Terrestrial ecosystem processes of Victoria Land, Antarctica

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Soil Biology & Biochemistry 38 (2006) 3019–3034 www.elsevier.com/locate/soilbio

Terrestrial ecosystem processes of Victoria Land, Antarctica J.E. Barretta,, R.A. Virginiaa, D.W. Hopkinsb, J. Aislabiec, R. Bargaglid, J.G. Bockheime, I.B. Campbellc, W.B. Lyonsf, D.L. Moorheadg, J.N. Nkemh, R.S. Sletteni, H. Steltzerh, D.H. Wallh, M.D. Wallensteinh a Environmental Studies Program, Dartmouth College, Hanover, NH 03755, USA School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UK c Landcare Research, Private Bag 3127, Hamilton, New Zealand d Department of Environmental Science, University of Siena, Via Mattioli 4, 53100 Siena, Italy e Department of Soil Science, University of Wisconsin, Madison, WI 53706-1299, USA f Byrd Polar Research Center, Ohio State University, Columbus, OH 43210, USA g Department of Earth, Ecological and Environmental Sciences, University of Toledo, Toledo, OH 43606, USA h Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO 80523, USA i Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA b

Received 23 November 2005; received in revised form 13 April 2006; accepted 19 April 2006 Available online 22 May 2006

Abstract Terrestrial environments of Victoria Land, Antarctica are ideal systems to test hypotheses about the sensitivity of ecosystem processes to climate variability, and the relationships between soil biodiversity and ecosystem functioning because of their high sensitivity to climate change and their limited diversity. This region is also considered among the most pristine of ecosystems, and therefore may serve as an indicator for detecting the response of other ecosystems to global environmental change. Rates and controls over key ecosystem processes remain poorly documented over much of Victoria Land, but it is generally held that the distribution and functioning of soil communities are most limited by the availability of liquid water and organic carbon. Here we review examples of ecosystem processes from several sites in North and South Victoria Land and develop a regional synthesis accounting for variation in the availability of soil resources (i.e. liquid water, organic matter, inorganic nutrients). Variation in soil microclimate, organic matter, moisture and salinity encountered over gradients of coastal to interior sites, latitude, and soil chronosequences are the primary controls over the structure of soil communities and their functioning. Imbalanced stoichiometric nutrient ratios frequently encountered in Victoria Land ecosystems also contribute to limited distribution of soil biota, and where they occur these elemental imbalances indicate lower biological activity and little biotic control over bulk element ratios in soils. Priorities and future directions of Victoria Land soil and ecosystem research are also discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Ecosystem functioning; Environmental gradients; Soil biodiversity; Soil ecosystems; Stoichiometry

1. Introduction Ecosystem processes in Victoria Land, Antarctica are constrained by combinations of extreme conditions including low temperatures, moisture and organic matter availability, and high salinity stress, resulting in both low diversity and biomass of organisms contributing to Corresponding author. Tel.: +1 603 646 1689; fax: +1 603 646 1682.

E-mail address: [email protected] (J.E. Barrett). 0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2006.04.041

ecosystem processes (Campbell and Claridge, 1987; Freckman and Virginia, 1997; Bargagli et al., 1999; Cowan et al., 2002; Sinclair et al., 2003; Wall, 2005; Adams et al., 2006). Understanding ecosystem dynamics in Victoria Land is thus challenging because of low rates and high spatial and temporal variability (Parsons et al., 2004; Elberling et al., 2006). Despite these difﬁculties there is intrinsic scientiﬁc value and opportunity in elucidating ecosystem dynamics of Victoria Land because: (1) ecosystem responses to climate variability are ampliﬁed in cold and high latitude

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regions, and may provide early evidence of responses anticipated for temperate ecosystems, and (2) the limited diversity and paucity of the soil organisms facilitates direct assessment of contributions of particular species (or genes) to ecosystem processes. The Victoria Land region encompasses a latitudinal gradient of 81, from the Darwin Glacier (791S) in the South, to Cape Adare (711S) in the north, where variation in energy balance, temperature, relative humidity and duration of melt season have a large potential to inﬂuence terrestrial ecosystem processes, as well as inform the development of hypotheses concerning the effects of climate change. A comprehensive synthesis of ecosystem dynamics for the region is not yet practical given the large gaps in our understanding of basic processes and drivers in even the most intensively studied regions (e.g. the McMurdo Dry Valleys, Terra Nova Bay region). The objective of this paper is to develop a conceptual model (Fig. 1) synthesizing what is known about controls over major ecosystem processes (i.e. C and N turnover) by drawing on examples from sites in South and North Victoria Land, with the intention of developing testable hypothesis for future study and to identify gaps in the current state of knowledge and suggest research priorities.

The scientiﬁc interest in polar regions that the International Polar Year (IPY; 2007–2008) is anticipated to promote, and the broad recognition of the importance of soil biodiversity in sustaining ecosystem functioning (Chapin et al., 2005; Wall, 2005), provide a unique opportunity for scientists working in Antarctica to demonstrate the relevance of the soil communities in Victoria Land to broader ecological questions worldwide. No other natural terrestrial ecosystems are known to maintain functioning soil food webs and ecosystem processing with so few taxa, and therefore no natural soil ecosystem, may be as sensitive to even small changes in diversity. Currently, little information of the role of Victoria Land soil communities in facilitating ecosystem processes is available, and their sensitivity to potential climate change remains largely unknown. Our coverage of Victoria Land is necessarily restricted to those areas where there have been scientiﬁc investigations (Table 1), and our consideration of ecosystem processes is limited to some of the major biogeochemical processes, such as carbon (C) transformations (primary production, soil respiration) and nitrogen (N) transformations (mineralization, nitriﬁcation and denitriﬁcation).

Aerosols 2Cl-, NO3-, SO4

Snow, ground ice

Radiant energy Latitude Day of year Slope Aspect Cloud cover

deposition melt f

Aeolian transport Legacy organic matter

Soil respiration

NPP

Ecosystem organic matter

f

Eolian transport, erosion

Lake inundation Marine incursions Ornithogenic inputs 3-

Soil nutrients (NH4+, NO3-, PO4 )

f

Lithology Weathering Mineralization Cryoturbation Proximity to coast Surface age Landscape history Ornithogenic inputs

Fig. 1. Conceptual model illustrating controls over the accumulation and turnover of organic matter in Victoria Land terrestrial ecosystems.

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Table 1 Location, elevation, estimated surface age and mean (7SD) organic C, N and P content of surface soils for sites in Victoria Land, Antarctica (data sources: Bargagli et al., 1998, 1999, Elberling et al., 2006, Barrett, unpublished; MCM-LTER, unpublished) Site Cape Hallett Luther Vale, Admiralty Mountains Edmonson Point Fryxell Basin, Taylor Valley Bonney Basin, Taylor Valley Beacon Valley Garwood Valley (polygons)

Lat.

Long. 0

72119 721220 741200 771370 771440 771500 781010

0

170113 1691530 1651080 1631150 1621190 1601400 1631530

Elev. (m)

SOCa

TNb

TPc

Est. surface age (kYr)

10 200 30 55 110 1050 400

19.4872.08 0.6170.09 4.1071.40 0.4370.07 0.1970.06 0.1670.06 1.6370.61

6.5271.16 0.2270.02 0.9870.60 0.0270.01 0.0370.03 0.3770.35 0.1170.05

NA NA 0.6070.30 0.6870.11 0.2870.03 0.4470.02 NA

NA NA NA 8–24d 75–98e 300–8000f NA

NA: Data not available. a Soil organic C (g/kg). b Total soil N (g/kg). c Total acid-soluble soil phosphorus (g/kg). d Hall and Denton (2000). e Denton et al. (1989) and Bockheim (2002). f Marchant et al. (1996) and Ng et al. (2005).

1.1. Conceptual framework Two categories of ecosystem processes operate in terrestrial environments; landscape development that results in the variation of physicochemical properties of soils which inﬂuences the distribution and activity of soil organisms, and the suite of more dynamic ecosystem processes directly controlled by the functioning of these organisms. The latter category includes processes such as photosynthesis, soil respiration, denitriﬁcation, methanogenesis, facilitated by soil biota and inﬂuenced by the physical environment, e.g. microclimate, soil moisture, redox, salinity, pH. The former category includes the geological processes of weathering and soil formation, which may be described by the major inputs, transformations, and outputs that occur over centuries to millennia and which may inﬂuence the contemporary concentration and composition of soil resources, e.g. soil C, N, P, and water necessary to sustain biological activity (Table 2). The spatial patterns of these soil properties are inﬂuenced by physical attributes of the soil environment as well as by legacies, or carry-overs from previous states in the history and landscape development of an ecosystem (Moorhead et al., 1999; Burkins et al., 2000), but the quantitative signiﬁcance of such inﬂuences is not always clear (Hopkins et al., 2006). The concept of ecological legacies as it has developed in the Antarctic literature refers to the accumulation (during discrete events or continuous inputs) of soil resources in the past, and the potential contribution these materials make to the contemporary functioning of soil ecosystems (Moorhead et al., 1999; Burkins et al., 2000). Contemporary processes such as respiration and photosynthesis may be inﬂuenced by these legacies, but are also subject to the dynamic physical environment inﬂuenced by diel to seasonal variation in solar radiation, temperature, moisture availability as well as any contemporary inputs through

Table 2 Factors affecting soil formation in Victoria Land, Antarctica Process

Examples

Ecosystem property/ process

Episodic geological events

Glaciation

Tills (various ages), lacustrine and marine soil organic matter and salts

Lake inundation Marine incursions Inputs

Sea birds Marine aerosols Dust Snow

Transformations (chemical)

Weathering

Ornithogenic organic matter Salt, metal accumulation Phosphorus redistribution Sublimation or melting Phosphorus and base cation availability

Mineralization Transformations (physical)

Cryoturbation Inﬂation Sublimation of subsurface ice

Patterned ground formations Desert pavement

Transformations (biological)

Weathering and exfoliation of sandstones by cryptoendoliths

Unknown

Outputs

Eolian redistribution Erosion Leaching Ammonia volatilization

Dune formation Salt layers Groundwater seeps Enrichment of 15N in soil N

eolian redistribution and atmospheric deposition (Fig. 1). The legacy model has emerged as the conceptual framework of research under the auspices of the McMurdo

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Long-Term Ecological Research project (MCM-LTER) to describe material linkages between aquatic and terrestrial landscapes over geologic time scales (1000–10,000 yr). Analogous long-term inﬂuences exist in all ecosystems, but because contemporary inputs of resources and energy are small, and biological transformation of the soil is very slow, the legacy of climatic events and geological processes assume much greater importance in Victoria Land. Thus, ecological legacies are imprinted on the contemporary landscape and inﬂuence the composition and activity of biotic communities (Moorhead et al., 1999). For example, in the McMurdo Dry Valleys, the absence of vascular plants and limited distribution of microbial mats and lichens generates a soil ecosystem shaped largely by geological processes (glaciation, lake inundation, cryoturbation), and contemporary soil organic matter is inﬂuenced by the legacies of paleolakes and marine incursions in low elevation valleys (Burkins et al., 2000). In contrast, the soils of coastal areas such as Edmonson Point and Cape Hallett receive substantial marine inputs, especially organic amendments in the form of guano from sea birds (Bargagli et al., 1999). Stable isotope composition of contemporary soil organic matter in these various systems reveals potential inputs of mixed source materials including lake, marine and terrestrial origins (Fig. 2, Burkins et al., 2000; Lawson et al., 2004). Eolian redistribution of microbial mat material from lakes and shallow ponds to surrounding soils may also be an important mechanism accounting for lacustrine inﬂuences on soil organic matter (Parker et al., 1982; Moorhead

35 30 25

δ 15N (‰)

20 15 10 5 0 -5 -10 -35

-30

-25

-20

-15

-10

δ 13C (‰) Taylor Valley

Cape Hallett 1

Luther Vale

Cape Hallett 2

Fig. 2. Isotope bi-plot of soils from Southern Victoria Land (Taylor Valley) and Northern Victoria Land (Luther Vale and Cape Hallett) (Barrett, unpublished data).

et al., 2003). Indeed, contemporary lake sources may be especially important in soils neighboring aquatic ecosystems, where the large pools of organic matter found in areas inundated by water are in effect subsidizing the surrounding C budgets of the less productive soils. A spatial subsidy model describing such a scenario for contemporary C dynamics of Garwood Valley is developed by Elberling et al. (2006); wherein the high rates of respiration and C turnover (relative to Taylor Valley) are linked to eolian redistribution of labile organic matter from the exposed surfaces of the lake and lake margin. In other parts of continental and maritime Antarctica, contemporary inputs from mosses, lichens and microbial mats contribute directly to the organic matter availability and habitat suitability for soil biota (Davey and Rothery, 1993; Schwarz et al., 1993). Legacies can inﬂuence the salt content of surface soils as well. For example, in soils developing on ancient tills, accumulation of atmospherically derived salts is a legacy of age, negligible leaching and limited biotic processing of solutes (e.g. Claridge and Campbell, 1968, 1977; Wada et al., 1981; Bockheim, 1997; Michalski et al., 2005) that can result in highly saline soils devoid of metazoan life (Nkem et al., 2006). The importance of these legacies on contemporary Victoria Land terrestrial ecosystems is a consequence of the limited biological inﬂuence over soil development. Carbon dynamics are the most fundamental of the biogeochemical cycles, and soil organic C is a useful element for integrating soil biota and ecosystem functioning. Understanding the sources of organic matter and controls over its accumulation and turnover is necessary for understanding the energetic basis of soil food webs. In Antarctica, extreme climate limits C ﬁxation by primary producers, including: mosses (Smith, 1999; Pannewitz et al., 2005), lichens (Kappen et al., 1991; Green et al., 1998), phototrophic microbes (Friedmann et al., 1993), and by chemoautotrophic bacteria (Hopkins et al., 2006); thus Antarctic soils have among the lowest organic carbon concentrations of any terrestrial ecosystem (Claridge et al., 2000; Burkins et al., 2001). Sources of contemporary organic C include microbial mats and mosses in intermittently saturated soils and sediments, rock-dwelling lichens, and endolithic microbial communities (Friedmann et al., 1993; Adams et al., 2006). The absence of higher plants and their lignin and cellulose, reduces the chemical complexity of soil organic matter, and contributes to highly labile organic pools that exhibit rapid rate kinetics under favorable conditions (Barrett et al., 2005; Hopkins et al., 2006). Rates of in situ soil respiration in Victoria Land ecosystems are nevertheless very low compared with temperate ecosystems because of environmental constraints, yet signify an ecologically signiﬁcant turnover of soil C, and indicate the presence of biologically functioning soil communities (Burkins et al., 2001; Parsons et al., 2004; Elberling et al., 2006). While carbon is usually limiting to terrestrial food webs in many Antarctic ecosystems, other elements such as N

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and P are often in high supply, resulting in elemental ratios that differ from the proportions generally recognized as necessary for biological metabolism and sustaining balanced growth (i.e. ecological stoichiometry or the Redﬁeld ratios of C:N:P ¼ 106:16:1, Redﬁeld, 1958). Ecosystem stoichiometry has proven to be a useful research approach in diverse marine, freshwater and terrestrial environments (Redﬁeld, 1958; Sterner and Elser, 2002; Vitousek, 2004). Stoichiometric approaches are based on the premise that organisms inﬂuence, and are inﬂuenced by the chemical composition of their environment, especially by the availability of essential nutrients (Redﬁeld, 1958). While distribution of nutrients (particularly N) have been shown to inﬂuence soil biota (Nkem et al., 2006), controls over the accumulation and biogeochemical cycling of N, P and other elements (e. g. Ca, K, Mg, Na, and S), are inﬂuenced less by soil biology and more by a suite of physical factors (Table 2) including the proximity of marine sources, soil age and the degree of mineral weathering, though weathering rates in Antarctic soils are extremely slow due to the dearth of available liquid water (Campbell and Claridge, 1987). Soil nutrient dynamics, may therefore be in the category of slow ecosystem processes such as landscape development, weathering and accumulation of soluble salts (Fig. 1). For example, high elevation, ice-free regions in continental Antarctica are among the oldest continually exposed surfaces on the planet with high salt concentrations from the long depositional history and negligible rates of soil leaching (Marchant et al., 1996; Bockheim, 2002). Nitrate (NO 3 ), usually found at low concentrations in productive ecosystems, can reach concentrations of 3% (w/w), resulting from low rates of atmospheric deposition over millennia and very low rates of biological cycling (Nkem et al., 2006). In contrast, soil phosphorus availability is typically greatest in geologically young soils, or soils inﬂuenced by ornithogenic inputs (Bargagli et al., 1998; Gudding, 2003) where N:P, and even C:P ratios are well below the Redﬁeld Ratios. The imbalanced stoichiometry frequently encountered in Victoria Land soil ecosystems contributes to limited distribution of soil biota (Barrett et al., accepted). For example, the high N content relative to C (low C:N) in soils occurring on ancient glacial tills of the McMurdo Dry Valleys, and in ornithogenic soils of coastal zones presents an additional limitation to many soil organisms and evidently prohibits colonization by some cryptogams and most metazoans (Bargagli et al., 1999; Porazinska et al., 2002; Sinclair et al., 2003; Nkem et al., in 2006).

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et al., 1994; Xie and Steinberger, 2001; Virginia and Wall, 1999). Considerations of the spatial distribution of soil factors is thus essential to understanding ecosystem dynamics in the polar deserts of Victoria Land where spatial variability is largely physically driven because of the absence of vascular plants and their inﬂuence over soils. Medium to broad scale spatial variability in physical properties, such as local climate, till composition and age are ﬁrst-order controls over the chemical composition and functioning of soil ecosystems from scales of 1 to 1000 km (Bockheim, 1997; Burkins et al., 2000; Parsons et al., 2004; Elberling et al., 2006). However, ﬁner scale (1–10 m or less) variability may be especially important considering the absence of conspicuous biological activity on the soil surface and the exceptionally ﬁne scales pertinent to the microscopic organisms comprising Antarctic soil communities. For example, variation in ground surface features (e.g. surface roughness, albedo, rock polish, etc.), controls the energy balance and surface soil temperature (Campbell et al., 1998; Campbell and Claridge, 2006), while patterned ground formations resulting from cryoturbation of the active layer (Sletten et al., 2003) signiﬁcantly inﬂuence the distribution of soil moisture, salts, organic matter and invertebrates over centimeter to meter scales (Barrett et al., 2004). Across Victoria Land, spatial variability in soil landforms encompasses ecosystems developing in relatively favorable habitats in the northern coastal zones where soil temperature, moisture and organic matter availability are relatively high (e.g. Edmonson Point, Cape Hallett), to ancient soils developing in high elevation valleys adjacent to the Polar Plateau where the combination of aridity, salinity and cold contribute to inhospitable soil environments (Nkem et al., 2006). Controls and constraints over ecosystem functioning vary markedly among these types of environments, but along with the limited temporal window when soil temperature and availability of liquid water can support biotic activity (Kennedy, 1995; Treonis et al., 2000; Sinclair et al., 2003), soil chemistry is a primary control over the establishment and functioning of biota (Barrett et al., 2004; Nkem et al., 2006). The source and composition of organic matter, and the salt content of soils impose strong limitations over the establishment of multi-trophic communities. Below we discuss examples of terrestrial ecosystem functioning drawing on studies conducted in a range of these types of environments sites in Southern and Northern Victoria Land. 2. Regional variability in ecosystem structure and functioning

1.2. Spatial variability 2.1. Southern Victoria Land Extreme terrestrial ecosystems are characterized by strong spatial patterning and heterogeneity in soil biogeochemistry and biodiversity (Charley and West, 1975; Freckman and Virginia, 1989; Schlesinger et al., 1990, 1996; Davey and Clarke, 1991; Virginia et al., 1992; Smith

The McMurdo Dry Valleys (761300 –781000 S, 0 0 160100 –165100 E) of Southern Victoria Land form the largest ice-free region on the Antarctic continent and have an extensive record of soil description and ecological

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2.1.1. Controls over soil C turnover in the McMurdo Dry Valleys Several mechanisms and potential source of organic matter have been proposed to account for contemporary

concentrations of soil organic matter observed in the McMurdo Dry Valleys ranging from contemporary, but exceptionally low rates of C ﬁxation and redistribution (Johnston and Vestal, 1991; Friedmann et al., 1993), to legacy sources ﬁxed by ancient autotrophic communities (Matsumoto et al., 1990; Burkins et al., 2000), and to spatial transfers from productive hot-spots (Elberling et al., 2006; Hopkins et al., in press). For example, the presence of long-chain, odd-numbered n-alkanes and even-numbered n-alkanoic acids in soils supports an ancient vascular plant source for some amount of soil organic matter (Matsumoto et al., 1990), but it remains unclear what proportion of soil organic C reservoirs are accounted for by these Permian aged materials. More recently, stable isotope signatures have shown that contemporary soil organic matter resembles the C and N in lake sediments and endolithic materials, with lacustrine-like signatures most evident in low elevation soils near the edges of

5 South Lake Fryxell

0 -5 -10 δ‰

research beginning in the International Geophysical Year (IGY; 1957–1958) and continuing through the current research programs of Antarctica New Zealand (ANZ) and the US National Science Foundation (NSF). Two notable ecosystem-level research programmes are the ANZ Latitudinal Gradient Project (LGP) and the US MCM-LTER. The LGP is an international collaboration examining broad spatial gradients along the Victoria Land coast, while the MCM-LTER focuses on temporal trends in ecosystem dynamics. Despite the long history of research in the region, certain key information is lacking, including a complete description of the carbon cycle that explicitly considers contemporary inputs (Barrett et al., 2006). Arid soils are the most extensive landform of the McMurdo Dry Valleys occupying 495% of glacier icefree surfaces below 1000 m (Burkins et al., 2001); however, over 50% of the soils have subsurface ice, either as buried massive ice or as ice-cemented soil (Bockheim, 2002). Dry valley soils are derived from tills enriched in granites, sandstones, dolerites and meta-sedimentary rocks that range from Holocene to Miocene in age. Soils are typically alkaline, coarse textured, extremely low in organic matter, and often have high concentrations of soluble salts (Campbell and Claridge, 1987; Bockheim, 1997). Due to low precipitation and air temperatures, liquid water content is restricted; most snow sublimes and only a small amount melts (Gooseff et al., 2003). Campbell and Claridge (1982) describe moisture movement mostly in the form of vapor with limited migration of meltwater from snow. Unfrozen water persists on mineral surfaces to very low temperatures (Banin and Anderson, 1975; Yong et al., 1979), and it is assumed that most biological activity and ion migration occurs in these thin water ﬁlms (Ugolini and Anderson, 1973; Claridge et al., 1999; Treonis et al., 2000; Cowan et al., 2002; Dickinson and Rosen, 2003). The depauperate soil communities of the McMurdo Dry Valleys are strikingly different from those of other terrestrial ecosystems. No vascular plants or vertebrates inhabit the McMurdo Dry Valleys, and soil food webs are composed of only cyanobacteria, algae, fungi (including lichens), a few moss species, bacteria, yeasts, protozoans and few taxa of metazoan invertebrates (Schwarz et al., 1993; Friedmann et al., 1993; Freckman and Virginia, 1997; Stevens and Hogg, 2002; Bamforth et al., 2005). While recent work has suggested greater microbial diversity than previously recognized (Cowan et al., 2002; Gregorich et al., 2006), the diversity within invertebrate taxa is extremely low, often with nematodes representing the sole metazoan taxa (Freckman and Virginia, 1997). In this way, Southern Victoria Land soil communities are distinct even from Northern Victoria Land where multi-trophic communities are more common (Sinclair et al., 2003).

-15 -20 -25 -30 0 South Lake Hoare -5 -10

δ‰

3024

-15 -20 -25 -30 0

50

100 Elevation

150

Soil 13C

Lake Mat 13C

Soil 15N

Lake Mat 15N

200

Fig. 3. Stable isotope values for soil organic matter and lake algal mats from various elevations in Taylor Valley. Re-plotted from Burkins et al. (2000) and Lawson et al. (2004).

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Table 3 Comparisons of soil respiration, organic C reservoirs and turnover (MRT) from Victoria Land and selected terrestrial ecosystems Ecosystem

Annual heterotrophic C ﬂux (g C m2 yr1)

Soil organic carbon (kg C m2)

Mean residence time (yr)

Study

Arctic Tundra (Toolik Lake LTER) Hot desert (Jornada LTER) Taylor Valley Garwood Valley Cape Hallett moss bed Cape Hallett abandoned rookery mound

6076 224738 6.575 — 26.2711 126.0752

20.4 5.8 0.15 0.2–2.0 2.7 3.5

340 26 23 30–123 104 29

Raich and Schlesinger (1992) Raich and Schlesinger (1992) Burkins et al. (2001) Elberling et al. (2006) Barrett (unpublished) Barrett (unpublished)

contemporary lakes (Fig. 3). However, the notion of a signiﬁcant ancient organic matter fraction (i.e. legacy) contributing to contemporary soils is difﬁcult to reconcile with the relatively rapid rates of in situ C turnover reported by several investigators (Burkins et al., 2001; Parsons et al., 2004; Hopkins et al., 2005; Elberling et al., 2006). Rates of observed soil respiration are among the lowest reported for terrestrial ecosystems (Table 3); however, because of the small pools of soil organic C, mean residence times (MRT ¼ pool/ﬂux) are shorter than expected for ecosystems thought to be controlled by legacies of ancient organic matter. Differences in composition and proximity of source pools contribute to wide variation in soil C content and potential microbial activity (Figs. 3 and 4). Soils closest to contemporary lake edges typically have the greatest concentrations of soil organic C and are thus capable of supporting higher rates of microbial activity (Fig. 4). In contrast, lacustrine materials are unlikely sources of organic matter in high elevation soils, distant from contemporary lake edges such as the Dais in Wright Valley (850 m a.s.l.), which has low concentrations of soil organic C and low biotic potential (Fig. 4). At intermediate positions, particularly on ancient lake terraces, such as those above the level of the modern Lake Vanda in the Wright Valley, signiﬁcant deposits of organic residues persist presumably derived from a period when the lake level was high (Hall et al., 2001, 2002). Soil communities at high-level sites, far away from either modern or ancient lacustrine inﬂuences may be supported by low rates of contemporary C inputs. Estimates of soil respiration in such sites are of the same order of magnitude as rates of net primary production predicted by models developed for endolithic communities (Friedmann et al., 1993; Barrett et al., 2005), suggesting that in situ phototrophs may contribute to contemporary soil C cycling. Modern sources of organic matter potentially include exfoliation of cryptoendolithic communities in Beacon sandstone rocks and sediments (Friedmann et al., 1993; Sun and Friedmann, 1999), redistribution of moss and microbial mats from hydrological margins of lakes, ponds and streams (Moorhead et al., 2003; Elberling et al., 2006; Hopkins et al., 2006), as well as soil dwelling moss, lichens

and microbial phototrophs in intermittently wet soils. Pannewitz et al. (2005) reported net photosynthetic rates of up to 12 mmol CO2 m2 s1 for Byrum spp. and Ceratodon purpureus mosses at Granite Harbor (771000 S, 1621250 E), a coastal site in South Victoria Land. For comparison, maximum rates of soil respiration are 0.15 and 0.8 mmol CO2 m2 s1 in Taylor (Parsons et al., 2004), and Garwood Valleys (Elberling et al., 2006), respectively. Aboveground autotrophic communities likely dominate C cycling and ecosystem processes where they occur, but are patchily distributed and not common through much of the McMurdo Dry Valleys (though their cryptic nature potentially contributes to an underestimate of their importance). Phototrophic microbes have also been observed on the sub-surfaces of marble and quartzite pebbles throughout Victoria Land (Hopkins et al., 2006). These hypolithic communities can contribute signiﬁcantly to the C balance of surface soils in hot deserts (Schlesinger et al., 2003), but their importance in the cold deserts of Victoria Land ecosystems remains unknown. It is probable that contemporary inputs of C are spatially and temporally discontinuous, relying on brief and episodic availability of sufﬁcient liquid water and solar energy. Elberling et al. (2006) propose a ‘‘spatial subsidy’’ model wherein C ﬁxation by microbial mats in hydrological dynamic and biologically productive aquatic ecosystems is an important source of organic matter to less productive areas of the dry valley landscape. Eolian redistribution of organic matter from such systems could be an important source of soil organic matter, particularly in small and active hydrological basins where aquatic ecosystems occupy a large proportion of the total land area, but cannot account for the total reservoir of C found in systems where soils occupy 495% of the surface area and 470% of the total C reservoir (e.g. Burkins et al., 2001). The legacy and spatial subsidy models are not mutually exclusive (Elberling et al., 2006); it is clear that ancient organic matter does account for some proportion of the contemporary soil C (Matsumoto et al., 1990), with paleolakes contributing to large proportions of organic matter in low elevation soils (Burkins et al., 2000), but alone are insufﬁcient to account for observed rates of soil respiration and C turnover in the McMurdo Dry Valleys

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100000

(a) Log10 soil organic C (µg Cg-1 soil)

10000 1000 100 10 1 10000 Log10 basal respiration (µg Cg-1 soil)

(b) 1000

100

10

Log10 substrate induced respiration (µg Cg-1 soil)

1 10000

(c) 1000

100

10

1 3

2 (H)

Shannon Wiener Index

(d)

1

No data

Ta oa yl re ar or-F wo r od yxe ll -h G i ar wo llslo G pe od ar -p wo o lyg od -s on an G d G ar d u wo arw oo ne od d -lo -d G we e ar wo r m lta od or -s ai n tre G ar am e wo ed od ge -la ke ed ge G

Ta y

lo

r-H

ry ab -L

rig ht W

W

rig

ht

-D

ai

nt h

s

0

Fig. 4. Variation in organic C and potential biotic activity and diversity in soils collected from the McMurdo Dry Valleys: (a) log-soil organic C; (b) logbasal respiration; (c) log-substrate induced respiration; and (d) Shannon–Wiener diversity determined from the composition of phospholipid fatty acids (PFLA) in soils collected from various locations in the McMurdo Dry Valleys (Hopkins, unpublished data).

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2.1.2. Nutrient cycling in the McMurdo Dry Valleys Nitrogen cycling in Victoria Land is strongly inﬂuenced by physical processes such as deposition, salt accumulation and ionic migration, whereas local biological transformation of N may occur in favorable soil environments (Wada et al., 1981; Barrett et al., 2002a; Michalski et al., 2005; Gregorich et al., 2006). Nitrogen ﬁxation is not thought to widely occur in Victoria Land soils, though cyanobacteria are present (Vincent, 1988; Eckford et al., 2002) and N2 ﬁxation has been observed in steam channels (HowardWilliams et al., 1989), since availability of reduced inorganic N in soils is typically greater than necessary to sustain biological activity (Barrett et al., 2005). Biological transformations of N have been reported for soils and sediments in hydrological margins of dry valley streams and lakes (Barrett et al., 2002a; Gregorich et al., 2006). Trends in the relative concentrations of NH+ 4 and NO3 indicate a signiﬁcant role of biota in N transformations in soils and sediment adjacent to a Taylor Valley stream (Barrett et al., 2002a) and snowpacks (Gooseff et al., 2003). Similarly, Gregorich et al. (2006) reported rates of denitriﬁcation in the range of 0.01–0.10 mmol m2 min1 for organic-rich lake margin sediments in the Garwood Valley, with the highest rates corresponding to the warmest and wettest microclimatic conditions. Hopkins et al. (2005, 2006) report detectable nitriﬁcation, with the largest potentials associated with wetter and N-rich soils. Investigations of N cycling in the dry valleys have focused almost exclusively on inorganic (NH+ 4 , NO2 and NO3 ) and total N; dissolved organic N is not thought to be a signiﬁcant fraction of the total N, as it can be elsewhere in

Antarctica on ornithogenic or moss-inﬂuenced soils (Jones et al., 2004). Low biotic potential of the soils contributes to large concentrations of NO 3 content in some surface soils of Southern Victoria Land (Fig. 5), associated mainly with the

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(Burkins et al., 2001; Barrett et al., 2006; Elberling et al., 2006). The relative importance of ancient vs. contemporary C to total soil budgets and perhaps more signiﬁcantly to contemporary soil food webs is still an open question. However, incubation experiments and models developed from these incubations (Barrett et al., 2005, 2006; Hopkins et al., 2006) suggest that a small, kinetically labile fraction of organic matter may be more important to contemporary ecosystem functioning than slowly cycling (i.e. a putative legacy pool) organic matter. The spatial subsidy and legacy models describe a C cycle where the ﬁxation of C by phototrophs, and the eventual heterotrophic consumption and decomposition of this organic matter are spatially and perhaps temporally decoupled. This decoupling of production and decomposition may contribute to food webs that are strongly C limited. For example, in a long-term manipulation experiment C amended plots showed signiﬁcant increases in soil respiration and nematode abundance relative to water and warming treatments (Burkins et al., 2001). This suggests that dry valley soil communities are capable of responding to exogenous C inputs. It is probable that multiple sources of organic matter contribute to observed levels of soil organic matter, but active, recently ﬁxed pools are the most important to contemporary food webs.

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local microclimate conditions, surface exposure age (Bockheim, 1997), distance inland (Keys and Williams, 1981) and soil moisture content (Barrett et al., 2002a). Soils in Beacon Valley, a valley in the Quartermain Mountains with exposure ages of hundreds of thousands to million of years (Marchant et al., 1996; Ng et al., 2005), have unusually large pools of NO 3 , that comprise the majority of the soil N reservoir (Fig. 5). These large accumulations of NO 3 are thought to be a legacy of atmospheric deposition over millennial time scales in an environment where physical losses by leaching and biological losses via denitriﬁcation are insigniﬁcant (Claridge and Campbell, 1977; Wada et al., 1981; Michalski et al., 2005). A comparison of the NO 3 depth proﬁles with a conservative tracer such as Cl is an index of the degree of biological control over N cycling since Cl content is not inﬂuenced by biota, while processes that create (nitriﬁcation) and consume NO 3 (microbial uptake, denitriﬁcation) are. The strong correlation between NO concentrations 3 and Cl 2 (r ¼ 0:83) in the oldest surface soils of the McMurdo Dry Valleys suggests that the vertical distribution of N is unaffected by biological transformation. It is notable that the correlation between NO 3 and Cl-proﬁles (index of physical dominance over N cycle) increases with relative age of the soil surface (Fig. 5). While dry ecosystems are particularly sensitive to breaks in the N cycle, leading to large accumulations of mineral N, especially NO 3 (Virginia and Jarrell, 1983; Ehleringer et al., 1992; Evans and Ehleringer, 1993), these unusual Antarctic N budgets are especially noteworthy because they suggest a negligible biotic inﬂuence over N cycling through a large area of the dry valley region. Phosphorus cycling in dry valley soils is also limited by the low biological activity, as well as by low rates of apatite weathering (Campbell and Claridge, 1987; Gudding, 2003; Blecker et al., 2006). In contrast to soil C, concentrations of total P are actually moderate high relative to many types of soils because of the large contributions of un-weathered calcium-bound fractions which dominate soil P budgets (Blecker et al., 2006). Concentrations of bulk P (HClsoluble) vary within and among valleys (Table 1), though total P, and the proportion of weathered fractions are greatest in soils developing on young tills of Taylor Valley adjacent to the Ross Sea (Gudding, 2003; Blecker et al., 2006). These concentrations of P, together with the low availability of organic C and variable levels of soil N contribute to imbalanced stoichiometry in dry valley soils. Ratios of C:P and N:P are often below 1, far below the proportions necessary to sustain balanced microbial growth. 2.2. Northern Victoria Land 2.2.1. Edmonson point Edmonson Point (741200 S, 1651080 E) is an approximately 6 km2 ice-free coastal area in the vicinity of Mt. Melbourne 75 km northeast of the Italian Research Station at Terra

Nova Bay. The terrain is composed of largely unconsolidated volcanic rock with generally undulating relief cut by numerous small drainages (Bargagli et al., 1999). The two principle sources of soil organic C are the extensive moss beds consisting of Byrum spp., C. purpureus and Hennediella heimii, and seabird guano, which contribute to differences in nutrient availability, stoichiometry and soil faunal community composition (Bargagli et al., 1999; Smith, 1999; Tosi et al., 2005). Soils from similar environments in West Antarctica exhibited high concentrations of dissolved organic N in pore water and rapid rates of amino acid turnover relative to inorganic pools of N (Jones et al., 2004). The abundance of melt-water, marine- and bird-derived nutrients make coastal zones such as Edmonson Point, among the most productive ecosystems in Victoria Land. This high productivity is evident in high concentrations of organic matter and nutrients relative to sites further south, even for soils unaffected by marine birds (Table 1). At Edmonson Point the distribution, composition and productivity of moss communities is dependent on the spatial proximity to water sources (Smith, 1999). Moss beds can modify the chemical environment of the surrounding soils, not only by ﬁxing C, but also through altering the nutrient availability and salt migration of the soils. For example, H. heimii appears to accumulate Ca, Cu, K, Mg and Na from the soils and enhances the upward migration of salt solutions, and NO 3 present in stream waters indicates seasonal nitriﬁcation and leaching from moss beds (Bargagli et al., 1999). 2.2.2. Cape Hallett and vicinity Cape Hallett (721190 S, 1701130 E) is a small ice-free area (72 ha) in Northern Victoria Land 100 km south of Cape Adare. Cape Hallett is occupied by numerous sea birds with about half of the total area occupied by an Adelie Penguin (Pygoscelis adeliae) rookery on the low-lying coastal areas and Skua Gull (Catharacta maccormicki) colonies on the lower portions of the scree slope. The Cape Hallett landscape consists of basalt screes and moraines colonized by dense moss beds (Byrum spp.), green algae (Praisola spp.) and conspicuous lichen colonies (Rudolph, 1963; Pannewitz et al., 2005). Hallett Station, a joint US/NZ research station, was built within the penguin rookery on Seabee Spit at Cape Hallett in 1956 in preparation for the IGY (1957–1958) and was operational from 1957 to 1973 (Gutheridge, 1983). As a consequence of human activities on site including; landscape modiﬁcation by excavation and removal of mounds from penguin rookeries, soil compaction, waste disposal and fuel spills, soils on the spit have been disturbed. Meteorological data collected during this time recorded mean annual temperatures of 15.31C and annual precipitation of 18.3 cm of water equivalent per year (Duphorn, 1981). Campbell and Claridge (1987) emphasize that there is higher moisture availability in soils at Cape Hallett compared to sites in Southern Victoria

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Land, because at Cape Hallett snow tends to melt rather than sublimes. Consequently ephemeral melt water ponds and streams are common and sub-surface soils are typically saturated after snowfall, with groundwater at between 8 and 80 cm below the soil surface during summer (Aislabie, unpublished data). Similar to Edmonson Point, and in contrast to the McMurdo Dry Valleys, factors affecting the patterning of soil organic matter and biota are largely marine. This is reﬂected in low C:N ratios and high concentrations of soil organic matter enriched in 15N relative to nearby noncoastal soils (Fig. 2). In these soils, ornithogenic deposits (i.e. penguin guano, and sea salts), inﬂuence the soil chemistry and biodiversity of soil ﬂoral and faunal communities occupying penguin rookeries (Smith, 1999; Porazinska et al., 2002; Sinclair et al., 2003; Wall, unpublished), and are distinct from those found in adjacent soils unaffected by ornithogenic inputs. Large concentrations of soil ammonium (4700 mg N g1 soil) in soils near active penguin mounds may inhibit the establishment of widespread lichen species such as Umbilicaria decussata or Pseudephebe minuscola and diverse metazoan communities, which are often substituted by bright-colored thalli of muscicolous lichens, e.g. Xanthoria elegans, X. mawsonii, and Candelariella flava (Bargagli, personal observation), as well as specialist nematode species such as Panagrolaimus davidi (Porazinska et al., 2002). In addition to the marine inﬂuences, the presence of extensive moss and lichen communities represent a large source of organic C to resident food webs (Rudolph, 1963; Smith, 1999; Pannewitz et al., 2005). Pannewitz et al. (2005) reported net photosynthetic rates of up to 4 mmol CO2 m2 s1 in Byrum spp. beds, where rates of C ﬁxation were controlled by the photon ﬂux density and the availability of water. Greater availability of organic matter (Table 1) and longer duration, warmer summer results in much higher rates of soil respiration (Fig. 6) and more diverse soil fauna communities in non-rookery soils relative

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to Southern Victoria Land (Sinclair et al., 2003; Adams et al., 2006). Other ice-free terrestrial environments in the vicinity of Cape Hallett occur in dry cirques and saddles of the Admiralty and Victoria Mountain Ranges that appear to be northern analogs of non-marine inﬂuenced terrestrial environments in Southern Victoria Land, i.e. the McMurdo Dry Valleys. Crater Cirque (Borghini et al., 2005), and an unnamed bowl-shaped valley northeast of Luther Peak are two such locations. The soils of the latter area, hereafter Luther Vale, occupy approximately 100 ha at 200 m a.s.l. and are underlain by metamorphic rock and melt-water from snowﬁelds on the northwestern side of Luther Peak. These higher elevation areas do not appear to receive signiﬁcant marine inputs and consequently the soil organic matter concentrations and isotopic signatures are more similar to inland soils from South Victoria Land than to the neighboring Cape Hallett, indicating a terrestrial source of organic matter, i.e. algae, moss and lichens (Fig. 2 and Table 1). Soil communities are similar to the McMurdo Dry Valleys, with the nematode Scottnema lindsayae dominating faunal populations (Freckman and Virginia, 1997; Barrett, unpublished). The similarities between Luther Vale, and the McMurdo Dry Valleys suggests that the source of C (microbial phototrophs versus ornithogenic) is a key factor in structuring soil communities. 2.3. Soil biodiversity and ecosystem functioning in Victoria Land Many studies have documented co-variation in spatial patterns of soil biodiversity and biogeochemistry in Victoria Land (Freckman and Virginia, 1997; Powers et al., 1998; Bargagli et al., 1999; Virginia and Wall, 1999; Courtright et al., 2001; Barrett et al., 2005; Connell et al., 2006). A growing amount of literature is providing support for linkages between soil biodiversity and ecosystem functioning driven by microbial, metazoan and cryptogam communities. For example, bacterial and microbial functional groups have been found to contribute to speciﬁc ecosystem processes such as hydrocarbon degradation and N2-ﬁxation (Eckford et al., 2002), and primary productivity in specialized lithic environments is facilitated by assemblages of microorganisms probably unique to Antarctica (Johnston and Vestal, 1991; Friedmann et al., 1993); while differences in diversity of soil microbial communities in the dry valleys (based upon analysis of phospholipid fatty acids) is associated with variation in soil C content and potential biotic activity (Fig. 5d). Similarly, variation in soil invertebrate communities encountered at landscape and watershed scales is associated with differences in soil respiration (Parsons et al., 2004) and N cycling (Treonis et al., 1999; Barrett et al., 2002a) in Taylor Valley. In above-ground communities, signiﬁcant differences in photosynthetic rates among moss species contribute to variation in C balance in sites from

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Southern and Northern Victoria Land (Smith, 1999; Pannewitz et al., 2005). Application of emerging molecular tools, along with the increased capability for in situ measurements of biogeochemical processes promise greater insight to the relationship between soil biodiversity and ecosystem functioning in Victoria Land. 2.4. Response of Victoria Land ecosystems to anthropogenic activities Victoria Land is a natural setting for testing hypothesis about climate change effects on ecosystem functioning. Antarctic systems are inherently sensitive to climate variability because even small changes in temperature can initiate phase changes in water and alter hydrology and biogeochemical cycling of Antarctic soils, which will potentially have myriad effects on ecosystem process. Victoria Land organisms are adapted to extreme environmental conditions but may be highly sensitive or intolerant to changes exceeding pre-existing thresholds. For example, the non-linear responses of physical conditions (lake ice thickness, stream ﬂow, soil moisture availability) associated with even small variations in temperature are linked to signiﬁcant changes in productivity and population declines of soil biota (Doran et al., 2002a). Polar vegetation and soil communities typically have higher apparent Q10’s than found in temperate ecosystems (Dalias et al., 2001; Mikan et al., 2002; Fierer et al 2005); those reported by Elberling et al. (2006) and Hopkins et al. (2006) from Garwood Valley are among the highest reported for all terrestrial ecosystems Other examples of environmental change include issues especially signiﬁcant in Antarctica (i.e. increased UV irradiation), as well as more widespread concerns that affect ecosystems worldwide (e.g. chemical pollutants, invasions). UV radiation is especially pertinent to Antarctica given the substantial decrease in stratospheric ozone column depth occurring within the past several decades (Madronich et al., 1998). Recently, Tosi et al. (2005) reported that UV radiation is the most important ecological factor for soil fungi in Victoria Land. Hydrocarbon spills, as a common anthropogenic disturbance, have the potential to cause a large environmental impact in Victoria Land ecosystems (Aislabie et al., 2004). Such spills occur mainly near scientiﬁc research stations where fuel is transported and stored in large quantities, and where aircraft and vehicles are refueled. Hydrocarbon contamination of Antarctica soils increases soil C, leading to changes in bacterial and fungal communities and may modify soil temperature and moisture regimes during summer (Aislabie et al., 2004). In addition to fuel spills other sources of hydrocarbons include emissions from diesel generators and incinerators or vehicles burning fuel (Lyons et al., 2000). Terrestrial ecosystems of Victoria Land may also serve as excellent systems for evaluating global transport of trace contaminants (Lyons et al., 1999; Bargagli, 2001). For

example, concentrations of heavy metals and organochlorine compounds in some Victoria Land soils are among the lowest yet reported, even for remote areas (Bargagli et al., 1998; Borghini et al., 2005). Some widespread species of Antarctic mosses and macro-lichens may also serve as reliable bio-monitors for evaluating global transport and deposition of persistent atmospheric contaminants in Victoria Land coastal ice-free areas (Bargagli, 2001). Concentrations of trace metals, organochlorine compounds and artiﬁcial radionuclides in Victoria Land soils and cryptogams are among the lowest yet reported for Earth’s remote areas (Bargagli, 2005; Borghini et al., 2005). However, the ﬁnding of enhanced mercury accumulation in terrestrial ecosystems adjacent to the Terra Nova Bay coastal polynya (Bargagli et al., 2005) supports speculations on the role of ice crystals (‘‘frost ﬂowers’’) growing in polynyas as source of bromine compounds which are involved in the oxidation and deposition of atmospheric mercury. Possible change in regional climate and sea ice coverage in the Southern Ocean may therefore increase the role of Victoria Land ecosystems as sinks in the global mercury cycle. 3. Priorities for future research Detecting and measuring ecosystem processes in extreme environments like Victoria Land soil ecosystems presents a signiﬁcant challenge that requires new approaches. Understanding the mechanisms underlying ecosystem functioning, and the magnitudes of key processes, e.g. net primary productivity (NPP), soil respiration, decomposition, nutrient mineralization and soil water dynamics is limited by the detection limits of analytical techniques and instrumentation in this environment. Thus developing novel, and modifying existing techniques for measuring key ecosystem functioning is a priority of the Victoria Land soil research community. For example, recently developed automated soil respiration measurement systems might provide an approach to understand controls over contemporary C cycling and help close the organic matter budgets of Victoria Land soil ecosystems. Together with automated climate monitoring stations (Doran et al., 2002b) and the proposed active-layer monitoring program (Guglielmin et al., 2003), long-term records of soil respiration in Victoria Land could provide one of the most sensitive indications of ecological responses to climate variability. The microbial base of Victoria Land ecosystems is both a challenge and an opportunity for researchers to integrate ecosystem process with biodiversity. One potential approach is to use biotic indicators as proxies for processes that are difﬁcult to detect. Ecosystem processes that integrate or directly express biotic activity (e.g. net primary production which integrates all autotrophic activity, or nitriﬁcation which is a direct function of speciﬁc nitrifying bacteria) may be directly linked to the functioning of organisms expressing speciﬁc genes. For example, nucleic acid-based techniques can be utilized to detect and quantify

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the presence of microbial groups associated with speciﬁc processes, such as ammonia oxidizers (Nicolaisen and Ramsing, 2002; Hermansson et al., 2004; Horz et al., 2004; see also Adams et al., 2006), or the presence or expression of functional genes involved in biogeochemical processes such as N2-ﬁxation or denitriﬁcation (Wallenstein and Vilgalys, 2005). The possibility of linking ecosystem function to expression of speciﬁc genes is not unique to Antarctic ecosystems, but the very small ﬂuxes encountered in these ecosystems make molecular tools a useful precursor to measuring ecosystem processes. In an environment where climate and organic substrate are limiting to biological processes, including molecular approaches in the ecosystem tool kit may aid researchers by allowing them to stratify ﬂux measurements by the presence of speciﬁc genes.

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4. Synthesis The general controls over the structure and functioning of Victoria Land soil communities are the availability of liquid water and organic matter. Variation in the distribution and activity of biological communities, and therefore ecosystem processes depend upon the proximity of water and the duration of liquid water availability. Environmental gradients are structured on local scales (1–1000 m) by variation in desert pavement (albedo, slope, aspect), the proximity of streams, lakes, snow-ﬁelds or glaciers, and regionally over the 81 of latitude in Victoria Land, according to latitudinal variation in energy balance. The physical factors (relative humidity, temperature, wind speed) that determine whether snow and ice melts or sublimates are critical in determining soil water availability (Fig. 1). Since proximity to open water is a strong inﬂuence over relative humidity (Doran et al., 2002b) coastal zones are the most productive ecosystems of Victoria Land, and therefore coastal to interior gradients may be as important as latitudinal gradients in structuring ecosystem functioning. Imbalanced stoichiometry of some soils indicates little biotic control over bulk element ratios in soil ecosystems. These imbalances are most strongly expressed in geologically young soils where P concentrations are highest (low N:P), in older soils where concentrations of N (as NO 3) exceed C concentrations, and in coastal soils where marine inputs (aerosols and bird guano) can exceed in situ biological or lithological sources of both N and P. Some soils of the McMurdo Dry Valleys have high concentrations of inorganic N, especially as NO 3 in surface soils relative to inland soils of Northern Victoria Land and dry temperate soils in general (Virginia et al., 1992; Barrett et al., 2002b), generating elemental ratios that depart widely from biological stoichiometry. Molar ratios of C:P based upon total P fractions are in the range of 0.9–1.9, and C:N vary from 20 to less than 1. The absence of multi-trophic communities in Victoria Land soils where the C:N ratios are far below the Redﬁeld Ratio of 6.6 reﬂects the inﬂuence of this imbalanced stoichiometry (Fig. 7). The only other terrestrial systems with comparably imbalanced stoichio-

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Invertebrates present Invertebrates absent Fig. 7. Soil organic C vs. N content of Victoria Land soils where invertebrate communities are present and absent. The solid line is the Redﬁeld Ratio of 6.625 C:N. The majority of points are from the McMurdo Dry Valleys; the points with the highest concentrations of C and N are from moss and rookery soils at Cape Hallett (MCM-LTER and Barrett, unpublished data).

metry (low C, high N, high P) are the extremely arid soils of the Atacama Desert (Ehleringer et al., 1992), which are a hot desert analog of Victoria Land ecosystems. Thus, imbalanced stoichiometry may be a general characteristic of extreme environments resulting from a lack of biotic inﬂuence over C, N and P cycling. The spatial heterogeneity and the diversity of landforms, soil types and sources of organic matter in Victoria Land exert signiﬁcant control over ecosystem processes. Sources of organic matter, whether in situ (i.e. moss, algae, lichens, endoliths), external or legacy inputs (e.g. lake or marine sediments, seabird guano), constrain the energetics and composition of soil communities. From a functional point of view, variation in reported rates of soil respiration, photosynthesis and C turnover are evidentially contingent upon the local climate, hydrological regime and source of organic matter. For example, there are signiﬁcant differences in rates of soil respiration and potential biotic activity even among geomorphically similar sites such as Garwood and Taylor Valley (Figs. 4 and 6). Considering the ranges of soil organic C content and turnover reviewed here, variation in microclimate encountered over environmental gradients along with the sources of organic matter may be the most important sources of variation in soil biodiversity and ecosystem functioning. Acknowledgements This paper is a product of the NSF sponsored workshop: Synthesis of Soil Biodiversity and Ecosystem Functioning

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in Victoria Land, Antarctica. We thank T. Seastedt for providing comments on an earlier version of the manuscript and Chris Frost, Breana Simmons, Patti Orth and Lily Hoffman for providing logistical support during the workshop. We are especially grateful to David Coleman for hosting the meeting and editing this volume. Two anonymous referees provided thoughtful reviews that have improved the organization and clarity of this manuscript.

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Terrestrial ecosystem processes of Victoria Land, Antarctica

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