Ross A. Virginia

Global patterns of foliar nitrogen isotopes and their relationships with climate, mycorrhizal fungi, foliar nutrient concentrations, and nitrogen availability.

Ratios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (delta(15)N), foliar N concentrations, mycorrhizal type and climate for over 11,000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar delta(15)N by 2 per thousand, 3.2 per thousand, 5.9 per thousand, respectively, relative to nonmycorrhizal plants. Foliar delta(15)N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT >or= -0.5 degrees C, but was invariant with MAT across sites with MAT < -0.5 degrees C. In independent landscape-level to regional-level studies, foliar delta(15)N increased with increasing N availability; at the global scale, foliar delta(15)N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar delta(15)N and ultimately global patterns in N cycling.

IMPACT OF DROUGHT ON DESERT SHRUBS: EFFECTS OF SEASONALITY AND DEGREE OF RESOURCE ISLAND DEVELOPMENT

Large areas of semiarid grasslands in the southwestern United States have been virtually replaced by shrubs during the past century. Understanding the causes and consequences of such vegetation dynamics requires that we elucidate the interplay between external forces of change (e.g., climate, human impacts) and the internal forces within these ecosystems that foster resilience and/or stability. Several conceptual models of arid ecosystems address this interplay by including the potential role of autogenic shrub effects on ecosystem processes, which lead to the formation of “resource islands” and tend to promote shrub persistence. Specifically, during the process of shrub establishment and maturation, the cycling of nutrients is progressively confined to the zones of litter accumulation beneath shrubs, while bare intershrub spaces become increasingly nutrient poor. As shrub resource islands develop, there is increased interception and stemflow by shrub canopies, confining infiltration of nutrient-enriched ...

Antarctic climate cooling and terrestrial ecosystem response

The average air temperature at the Earths surface has increased by 0.06 °C per decade during the 20th century, and by 0.19 °C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarcticas peninsula region over the second half of the 20th century. Although previous reports suggest slight recent continental warming, our spatial analysis of Antarctic meteorological data demonstrates a net cooling on the Antarctic continent between 1966 and 2000, particularly during summer and autumn. The McMurdo Dry Valleys have cooled by 0.7 °C per decade between 1986 and 2000, with similar pronounced seasonal trends. Summer cooling is particularly important to Antarctic terrestrial ecosystems that are poised at the interface of ice and water. Here we present data from the dry valleys representing evidence of rapid terrestrial ecosystem response to climate cooling in Antarctica, including decreased primary productivity of lakes (6–9% per year) and declining numbers of soil invertebrates (more than 10% per year). Continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change.

LOW-DIVERSITY ANTARCTIC SOIL NEMATODE COMMUNITIES: DISTRIBUTION AND RESPONSE TO DISTURBANCE

We are studying the distribution, biodiversity, and abundance of nematodes in the most extreme terrestrial environment on earth, the Dry Valley region of Antarctica. Here we report that the nematode community structure of 1–3 species in two functional groups may be the simplest soil food web of any terrestrial ecosystem. Nematodes were widespread and not correlated with moisture, C, or N, factors that define soil biotic complexity elsewhere. In a field experiment, treatments increasing soil water, carbon, and temperature, alone or in combination, generally decreased the abundance of the single omnivore-predator species and increased the abundance of its microbivorous prey species. These low-diversity nematode communities, limited to ≤3 species, apparently lack species redundancy and appear sensitive to environmental change. Our findings suggest that Antarctic soil ecosystems are sensitive to anthropogenic disturbance.

Controls on soil biodiversity: insights from extreme environments

Abstract Research in low biodiversity extreme environments allows separation of the climatic, soil and biological interactions that determine soil biodiversity and community structure. Studies focused on the response of low diversity communities in soils of the Antarctic Dry Valleys and the Chihuahuan Desert of the southwestern USA, to manipulations of soil resources and climate, offer the best opportunity to learn about the environmental controls on soil biodiversity and the role of biodiversity in soil functioning. We propose that insights based on research in these extreme environments should be applicable to understanding soil biodiversity in more complex, temperate and tropical ecosystems. The study of extreme soil ecosystems may also provide information on the response of soil biodiversity to increasing occurrences of environmental extremes that are predicted to occur from global change models. Studies from hot and cold deserts show that decomposition-based food webs can be very simple, that aridity produces similar mechanisms for survival and dispersal of organisms in temperate and polar systems, that suitable soil habitats are patchily distributed in arid environments, and the low biodiversity of extreme soil ecosystems creates little or no functional redundancy making these systems susceptible to disturbance. We suggest that species within the same functional group can have small differences in ecology that are sufficient to affect ecosystem processes. When this occurs, differential responses of species to disturbance within a functional group will not stabilise the soil ecosystem, but rather lead to dramatic changes in community composition and ecosystem process rates.

Carbon isotopes reveal soil organic matter dynamics following arid land shrub expansion

Abstract Over the past century, overgrazing and drought in New Mexico’s Jornada Basin has promoted the replacement of native black grama (Bouteloua eriopoda Torr.) grass communities by shrubs, primarily mesquite (Prosopis glandulosa Torr.). We investigated the effects of shrub expansion on the distribution, origin, turnover, and quality of light (LFC) and heavy (HFC) soil organic matter (SOM) fractions using δ13C natural abundance to partition SOM into C4 (grass) and C3 (shrub) sources. Soil organic matter beneath grasses and mesquite was isotopically distinct from associated plant litter, providing evidence of both recent shrub expansion and Holocene plant community changes. Our δ13C analyses indicated that SOM derived from mesquite was greatest beneath shrub canopies, but extended at least 3 m beyond canopy margins, similar to the distribution of fine roots. Specific 14C activities of LFC indicated that root litter is an important source of SOM at depth. Comparison of turnover rates for surface LFC pools in grass (7 or 40 years) and mesquite (11 or 28 years) soils and for HFC pools by soil depth (∼150–280 years), suggest that mesquite may enhance soil C storage relative to grasses. We conclude that the replacement of semiarid grasslands by woody shrubs will effect changes in root biomass, litter production, and SOM cycling that influence nutrient availability and long-term soil C sequestration at the ecosystem level.

ORIGIN AND DISTRIBUTION OF SOIL ORGANIC MATTER IN TAYLOR VALLEY, ANTARCTICA

Soil organic matter in the dry valleys of the McMurdo Sound Region sustains a unique terrestrial microbial community in one of the most extreme climates on earth. The origin of this organic material has long been in question, given the absence of higher plants in this arid ecosystem. We examined the sources and distribution of organic matter from six elevational transects extending along a 30-km section of Taylor Valley (163° E, 77.35° S), from the head of the valley to the Ross Sea. The δ13C and δ15N of soil were determined both to identify potential sources of soil organic matter (i.e., marine, lacustrine, and/or terrestrial) and to map C and N source distribution in the valley. Results suggest that the primary source for soil organic matter is not the wind transport of organic material from modern aquatic systems (a long-held hypothesis). Instead, our data indicate that modern distributional patterns of low-elevation (<150 m above sea level [a.s.l.]) soil organic matter (SOM) in Taylor Dry Valley are s...

Invertebrate biodiversity in Antarctic dry valley soils and sediments

ABSTRACT We studied invertebrate communities across a transition zone between soils and stream sediments in the cold desert landscape of Taylor Valley, Antarctica. We hypothesized that hydrological and biogeochemical linkages in the functionally important transition zone between streams and surrounding soils should be important in structuring invertebrate communities. We compared invertebrate communities along transects beginning in the saturated sediments under flowing stream water and extending laterally through the hyporheic zone to the dry soils that characterize most of the dry valley landscape. Nematodes, rotifers, and tardigrades assembled into different communities in soils and sediments, but there was no relationship between the total abundance of invertebrates and moisture. Community diversity was, however, influenced by the moisture and salinity gradients created with distance from flowing waters. The wet, low-salinity sediments in the center of the stream contained the most invertebrates and had the highest taxonomic diversity. Adjacent to the stream, communities in the hyporheic zone were influenced strongly by salt deposition. Abundance of invertebrates was low in the hyporheic zone, but this area contained the most co-occurring nematode species (three species). In dry soils, communities were composed almost entirely of a single species of nematode, Scottnema lindsayae, an organism not found in the stream center. These results suggest spatially-partitioned niches for invertebrates in soils and sediments in the dry valley landscape based on proximity to sources of moisture and the interactive effects of salinity.

The Biodiversity and Biogeochemistry of Cryoconite Holes from McMurdo Dry Valley Glaciers, Antarctica

Abstract Once thought of as inert, ice has been increasingly recognized as a habitat suitable for life. The landscape of the MCMurdo Dry Valleys (MCM) of Antarctica is dominated by glaciers, and glacier melt is the primary water source for life in soils, streams, and lakes. The glaciers, despite their cold and lifeless appearance, offer functioning habitats for life. The major objective of this study was to examine biogeochemical characteristics of miniecosystems present in cryoconite holes and to determine links to other components (soils, streams, and lakes) of the dry valley landscape. We examined cryoconite holes from 5 glaciers spanning the length of Taylor Valley, one of many valleys in the MCM. Cryoconite biotic communities were composed of the same species observed in streams and lakes, namely, cyanobacteria (Chlorococcus, Chroococcus, Crinalium, Oscillatoria, Nostoc, and Sprirulina), rotifers (Philodina gregaria and Cephalodella catellina), tardigrades (Acutuncus antarcticus and Hypsibius spp.), and ciliates. Biotic communities did not reflect the composition of the immediately surrounding environments, suggesting the effects of eolian mixing and transport of sediments and biota across the valley. Gradients of chemistry and biotic abundance in cryoconite holes reflected the position of each glacier in the valley. Nitrogen and organic carbon concentration patterns across glaciers potentially resulted from biological activities in cryoconite holes. Properties of holes were stable from one to the next sampling season, suggesting that changes of cryoconite hole properties develop on longer than yearly time scales.

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.