Seasonal Dynamics of Microbial Community Composition and Function in Oak Canopy and Open Grassland Soils

Soil microbial communities are closely associated with aboveground plant communities, with multiple potential drivers of this relationship. Plants can affect available soil carbon, temperature, and water content, which each have the potential to affect microbial community composition and function. These same variables change seasonally, and thus plant control on microbial community composition may be modulated or overshadowed by annual climatic patterns. We examined microbial community composition, C cycling processes, and environmental data in California annual grassland soils from beneath oak canopies and in open grassland areas to distinguish factors controlling microbial community composition and function seasonally and in association with the two plant overstory communities. Every 3 months for up to 2 years, we monitored microbial community composition using phospholipid fatty acid (PLFA) analysis, microbial biomass, respiration rates, microbial enzyme activities, and the activity of microbial groups using isotope labeling of PLFA biomarkers (¹³C-PLFA). Distinct microbial communities were associated with oak canopy soils and open grassland soils and microbial communities displayed seasonal patterns from year to year. The effects of plant species and seasonal climate on microbial community composition were similar in magnitude. In this Mediterranean ecosystem, plant control of microbial community composition was primarily due to effects on soil water content, whereas the changes in microbial community composition seasonally appeared to be due, in large part, to soil temperature. Available soil carbon was not a significant control on microbial community composition. Microbial community composition (PLFA) and ¹³C-PLFA ordination values were strongly related to intra-annual variability in soil enzyme activities and soil respiration, but microbial biomass was not. In this Mediterranean climate, soil microclimate appeared to be the master variable controlling microbial community composition and function.     

Fungal community responses to precipitation

Understanding how fungal communities are affected by precipitation is an essential aspect of predicting soil functional responses to future climate change and the consequences of those responses for the soil carbon cycle. We tracked fungal abundance, fungal community composition, and soil carbon across 4 years in long‐term field manipulations of rainfall in northern California. Fungi responded directly to rainfall levels, with more abundant, diverse, and consistent communities predominating under drought conditions, and less abundant, less diverse, and more variable communities emerging during wetter periods and in rain‐addition treatments. Soil carbon storage itself did not vary with rainfall amendments, but increased decomposition rates foreshadow longer‐term losses of soil carbon under conditions of extended seasonal rainfall. The repeated recovery of fungal diversity and abundance during periodic drought events suggests that species with a wide range of environmental tolerances coexist in this community, consistent with a storage effect in soil fungi. Increased diversity during dry periods further suggests that drought stress moderates competition among fungal taxa. Based on the responses observed here, we suggest that there may be a relationship between the timescale at which soil microbial communities experience natural environmental fluctuations and their ability to respond to future environmental change.     

Soil Water Content and Organic Carbon Availability Are Major Determinants of Soil Microbial Community Composition

Exploration of environmental factors governing soil microbial community composition is long overdue and now possible with improved methods for characterizing microbial communities. Previously, we observed that rice soil microbial communities were distinctly different from tomato soil microbial communities, despite management and seasonal variations within soil type. Potential contributing factors included types and amounts of organic inputs, organic carbon content, and timing and amounts of water inputs. Of these, both soil water content and organic carbon availability were highly correlated with observed differences in composition. We examined how organic carbon amendment (compost, vetch, or no amendment) and water additions (from air dry to flooded) affect microbial community composition. Using canonical correspondence analysis of phospholipid fatty acid data, we determined flooded, carbon-amended (+C) microcosm samples were distinctly different from other +C samples and unamended (–C) samples. Although flooding without organic carbon addition influenced composition some, organic carbon addition was necessary to substantially alter community composition. Organic carbon availability had the same general effects on microbial communities regardless of whether it was compost or vetch in origin. In addition, flooded samples, regardless of organic carbon inputs, had significantly lower ratios of fungal to bacterial biomarkers, whereas under drier conditions and increased organic carbon availability the microbial communities had higher proportions of fungal biomass. When comparing field and microcosm soil, flooded +C microcosm samples were most similar to field-collected rice soil, whereas all other treatments were more similar to field-collected tomato soil. Overall, manipulating water and carbon content selected for microbial communities similar to those observed when the same factors were manipulated at the field scale.     

Soil Microbial Responses to Temporal Variations of Moisture and Temperature in a Chihuahuan Desert Grassland

Global climate change models indicate that storm magnitudes will increase in many areas throughout southwest North America, which could result in up to a 25% increase in seasonal precipitation in the Big Bend region of the Chihuahuan Desert over the next 50 years. Seasonal precipitation is a key limiting factor regulating primary productivity, soil microbial activity, and ecosystem dynamics in arid and semiarid regions. As decomposers, soil microbial communities mediate critical ecosystem processes that ultimately affect the success of all trophic levels, and the activity of these microbial communities is primarily regulated by moisture availability. This research is focused on elucidating soil microbial responses to seasonal and yearly changes in soil moisture, temperature, and selected soil nutrient and edaphic properties in a Sotol Grassland in the Chihuahuan Desert at Big Bend National Park. Soil samples were collected over a 3-year period in March and September (2004-2006) at 0-15 cm soil depth from 12 3 x 3 m community plots. Bacterial and fungal carbon usage (quantified using Biolog 96-well micro-plates) was related to soil moisture patterns (ranging between 3.0 and 14%). In addition to soil moisture, the seasonal and yearly variability of soil bacterial activity was most closely associated with levels of soil organic matter, extractable NH(4)-N, and soil pH. Variability in fungal activity was related to soil temperatures ranging between 13 and 26 degrees C. These findings indicate that changes in soil moisture, coupled with soil temperatures and resource availability, drive the functioning of soil-microbial dynamics in these desert grasslands. Temporal patterns in microbial activity may reflect the differences in the ability of bacteria and fungi to respond to seasonal patterns of moisture and temperature. Bacteria were more able to respond to moisture pulses regardless of temperature, while fungi only responded to moisture pulses during cooler seasons with the exception of substantial increased magnitudes in precipitation occurring during warmer months. Changes in the timing and magnitude of precipitation will alter the proportional contribution of bacteria and fungi to decomposition and nitrogen mineralization in this desert grassland.     

Temporal variation overshadows the response of leaf litter microbial communities to simulated global change

Bacteria and fungi drive the decomposition of dead plant biomass (litter), an important step in the terrestrial carbon cycle. Here we investigate the sensitivity of litter microbial communities to simulated global change (drought and nitrogen addition) in a California annual grassland. Using 16S and 28S rDNA amplicon pyrosequencing, we quantify the response of the bacterial and fungal communities to the treatments and compare these results to background, temporal (seasonal and interannual) variability of the communities. We found that the drought and nitrogen treatments both had significant effects on microbial community composition, explaining 2–6% of total compositional variation. However, microbial composition was even more strongly influenced by seasonal and annual variation (explaining 14–39%). The response of microbial composition to drought varied by season, while the effect of the nitrogen addition treatment was constant through time. These compositional responses were similar in magnitude to those seen in microbial enzyme activities and the surrounding plant community, but did not correspond to a consistent effect on leaf litter decomposition rate. Overall, these patterns indicate that, in this ecosystem, temporal variability in the composition of leaf litter microorganisms largely surpasses that expected in a short-term global change experiment. Thus, as for plant communities, future microbial communities will likely be determined by the interplay between rapid, local background variability and slower, global changes.     

Soil Microbial Community Response to Drought and Precipitation Variability in the Chihuahuan Desert

Increases in the magnitude and variability of precipitation events have been predicted for the Chihuahuan Desert region of West Texas. As patterns of moisture inputs and amounts change, soil microbial communities will respond to these alterations in soil moisture windows. In this study, we examined the soil microbial community structure within three vegetation zones along the Pine Canyon Watershed, an elevation and vegetation gradient in Big Bend National Park, Chihuahuan Desert. Soil samples at each site were obtained in mid-winter (January) and in mid-summer (August) for 2 years to capture a component of the variability in soil temperature and moisture that can occur seasonally and between years along this watershed. Precipitation patterns and amounts differed substantially between years with a drought characterizing most of the second year. Soils were collected during the drought period and following a large rainfall event and compared to soil samples collected during a relatively average season. Structural changes within microbial community in response to site, season, and precipitation patterns were evaluated using fatty acid methyl ester (FAME) and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analyses. Fungal FAME amounts differed significantly across seasons and sites and greatly outweighed the quantity of bacterial and actinomycete FAME levels for all sites and seasons. The highest fungal FAME levels were obtained in the low desert scrub site and not from the high elevation oak–pine forests. Total bacterial and actinomycete FAME levels did not differ significantly across season and year within any of the three locations along the watershed. Total bacterial and actinomycete FAME levels in the low elevation desert-shrub and grassland sites were slightly higher in the winter than in the summer. Microbial community structure at the high elevation oak–pine forest site was strongly correlated with levels of NH₄ ⁺–N, % soil moisture, and amounts of soil organic matter irrespective of season. Microbial community structure at the low elevation desert scrub and sotol grasslands sites was most strongly related to soil pH with bacterial and actinobacterial FAME levels accounting for site differences along the gradient. DGGE band counts of amplified soil bacterial DNA were found to differ significantly across sites and season with the highest band counts found in the mid-elevation grassland site. The least number of bands was observed in the high elevation oak–pine forest following the large summer-rain event that occurred after a prolonged drought. Microbial responses to changes in precipitation frequency and amount due to climate change will differ among vegetation zones along this Chihuahuan Desert watershed gradient. Soil bacterial communities at the mid-elevation grasslands site are the most vulnerable to changes in precipitation frequency and timing, while fungal community structure is most vulnerable in the low desert scrub site. The differential susceptibility of the microbial communities to changes in precipitation amounts along the elevation gradient reflects the interactive effects of the soil moisture window duration following a precipitation event and differences in soil heat loads. Amounts and types of carbon inputs may not be as important in regulating microbial structure among vegetation zones within in an arid environment as is the seasonal pattern of soil moisture and the soil heat load profile that characterizes the location.     

Soil microbial and nutrient responses to 7 years of seasonally altered precipitation in a Chihuahuan Desert grassland

Soil microbial communities in Chihuahuan Desert grasslands generally experience highly variable spatiotemporal rainfall patterns. Changes in precipitation regimes can affect belowground ecosystem processes such as decomposition and nutrient cycling by altering soil microbial community structure and function. The objective of this study was to determine if increased seasonal precipitation frequency and magnitude over a 7‐year period would generate a persistent shift in microbial community characteristics and soil nutrient availability. We supplemented natural rainfall with large events (one/winter and three/summer) to simulate increased precipitation based on climate model predictions for this region. We observed a 2‐year delay in microbial responses to supplemental precipitation treatments. In years 3–5, higher microbial biomass, arbuscular mycorrhizae abundance, and soil enzyme C and P acquisition activities were observed in the supplemental water plots even during extended drought periods. In years 5–7, available soil P was consistently lower in the watered plots compared to control plots. Shifts in soil P corresponded to higher fungal abundances, microbial C utilization activity, and soil pH. This study demonstrated that 25% shifts in seasonal rainfall can significantly influence soil microbial and nutrient properties, which in turn may have long‐term effects on nutrient cycling and plant P uptake in this desert grassland.     

Linking Microbial Community Structure and Function to Seasonal Differences in Soil Moisture and Temperature in a Chihuahuan Desert Grassland

Global and regional climate models predict higher air temperature and less frequent, but larger precipitation events in arid regions within the next century. While many studies have addressed the impact of variable climate in arid ecosystems on plant growth and physiological responses, fewer studies have addressed soil microbial community responses to seasonal shifts in precipitation and temperature in arid ecosystems. This study examined the impact of a wet (2004), average (2005), and dry (2006) year on subsequent responses of soil microbial community structure, function, and linkages, as well as soil edaphic and nutrient characteristics in a mid-elevation desert grassland in the Chihuahuan Desert. Microbial community structure was classified as bacterial (Gram-negative, Gram-positive, and actinomycetes) and fungal (saprophytic fungi and arbuscular mycorrhiza) categories using (fatty acid methyl ester) techniques. Carbon substrate use and enzymic activity was used to characterize microbial community function annually and seasonally (summer and winter). The relationship between saprophytic fungal community structure and function remained consistent across season independent of the magnitude or frequency of precipitation within any given year. Carbon utilization by fungi in the cooler winter exceeded use in the warmer summer each year suggesting that soil temperature, rather than soil moisture, strongly influenced fungal carbon use and structure and function dynamics. The structure/function relationship for AM fungi and soil bacteria notably changed across season. Moreover, the abundance of Gram-positive bacteria was lower in the winter compared to Gram-negative bacteria. Bacterial carbon use, however, was highest in the summer and lower during the winter. Enzyme activities did not respond to either annual or seasonal differences in the magnitude or timing of precipitation. Specific structural components of the soil microbiota community became uncoupled from total microbial function during different seasons. This change in the microbial structure/function relationship suggests that different components of the soil microbial community may provide similar ecosystem function, but differ in response to seasonal temperature and precipitation. As soil microbes encounter increased soil temperatures and altered precipitation amounts and timing that are predicted for this region, the ability of the soil microbial community to maintain functional resilience across the year may be reduced in this Chihuahuan Desert ecosystem.     

Soil extracellular enzyme activities correspond with abiotic factors more than fungal community composition

Soil extracellular enzymes are the proximal drivers of decomposition. However, the relative influence of climate, soil nutrients and edaphic factors compared to microbial community composition on extracellular enzyme activities (EEA) is poorly resolved. Determining the relative effects of these factors on soil EEA is critical since changes in climate and microbial species composition may have large impacts on decomposition. We measured EEA from five sites during the growing season in March and 17 sites during the dry season in July throughout southern California and simultaneously collected data on climate, soil nutrients, soil edaphic factors and fungal community composition. The concentration of carbon and nitrogen in the soil and soil pH were most related to hydrolytic EEA. Conversely, oxidative EEA was mostly related to mean annual precipitation. Fungal community composition was not correlated with EEA at the species, genus, family or order levels. The hyphal length of fungi was correlated with EEA during the growing season while relative abundance of taxa within fungal phyla, in particular Chytridiomycota, was correlated with the EEA of beta-glucosidase, cellobiohydrolase, acid phosphatase and beta-xylosidase in the dry season. Overall, in the dry season, 35.3 % of the variation in all enzyme activities was accounted for by abiotic variables, while fungal composition accounted for 27.4 %. Because global change is expected to alter precipitation regimes and increase nitrogen deposition in soils, EEA may be affected, with consequences for decomposition.     

Drought‐resistant fungi control soil organic matter decomposition and its response to temperature

Microbial‐mediated decomposition of soil organic matter (SOM) ultimately makes a considerable contribution to soil respiration, which is typically the main source of CO₂ arising from terrestrial ecosystems. Despite this central role in the decomposition of SOM, few studies have been conducted on how climate change may affect the soil microbial community and, furthermore, on how possible climate‐change induced alterations in the ecology of microbial communities may affect soil CO₂ emissions. Here we present the results of a seasonal study on soil microbial community structure, SOM decomposition and its temperature sensitivity in two representative Mediterranean ecosystems where precipitation/throughfall exclusion has taken place during the last 10 years. Bacterial and fungal diversity was estimated using the terminal restriction fragment length polymorphism technique. Our results show that fungal diversity was less sensitive to seasonal changes in moisture, temperature and plant activity than bacterial diversity. On the other hand, fungal communities showed the ability to dynamically adapt throughout the seasons. Fungi also coped better with the 10 years of precipitation/throughfall exclusion compared with bacteria. The high resistance of fungal diversity to changes with respect to bacteria may open the controversy as to whether future ‘drier conditions’ for Mediterranean regions might favor fungal dominated microbial communities. Finally, our results indicate that the fungal community exerted a strong influence over the temporal and spatial variability of SOM decomposition and its sensitivity to temperature. The results, therefore, highlight the important role of fungi in the decomposition of terrestrial SOM, especially under the harsh environmental conditions of Mediterranean ecosystems, for which models predict even drier conditions in the future.     

Links between soil microbial communities and plant traits in a species‐rich grassland under long‐term climate change

Climate change can influence soil microorganisms directly by altering their growth and activity but also indirectly via effects on the vegetation, which modifies the availability of resources. Direct impacts of climate change on soil microorganisms can occur rapidly, whereas indirect effects mediated by shifts in plant community composition are not immediately apparent and likely to increase over time. We used molecular fingerprinting of bacterial and fungal communities in the soil to investigate the effects of 17 years of temperature and rainfall manipulations in a species‐rich grassland near Buxton, UK. We compared shifts in microbial community structure to changes in plant species composition and key plant traits across 78 microsites within plots subjected to winter heating, rainfall supplementation, or summer drought. We observed marked shifts in soil fungal and bacterial community structure in response to chronic summer drought. Importantly, although dominant microbial taxa were largely unaffected by drought, there were substantial changes in the abundances of subordinate fungal and bacterial taxa. In contrast to short‐term studies that report high resistance of soil fungi to drought, we observed substantial losses of fungal taxa in the summer drought treatments. There was moderate concordance between soil microbial communities and plant species composition within microsites. Vector fitting of community‐weighted mean plant traits to ordinations of soil bacterial and fungal communities showed that shifts in soil microbial community structure were related to plant traits representing the quality of resources available to soil microorganisms: the construction cost of leaf material, foliar carbon‐to‐nitrogen ratios, and leaf dry matter content. Thus, our study provides evidence that climate change could affect soil microbial communities indirectly via changes in plant inputs and highlights the importance of considering long‐term climate change effects, especially in nutrient‐poor systems with slow‐growing vegetation.     

Key Edaphic Properties Largely Explain Temporal and Geographic Variation in Soil Microbial Communities across Four Biomes

Soil microbial communities play a critical role in nutrient transformation and storage in all ecosystems. Quantifying the seasonal and long-term temporal extent of genetic and functional variation of soil microorganisms in response to biotic and abiotic changes within and across ecosystems will inform our understanding of the effect of climate change on these processes. We examined spatial and seasonal variation in microbial communities based on 16S rRNA gene sequencing and phospholipid fatty acid (PLFA) composition across four biomes: a tropical broadleaf forest (Hawaii), taiga (Alaska), semiarid grassland-shrubland (Utah), and a subtropical coniferous forest (Florida). In this study, we used a team-based instructional approach leveraging the iPlant Collaborative to examine publicly available National Ecological Observatory Network (NEON) 16S gene and PLFA measurements that quantify microbial diversity, composition, and growth. Both profiling techniques revealed that microbial communities grouped strongly by ecosystem and were predominately influenced by three edaphic factors: pH, soil water content, and cation exchange capacity. Temporal variability of microbial communities differed by profiling technique; 16S-based community measurements showed significant temporal variability only in the subtropical coniferous forest communities, specifically through changes within subgroups of Acidobacteria. Conversely, PLFA-based community measurements showed seasonal shifts in taiga and tropical broadleaf forest systems. These differences may be due to the premise that 16S-based measurements are predominantly influenced by large shifts in the abiotic soil environment, while PLFA-based analyses reflect the metabolically active fraction of the microbial community, which is more sensitive to local disturbances and biotic interactions. To address the technical issue of the response of soil microbial communities to sample storage temperature, we compared 16S-based community structure in soils stored at -80°C and -20°C and found no significant differences in community composition based on storage temperature. Free, open access datasets and data sharing platforms are powerful tools for integrating research and teaching in undergraduate and graduate student classrooms. They are a valuable resource for fostering interdisciplinary collaborations, testing ecological theory, model development and validation, and generating novel hypotheses. Training in data analysis and interpretation of large datasets in university classrooms through project-based learning improves the learning experience for students and enables their use of these significant resources throughout their careers.     

Soil Bacterial Community Structure Responses to Precipitation Reduction and Forest Management in Forest Ecosystems across Germany

Soil microbial communities play an important role in forest ecosystem functioning, but how climate change will affect the community composition and consequently bacterial functions is poorly understood. We assessed the effects of reduced precipitation with the aim of simulating realistic future drought conditions for one growing season on the bacterial community and its relation to soil properties and forest management. We manipulated precipitation in beech and conifer forest plots managed at different levels of intensity in three different regions across Germany. The precipitation reduction decreased soil water content across the growing season by between 2 to 8% depending on plot and region. T-RFLP analysis and pyrosequencing of the 16S rRNA gene were used to study the total soil bacterial community and its active members after six months of precipitation reduction. The effect of reduced precipitation on the total bacterial community structure was negligible while significant effects could be observed for the active bacteria. However, the effect was secondary to the stronger influence of specific soil characteristics across the three regions and management selection of overstorey tree species and their respective understorey vegetation. The impact of reduced precipitation differed between the studied plots; however, we could not determine the particular parameters being able to modify the response of the active bacterial community among plots. We conclude that the moderate drought induced by the precipitation manipulation treatment started to affect the active but not the total bacterial community, which points to an adequate resistance of the soil microbial system over one growing season.     

Shifts in microbial biomass and the bacteria: fungi ratio occur under field conditions within 3 h after rainfall

Increases in the frequency of soil drying and extreme precipitation projected by climate models may have important consequences for soil microbial community composition. However, the microbial response may occur over short time scales not captured by traditional sampling methods. Following a 2-year rainfall exclusion experiment in a pine forest ecosystem, we used phospholipid fatty acid profiling to measure the hourly, daily, and weekly-scale response of soil microbial biomass and the bacteria/fungi ratio to a precipitation event. We compared this response to the rewetting of un-manipulated plots. Within 3 h of watering, we detected increases in fungal and bacterial biomass of 125% and 66%, respectively, in un-manipulated plots, but only small increases in biomass within drought plots. We detected a decrease in the bacteria/fungi ratio in un-manipulated plots and an increase in this ratio in the drought plots. This surprising result was likely caused by root mortality (resulting from the previous 2-year rain exclusion) and an increase in ammonium pools in the drought plots, both of which could have suppressed fungal growth. Whereas past research suggests that soil microbes are resistant to drying–rewetting stress and to changes in annual precipitation patterns, here we show that microbes are sensitive to soil drying, but highly resilient, recovering within hours or days of a rain event. We propose that more emphasis be placed on hourly-scale field measurements of soil microbial community structure in future climate change studies.     

Soil Microbial Community Responses to Multiple Experimental Climate Change Drivers

Researchers agree that climate change factors such as rising atmospheric [CO₂] and warming will likely interact to modify ecosystem properties and processes. However, the response of the microbial communities that regulate ecosystem processes is less predictable. We measured the direct and interactive effects of climatic change on soil fungal and bacterial communities (abundance and composition) in a multifactor climate change experiment that exposed a constructed old-field ecosystem to different atmospheric CO₂ concentration (ambient, +300 ppm), temperature (ambient, +3°C), and precipitation (wet and dry) might interact to alter soil bacterial and fungal abundance and community structure in an old-field ecosystem. We found that (i) fungal abundance increased in warmed treatments; (ii) bacterial abundance increased in warmed plots with elevated atmospheric [CO₂] but decreased in warmed plots under ambient atmospheric [CO₂]; (iii) the phylogenetic distribution of bacterial and fungal clones and their relative abundance varied among treatments, as indicated by changes in 16S rRNA and 28S rRNA genes; (iv) changes in precipitation altered the relative abundance of Proteobacteria and Acidobacteria, where Acidobacteria decreased with a concomitant increase in the Proteobacteria in wet relative to dry treatments; and (v) changes in precipitation altered fungal community composition, primarily through lineage specific changes within a recently discovered group known as soil clone group I. Taken together, our results indicate that climate change drivers and their interactions may cause changes in bacterial and fungal overall abundance; however, changes in precipitation tended to have a much greater effect on the community composition. These results illustrate the potential for complex community changes in terrestrial ecosystems under climate change scenarios that alter multiple factors simultaneously.Soil microbial communities are responsible for the cycling of carbon (C) and nutrients in ecosystems, and their activities are regulated by biotic and abiotic factors such as the quantity and quality of litter inputs, temperature, and moisture. Atmospheric and climatic changes will impact both abiotic and biotic drivers in ecosystems and the response of ecosystems to these changes. Feedbacks from ecosystem to the atmosphere may also be regulated by soil microbial communities (3). Although microbial communities regulate important ecosystem processes, it is often unclear how the abundance and composition of microbial communities correlate with climatic perturbations and interact to effect ecosystem processes. As such, much of the ecosystem climate change research conducted to date has focused on macroscale responses to climatic change such as changes in plant growth (43, 44), plant community composition (2, 37), and coarse scale soil processes (14, 18, 21, 26), many of which may also indirectly interact to effect microbial processes. Studies that have addressed the role of microbial communities and processes have most often targeted gross parameters, such as microbial biomass, enzymatic activity, or basic microbial community profiles in response to single climate change factors (22, 28, 29, 33, 61, 63).Climate change factors such as atmospheric CO₂ concentrations, warming, and altered precipitation regimes can potentially have both direct and indirect impacts on soil microbial communities. However, the direction and magnitude of these responses is uncertain. For example, the response of soil microbial communities to changes in atmospheric CO₂ concentrations can be positive or negative, and consistent overall trends between sites and studies have not been observed (1, 28, 34-36). Further, depending on what limits ecosystem productivity, precipitation and soil moisture changes may increase or decrease the ratio of bacteria and fungi, as well as shift their community composition (8, 50, 58). Increasing temperatures can increase in microbial activity, processing, and turnover, causing the microbial community to shift in favor of representatives adapted to higher temperatures and faster growth rates (7, 46, 60, 64, 65). Atmospheric and climatic changes are happening in concert with one another so that ecosystems are experiencing higher levels of atmospheric CO₂, warming, and changes in precipitation regimes simultaneously. Although the many single factor climate change studies described above have enabled a better understanding of how microbial communities may respond to any one factor, understanding how multiple climate change factors interact with each other to influence microbial community responses is poorly understood. For example, elevated atmospheric [CO₂] and precipitation changes might increase soil moisture in an ecosystem, but this increase may be counteracted by warming (10). Similarly, warming may increase microbial activity in an ecosystem, but this increase may be eliminated if changes in precipitation lead to a drier soil condition or reduced litter quantity, quality, and turnover. Such interactive effects of climate factors in a multifactorial context have been less commonly studied even in plant communities (45), and detailed studies are rarer still in soil microbial communities (25). Clearly, understanding how microbial communities will respond to these atmospheric and climate change drivers is important to make accurate predications of how ecosystems may respond to future climate scenarios.To address how multiple climate change drivers will interact to shape soil microbial communities, we took advantage of a multifactor climatic change experiment that manipulated atmospheric CO₂ (+300 ppm, ambient), warming (+3°C, ambient) and precipitation (wet and dry) in a constructed old-field ecosystem that had been ongoing for 3.5 years at the time of sampling. Previous work on this project has demonstrated direct and interactive effects of the treatments on plant community composition and biomass (15, 30), soil respiration (56), microbial activity (30), nitrogen fixation (21), and soil carbon stocks (20). These results led us to investigations of how the soil bacterial and fungal communities, important regulators of some of these processes, were responding using culture-independent molecular approaches. Our research addresses two overarching questions. (i) Do climatic change factors and their interactions alter bacterial and fungal abundance and diversity? (ii) Do climatic change factors and their interactions alter bacterial or fungal community composition?     

Moderate Warming in Microcosm Experiment Does Not Affect Microbial Communities in Temperate Vineyard Soils

Changes in the soil microbial community structure can lead to dramatic changes in the soil ecosystem. Temperature, which is projected to increase with climate change, is commonly assumed to affect microbial communities, but its effects on agricultural soils are not fully understood. We collected soil samples from six vineyards characterised by a difference of about 2 °C in daily soil temperature over the year and simulated in a microcosm experiment different temperature regimes over a period of 1 year: seasonal fluctuations in soil temperature based on the average daily soil temperature measured in the field; soil temperature warming (2 °C above the normal seasonal temperatures); and constant temperatures normally registered in these temperate soils in winter (3 °C) and in summer (20 °C). Changes in the soil bacterial and fungal community structures were analysed by automated ribosomal intergenic spacer analysis (ARISA). We did not find any effect of warming on soil bacterial and fungal communities, while stable temperatures affected the fungal more than the bacterial communities, although this effect was soil dependent. The soil bacterial community exhibited soil-dependent seasonal fluctuations, while the fungal community was mainly stable. Each soil harbours different microbial communities that respond differently to seasonal temperature fluctuations; therefore, any generalization regarding the effect of climate change on soil communities should be made carefully.     

土壤微生物群落对枯落物输入的响应

枯落物分解在陆地生态系统物质循环能量流动中起着关键性作用,明确枯落物输入对土壤微生物群落的影响有助于理解土壤微生物生物多样性和陆地生态系统功能的相互关系。本文采用整合分析方法,以中国为研究区域,以不添加枯落物为对照组,探究土壤微生物(真菌、细菌、放线菌)及微生物生物量碳、生物量氮对枯落物输入的响应。结果表明:与不添加枯落物相比,添加枯落物后土壤微生物生物量碳、生物量氮分别显著增加3.9%和4.4%;土壤真菌PLFA、细菌PLFA及总微生物PLFA分别增加4.0%、3.1%和2.4%。枯落物输入对土壤微生物的影响受到气候条件、年降水量、植被类型及土壤酸碱度等因素的显著影响;不同气候类型下,土壤微生物对枯落物输入的响应呈现出亚热带季风气候区>温带季风气候区>温带大陆气候区的趋势,以及随着年降水量的增加呈现出先升高后降低的趋势;不同植被类型下,土壤微生物对枯落物输入的响应呈现出阔叶林>草地≈混交林>针叶林的趋势。     

Enhanced summer warming reduces fungal decomposer diversity and litter mass loss more strongly in dry than in wet tundra

Many Arctic regions are currently experiencing substantial summer and winter climate changes. Litter decomposition is a fundamental component of ecosystem carbon and nutrient cycles, with fungi being among the primary decomposers. To assess the impacts of seasonal climatic changes on litter fungal communities and their functioning, Betula glandulosa leaf litter was surface‐incubated in two adjacent low Arctic sites with contrasting soil moisture regimes: dry shrub heath and wet sedge tundra at Disko Island, Greenland. At both sites, we investigated the impacts of factorial combinations of enhanced summer warming (using open‐top chambers; OTCs) and deepened snow (using snow fences) on surface litter mass loss, chemistry and fungal decomposer communities after approximately 1 year. Enhanced summer warming significantly restricted litter mass loss by 32% in the dry and 17% in the wet site. Litter moisture content was significantly reduced by summer warming in the dry, but not in the wet site. Likewise, fungal total abundance and diversity were reduced by OTC warming at the dry site, while comparatively modest warming effects were observed in the wet site. These results suggest that increased evapotranspiration in the OTC plots lowered litter moisture content to the point where fungal decomposition activities became inhibited. In contrast, snow addition enhanced fungal abundance in both sites but did not significantly affect litter mass loss rates. Across sites, control plots only shared 15% of their fungal phylotypes, suggesting strong local controls on fungal decomposer community composition. Nevertheless, fungal community functioning (litter decomposition) was negatively affected by warming in both sites. We conclude that although buried soil organic matter decomposition is widely expected to increase with future summer warming, surface litter decay and nutrient turnover rates in both xeric and relatively moist tundra are likely to be significantly restricted by the evaporative drying associated with warmer air temperatures.     

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