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The gut microbiota is an emerging frontier in wildlife research and its importance to vertebrate health and physiology is becoming ever more apparent. Reptiles, in particular snakes, have not received the same attention given to other vertebrates and the composition of their wild gut microbiome remains understudied. The primary goal of this work was to describe the cloacal microbiota of two Colubrids, the Eastern Gartersnake (Thamnophis sirtalis sirtalis) and the Northern Watersnake (Nerodia sipedon sipedon), and if their cloacal microbiota differed as well as if it did between a wetland and upland population of the former species. We utilized next-generation sequencing of cloacal swabs—a non-destructive proxy for the gut microbiota. The cloacal microbiome of Eastern Gartersnakes (N = 9) was like those of other snakes being comprised of Proteobacteria, Bacteroidetes, and Firmicutes, while that of Northern Watersnakes (N = 6) was dominated by Tenericutes. Seven microbial operational taxonomic units (OTUs), all members of Proteobacteria, were shared among all individuals and were indicative of a core microbiome in Eastern Gartersnakes, but these OTUs were not particularly relevant to Northern Watersnakes. The latter had greater OTU richness than did Eastern Gartersnakes, and habitat did not have any apparent effect on the microbial community composition in Eastern Gartersnakes. Our findings suggest host taxonomy to be a determining factor in the cloacal microbiota of snakes and that Tenericutes are associated with aquatic habitats. This is the first report to examine the cloacal microbiome of these species and provides a useful foundation for future work to build upon.

Nitrogen (N) from anthropogenic sources has dramatically increased in terrestrial ecosystems globally. Although belowground microbial processes and events that release N into the atmosphere, such as fire, could support ecosystem resilience to eutrophication, little is known about how these factors might affect N loss following chronic fertilization, thus promoting ecosystem recovery. We studied how N pools, N-cycling potential rates and their affiliated microbial populations, and microbial community composition responded to the factorial effects of cessation of 30 years of chronic N fertilization and annual burning at a tallgrass prairie. Soil N availability in previously fertilized plots recovered by 86% (using never-fertilized control plots as a 100% recovery reference), while plant and microbial biomass did not change. Nitrification potential recovered (80%), and denitrification potential partially recovered (50%), in previously fertilized plots. There were differential responses among key groups of nitrifiers (archaea vs. bacteria) and denitrifiers (nosZ clade I vs. clade II from Anaeromyxobacter dehalogenans) to long-term N fertilization and cessation despite the whole microbial community composition not shifting significantly after fertilization ceased. Also, N-cycling potentials were consistently higher in unburned prairie. Together, results suggest that fire is a prominent mechanism for ecosystem N removal in annually burned prairie, while N-cycling microbes will have an important role in the absence of fire; however, the recovery to pre-fertilized condition, with or without fire, will take longer than 1 year. Overall, differential resilience of biotic populations and processes can potentially shape different outcomes of soil N loss and tallgrass prairie ecosystem recovery from long-term N fertilization.

Fire can impact terrestrial ecosystems by changing abiotic and biotic conditions. Short fire intervals maintain grasslands and communities adapted to frequent, low-severity fires. Shrub encroachment that follows longer fire intervals accumulates fuel and can increase fire severity. This patchily distributed biomass creates mosaics of burn severities in the landscape—pyrodiversity. Afforded by a scheduled burn of a watershed protected from fires for 27 years, we investigated effects of woody encroachment and burn severity on soil chemistry and soil-inhabiting bacteria and fungi. We compared soils before and after fire within the fire-protected, shrub-encroached watershed and soils in an adjacent, annually burned and non-encroached watershed. Organic matter and nutrients accumulated in the fire-protected watershed but responded less to woody encroachment within the encroached watershed. Bioavailable nitrogen and phosphorus and fungal and bacterial communities responded to high-severity burn regardless of encroachment. Low-severity fire effects on soil nutrients differed, increased bacterial but decreased fungal diversity and effects of woody encroachment within the encroached watershed were minimal. High-severity burns in the fire-protected watershed led to a novel soil system state distinct from non-encroached and encroached soil systems. We conclude that severe fires may open grassland restoration opportunities to manipulate soil chemistry and microbial communities in shrub-encroached habitats.

In grasslands, fire management and fertilization are established drivers of plant community change, but associated soil fungal responses are less well defined. We predicted that soil fungal communities would change seasonally, that decades of fire cessation and nitrogen (N) fertilization would alter fungal distributions, and that plant and fungal community change would be correlated. Surface soils were sampled monthly for 1 y from a 30-y fire by fertilization experiment to evaluate fungal community dynamics and assess correlation with plant community heterogeneity. ITS gene community composition was seasonally stable, excepting increased arbuscular mycorrhizal fungal summer abundance in the burned, fertilized treatment. Long-term treatments affected soil fungal and plant communities, with correlated heterogeneity patterns. Despite woody encroachment in the fire cessation treatment, soil fungal communities did not resemble those of forests. This study provides evidence supporting the strength of feedbacks between fungal and plant community change in response to long-term grassland fire and N management changes.

A history of drought exposure promoted by variable precipitation regimes can select for drought-tolerant soil microbial taxa, but the mechanisms of survival and death of microbial populations through the selective stresses of soil drying and re-wet are not well understood. We subjected soils collected from a 15-year field drought experiment ("Altered" precipitation history with extended dry periods, versus the "Ambient" field control) to a laboratory drying/re-wetting experiment, to learn whether selective population survival, death, or maintenance of protein synthesis potential and microbial respiration through variable soil water conditions was affected by field drought legacy. Microbial community composition, as measured by Illumina MiSeq sequencing of the 16S rRNA and 16S rRNA gene, shifted with laboratory drying/re-wet and field drought treatments. In Ambient soils, there was a higher proportion of reduced OTU abundance (indicative of mortality) during re-wet, whereas Altered soils had a greater proportion of stable OTU populations that did not change in abundance (indicative of survival) through drying/re-wet. Altered soils also had a lower proportion of rRNA:rRNA genes (lower protein synthesis potential) during dry-down, a greater weighted mean rRNA operon number (potential growth rate and r-selection) which was associated with higher abundance of Firmicutes (order Bacillales), and lower average microbial respiration rates. These data demonstrate that soils with a weaker historical drought legacy exhibit a higher prevalence of microbial water-stress mortality and differential survival and death at OTU levels following short-term dryingand re-wetting, concurrent with higher carbon loss potential. This work provides novel insight into the mechanisms and consequences of soil microbial changes resulting from extended drought conditions.

Colonization of gut microbiomes during early life can shape metabolism and immunity of adult animals. However, most data are derived from antibiotic-treated or germ-free laboratory mammals. Furthermore, few studies have explored how microbial colonization during critical windows influences a suite of other fitness-related traits in wild animals. This study tested whether hatching constitutes a critical developmental window for gut microbiome colonization in wild-caught amphibians and whether perturbations to gut microbiota at hatching shape fitness-related traits of larval growth, metabolism, metamorphosis and disease susceptibility. We sterilized wood frog eggs and then inoculated them with microbes from differing sources, including from another species (bullfrogs) that differ in disease resistance and life history. We measured development, growth and metabolic rates through metamorphosis among individuals from each microbial treatment. A separate group was exposed to an LD50 dose of ranavirus-an emerging disease-to test for microbiome effects on disease susceptibility. We also quantified rates of deformities to test for microbial treatment effects on overall health. Manipulation of microbiota on eggs altered the trajectory of gut microbiome communities across larval ontogeny, though disruption appeared to be transitory. While microbiome structure converged among the treatments by metamorphosis, the effects of disruption on host phenotypes persisted. Larvae inoculated with the bullfrog gut microbiota exhibited accelerated growth and development rates compared to controls. By contrast, sterilized larvae maintained in sterile water for several days after hatching exhibited greater disruption to their gut microbiota across ontogeny, as well as altered metabolism, more tail deformities, and were more likely to die when exposed to an LD50 dose of ranavirus compared to the other treatments. These results suggest perturbations to the microbiota during critical developmental windows can alter the trajectory of the gut microbiome, and have long-term effects on fitness-related traits in larval amphibians. These results suggest that explicit tests of how changes in the composition and abundance of the microbial community shape phenotypes across ontogeny in amphibians could shed light on host-microbe interactions in wildlife, as well as inform conservation efforts to mitigate emerging diseases.

Microbiota in streams drive many ecosystem functions, including whole-stream metabolism, nitrogen (N) cycling, and the production of basal resources that fuel stream food webs. Interactions...

Nitrogen (N) availability is a driver of soil microbial diversity and function, and is affected by prescribed burning (N removal through volatilization) and fertilization (N addition). Because soil microbes control critical feedbacks to ecosystem function, it is important to understand the dynamics and responses of microbial populations under conditions of contrasting N availability. This study took place at a long-term field manipulation in which native tallgrass prairie was annually burned or not burned, and annually fertilized or not fertilized, in a factorial design, since 1986. Composite surface soil samples (0–15 cm) were collected monthly between November 2014 and December 2015 from replicate plots to evaluate event-based (post-fire, post-fertilization), seasonal, and long-term responses of soil microbial communities to management and environmental changes. Bacterial 16S rRNA gene and fungal ITS population sizes were estimated using qPCR, and bacterial community composition (BCC) was measured using Illumina MiSeq sequencing of 16S rRNA genes. We expected seasonal and event-based change in all parameters, and that total microbial population sizes and diversity would be lower in soils with higher N availability, due to greater competitive dominance of nitrophilic or copiotrophic taxa. Bacterial and fungal population sizes varied significantly by sampling month, in that bacterial populations were approximately 10× greater in summer (June–August), but did not change in response to management events or long-term treatments. In contrast, very few individual taxonomic groups displayed seasonal or event-based responses, and there was no significant whole-community turnover on weekly or monthly time-scales; instead, BCC was strongly impacted by both the long-term fire and fertilization treatments. Specifically, there were increases and decreases in putatively "copiotrophic" and "oligotrophic" prokaryotic Phyla in response to long-term N fertilization, which were significantly stronger and more predictable in soils following long-term fire suppression. These results reveal that while long-term grassland management changes BCC beyond the detected range of seasonal variability, total bacterial populations change coherently month-to-month, potentially due to significant plant inputs of labile carbon during the growing season. Furthermore, because prescribed burning reduces soil N availability, the interactive responses to fire suppression plus fertilization suggest that higher background levels of soil N availability may increase the magnitude of soil microbial sensitivity to N fertilization.

Through litter and root inputs, plants modulate soil properties and the soil microbial communities around them. In turn, these changes in soil properties and microbial community composition can impact plant performance (i.e., plant-soil feedbacks). Many studies have focused on how plant-soil feedbacks affect plant performance and successional patterns, but studies on the impact of plant-soil feedbacks on ecosystem processes are rare. Our research focused on the potential of species-specific plant-soil feedbacks to affect rates of soil organic matter (SOM) decomposition. We conducted a “home vs. away” plant-soil feedback greenhouse experiment using two C3 grass species (Bromus inermis and Pascopyrum smithii) grown in C4 tallgrass prairie soil. We used a closed-circuit CO2 trapping method and isotopic analysis to differentiate between root-derived and SOM-derived CO2 production. Contrary to our predictions, plant-soil feedbacks on plant biomass were independent of the effects of plant-soil interactions on SOM-derived CO2 production, but we did detect a significant legacy of conditioning by B. inermis on subsequent total belowground respiration (i.e., total belowground respiration was higher in soils originally conditioned by B. inermis regardless of which plant species was currently growing in those soils). We attribute these results to the differential effects of these plant species on soil chemistry and soil microbes during the original conditioning phase. This is supported by the observation that differences in soil chemistry and bacterial community composition persisted in soils conditioned by different plant species throughout the entire experiment. Together these results suggest that plant-soil history is important for soil respiration and that differences in soil microbial communities induced by conditioning with different plant species may have lasting effects on ecosystem processes.

Exploration of the importance of developmental windows for microbial colonization in diverse animal taxa, and tests of how these shape both animal microbiomes as well as host phenotypes promise to shed needed light on host-microbe interactions. The aims of this study were to explore how gut microbiota diversity of larval amphibians varies among species and across ontogeny, and to test if manipulation of gut colonization can reveal how microbiomes develop. We found that gut microbiomes differ among species and change across larval ontogeny, with distinctive differences between larvae, metamorphic animals, and juvenile frogs. Through applying a gnotobiotic protocol to eggs and cross-inoculating gut microbiomes between species, we demonstrated that microbiota can be transplanted among species and developmental stages. These results also demonstrated that microbial colonization at hatching is potentially formative for long term composition and function of amphibian gut microbiomes, suggesting that hatching may be a critical developmental window for colonization, similar to the effects of birth mode on human microbiomes. Specifically, our results suggest that either the egg jelly and/or capsules surrounding amphibian eggs are likely important sources for initial microbiome inoculation. Furthermore, we speculate these results suggest that vertical transmission may be important to amphibian microbiome establishment and development, as is common among many animal taxa. Taken together, our results suggest that explicit tests of how host developmental windows influence microbial colonization, and shape amphibian microbiomes across life stages promise to provide insight into the ecological and evolutionary dynamics of host-microbe interactions.

In August 2008, Kasatochi volcano erupted and buried a small island in pyroclastic deposits and fine ash; since then, microbes, plants and birds have begun to re-colonize the initially sterile surface. Five years post-eruption, bacterial 16S rRNA gene and fungal internal transcribed spacer (ITS) copy numbers and extracellular enzyme activity (EEA) potentials were one to two orders of magnitude greater in pyroclastic materials with organic matter (OM) inputs relative to those without, despite minimal accumulation of OM (< 0.2%C). When normalized by OM levels, post-eruptive surfaces with OM inputs had the highest β-glucosidase, phosphatase, NAGase and cellobiohydrolase activities, and had microbial population sizes approaching those in reference soils. In contrast, the strongest factor determining bacterial community composition was the dominance of plants versus birds as OM input vectors. Although soil pH ranged from 3.9 to 7.0, and %C ranged 100×, differentiation between plant- and bird-associated microbial communities suggested that cell dispersal or nutrient availability are more likely drivers of assembly than pH or OM content. This study exemplifies the complex relationship between microbial cell dispersal, soil geochemistry, and microbial structure and function; and illustrates the potential for soil microbiota to be resilient to disturbance.

Microbial diversity and function in intermittent rivers and ephemeral streams (IRES) are tightly linked to specific habitat availability and hydrological phases. The intensity and frequency of the different phases (especially drying and rewetting) affect community composition and key functions, mainly linked to biogeochemical processes. Resistance and resilience strategies are distinct among microorganism groups-bacteria, archaea, fungi, protozoans-and strongly depend on different types of microhabitat or refuge available. The biodiversity of prokaryotes in IRES is strongly affected by hydrology but microhabitat conditions and type of benthic substrate significantly affect their community composition. Fungi are very sensitive to drying but use several refuges, including the terrestrial habitat, and resistance strategies. Protozoans show a wide range of survival strategies and several species can resist harsh conditions such as anoxia in drying pools. Thus, they become especially relevant for ecosystem functions when other organisms are inhibited. This sensitivity causes "waves" of microbial functions and biodiversity to covary with hydrological phases, potentially affecting ecosystem functioning and higher trophic levels. Microbially mediated functions in IRES are perhaps the most critical to freshwater ecosystem services such as nitrogen and carbon cycling. Therefore, efforts to manage and restore IRES will depend on improved understanding of hydrological controls on microbial communities and functions across space and time. © 2017 Copyright

Enzyme activities in soil indicate the relative activities of microbes, which include bacteria, fungi, algae, and other organisms. Changes in soil management alter the composition and activity of soil microbes. Plants rely on soil microbes to break down soil nutrients, and make those nutrients available for plant growth. Symbiotic relationships between soil microbes and plants enhance plant growth and productivity. Alternatively, antagonistic relationships between the soil microbial community and plants limit plant production. Soil dwellers such as nematodes or disease-causing fungi such as Macrophomina phaseolina (the fungus responsible for charcoal rot) can be particularly deleterious to crop growth and yield. Changes in the soil microbial community impact crop performance through the synergistic or antagonistic relationships between crop plants and soil biological activity. Our research is designed to explore soil microbial activity, assess changes in the potential activities of hydrolytic and oxidative enzymes involved in nutrient acquisition in the soil, and determine their potential impact on the productive capacity of soil.

Amino sugar dynamics represent an important but under-investigated component of the carbon (C) and nitrogen (N) cycles in old-growth Douglas-fir forest soils. Because fungal biomass is high in these soils, particularly in areas colonized by rhizomorphic ectomycorrhizal fungal mats, organic matter derived from chitinous cell wall material (or the monomeric building block of chitin, N-acetylglucosamine (NAG)) could be a significant source of C or N to the soil microbiota, and thus an important driver of microbial C and N processing. This paper reports the results of incubation experiments initiated to measure chitin degradation, NAG utilization, and the contribution of these substrates to soil respiration and N mineralization rates in mat-colonized and non-mat soil organic horizons. Amendments of chitin and NAG stimulated respiration, N mineralization, and biomass accumulation in mat and non-mat soils, and responses to NAG amendment were stronger than to chitin amendment. NAG-induced respiration was consistently two-fold higher in mat soils than non-mat soils, but induced N mineralization was similar between the two soil patch types. Assimilation of both C and N into microbial biomass was apparent, biomass C:N ratio decreased in all treatments, and microbial N use efficiency (treatment means 0.25 ± 0.06–0.50 ± 0.05) was greater than C use efficiency (treatment means 0.12 ± 0.04–0.32 ± 0.02). NAGase enzyme response was non-linear and showed the same pattern in chitin and NAG amendments. Responses to NAG and chitin amendment differed between mat and non-mat soils, indicating different mechanisms driving NAG and chitin utilization or differences in saprotrophic community composition between the two soil patch types. Net chitin and NAG processing rates were 0.08–3.4 times the basal respiration rates and 0.07–14 times the ambient net N mineralization rates, high enough for the turnover of total soil amino sugars to potentially occur in days to weeks. The results support the hypotheses that amino sugars are important microbial C and N sources and drivers of C and N cycling in these soils.

Ammonia-oxidizing bacteria and ammonia-oxidizing archaea are commonly found together in soils, yet the factors influencing their relative distribution and activity remain unclear. We examined archaeal and bacterial amoA gene distribution, and used a novel bioassay to assess archaeal and bacterial contributions to nitrification potentials in soils spanning a range of land uses (forest, pasture, cultivated and long-term fallowed cropland) along a 10 km transect. The assay, which quantifies the extent to which acetylene-inactivated soil nitrification potential recovers (RNP) in the presence of bacterial protein synthesis inhibitors, indicated a significant archaeal contribution to the nitrification potentials of the pasture and long-term fallowed soils. Archaeal amoA gene abundance did not vary significantly among the soils, but bacterial amoA gene abundance did, resulting in archaeal : bacterial amoA abundance ratios ranging from 1.1 ± 0.8 in cultivated soils to 396 ± 176 in pasture soils. Both archaeal and bacterial amoA gene compositions were heterogeneous across the landscape, but differed in their patterns of variability. Archaeal amoA gene distributions were distinct among each of the three main land-use types: forest, pasture and cropland soils. In contrast, bacterial amoA gene composition was distinct in forest and in cultivated cropland, while pasture and long-term fallowed cropland soils were similar. In both pasture and long-term fallowed cropland soils, one phylotype of Nitrosospira cluster 3a was highly abundant. This distinct bacterial amoA gene fingerprint correlated with significant contributions of archaea to RNP of both soils, despite differences in archaeal amoA gene composition between the pasture and fallowed soils. This observation suggests that the factors driving the development of ammonia-oxidizing bacteria community composition might influence the extent of archaeal contribution to soil nitrification.

Microorganisms inhabiting stream sediments mediate biogeochemical processes of importance to both aquatic and terrestrial ecosystems. In deserts, the lateral margins of ephemeral stream channels (parafluvial sediments) are dried and rewetted, creating periodically wet conditions that typically enhance microbial activity. However, the influence of water content on microbial community composition and diversity in desert stream sediments is unclear. We sampled stream margins along gradients of wet to dry sediments, measuring geochemistry and bacterial 16S rRNA gene composition, at streams in both a cold (McMurdo Dry Valleys, Antarctica) and hot (Chihuahuan Desert, New Mexico, USA) desert. Across the gradients, sediment water content spanned a wide range (1.6-37.9% w/w), and conductivity was highly variable (12.3-1,380 μS cm(-2)). Bacterial diversity (at 97% sequence similarity) was high and variable, but did not differ significantly between the hot and cold desert and was not correlated with sediment water content. Instead, conductivity was most strongly related to diversity. Water content was strongly related to bacterial 16S rRNA gene community composition, though samples were distributed in wet and dry clusters rather than as assemblages shifting along a gradient. Phylogenetic analyses showed that many taxa from wet sediments at the hot and cold desert site were related to, respectively, halotolerant Gammaproteobacteria, and one family within the Sphingobacteriales (Bacteroidetes), while dry sediments at both sites contained a high proportion of taxa related to the Acidobacteria. These results suggest that bacterial diversity and composition in desert stream sediments is more strongly affected by hydrology and conductivity than temperature.

A method was developed to determine the contributions of ammonia-oxidizing archaea (AOA) and ammoniaoxidizing bacteria (AOB) to the nitrification potentials (NPs) of soils taken from forest, pasture, cropped, and fallowed (19 years) lands. Soil slurries were exposed to acetylene to irreversibly inactivate ammonia monooxygenase, and upon the removal of acetylene, the recovery of nitrification potential (RNP) was monitored in the presence and absence of bacterial or eukaryotic protein synthesis inhibitors. For unknown reasons, and despite measureable NPs, RNP did not occur consistently in forest soil samples; however, pasture, cropped, and fallowed soil RNPs commenced after lags that ranged from 12 to 30 h after acetylene removal. Cropped soil RNP was completely prevented by the bacterial protein synthesis inhibitor kanamycin (800 g/ml), whereas a combination of kanamycin plus gentamicin (800 g/ml each) only partially prevented the RNP (60%) of fallowed soils. Pasture soil RNP was completely insensitive to either kanamycin, gentamicin, or a combination of the two. Unlike cropped soil, pasture and fallowed soil RNPs occurred at both 30°C and 40°C and without supplemental NH4 (

The effects of global N enrichment on soil processes in grassland ecosystems have received relatively little study. We assessed microbial community response to experimental increases in N availability by measuring extracellular enzyme activity (EEA) in soils from three grasslands with contrasting edaphic and climatic characteristics: a semiarid grassland at the Sevilleta National Wildlife Refuge, New Mexico, USA (SEV), and mesic grasslands at Konza Prairie, Kansas, USA (KNZ) and Ukulinga Research Farm, KwaZulu-Natal, South Africa (SAF). We hypothesized that, with N enrichment, soil microbial communities would increase C and P acquisition activity, decrease N acquisition activity, and reduce oxidative enzyme production (leading to recalcitrant soil organic matter [SOM] accumulation), and that the magnitude of response would decrease with soil age (due to higher stabilization of enzyme pools and P limitation of response). Cellulolytic activities followed the pattern predicted, increasing 35-52% in the youngest soil (SEV), 10-14% in the intermediate soil (KNZ) and remaining constant in the oldest soil (SAF). The magnitude of phosphatase response did not vary among sites. N acquisition activity response was driven by the enzyme closest to its pH optimum in each soil: i.e., leucine aminopeptidase in alkaline soil, beta-N-acetylglucosaminidase in acidic soil. Oxidative enzyme activity varied widely across ecosystems, but did not decrease with N amendment at any site. Likewise, SOM and %C pools did not respond to N enrichment. Between-site variation in both soil properties and EEA exceeded any treatment response, and a large portion of EEA variability (leucine aminopeptidase and oxidative enzymes), 68% as shown by principal components analysis, was strongly related to soil pH (r = 0.91, P0.001). In these grassland ecosystems, soil microbial responses appear constrained by a molecular-scale (pH) edaphic factor, making potential breakdown rates of SOM resistant to N enrichment.

The importance of microbial activity to ecosystem function in aquatic ecosystems is well established, but microbial diversity has been less frequently addressed. This review and synthesis of the hundreds of published studies on stream microbial diversity shows that factors known to drive ecosystem processes, such as nutrient availability, hydrology, metal contamination, contrasting land-use and temperature, also cause heterogeneity in bacterial diversity. Temporal heterogeneity in stream bacterial diversity was frequently observed, reflecting the dynamic nature of both stream ecosystems and microbial community composition. However, within-stream spatial differences in stream bacterial diversity were more commonly observed, driven specifically by different organic matter compartments. Bacterial phyla showed similar patterns in relative abundance with regard to compartment type across different streams. For example, surface water contained the highest relative abundance of Actinobacteria, while epilithon contained the highest relative abundance of Cyanobacteria and Bacteroidetes. This suggests that contrasting physical and/or nutritional habitats characterized by different stream organic matter compartment types may select for certain bacterial lineages. When comparing the prevalence of physicochemical effects on stream bacterial diversity, effects of changing metal concentrations were most, while effects of differences in nutrient concentrations were least frequently observed. This may indicate that although changing nutrient concentrations do tend to affect microbial diversity, other environmental factors are more likely to alter stream microbial diversity and function. The common observation of connections between ecosystem process drivers and microbial diversity suggests that microbial taxonomic turnover could mediate ecosystem-scale responses to changing environmental conditions, including both microbial habitat distribution and physicochemical factors.

Understanding controls over the distribution of soil bacteria is a fundamental step toward describing soil ecosystems, understanding their functional capabilities, and predicting their responses to environmental change. This study investigated the controls on the biomass, species richness, and community structure and composition of soil bacterial communities in the McMurdo Dry Valleys, Antarctica, at local and regional scales. The goals of the study were to describe the relationships between abiotic characteristics and soil bacteria in this unique, microbially dominated environment, and to test the scale dependence of these relationships in a low complexity ecosystem. Samples were collected from dry mineral soils associated with snow patches, which are a significant source of water in this desert environment, at six sites located in the major basins of the Taylor and Wright Valleys. Samples were analyzed for a suite of characteristics including soil moisture, pH, electrical conductivity, soil organic matter, major nutrients and ions, microbial biomass, 16 S rRNA gene richness, and bacterial community structure and composition. Snow patches created local biogeochemical gradients while inter-basin comparisons encompassed landscape scale gradients enabling comparisons of microbial controls at two distinct spatial scales. At the organic carbon rich, mesic, low elevation sites Acidobacteria and Actinobacteria were prevalent, while Firmicutes and Proteobacteria were dominant at the high elevation, low moisture and biomass sites. Microbial parameters were significantly related with soil water content and edaphic characteristics including soil pH, organic matter, and sulfate. However, the magnitude and even the direction of these relationships varied across basins and the application of mixed effects models revealed evidence of significant contextual effects at local and regional scales. The results highlight the importance of the geographic scale of sampling when determining the controls on soil microbial community characteristics.