Dispersal alters soil microbial community response to drought

Microbial communities will experience novel climates in the future. Dispersal is now recognized as a driver of microbial diversity and function, but our understanding of how dispersal influences responses to novel climates is limited. We experimentally tested how the exclusion of aerially dispersed fungi and bacteria altered the compositional and functional response of soil microbial communities to drought. We manipulated dispersal and drought by collecting aerially deposited microbes after precipitation events and subjecting soil mesocosms to either filter-sterilized rain (no dispersal) or unfiltered rain (dispersal) and to either drought (25% ambient) or ambient rainfall for 6 months. We characterized community composition by sequencing 16S and ITS rRNA regions and function using community-level physiological profiles. Treatments without dispersal had lower soil microbial biomass and metabolic diversity but higher bacterial and fungal species richness. Dispersal also altered soil community response to drought; drought had a stronger effect on bacterial (but not fungal) community composition, and induced greater functional loss, when dispersal was present. Surprisingly, neither immigrants nor drought-tolerant taxa had higher abundance in dispersal treatments. We show experimentally that natural aerial dispersal rate alters soil microbial responses to disturbance. Changes in dispersal rates should be considered when predicting microbial responses to climate change.     

Recent research progress on soil microbial responses to drying–rewetting cycles

Climatic change, such as increases in extreme drought and rainfall events and changes in rainfall intensity and pattern, has been strongly influencing soil moisture. The climatic change impact is particularly common in arid, semi-arid and Mediterranean regions, which is causing dramatic changes in the intensity and frequency of soil drying–rewetting cycles. The soil drying–rewetting cycle is a natural phenomenon that the soil experiences drying, then wetting, and then drying and rewetting again and again. When a dry soil is being rewetted, the amount of soil microbial biomass and its activity can be sharply increasing in a short time period, and then a large amount of gaseous carbon (C) and nitrogen (N) erupts from the soil. The sudden release of gaseous C and N is caused by the stimulation of the soil microbes. Such a phenomenon is called “Birch effect”. The drying–rewetting cycles have direct and indirect effects on soil microbes, and soil microbial responses to the drying and rewetting events play an important role in the feedbacks of terrestrial ecosystems. From aspects of soil microbial biomass, microbial activities and microbial structure, we review recent advances on studies regarding microbial responses to soil drying–rewetting cycles. We interpret the microbial responses using five different types of mechanisms: (1) Microbial stress mechanism: when a soil becomes dry, microorganisms must accumulate compatible solutes such as carbohydrates and aminoacids so that the soil microbes can equilibrate with their environment in order to avoid dehydrating and being killed. When the soil is rewetted, soil microbes must dispose of those osmolytes rapidly by transforming them into carbon dioxide (CO₂), dissolved organic carbon (DOC) and nutrients in order to prevent water from being flowing into the cells. (2) Substrate supply mechanism: low soil moisture may result in the physical disruption of soil aggregates which leads to the exposure of new soil surfaces and of previously protected organic matter. When the soil is rewetted, its physical structure is further disrupted by swelling. The increased new soil surfaces and previously protected organic matter will improve the microorganism’s nutrient availability. (3) Soil hydrophobicity mechanism: soil hydrophobicity can cause the reduction of soil moisture and nutrient availability and inhibition of microbial decomposition of soil organic matter. Therefore, soil hydrophobicity is an important factor of explaining the activity of microorganism in drying and rewetting events. (4) Diffusive limitations mechanism: transportation of the soil microbe is limited in a dry soil. When soil moisture is increasing, soil microbial activity is enhanced along with the increased availability of substrate nutrients. (5) Predation mechanism: a moist soil is usually conducive to the increase of bacteria and fungi populations. In response, protozoa and nematodes also increase, leading to the fluctuation of the soil microbial community structure. On the basis of the literature review, we propose five important aspects to be considered in the future: (1) assessing soil microbes’ concrete adapting ways to the drying–rewetting cycles, (2) evaluating the microbial responses to the drying–rewetting cycles based on suitable indicators, (3) interpreting microbial responses to the drying–rewetting cycles by combining field investigation and laboratory controlling experiment, (4) investigating the microbial responses to the drying–rewetting cycles at different temporal and spatial scales.     

Drying–Rewetting Cycles Affect Fungal and Bacterial Growth Differently in an Arable Soil

Drying and rewetting is a frequent physiological stress for soil microbial communities; a stress that is predicted to grow more influential with future climate change. We investigated the effect of repeated drying–rewetting cycles on bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, on the biomass concentration and composition (PLFA), and on the soil respiration. Using different plant material amendments, we generated soils with different initial fungal:bacterial compositions that we exposed to 6–10 repetitions of a drying–rewetting cycle. Drying–rewetting decreased bacterial growth while fungal growth remained unaffected, resulting in an elevated fungal:bacterial growth ratio. This effect was found irrespective of the initial fungal:bacterial biomass ratio. Many drying–rewetting cycles did not, however, affect the fungal:bacterial growth ratio compared to few cycles. The biomass response of the microbial community differed from the growth response, with fungal and total biomass only being slightly negatively affected by the repeated drying–rewetting. The discrepancy between growth- and biomass-based assessments underscores that microbial responses to perturbations might previously have been misrepresented with biomass-based assessments. In light of this, many aspects of environmental microbial ecology may need to be revisited with attention to what measure of the microbial community is relevant to study.     

Response of the Soil Microbial Community to Changes in Precipitation in a Semiarid Ecosystem

Microbial communities regulate many belowground carbon cycling processes; thus, the impact of climate change on the structure and function of soil microbial communities could, in turn, impact the release or storage of carbon in soils. Here we used a large-scale precipitation manipulation (+18%, −50%, or ambient) in a piñon-juniper woodland (Pinus edulis-Juniperus monosperma) to investigate how changes in precipitation amounts altered soil microbial communities as well as what role seasonal variation in rainfall and plant composition played in the microbial community response. Seasonal variability in precipitation had a larger role in determining the composition of soil microbial communities in 2008 than the direct effect of the experimental precipitation treatments. Bacterial and fungal communities in the dry, relatively moisture-limited premonsoon season were compositionally distinct from communities in the monsoon season, when soil moisture levels and periodicity varied more widely across treatments. Fungal abundance in the drought plots during the dry premonsoon season was particularly low and was 4.7 times greater upon soil wet-up in the monsoon season, suggesting that soil fungi were water limited in the driest plots, which may result in a decrease in fungal degradation of carbon substrates. Additionally, we found that both bacterial and fungal communities beneath piñon pine and juniper were distinct, suggesting that microbial functions beneath these trees are different. We conclude that predicting the response of microbial communities to climate change is highly dependent on seasonal dynamics, background climatic variability, and the composition of the associated aboveground community.     

Differential sensitivity of total and active soil microbial communities to drought and forest management

Climate change will affect semiarid ecosystems through severe droughts that increase the competition for resources in plant and microbial communities. In these habitats, adaptations to climate change may consist of thinning—that reduces competition for resources through a decrease in tree density and the promotion of plant survival. We deciphered the functional and phylogenetic responses of the microbial community to 60 years of drought induced by rainfall exclusion and how forest management affects its resistance to drought, in a semiarid forest ecosystem dominated by Pinus halepensis Mill. A multiOMIC approach was applied to reveal novel, community‐based strategies in the face of climate change. The diversity and the composition of the total and active soil microbiome were evaluated by 16S rRNA gene (bacteria) and ITS (fungal) sequencing, and by metaproteomics. The microbial biomass was analyzed by phospholipid fatty acids (PLFAs), and the microbially mediated ecosystem multifunctionality was studied by the integration of soil enzyme activities related to the cycles of C, N, and P. The microbial biomass and ecosystem multifunctionality decreased in drought‐plots, as a consequence of the lower soil moisture and poorer plant development, but this decrease was more notable in unthinned plots. The structure and diversity of the total bacterial community was unaffected by drought at phylum and order level, but did so at genus level, and was influenced by seasonality. However, the total fungal community and the active microbial community were more sensitive to drought and were related to ecosystem multifunctionality. Thinning in plots without drought increased the active diversity while the total diversity was not affected. Thinning promoted the resistance of ecosystem multifunctionality to drought through changes in the active microbial community. The integration of total and active microbiome analyses avoids misinterpretations of the links between the soil microbial community and climate change.     

Soil microbial community response to drying and rewetting stress: does historical precipitation regime matter?

Climate models project that precipitation patterns will likely intensify in the future, resulting in increased duration of droughts and increased frequency of large soil rewetting events, which are stressful to the microorganisms that drive soil biogeochemical cycling. Historical conditions can affect contemporary microbial responses to environmental factors through the persistence of abiotic changes or through the selection of a more tolerant microbial community. We examined how a history of intensified rainfall would alter microbial functional response to drying and rewetting events, whether this historical legacy was mediated through altered microbial community composition, and how long community and functional legacies persisted under similar conditions. We collected soils from a long-term field manipulation (Rainfall Manipulation Plot Study) in Kansas, USA, where rainfall variability was experimentally amplified. We measured respiration, microbial biomass, fungal:bacterial ratios and bacterial community composition after collecting soils from the field experiment, and after subjecting them to a series of drying–rewetting pulses in the lab. Although rainfall history affected respiration and microbial biomass, the differences between field treatments did not persist throughout our 115-day drying–rewetting incubation. However, soils accustomed to more extreme rainfall did change less in response to lab moisture pulses. In contrast, bacterial community composition did not differ between rainfall manipulation treatments, but became more dissimilar in response to drying–rewetting pulses depending on their previous field conditions. Our results suggest that environmental history can affect contemporary rates of biogeochemical processes both through changes in abiotic drivers and through changes in microbial community structure. However, the extremity of the disturbance and the mechanism through which historical legacies occur may influence how long they persist, which determines the importance of these effects for biogeochemical cycling.     

Drought turns a Central European Norway spruce forest soil from an N2O source to a transient N2O sink

Based on current climate scenarios, a higher frequency of summer drought periods followed by heavy rainfall events is predicted for Central Europe. It is expected that drying/rewetting events induce an increased matter cycling in soils and may contribute considerably to increased emissions of the greenhouse gas N₂O on annual scales. To investigate the influence of drying/rewetting events on N₂O emissions in a mature Norway spruce forest in the Fichtelgebirge area (NE Bavaria, Germany), a summer drought period of 46 days was induced by roof installations on triplicate plots, followed by a rewetting event of 66 mm experimental rainfall in 2 days. Three nonmanipulated plots served as controls. The experimentally induced soil drought was accompanied by a natural drought. During the drought period, the soil of both the throughfall exclusion and control plots served as an N₂O sink. This was accompanied by subambient N₂O concentrations in upper soil horizons. The sink strength of the throughfall exclusion plots was doubled compared with the control plots. We conclude that the soil water status together with the soil nitrate availability was an important driving factor for the N₂O sink strength. Rewetting quickly turned the soil into a source for atmospheric N₂O again, but it took almost 4 months to turn the cumulative soil N₂O fluxes from negative (sink) to positive (source) values. N₂O concentration and isotope analyses along soil profiles revealed that N₂O produced in the subsoil was subsequently consumed during upward diffusion along the soil profile throughout the entire experiment. Our results show that long drought periods can lead to drastic decreases of N₂O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N₂O sinks. Accumulation of more field‐scale data on soil N₂O uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global N₂O budget.     

杉木人工林土壤微生物生物量和碳源利用能力对模拟氮沉降和干旱的响应

杉木林是我国亚热带地区最主要的人工林和重要的碳汇。本研究以杉木人工林为对象,通过设置氮素添加(40 kg N·hm^-2·a^-1)和隔离降雨(-50%)试验分别模拟氮沉降和干旱,在夏季(7月)和冬季(1月)采集表层土壤,通过磷脂脂肪酸、平板计数、Biolog等方法综合分析土壤微生物生物量、数量及碳源利用能力。结果表明: 土壤微生物生物量及微生物组成在两季节间存在显著差异;氮添加和隔离降雨可在一定程度上减少可培养细菌和真菌的数量,细菌数量较真菌数量对氮添加和隔离降雨的响应更敏感。隔离降雨显著抑制了土壤中微生物的碳源利用能力,而氮添加无显著影响。土壤细菌数量与微生物碳源利用能力呈显著正相关,表明可培养细菌是介导微生物碳转化的关键组分。本研究强调了氮沉降和干旱对亚热带杉木人工林表层土壤微生物的影响,可为评估未来全球变化情景下亚热带森林生态系统的土壤微生物生态功能提供科学依据。     

Consequences of exclusion of precipitation on microorganisms and microbial consumers in montane tropical rainforests

The structure and functioning of decomposer systems heavily relies on soil moisture. However, this has been primarily studied in temperate ecosystems; little is known about how soil moisture affects the microfaunal food web in tropical regions. This lack of knowledge is surprising, since the microfaunal food web controls major ecosystem processes. To evaluate the role of precipitation in the structure of soil food web components (i.e., microorganisms and testate amoebae), we excluded water input by rain in montane rainforests at different altitudes in Ecuador. Rain exclusion strongly reduced microbial biomass and respiration by about 50?%, and fungal biomass by 23?%. In testate amoebae, rain exclusion decreased the density of live cells by 91?% and caused a shift in species composition at each of the altitudes studied, with ergosterol concentrations, microbial biomass, and water content explaining 25?% of the variation in species data. The results document that reduced precipitation negatively affects soil microorganisms, but that the response of testate amoebae markedly exceeds that of bacteria and fungi. This suggests that, in addition to food, low precipitation directly affects the community structure of testate amoebae, with the effect being more pronounced at lower altitudes. Overall, the results show that microorganisms and testate amoebae rapidly respond to a reduction in precipitation, with testate amoebae—representatives of higher trophic levels—being more sensitive. The results imply that precipitation and soil moisture in tropical rainforests are the main factors regulating decomposition and nutrient turnover.     

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.     

Disentangling the Litter Quality and Soil Microbial Contribution to Leaf and Fine Root Litter Decomposition Responses to Reduced Rainfall

Climate change-induced rainfall reductions in Mediterranean forests negatively affect the decomposition of plant litter through decreased soil moisture. However, the indirect effects of reduced precipitation on litter decomposition through changes in litter quality and soil microbial communities are poorly studied. This is especially the case for fine root litter, which contributes importantly to forests plant biomass. Here we analyzed the effects of long-term (11 years) rainfall exclusion (29% reduction) on leaf and fine root litter quality, soil microbial biomass, and microbial community-level physiological profiles in a Mediterranean holm oak forest. Additionally, we reciprocally transplanted soils and litter among the control and reduced rainfall treatments in the laboratory, and analyzed litter decomposition and its responses to a simulated extreme drought event. The decreased soil microbial biomass and altered physiological profiles with reduced rainfall promoted lower fine root—but not leaf—litter decomposition. Both leaf and root litter, from the reduced rainfall treatment, decomposed faster than those from the control treatment. The impact of the extreme drought event on fine root litter decomposition was higher in soils from the control treatment compared to soils subjected to long-term rainfall exclusion. Our results suggest contrasting mechanisms driving drought indirect effects on above-(for example, changes in litter quality) and belowground (for example, shifts in soil microbial community) litter decomposition, even within a single tree species. Quantifying the contribution of these mechanisms relative to the direct soil moisture-effect is critical for an accurate integration of litter decomposition into ecosystem carbon dynamics in Mediterranean forests under climate change.     

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.     

Soil microbial moisture dependences and responses to drying–rewetting: The legacy of 18 years drought

Climate change will alter precipitation patterns with consequences for soil C cycling. An understanding of how fluctuating soil moisture affects microbial processes is therefore critical to predict responses to future global change. We investigated how long‐term experimental field drought influences microbial tolerance to lower moisture levels (“resistance”) and ability to recover when rewetted after drought (“resilience”), using soils from a heathland which had been subjected to experimental precipitation reduction during the summer for 18 years. We tested whether drought could induce increased resistance, resilience, and changes in the balance between respiration and bacterial growth during perturbation events, by following a two‐tiered approach. We first evaluated the effects of the long‐term summer drought on microbial community functioning to drought and drying–rewetting (D/RW), and second tested the ability to alter resistance and resilience through additional perturbation cycles. A history of summer drought in the field selected for increased resilience but not resistance, suggesting that rewetting after drought, rather than low moisture levels during drought, was the selective pressure shaping the microbial community functions. Laboratory D/RW cycles also selected for communities with a higher resilience rather than increased resistance. The ratio of respiration to bacterial growth during D/RW perturbation was lower for the field drought‐exposed communities and decreased for both field treatments during the D/RW cycles. This suggests that cycles of D/RW also structure microbial communities to respond quickly and efficiently to rewetting after drought. Our findings imply that microbial communities can adapt to changing climatic conditions and that this might slow the rate of soil C loss predicted to be induced by future cyclic drought.     

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。     

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 CO<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript> efflux and fungal and bacterial biomass in a plantation and a secondary forest in wet tropics in Puerto Rico

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO₂ efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO₂ efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO₂ efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO₂ efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO₂ efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO₂ efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO₂ efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO₂ efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R² > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests.     

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.     

Microbial Nonlinear Response to a Precipitation Gradient in the Northeastern Tibetan Plateau

The response of soil microbes to global warming, especially their response to precipitation, remains poorly known. The Tibetan Plateau is very sensitive to climate change. In particular, the northeastern margin of the Tibetan Plateau is an interesting area to test the response of soil microbial communities to precipitation, as there is a distinct gradient in annual precipitation from east to west. We collected soil samples along a precipitation gradient in arid and semi-arid areas of the northeastern Tibetan Plateau. Phospholipid fatty acid (PLFA) technology was used to analyze the microbial community structure and total microbial biomass. With declining precipitation, bacterial biomass decreased significantly, whereas fungal biomass did not show an obvious trend; this result indicates that bacteria are more sensitive to mean annual precipitation (MAP). Overall, the biomass of Gram-negative (G^?) bacteria represented up to 82% of the total bacterial biomass. In the high (260–394 mm yr^?1) MAP areas, bacterial biomass was mainly concentrated at the surface and decreased with increasing soil depth (0–40 cm). In contrast, in the low (36–260 mm yr^?1) MAP areas, bacterial biomass was mainly concentrated in the deep soils. The mean annual precipitation was strongly correlated with soil microbial community in space, with microbial communities in the 0–10-cm soil depth most affected by precipitation. Groundwater may impact microbial communities in the 20–40-cm soil depth of this arid and semiarid region. The clustering of the microbial communities was significantly grouped according to the MAP gradient, revealing that MAP is a major driving force of microbial communities in this arid and semi-arid area. The decline in MAP led to a shift in the structure of the microbial community and an overall reduction in microbial biomass.     

Soil microbial responses to experimental warming and clipping in a tallgrass prairie

Global surface temperature is predicted to increase by 1.4–5.8°C by the end of this century. However, the impacts of this projected warming on soil C balance and the C budget of terrestrial ecosystems are not clear. One major source of uncertainty stems from warming effects on soil microbes, which exert a dominant influence on the net C balance of terrestrial ecosystems by controlling organic matter decomposition and plant nutrient availability. We, therefore, conducted an experiment in a tallgrass prairie ecosystem at the Great Plain Apiaries (near Norman, OK) to study soil microbial responses to temperature elevation of about 2°C through artificial heating in clipped and unclipped field plots. While warming did not induce significant changes in net N mineralization, soil microbial biomass and respiration rate, it tended to reduce extractable inorganic N during the second and third warming years, likely through increasing plant uptake. In addition, microbial substrate utilization patterns and the profiles of microbial phospholipid fatty acids (PLFAs) showed that warming caused a shift in the soil microbial community structure in unclipped subplots, leading to the relative dominance of fungi as evidenced by the increased ratio of fungal to bacterial PLFAs. However, no warming effect on soil microbial community structure was found in clipped subplots where a similar scale of temperature increase occurred. Clipping also significantly reduced soil microbial biomass and respiration rate in both warmed and unwarmed plots. These results indicated that warming‐led enhancement of plant growth rather than the temperature increase itself may primarily regulate soil microbial response. Our observations show that warming may increase the relative contribution of fungi to the soil microbial community, suggesting that shifts in the microbial community structure may constitute a major mechanism underlying warming acclimatization of soil respiration.