| In soils of glacier forelands and deserts, microbial communities can be important contributors to ecosystem processes because vegetation is sparse or absent. These ecosystems are also expanding geographically due to global warming. In spite of this, the role of microbes, particularly with regard to carbon (C) and nitrogen (N) cycling, has not been well characterized. Therefore, I examined the mechanisms and patterns of soil C and N cycling in these two contrasting environments using a combination of standard biogeochemical parameter and rate measurements and DNA-based molecular fingerprinting of microbial communities. The glacier foreland site on the west coast of the Antarctic Peninsula had been deglaciated for a maximum of approximately 30 years. Soil moisture declined with distance from the glacier and may explain why litter decomposition rates declined and soil nitrate concentrations increased with distance. Microbial communities were able to colonize exposed ground soon after glacial recession (i.e. within the first growing season). Soil C inputs at this site were dominated by cyanobacterial primary production. Unexpectedly, biological N-fixation was carried out predominantly by beta-proteobacteria rather than cyanobacteria. Rates of other N transformations, including denitrification and aerobic and anaerobic ammonia-oxidation, while present, were very low relative to N-fixation rates. The presence of vascular plants (Deschampsia antarctica Antarctic hairgrass) that established 30 m from the glacier during the past 5 years, did not lead to detectable changes in localized pools of soil C and N. As was the case in the glacier foreland, the N cycle of biological soil crusts (BSCs) of US Southwestern deserts was also incomplete, with negligible denitrification and anaerobic ammonia-oxidation rates, regardless of their geographical location. In contrast to the glacier foreland, aerobic ammonia-oxidation was a consistently significant transformation in addition to N-fixation in BSCs. These results confirm the importance of microbial communities as C and N providers in these extreme environments. |
