Functional gene diversity of soil microbial communities from five oil-contaminated fields in China

To compare microbial functional diversity in different oil-contaminated fields and to know the effects of oil contaminant and environmental factors, soil samples were taken from typical oil-contaminated fields located in five geographic regions of China. GeoChip, a high-throughput functional gene array, was used to evaluate the microbial functional genes involved in contaminant degradation and in other major biogeochemical/metabolic processes. Our results indicated that the overall microbial community structures were distinct in each oil-contaminated field, and samples were clustered by geographic locations. The organic contaminant degradation genes were most abundant in all samples and presented a similar pattern under oil contaminant stress among the five fields. In addition, alkane and aromatic hydrocarbon degradation genes such as monooxygenase and dioxygenase were detected in high abundance in the oil-contaminated fields. Canonical correspondence analysis indicated that the microbial functional patterns were highly correlated to the local environmental variables, such as oil contaminant concentration, nitrogen and phosphorus contents, salt and pH. Finally, a total of 59% of microbial community variation from GeoChip data can be explained by oil contamination, geographic location and soil geochemical parameters. This study provided insights into the in situ microbial functional structures in oil-contaminated fields and discerned the linkages between microbial communities and environmental variables, which is important to the application of bioremediation in oil-contaminated sites.     

不同林型土壤微生物有机碳降解基因的多样性

应用寡聚核苷酸基因芯片,分析了米亚罗林区冷杉原始林（M—Y）和20世纪60年代云杉人工林（M-60）土壤微生物的功能基因多样性。该功能基因芯片含有与有机碳降解、碳固定、氮、磷、硫循环和金属抗性相关的1961个基因探针。在M—Y和M-60样地中分别检测到39和62个具有较强杂交信号（SNR≥2）的功能基因,其基因多样性水平指数分别为3．59和4．04,杂交信号强度总值分别为480280和630560。M—Y和M-60样地中分别检测到32个和37个有机碳降解基因,占总基因的82％和60％,这些基因分属于22个不同的基因类群,分别参与木质素、木聚糖、几丁质等有机碳的降解过程。有机碳降解基因在两个样地中存在较大的多样性和丰度差异。这些结果说明了大多数的土壤微生物直接参与了土壤有机碳的降解,同时,林型不同显著影响了土壤微生物群落结构和有机碳降解微生物的多样性。     

恩诺沙星对土壤微生物群落功能多样性的影响

为了评价恩诺沙星(enrofloxacin)对土壤微生物群落的影响,借助BIOLOG检测法比较了不同浓度恩诺沙星影响下的土壤微生物的群落特征。结果表明,各添加药物组与空白对照组的土壤微生物群落代谢多样性差异显著(P<0.05),空白对照组土壤微生物在BIOLOG微平板上的平均每孔颜色变化率(AWCD)和微生物代谢多样性指数(丰富度和多样性)显著高于添加药物组;药物显著影响微生物群落对BIOLOG微平板中碳源的利用能力,较低浓度的恩诺沙星(0.01μg/g)就显著影响了土壤微生物群落对α-Ketobutyric Acid、L-Phenylalanine、1-Erythritol、D-Xylose等碳源的利用代谢能力(P<0.05),且土壤微生物群落利用各类碳源的能力随药物浓度加大而降低。由此可见,恩诺沙星在土壤中残留时对土壤微生物的影响不容忽视。     

Huanglongbing alters the structure and functional diversity of microbial communities associated with citrus rhizosphere

The diversity and stability of bacterial communities present in the rhizosphere heavily influence soil and plant quality and ecosystem sustainability. The goal of this study is to understand how ‘Candidatus Liberibacter asiaticus'' (known to cause Huanglongbing, HLB) influences the structure and functional potential of microbial communities associated with the citrus rhizosphere. Clone library sequencing and taxon/group-specific quantitative real-time PCR results showed that ‘Ca. L. asiaticus'' infection restructured the native microbial community associated with citrus rhizosphere. Within the bacterial community, phylum Proteobacteria with various genera typically known as successful rhizosphere colonizers were significantly greater in clone libraries from healthy samples, whereas phylum Acidobacteria, Actinobacteria and Firmicutes, typically more dominant in the bulk soil were higher in ‘Ca. L. asiaticus''-infected samples. A comprehensive functional microarray GeoChip 3.0 was used to determine the effects of ‘Ca. L. asiaticus'' infection on the functional diversity of rhizosphere microbial communities. GeoChip analysis showed that HLB disease has significant effects on various functional guilds of bacteria. Many genes involved in key ecological processes such as nitrogen cycling, carbon fixation, phosphorus utilization, metal homeostasis and resistance were significantly greater in healthy than in the ‘Ca. L. asiaticus''-infected citrus rhizosphere. Our results showed that the microbial community of the ‘Ca. L. asiaticus''-infected citrus rhizosphere has shifted away from using more easily degraded sources of carbon to the more recalcitrant forms. Overall, our study provides evidence that the change in plant physiology mediated by ‘Ca. L. asiaticus'' infection could elicit shifts in the composition and functional potential of rhizosphere microbial communities. In the long term, these fluctuations might have important implications for the productivity and sustainability of citrus-producing agro-ecosystems.     

凋落物对土壤有机碳与微生物功能多样性的影响

森林凋落物是影响土壤微生物群落和有机碳含量的重要因素,但其作用的程度和机制尚不清楚,研究该问题对于分析森林生态系统碳循环和资源管理具有重要意义。研究凋落物去除与添加处理下土壤有机碳含量与土壤微生物对碳源利用的差异,明确凋落物去除与添加对土壤微生物群落代谢功能及其多样性的影响,探究不同处理下SOC含量变化的土壤微生物群落代谢机理。选取承德市雾灵山1405-1435 m海拔范围内核桃楸-蒙古栎混交林的表层土壤,采用室内培养结合Biolog-ECO方法,测定了培养第21天的土壤有机碳（soil organic carbon,SOC）含量及微生物群落的AWCD值、Shannon-Wiener多样性指数、Simpson优势度指数、McIntosh均匀度指数、Pielou丰富度指数,分析培养期内凋落物的不同处理下SOC含量与微生物功能多样性的变化特征。结果表明：1）不同凋落物处理对SOC含量与土壤微生物群落多样性具有显著影响（P<0.05）,DL > HL > NL > CK;2）不同凋落物处理下土壤微生物群落代谢活性和土壤微生物对碳源的利用程度具有显著差异（P<0.05）,碳水化合物类和氨基酸类是土壤微生物的主要碳源;3）不同处理的SOC含量与土壤微生物多样性具有正相关关系。双倍凋落物添加在短期内对土壤微生物多样性影响难以达到显著水平且在一定程度上对土壤微生物的代谢活性具有抑制作用,土壤微生物群落功能多样性对SOC含量具有重要影响。     

Relating belowground microbial composition to the taxonomic,phylogenetic, and functional trait distributions of trees in a tropical forest

The complexities of the relationships between plant and soil microbial communities remain unresolved. We determined the associations between plant aboveground and belowground (root) distributions and the communities of soil fungi and bacteria found across a diverse tropical forest plot. Soil microbial community composition was correlated with the taxonomic and phylogenetic structure of the aboveground plant assemblages even after controlling for differences in soil characteristics, but these relationships were stronger for fungi than for bacteria. In contrast to expectations, the species composition of roots in our soil core samples was a poor predictor of microbial community composition perhaps due to the patchy, ephemeral, and highly overlapping nature of fine root distributions. Our ability to predict soil microbial composition was not improved by incorporating information on plant functional traits suggesting that the most commonly measured plant traits are not particularly useful for predicting the plot‐level variability in belowground microbial communities.     

A standardized assessment of forest mammal communities reveals consistent functional composition and vulnerability across the tropics

The understanding of global diversity patterns has benefitted from a focus on functional traits and how they relate to variation in environmental conditions among assemblages. Distant communities in similar environments often share characteristics, and for tropical forest mammals, this functional trait convergence has been demonstrated at coarse scales (110–200 km resolution), but less is known about how these patterns manifest at fine scales, where local processes (e.g. habitat features and anthropogenic activities) and biotic interactions occur. Here, we used standardized camera trapping data and a novel analytical method that accounts for imperfect detection to assess how the functional composition of terrestrial mammal communities for two traits – trophic guild and body mass – varies across 16 protected areas in tropical forests and three continents, in relation to the extent of protected habitat and anthropogenic pressures. We found that despite their taxonomic differences, communities generally have a consistent trophic guild composition, and respond similarly to these factors. Insectivores were found to be sensitive to the size of protected habitat and surrounding human population density. Body mass distribution varied little among communities both in terms of central tendency and spread, and interestingly, community average body mass declined with proximity to human settlements. Results indicate predicted trait convergence among assemblages at the coarse scale reflects consistent functional composition among communities at the local scale, suggesting that broadly similar habitats and selective pressures shaped communities with similar trophic strategies and responses to drivers of change. These similarities provide a foundation for assessing assemblages under anthropogenic threats and sharing conservation measures.     

热带次生林刀耕火种过程中土壤节肢动物群落结构及多样性的变化

通过模拟刀耕火种过程,对热带次生林土壤节肢运动群薄结构及多样性在刀耕火种过程中变化进行了初步研究,结果表明,火烧1周后,土壤节肢运行类群数和个体数从次生林地的14类,564头降为7类,164头,并且在群落组成上发生了较大变化,膜翅目的烧后1天成为突出优势类群,蜱蟥目在烧后1周成为唯一的优势类群,而在多样性方面,土壤节肢运动群落类群数（丰度）,个体数（多度）和DG多样性指数在刀耕火种过程中均降低,土壤节肢运动的垂直分布因刀耕火种干扰发生了很大变化,尚存土壤节肢运动随干扰强度的加剧,出现逐步由土壤表层向深层迁移的现象,一些土壤节肢运动在土层间做垂直迁移活动,说明土壤层在一定程度上减轻了刀耕火种对它们的破坏作用,维持了某些类群数量及群落多样性。     

亚热带森林转换对土壤氮转化关键功能微生物群落的影响

近年来,由于林地开发和商品林建设等原因,我国亚热带地区大量天然林和次生林经皆伐改造为林分结构简单、树种单一的人工林。氮(N)素是维持森林植被生长和系统初级生产力的重要因子,土壤微生物驱动了森林土壤N转化的关键过程。然而,目前亚热带森林转换对土壤N转化微生物群落的影响仍不清晰。以湖南芦头森林生态系统国家定位观测研究站内典型次生林(CS)及由其转换而成的油茶(YC)、黄桃(HT)、杨梅(YM)和杉木(SM)四种人工林为研究对象,采用实时荧光定量PCR和高通量测序等方法,研究了各林分土壤性质、固N菌和氨氧化微生物功能基因丰度、群落特征及相互关系,旨在探讨亚热带森林转换后土壤N转化关键过程(固N和氨氧化作用)的功能微生物群落变化及驱动因素。结果表明:森林转换显著改变了土壤碳(C)、N含量,降低了土壤nifH基因丰度、固N菌和氨氧化细菌的群落α多样性,但提高了氨氧化微生物amoA基因丰度和氨氧化古菌的群落α多样性;并且,森林转换通过改变各功能微生物优势菌群(如变形菌、蓝细菌、泉古菌和奇古菌等)的相对丰度,显著影响了土壤固N菌和氨氧化微生物的群落组成;冗余分析和结构方程模型表明,土壤有机碳、全氮 、铵态氮含量和pH是驱动土壤固N菌和氨氧化微生物群落变化的关键因素。森林转换后,合理的施肥方式有利于人工林土壤固N菌和氨氧化微生物的群落恢复。研究结果为转换后单一人工林土壤养分恢复、生产力的提高和可持续经营提供了科学依据。     

Land use change has stronger effects on functional diversity than taxonomic diversity in tropical Andean hummingbirds

Land use change modifies the environment at multiple spatial scales, and is a main driver of species declines and deterioration of ecosystem services. However, most of the research on the effects of land use change has focused on taxonomic diversity, while functional diversity, an important predictor of ecosystem services, is often neglected. We explored how local and landscape scale characteristics influence functional and taxonomic diversity of hummingbirds in the Andes Mountains in southern Ecuador. Data was collected in six landscapes along a land use gradient, from an almost intact landscape to one dominated by cattle pastures. We used point counts to sample hummingbirds from 2011 to 2012 to assessed how local factors (i.e., vegetation structure, flowering plants richness, nectar availability) and landscape factors (i.e., landscape heterogeneity, native vegetation cover) influenced taxonomic and functional diversity. Then, we analyzed environment – trait relationships (RLQ test) to explore how different hummingbird functional traits influenced species responses to these factors. Taxonomic and functional diversity of hummingbirds were positively associated with landscape heterogeneity but only functional diversity was positively related to native vegetation coverage. We found a weak response of taxonomic and functional diversity to land use change at the local scale. Environment‐trait associations showed that body mass of hummingbirds likely influenced species sensitivity to land use change. In conclusion, landscape heterogeneity created by land use change can positively influence hummingbird taxonomic and functional diversity; however, a reduction of native vegetation cover could decrease functional diversity. Given that functional diversity can mediate ecosystem services, the conservation of native vegetation cover could play a key role in the maintenance of hummingbird pollination services in the tropical Andes. Moreover, there are particular functional traits, such as body mass, that increase a species sensitivity to land use change.     

15N natural abundance in forest and pasture soils of the Brazilian Amazon Basin

The natural abundance of ¹⁵N was examined in soil profiles from forests and pastures of the Brazilian Amazon Basin to compare tropical forests on a variety of soil types and to investigate changes in the sources of nitrogen to soils following deforestation for cattle ranching. Six sites in the state of Rondônia, two sites in Pará and one in Amazonas were studied. All sites except one were chronosequences and contained native forest and one or more pastures ranging from 2 to 27 years old. Forest soil ¹⁵N values to a depth of 1 m ranged from 8 to 23 and were higher than values typically found in temperate forests. A general pattern of increasing ¹⁵N values with depth near the soil surface was broadly similar to patterns in other forests but a decrease in ¹⁵N values in many forest profiles between 20 and 40 cm suggests that illuviation of ¹⁵N-depleted nitrate may influence total soil ¹⁵N values in deeper soil where total N concentrations are low. In four chronosequences in Rondônia, the ¹⁵N values of surface soil from pastures were lower than in the original forest and ¹⁵N values were increasingly depleted in older pastures. Inputs of atmospheric N by dinitrogen fixation could be an important N source in these pastures. Other pastures in Amazonas and Pará and Rondônia showed no consistent change from forest values. The extent of fractionation that leads to ¹⁵N enrichment in soils was broadly similar over a wide range of soil textures and indicated that similar processes control N fractionation and loss under tropical forest over a broad geographic region. Forest ¹⁵N profiles were consistent with conceptual models that explain enrichment of soil ¹⁵N values by selective loss of ¹⁴N during nitrification and denitrification.     

Fire affects the taxonomic and functional composition of soil microbial communities,with cascading effects on grassland ecosystem functioning

Fire is a crucial event regulating the structure and functioning of many ecosystems. Yet few studies have focused on how fire affects taxonomic and functional diversities of soil microbial communities, along with changes in plant communities and soil carbon (C) and nitrogen (N) dynamics. Here, we analyze these effects in a grassland ecosystem 9 months after an experimental fire at the Jasper Ridge Global Change Experiment site in California, USA. Fire altered soil microbial communities considerably, with community assembly process analysis showing that environmental selection pressure was higher in burned sites. However, a small subset of highly connected taxa was able to withstand the disturbance. In addition, fire decreased the relative abundances of most functional genes associated with C degradation and N cycling, implicating a slowdown of microbial processes linked to soil C and N dynamics. In contrast, fire stimulated above‐ and belowground plant growth, likely enhancing plant–microbe competition for soil inorganic N, which was reduced by a factor of about 2. To synthesize those findings, we performed structural equation modeling, which showed that plants but not microbial communities were responsible for significantly higher soil respiration rates in burned sites. Together, our results demonstrate that fire ‘reboots’ the grassland ecosystem by differentially regulating plant and soil microbial communities, leading to significant changes in soil C and N dynamics.     

降水变化对红松阔叶林土壤微生物功能多样性的影响

红松阔叶林生态系统是中国东北地带性顶级植被,具有重要的生态学意义;长白山是研究温带森林对大气降水变化正负反馈的理想地带。以长白山红松阔叶林0—5cm、5—10cm表层土和红松根际土为研究对象,基于Biolog生态平板法,分析了不同降水量条件下土壤微生物功能多样性的变化规律及机制。结果表明,平均颜色变化率(Average well color development,AWCD)随培养时间的延长而增加,在培养初期的表现为降水减少30%样地降水增加30%对照样地,培养的末期,AWCD值的变化没有表现出明显的规律性。各降水量条件下根际土土壤微生物的AWCD值均高于表层土。Shannon多样性指数、Simpson优势度指数和McIntosh均匀度指数在不同降水量样地间均未表现出显著的差异,即降水量增减30%的情况下,未对红松林表层土和红松根际土的微生物功能多样性产生显著影响。土壤微生物利用率最高的碳源是氨基类,其次是碳水类和多聚类,其中氨基类和碳水类也是不同降水条件下微生物功能多样性表现出异质性的主要碳源。     

Historical precipitation predictably alters the shape and magnitude of microbial functional response to soil moisture

Soil moisture constrains the activity of decomposer soil microorganisms, and in turn the rate at which soil carbon returns to the atmosphere. While increases in soil moisture are generally associated with increased microbial activity, historical climate may constrain current microbial responses to moisture. However, it is not known if variation in the shape and magnitude of microbial functional responses to soil moisture can be predicted from historical climate at regional scales. To address this problem, we measured soil enzyme activity at 12 sites across a broad climate gradient spanning 442–887 mm mean annual precipitation. Measurements were made eight times over 21 months to maximize sampling during different moisture conditions. We then fit saturating functions of enzyme activity to soil moisture and extracted half saturation and maximum activity parameter values from model fits. We found that 50% of the variation in maximum activity parameters across sites could be predicted by 30‐year mean annual precipitation, an indicator of historical climate, and that the effect is independent of variation in temperature, soil texture, or soil carbon concentration. Based on this finding, we suggest that variation in the shape and magnitude of soil microbial response to soil moisture due to historical climate may be remarkably predictable at regional scales, and this approach may extend to other systems. If historical contingencies on microbial activities prove to be persistent in the face of environmental change, this approach also provides a framework for incorporating historical climate effects into biogeochemical models simulating future global change scenarios.     

中亚热带几种针、阔叶树种凋落物混合分解对土壤微生物群落碳代谢多样性的影响

土壤微生物群落功能如何响应凋落物物种组成的改变,对于指导我国的人工林种植具有重要意义。通过小盆模拟试验,用BIOLOG微平板培养的方法研究了南方红壤丘陵区典型物种马尾松和湿地松的凋落物分别与白栎和青冈的凋落物混合,与单一针叶凋落物分解时相比,针阔混合凋落物分解过程中土壤微生物群落功能多样性的差异。结果表明：(1)针叶和阔叶混合分解提高了土壤微生物群落碳代谢的强度、丰富度以及多样性,针阔混合凋落物分解时土壤微生物群落的碳源代谢方式与单一针叶分解时差异显著;(2)凋落物分解前期土壤微生物群落碳代谢强度、丰富度和多样性与凋落物初始C/N呈显著负相关,分解后期土壤微生物群落碳代谢强度与凋落物初始木质素/N比呈显著负相关,碳代谢多样性与初始P含量呈显著正相关;(3)通过改变凋落物物种组成,进而改变凋落物元素组成(C/N、木质素/N和P),能显著影响土壤微生物群落的碳代谢强度和代谢多样性。在南方红壤丘陵区,针-阔叶混合的凋落物比单一针叶凋落物更有利于提高土壤微生物群落的功能。     

萘对川西亚高山森林土壤微生物量、丰度和磷脂脂肪酸的影响

萘作为土壤动物化学抑制剂已在土壤动物生态功能的研究中广泛使用,但其非目标效应使其应用仍存在很大的不确定性。为了解在亚高山森林土壤应用萘抑制土壤动物群落的非目标效应,以川西亚高山森林土壤为研究对象,采用微缩实验研究了土壤微生物生物量、丰度和磷脂脂肪酸对萘胁迫的短期响应。结果表明,萘处理和对照的土壤微生物生物量碳(MBC)、真菌丰度以及细菌、真菌、革兰氏阳性菌(G~+)和革兰氏阴性菌(G~-)PLFAs含量在整个培养期间表现为降低的变化趋势,二者的土壤微生物生物量碳和G~+PLFAs含量以培养52d最低,细菌、真菌和G~-PLFAs含量以培养的45d最低。萘处理和对照的微生物生物量氮(MBN)含量表现出先升高后降低的动态,微生物生物量碳氮比(MBC/MBN)则表现为相反趋势。对照的真菌/细菌PLFAs比值呈现先升高后降低的动态,以培养的17d最高,但萘处理的真菌/细菌PLFAs比值无明显变化规律;萘处理的G~+/G~-PLFAs比值表现为降低的变化趋势,对照的G~+/G~-PLFAs比值表现为先降低后升高的趋势。萘处理仅显著影响了G~+/G~-PLFAs比值,但萘处理和采样时间的交互作用显著影响MBC/MBN、细菌丰度、真菌/细菌丰度比以及细菌、真菌的PLFAs含量、真菌/细菌PLFAs比值、G~+/G~-PLFAs比值。萘作为土壤动物抑制剂对川西亚高山森林土壤微生物群落的非目标效应具有时间变异性。     

热带次生林、旱稻种植地和火烧迹地土壤节肢动物群落结构特征及季节变化

通过模拟刀耕火种过程 ,对刀耕火种前后的次生林、旱稻地 (第 2年 )和火烧迹地 (火烧后直接撂荒地第 2年 )土壤节肢动物群落结构特征及季节变化进行了调查研究。结果显示 :3块样地土壤节肢动物群落的优势类群组成相同 ,均为蜱螨目、膜翅目和弹尾目 ,但不同生境样地中各优势类群所占群落总数的比例不同 ,并且 3样地常见和稀有类群的组成差异较大 ;土壤节肢动物类群数、个体数和 DG多样性指数表现为次生林高于其它 2块样地 ,而旱稻地和火烧迹地则无较大差异 ,但一些类群在旱稻地、火烧迹地的数量分布与次生林具有差异 ,且在土壤层的表现较为突出 ;3块样地土壤节肢动物群落具有较好相似性 ,其中旱稻地与火烧迹地达到极相似水平 (D、DS>0 .9)。3种不同类型生境土壤节肢动物群落在类群数、个体数和多样性指数的季节变化总体呈现出雨量少的干季或雨季初末期高于雨量最大的雨季中期 ,与当地降雨量和气温变化有密切关系 ,同时各样地土壤节肢动物群落因生境条件不同及人为活动干扰强弱而形成各自的季节消长特点。研究表明刀耕火种后的旱稻种植对土壤节肢动物群落的恢复和发展在一定限制条件 (面积、周围次生林和坡度 )下无破坏性影响 ,但植被改变、农事活动等对直接撂荒地和旱稻地土壤节肢动物群落的季节消长产生     

Land use change emission scenarios: anticipating a forest transition process in the Brazilian Amazon

Following an intense occupation process that was initiated in the 1960s, deforestation rates in the Brazilian Amazon have decreased significantly since 2004, stabilizing around 6000 km² yr^?1 in the last 5 years. A convergence of conditions contributed to this, including the creation of protected areas, the use of effective monitoring systems, and credit restriction mechanisms. Nevertheless, other threats remain, including the rapidly expanding global markets for agricultural commodities, large‐scale transportation and energy infrastructure projects, and weak institutions. We propose three updated qualitative and quantitative land‐use scenarios for the Brazilian Amazon, including a normative ‘Sustainability’ scenario in which we envision major socio‐economic, institutional, and environmental achievements in the region. We developed an innovative spatially explicit modelling approach capable of representing alternative pathways of the clear‐cut deforestation, secondary vegetation dynamics, and the old‐growth forest degradation. We use the computational models to estimate net deforestation‐driven carbon emissions for the different scenarios. The region would become a sink of carbon after 2020 in a scenario of residual deforestation (~1000 km² yr^?1) and a change in the current dynamics of the secondary vegetation – in a forest transition scenario. However, our results also show that the continuation of the current situation of relatively low deforestation rates and short life cycle of the secondary vegetation would maintain the region as a source of CO₂ – even if a large portion of the deforested area is covered by secondary vegetation. In relation to the old‐growth forest degradation process, we estimated average gross emission corresponding to 47% of the clear‐cut deforestation from 2007 to 2013 (using the DEGRAD system data), although the aggregate effects of the postdisturbance regeneration can partially offset these emissions. Both processes (secondary vegetation and forest degradation) need to be better understood as they potentially will play a decisive role in the future regional carbon balance.     

Relationships between soil hydrology and forest structure and composition in the southern Brazilian Amazon

Question: Is soil hydrology an important niche‐based driver of biodiversity in tropical forests? More specifically, we asked whether seasonal dynamics in soil water regime contributed to vegetation partitioning into distinct forest types. Location: Tropical rain forest in northwestern Mato Grosso, Brazil. Methods: We investigated the distribution of trees and lianas ≥ 1 cm DBH in ten transects that crossed distinct hydrological transitions. Soil water content and depth to water table were measured regularly over a 13‐month period. Results: A detrended correspondence analysis (DCA) of 20 dominant species and structural attributes in 10 × 10 m subplots segregated three major forest types: (1) high‐statured upland forest with intermediate stem density, (2) medium‐statured forest dominated by palms, and (3) low‐statured campinarana forest with high stem density. During the rainy season and transition into the dry season, distinct characteristics of the soil water regime (i.e. hydro‐indicators) were closely associated with each vegetation community. Stand structural attributes and hydro‐indicators were statistically different among forest types. Conclusions: Some upland species appeared intolerant of anaerobic conditions as they were not present in palm and campinarana sites, which experienced prolonged periods of saturation at the soil surface. A shallow impermeable layer restricted rooting depth in the campinarana community, which could heighten drought stress during the dry season. The only vegetation able to persist in campinarana sites were short‐statured trees that appear to be well‐adapted to the dual extremes of inundation and drought.     

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.