and different plant groups. However, there is evidence that the associations are highly specific. Hence, we aimed to determine the specificity of plant-microbe relationships amongst co-occurring grass species in a temperate grassland.

Methods and results

We examined the broad microbial groups of bacteria and fungi as well as a specific fungal group, the arbuscular mycorrhizal community amongst two dominant C₃ and C₄ species and one sub-dominant C₃ species using terminal restriction fragment length polymorphism (T-RFLP) analysis. We found that the two dominant species were more similar to each other in their bacterial and arbuscular mycorrhizal community composition than either was to the sub-dominant species, but not in their fungal community composition. We also found no clear evidence that those differences were directly linked to soil chemical properties.

Conclusions

Our results demonstrate that co-occurring grass species have a distinct soil microbial community and T-RFLP analysis is able to detect plant species effect on the microbial community composition on an extremely local scale, providing an insight into the differences in the response of bacterial, fungal and arbuscular mycorrhizal communities to different, but similar and co-occurring, plant species.     

Land use and climatic factors structure regional patterns in soil microbial communities

Aim  Although patterns are emerging for macroorganisms, we have limited understanding of the factors determining soil microbial community composition and productivity at large spatial extents. The overall objective of this study was to discern the drivers of microbial community composition at the extent of biogeographical provinces and regions. We hypothesized that factors associated with land use and climate would drive soil microbial community composition and biomass.
Location  Great Basin Province, Desert Province and California Floristic Province, California, USA.
Methods  Using phospholipid fatty acid analysis, we compared microbial communities across eight land-use types sampled throughout the State of California, USA ( n = 1117).
Results  The main factor driving composition and microbial biomass was land-use type, especially as related to water availability and disturbance. Dry soils were more enriched in Gram-negative bacteria and fungi, and wetter soils were more enriched in Gram-positive, anaerobic and sulphate-reducing bacteria. Microbial biomass was lowest in ecosystems with the wettest and driest soils. Disturbed soils had less fungal and more Gram-positive bacterial biomass than wildland soils. However, some factors known to influence microbial communities, such as soil pH and specific plant taxa, were not important here.
Main conclusions  Distinct microbial communities were associated with land-use types and disturbance at the regional extent. Overall, soil water availability was an important determinant of soil microbial community composition. However, because of the inclusion of managed and irrigated agricultural ecosystems, the effect of precipitation was not significant. Effects of environmental and management factors, such as flooding, tillage and irrigation, suggest that agricultural management can have larger effects on soil microbial communities than elevation and precipitation gradients.     

Plant functional traits and soil microbial biomass in different vegetation zones on the Loess Plateau

Plant functional traits built the relationships between plant diversity, species composition, and physiology along with the environmental changes, thus influencing soil microbial community. As the sensitivity indicators, soil microbial biomass and plant functional traits responses soil micro-organism and plant characteristics in direct way. Ten plant functional traits of 149 species and soil microbial biomass (carbon, nitrogen, and phosphorus) were analyzed across the different vegetation types (forest, forest-steppe, and steppe) that are divided by environmental gradient (temperature and precipitation), aimed to find the correlations among them. Our results confirmed the greatest values of plant functional traits (except the leaf density and the fine root density) that were distributed in the steppe zone, mainly due to the different mean annual temperature and mean annual precipitation conditions. For different plant growth forms, the plant functional traits were significant differences among the vegetation zones. The advantages of higher rate nutrient cycling, plentiful biomass supplements, and favorite habit conditions lead to the forest-steppe zone with the highest C_mic and N_mic concentrations. The canonical correlation analysis indicated that leaf nitrogen, root nitrogen, and fine root densities were correlated with root exudate and tissue which affected the concentrations of soil organic carbon (SOC) and total nitrogen (N), consequently impacting soil microbial biomass carbon (C_mic) and soil microbial biomass nitrogen (N_mic). Soil is the medium that connects micro-organism and plant root system that influenced leaf nitrogen, root nitrogen, and fine root density of plant functional traits, the concentrations of SOC and total N that plant feedback are consequently influencing C_mic and N_mic.     

Unrecognized controls on microbial functioning in Blue Carbon ecosystems: The role of mineral enzyme stabilization and allochthonous substrate supply

Tidal wetlands are effective carbon sinks, mitigating climate change through the long‐term removal of atmospheric CO₂. Studies along surface‐elevation and thus flooding‐frequency gradients in tidal wetlands are often used to understand the effects of accelerated sea‐level rise on carbon sequestration, a process that is primarily determined by the balance of primary production and microbial decomposition. It has often been hypothesized that rates of microbial decomposition would increase with elevation and associated increases in soil oxygen availability; however, previous studies yield a wide range of outcomes and equivocal results. Our mechanistic understanding of the elevation–decomposition relationship is limited because most effort has been devoted to understanding the terminal steps of the decomposition process. A few studies assessed microbial exo‐enzyme activities (EEAs) as initial and rate‐limiting steps that often reveal important insight into microbial energy and nutrient constraints. The present study assessed EEAs and microbial abundance along a coastal ecotone stretching a flooding gradient from tidal flat to high marsh in the European Wadden Sea. We found that stabilization of exo‐enzymes to mineral sediments leads to high specific EEAs at low substrate concentrations in frequently flooded, sediment‐rich zones of the studied ecotone. We argue that the high background activity of a mineral‐associated enzyme pool provides a stable decomposition matrix in highly dynamic, frequently flooded zones. Furthermore, we demonstrate that microbial communities are less nutrient limited in frequently flooded zones, where inputs of nutrient‐rich marine organic matter are higher. This was reflected in both increasing exo‐enzymatic carbon versus nutrient acquisition and decreasing fungal versus bacterial abundance with increasing flooding frequency. Our findings thereby suggest two previously unrecognized mechanisms that may contribute to stimulated microbial activity despite decreasing oxygen availability in response to accelerated sea‐level rise.     

Loss of microbial diversity in soils is coincident with reductions in some specialized functions

Loss of microbial diversity is considered a major threat because of its importance for ecosystem functions, but there is a lack of conclusive evidence that diversity itself is reduced under anthropogenic stress, and about the consequences of diversity loss. Heavy metals are one of the largest, widespread pollutant types globally, and these represent a significant environmental stressor for terrestrial microbial communities. Using combined metagenomics and functional assays, we show that the compositional and functional response of microbial communities to long‐term heavy metal stress results in a significant loss of diversity. Our results indicate that even at a moderate loss of diversity, some key specialized functions (carried out by specific groups) may be compromised. Together with previous work, our data suggest disproportionate impact of contamination on microbes that carry out specialized, but essential, ecosystem functions. Based on these findings, we propose a conceptual framework to explicitly consider diversity of functions and microbial functional groups to test the relationship between biodiversity and soil functions.     

Effect of above-ground plant species on soil microbial community structure and its impact on suppression of Rhizoctonia solani AG3

The extent of soil microbial diversity is seen to be critical to the maintenance of soil health and quality. Different agricultural practices are able to affect soil microbial diversity and thus the level of suppressiveness of plant diseases. In a 4-year field experiment, we investigated the microbial diversity of soil under different agricultural regimes. We studied permanent grassland, grassland turned into arable land, long-term arable land and arable land turned into grassland. The diversity of microbial communities was described by using cultivation-based and cultivation-independent methods. Both types of methods revealed differences in the diversities of soil microbial communities between different treatments. The treatments with higher above-ground biodiversity generally maintained higher levels of microbial diversity. Moreover, a positive correlation between suppression of Rhizoctonia solani AG3 and microbial diversity was observed. Permanent (species-rich) grassland and grassland turned into maize stimulated higher microbial diversities and higher levels of suppressiveness of R. solani AG3 compared with the long-term arable land. Effects of agricultural practices on Bacillus and Pseudomonas communities were also observed and clear correlations between the levels of suppressiveness and the diversities of these bacterial groups were found. This study highlighted the importance of agricultural management regime for soil microbial community structure and diversity as well as the level of soil suppressiveness.