L.G. Greenfield

Controls on the distribution of productivity and organic resources in Antarctic Dry Valley soils

The Antarctic Dry Valleys are regarded as one of the harshest terrestrial habitats on Earth because of the extremely cold and dry conditions. Despite the extreme environment and scarcity of conspicuous primary producers, the soils contain organic carbon and heterotrophic micro-organisms and invertebrates. Potential sources of organic compounds to sustain soil organisms include in situ primary production by micro-organisms and mosses, spatial subsidies from lacustrine and marine-derived detritus, and temporal subsidies (‘legacies’) from ancient lake deposits. The contributions from these sources at different sites are likely to be influenced by local environmental conditions, especially soil moisture content, position in the landscape in relation to lake level oscillations and legacies from previous geomorphic processes. Here we review the abiotic factors that influence biological activity in Dry Valley soils and present a conceptual model that summarizes mechanisms leading to organic resources therein.

Isotopic evidence for the provenance and turnover of organic carbon by soil microorganisms in the Antarctic dry valleys.

The extremely cold and arid Antarctic dry valleys are one of the most environmentally harsh terrestrial ecosystems supporting organisms in which the biogeochemical transformations of carbon are exclusively driven by microorganisms. The natural abundance of (13)C and (15)N in source organic materials and soils have been examined to obtain evidence for the provenance of the soil organic matter and the C loss as CO(2) during extended incubation (approximately 1200 days at 10 degrees C under moist conditions) has been used to determine the potential decay of soil organic C. The organic matter in soils remote from sources of liquid water or where lacustrine productivity was low had isotope signatures characteristic of endolithic (lichen) sources, whereas at more sheltered and productive sites, the organic matter in the soils that was a mixture mainly lacustrine detritus and moss-derived organic matter. Soil organic C declined by up to 42% during extended incubation under laboratory conditions (equivalent to 50-73 years in the field on a thermal time basis), indicating relatively fast turnover, consistent with previous studies indicating mean residence times for soil organic C in dry valley soils in the range 52-123 years and also with recent inputs of relatively labile source materials.

Release of mineral nitrogen from plant root nodules

Abstract Root nodules from a variety of nodulated leguminous and non-leguminous plants were added to soil samples and the net release of mineral N was monitored periodically for 56 or 98 days. Over these periods, various nodules demonstrated either net N immobilisation, mineralisation, or immobilisation followed by mineralisation. No nodular tissue mineralised > 30% of its organic N content. Mineral N release from the nodules was not significantly related to their C to N ratios or organic N content ( r = 0.410). Nitrogen fractionation data from these nodules indicated that the main forms of N present in combined form were α-amino N and hydrolysable unidentified N(HUN). Attempts to relate mineral N release to the forms of N present in the nodules indicated that nodule N mineralisation was significantly related to nodule concentration of NH 4 -N and hexosamine-N in combination ( r =0.741). It was concluded that these two components are probably highly important in regulating mineral N release from decomposing root nodules.

Retention of precipitation nitrogen by Antarctic mosses, lichens and fellfield soils

Retention of mineral elements in precipitation by mosses and lichens involving ion exchange and chelation mechanisms is a source of nutrients for these biota growing on rocks and nutrient poor soils (Brown 1987, Crittenden 1989). In qualitative work not involving nitrogen (N) Allen et al. (1967) demonstrated that fresh Antarctic mosses treated with hydrochloric acid could retain Na, P, Ca and K after leaching with concentrated solution of these elements. Ahumic fellfield soils are widespread in Antarctica and support sparse plant growth. This short note reports the results of work designed to show that fellfield soils and plants may obtain most of their N from atmospheric precipitation.

Microbial responses to carbon and nitrogen supplementation in an Antarctic dry valley soil

Abstract The soils of the McMurdo Dry Valleys are exposed to extremely dry and cold conditions. Nevertheless, they contain active biological communities that contribute to the biogeochemical processes. We have used ester-linked fatty acid (ELFA) analysis to investigate the effects of additions of carbon and nitrogen in glucose and ammonium chloride, respectively, on the soil microbial community in a field experiment lasting three years in the Garwood Valley. In the control treatment, the total ELFA concentration was small by comparison with temperate soils, but very large when expressed relative to the soil organic carbon concentration, indicating efficient conversion of soil organic carbon into microbial biomass and rapid turnover of soil organic carbon. The ELFA concentrations increased significantly in response to carbon additions, indicating that carbon supply was the main constraint to microbial activity. The large ELFA concentrations relative to soil organic carbon and the increases in ELFA response to organic carbon addition are both interpreted as evidence for the soil microbial community containing organisms with efficient scavenging mechanisms for carbon. The diversity of the ELFA profiles declined in response to organic carbon addition, suggesting the responses were driven by a portion of the community increasing in dominance whilst others declined.