John E. Barrett

On the rocks: the microbiology of Antarctic Dry Valley soils

The arid soils of the Antarctic Dry Valleys constitute some of the oldest, coldest, driest and most oligotrophic soils on Earth. Early studies suggested that the Dry Valley soils contained, at best, very low levels of viable microbiota. However, recent applications of molecular methods have revealed a dramatically contrasting picture — a very wide diversity of microbial taxa, many of which are uncultured and taxonomically unique, and a community that seems to be structured solely by abiotic processes. Here we review our understanding of these extreme Antarctic terrestrial microbial communities, with particular emphasis on the factors that are involved in their development, distribution and maintenance in these cold desert environments.

Potential nitrogen immobilization in grassland soils across a soil organic matter gradient

Nitrogen additions to grasslands have increased historically and are likely to continue increasing given the current and projected land use patterns, urbanization and fossil fuel use. Nitrogen retention in both grassland and forest soils is often limited by organic substrate availability, but few studies have explicitly tested the relationship between soil carbon content and nitrogen retention. We initiated a laboratory study to directly assess the influence of soil organic matter content on potential nitrogen immobilization and turnover for soils collected from across a temperature gradient in the Great Plains region of the U.S. We measured soil organic carbon, total nitrogen and carbon‐nitrogen ratios and estimated carbon mineralization and net nitrogen mineralization over 5- and 30-day laboratory incubations. We used the 15 N pool dilution assay to estimate gross nitrogen immobilization and nitrogen turnover for 5 day laboratory incubations. Soil organic carbon concentration and soil carbon‐ nitrogen ratios were negatively correlated with mean annual temperature in a linear regression model that accounted for 46‐ 56% of the variability, respectively. Regional patterns in soil organic carbon content and small scale variability in substrate availability imposed by discontinuous plant cover together strongly influenced potential nitrogen immobilization. Potential carbon mineralization and nitrogen immobilization increased with increasing soil organic matter content. Soil organic carbon content accounted for 58% of the variation in potential rates of N immobilization. A strong correlation between nitrogen immobilization and carbon mineralization further suggests that rapid stabilization of nitrogen is facilitated by an active microbial community and the availability of a readily mineralizable organic substrate. 7 2000 Elsevier Science Ltd. All rights reserved.

Hydrologic Response to Extreme Warm and Cold Summers in the McMurdo Dry Valleys, East Antarctica

Abstract The meteorological characteristics and hydrological response of an extreme warm, and cold summer in the McMurdo Dry Valleys are compared. The driver behind the warmer summer conditions was the occurrence of down-valley winds, which were not present during the colder summer. Occurrence of the summer down-valley winds coincided with lower than typical mean sea level pressure in the Ross Sea region. There was no significant difference in the amount of solar radiation received during the two summers. Compared to the cold summer, glaciological and hydrological response to the warm summer in Taylor Valley included significant glacier mass loss, and 3- to nearly 6000-fold increase in annual streamflow. Lake levels decreased slightly during the cold summer, and increased between 0.54 and 1.01 m during the warm summer, effectively erasing the prior 14 years of lake level lowering in a period of three months. Lake level rise during the warm summer was shown to be strongly associated with and increase in degree days above freezing at higher elevations. We suggest that strong summer down-valley winds may have been responsible for the generation of large glacial lakes during the Last Glacial Maximum when ice core records recorded annual temperatures significantly colder than present.

Microbial community composition in soils of Northern Victoria Land, Antarctica

Biotic communities and ecosystem dynamics in terrestrial Antarctica are limited by an array of extreme conditions including low temperatures, moisture and organic matter availability, high salinity, and a paucity of biodiversity to facilitate key ecological processes. Recent studies have discovered that the prokaryotic communities in these extreme systems are highly diverse with patchy distributions. Investigating the physical and biological controls over the distribution and activity of microbial biodiversity in Victoria Land is essential to understanding ecological functioning in this region. Currently, little information on the distribution, structure and activity of soil communities anywhere in Victoria Land are available, and their sensitivity to potential climate change remains largely unknown. We investigated soil microbial communities from low- and high-productivity habitats in an isolated Antarctic location to determine how the soil environment impacts microbial community composition and structure. The microbial communities in Luther Vale, Northern Victoria Land were analysed using bacterial 16S rRNA gene clone libraries and were related to soil geochemical parameters and classical morphological analysis of soil metazoan invertebrate communities. A total of 323 16S rRNA gene sequences analysed from four soils spanning a productivity gradient indicated a high diversity (Shannon-Weaver values > 3) of phylotypes within the clone libraries and distinct differences in community structure between the two soil productivity habitats linked to water and nutrient availability. In particular, members of the Deinococcus/Thermus lineage were found exclusively in the drier, low-productivity soils, while Gammaproteobacteria of the genus Xanthomonas were found exclusively in high-productivity soils. However, rarefaction curves indicated that these microbial habitats remain under-sampled. Our results add to the recent literature suggesting that there is a higher biodiversity within Antarctic soils than previously expected.

Co-variation in soil biodiversity and biogeochemistry in northern and southern Victoria Land, Antarctica

Data from six sites in Victoria Land (72–77°S) investigating co-variation in soil communities (microbial and invertebrate) with biogeochemical properties showthe influence of soil properties on habitat suitability varied among local landscapes as well as across climate gradients. Species richness of metazoan invertebrates (Nematoda, Tardigrada and Rotifera) was similar to previous descriptions in this region, though identification of three cryptic nematode species of Eudorylaimus through DNA analysis contributed to the understanding of controls over habitat preferences for individual species. Denaturing Gradient Gel Electrophoresis profiles revealed unexpectedly high diversity of bacteria. Distribution of distinct bacterial communities was associated with specific sites in northern and southern Victoria Land, as was the distribution of nematode and tardigrade species. Variation in soil metazoan communities was related to differences in soil organic matter, while bacterial diversity and community structure were not strongly correlated with any single soil property. There were no apparent correlations between metazoan and bacterial diversity, suggesting that controls over distribution and habitat suitability are different for bacterial and metazoan communities. Our results imply that top-down controls over bacterial diversity mediated by their metazoan consumers are not significant determinants of bacterial community structure and biomass in these ecosystems.

Soil Carbon Dioxide Flux in Antarctic Dry Valley Ecosystems

The Antarctic dry valleys of southern Victoria Land are extreme desert environments where abiotic factors, such as temperature gradients, parent material, and soil water dynamics, may have a significant influence on soil carbon dioxide (CO2) flux. Previous measurements of soil respiration have demonstrated very low rates of CO2 efflux, barely above detection limits. We employed a modified infrared gas-analyzer system that enabled detection of smaller changes in CO2 concentration in the field than previously possible. We measured diel CO2 fluxes and monitored soil microclimate at three sites in Taylor Valley. Soil CO2 flux ranged from −0.1 to 0.15 μmol m−2 s−1. At two of the three sites, we detected a physically driven flux associated with diel variability in soil temperature. At these sites, CO2 uptake (negative flux) was associated with dropping soil temperatures, whereas CO2 evolution (positive flux) was associated with increases in soil temperature. These observations are corroborated by laboratory experiments that suggest that CO2 flux is influenced by physically driven processes. We discuss four potential mechanisms that may contribute to physically driven gas exchange. Our results suggest there are strong interactions between biological and abiotic controls over soil CO2 flux in terrestrial ecosystems of the Antarctic dry valleys, and that the magnitude of either may dominate depending on the soil environment and biological activity.

VARIATION IN BIOGEOCHEMISTRY AND SOIL BIODIVERSITY ACROSS SPATIAL SCALES IN A POLAR DESERT ECOSYSTEM

Desert ecosystems are characterized by distinct spatial patterning in soil biogeochemistry and biodiversity. In the Antarctic Dry Valleys, soil polygons are prominent features of the landscape and may be key units for scaling local ecological information to the greater region. We examined polygon soils in each of the three basins of Taylor Valley, Antarctica. Our objectives were to characterize variability in soil biogeochemistry and biodiversity at local to regional scales, and to test the influence of soil properties upon invertebrate communities. We found that soil biogeochemical properties and biodiversity vary over multiple spatial scales from fine ( 10 km) scales. Differences in biogeochemistry were most pronounced at broad scales among the major lake basins of Taylor Valley corresponding to differences in geology and microclimate, while variation in invertebrate biodiversity and abundance occurred at landscape scales of 10–500 m, and within individual soil polygons. Variation in biogeochemistry and invertebrate communities across these scales reflects the influence of physical processes and landscape development over ecosystem structure in the dry valleys. The development of soil polygons influences the spatial patterning of soil properties such as soil organic matter, salinity, moisture, and invertebrate habitat suitability. Nematode abundance and life history data indicate that polygon interiors are more suitable habitats than soils in the troughs at the edges of polygons. These data suggest that physical processes (i.e., polygon development) and biogeochemistry are important influences on the spatial variability of biotic communities in dry valley soil ecosystems.

Biogeochemical stoichiometry of Antarctic Dry Valley ecosystems

operate over 10–10 6 years. The simple organisms (microbial and metazoan) comprising dry valley foodwebs adhere to strict biochemical requirements in the composition of their biomass, and when activated by availability of liquid water, they influence the chemical composition of their environment according to these ratios. Nitrogen and phosphorus varied significantly in terrestrial and aquatic ecosystems occurring on landscape surfaces across a wide range of exposure ages, indicating strong influences of landscape development and geochemistry on nutrient availability. Biota control the elemental ratio of stream waters, while geochemical stoichiometry (e.g., weathering, atmospheric deposition) evidently limits the distribution of soil invertebrates. We present a conceptual model describing transformations across dry valley landscapes facilitated by exchanges of liquid water and biotic processing of dissolved nutrients. We conclude that contemporary ecosystem stoichiometry of Antarctic Dry Valley soils, glaciers, streams, and lakes results from a combination of extant biological processes superimposed on a legacy of landscape processes and previous climates.

Snow-Patch Influence on Soil Biogeochemical Processes and Invertebrate Distribution in the McMurdo Dry Valleys, Antarctica

Abstract The McMurdo Dry Valleys is the largest of the ice-free areas in Antarctica. Precipitation events in excess of 1 cm of snow accumulation are rare. During the winter, snow is transported by strong katabatic winds blowing from the polar plateau, and deposited into the lee of topographic features (e.g., stream channels and other topographic depressions). At the start of the austral summer (early October), as much as 10% of the valley soils may be covered by distributed snow patches. Because liquid water is the primary driver of biological, physical, and chemical processes in this polar desert, quantifying fluxes of water from snow patches is important to understanding the influence of hydrology on soil biology and nutrient cycling. During the austral summer of 1999–2000, four snow patches that had developed during the previous winter in Taylor Valley were studied. We measured snow-patch area, depth, and snow water equivalent, as well as subnivian (under snow) and nearby exposed (control) soil temperature, light intensity, soil moisture, invertebrate abundance, soil organic matter content, and 95-d labile pools of C and N. Subnivian soils differed from exposed soils being as much as 26.8°C colder than exposed soils; average soil moisture ranging from 6.9 to 13.6% compared to 0.4% in exposed soils; soil invertebrate populations exceeding 7900 individuals kg−1 dry soil versus less than 1200 individuals kg−1 dry soil in exposed soils; and soil invertebrate species richness values greater than 2 taxa, compared to 1.3 taxa in exposed soils. The results of this study show that these seasonal, sparse snow patches may be an important source of moisture and control habitat of soil ecosystems in this extreme environment.

The Influence of Soil Geochemistry on Nematode Distribution, Mcmurdo Dry Valleys, Antarctica

ABSTRACT Soils of the McMurdo Dry Valleys are among the most extreme terrestrial environments, hosting low-diversity food webs of microbes, protozoa, and metazoan invertebrates. Distribution of soil invertebrates, particularly nematodes, is related to the highly variable soil geochemistry of the valleys. Bull Pass is a glacially carved area within the McMurdo Dry Valleys where a broad range of geochemical conditions occurs along a continuous soil gradient. This site provides the opportunity to investigate how soil geochemistry controls nematode distribution on a local scale, and to establish correlations that may also be relevant at regional scales. At Bull Pass, two nematode species were present, with the dominant Scottnema lindsayae occurring in >30% of the samples. There were significant negative correlations between live nematode abundance and soil nitrate concentration and salinity, consistent with experiments showing strong salinity effects on nematode survival. A logistic regression model based on data sets from across the McMurdo Dry Valleys showed a strong negative relationship between soil salinity and the probability of live nematodes occurring. Soil chemistry and nematode distribution from the Bull Pass transect are compared with model results and suggest that the larger-scale distribution of nematodes across the McMurdo Dry Valleys may be reflected in the smaller-scale chemical and biological gradients at Bull Pass.