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The McMurdo Dry Valleys of southern Victoria Land, Antarctica, represent a unique environment where climatic extremes limit the development of complex and diverse soil communities. In these soils, the microbial feeding nematode is the most abundant and widespread invertebrate. The distribution of this nematode is highly patchy and is related to soil properties such as salinity and pH (Freckman and Virginia in press). The primary source of soil organic matter (SOM) sustaining these low-diversity dry valley soil ecosystems is not obvious given the virtual absence of above-ground plant biomass. The photosynthetic capacity of the soils may be inadequate to account for observed levels of SOM, implicating other sources of organic matter such as windborne particulates from biologically richer lakes and streams, cryptoendolithic communities, and the Ross Sea (Wynn-Williams 1990, pp. 71-146). An understanding of dry valley soil communities and the cycling of carbon and other nutrients in these ecosystems requires information about the sources, quality, and distribution of SOM. In this article, we report first-year results of a study to examine the sources of organic matter to Taylor Valley using stable isotopes of carbon and nitrogen. This work is part of a larger effort to determine the factors controlling the distribution, diversity, and function of soil biota in the McMurdo Dry Valleys (Freckman and Virginia in press; Powers, Freckman, and Virginia 1995).
In austral summer 1994, we began an intensive stable isotopic study of Taylor Dry Valley soils. Because the isotopic fractionation of soil nitrogen reactions in Antarctica is known to be large (Wada, Shibata, and Torii 1981), we hypothesized that potential organic matter sources to dry valley soils would have distinct nitrogen isotopic abundance [ratio of nitrogen-15/nitrogen-14 (15N/14N)]. This information coupled with data on the carbon-13/carbon-12 (13C/12C) ratio of organic matter, also known to vary between marine and terrestrial sources, might allow us to distinguish between potential sources of organic matter to soil by, first, determining the carbon and nitrogen isotopes of sources and, second, examining isotopes and SOM concentrations along gradients (distance, elevation) from potential sources of organic matter to study mixing of sources and their influence on the isotopic signature of SOM.
Potential organic matter sources (marine and lake algae, rock infected with cryptoendolithic organisms, penguin rookery soil, and bird remains) from dry valley lakes to Ross Island penguin rookeries were sampled. Anthracite coal from the surrounding Beacon Supergroup lithologies was also sampled because of its high organic carbon content and its possible dissemination in the glacial tills upon which most dry valley soils form (Campbell and Claridge 1987). To characterize the potential range of isotopic variation in the SOM of dry valley soils themselves, 41 soil sites were systematically sampled throughout Taylor Dry Valley. Samples were taken along six elevational transects perpendicular to the length of the valley, from the head of the valley to the Ross Sea, areas roughly corresponding to the three major drainage basins for Lakes Bonney, Hoare, and Fryxell, respectively. The relative abundance of 13C and 15N of organic matter in all samples was analyzed using combustion and cryogenic purification at the Dartmouth Light Isotope Tracers in the Environment Laboratory. Isotopic measurements are expressed in parts per thousand difference from a standard using the equation:
δ13C or δ15N = [(Rsample-Rstandard)/(Rstandard)]x1000
where R=13C/12C or 15N/14N. The standard for carbon is the PeeDee Belemnite (PDB) and, for nitrogen, atmospheric N2.
Based on the samples of source materials collected in our study and on literature reports, organic matter derived from penguin rookeries, marine algae, lacustrine, and cryptoendolithic sources have sufficiently distinct carbon and nitrogen isotope signatures for measurements of SOM to provide data on the source or sources of organic matter to a particular location ( figure). The range of variation for the 41 soils was about 22 δ15N and 10 δ13C units. The data show that distant Ross Island penguin rookeries are not a likely source of SOM to Taylor Valley because the characteristic 15N-enriched isotopic signature of penguin rookeries (Mizutani and Wada 1988) was not found. The soil isotopic data do not rule out the other hypothesized sources, however, so sources of SOM to Taylor Valley may be multiple. Isotopically, SOM in Taylor Valley was shown to share isotopic signatures with anthracite coal, marine-derived organic matter, lacustrine-derived organic matter, and cryptoendolithically derived organic matter. Coal is probably not an important source of carbon to the Taylor Valley soils we sampled because samples falling within the isotopic range of both coal- and marine-derived organic matter (see stippled area, figure) have an organic carbon-to-nitrogen ratio of 11.6±4.6. In contrast, soils with significant coal content would be expected to have organic carbon-to-nitrogen ratios greater than 100 (see Campbell and Claridge 1987). Petrographic and SEM research is currently being done to verify this result.
The pattern of the soil isotope data also suggests a mixing of sources in Taylor Valley SOM. Sites at elevations more than 150 meters above sea level contained organic matter isotopically most similar to cryptoendolithic systems, whereas sites at the valley floor contained organic matter more similar to that of lacustrine and/or marine systems. Our early results indicate measurements of carbon and nitrogen isotopes hold considerable promise for organic matter source identification in antarctic soils.
We thank the McMurdo Dry Valley Long-Term Ecological Research (LTER) project and Jim Raymond for lacustrine and marine samples. We are also grateful for the excellent logistic support given by the VXE-6 Squadron of the United States Navy, and the Royal New Zealand Air Force helicopter crews. The help of L.E. Powers and M. Ho, as well as E. Courtright, R. Alward, M. Roberts, and E. Marlies was invaluable. This work is supported by National Science Foundation grant OPP 91-20123 to R.A. Virginia and D.W. Freckman and is a contribution to the National Science Foundation McMurdo LTER Program.
References
Campbell, I.B., and G.G.C. Claridge. 1987. Antarctica: Soils, weathering processes and environment. New York: Elsevier Science Publishers.
Freckman, D.W., and R.A. Virginia. In press. Low diversity antarctic soil nematode communities: Distribution and response to disturbance. Ecology.
Powers, L.P., D.W. Freckman, and R.A. Virginia. 1995. Spatial distribution of nematodes in polar desert soils of Antarctica. Polar Biology, 15, 325-333.
Mizutani, H., and E. Wada. 1988. Nitrogen and carbon isotope ratios in seabird rookeries and their ecological implications. Ecology, 69, 340-349.
Wada, E., R. Shibata, and T. Torii. 1981. 15N abundance in Antarctica: Origin of soil nitrogen and ecological implications. Nature, 292, 327-329.
Wynn-Williams, D.D. 1990. Ecological aspects of antarctic microbiology. In K.C. Marshall (Ed.), Advances in microbial ecology (Vol. 2). New York: Plenum Press.