Wednesday, August 8, 2007

PS 40-5: Iron reduction as a dominant soil respiratory process in an Arctic ecosystem

Mony Jha and David A Lipson. San Diego State University

Global warming is predicted to have relatively strong effects on arctic ecosystems, including potentially dramatic changes in soil water. Such changes could alter the balance between aerobic and anaerobic respiration, leading to major alterations of the arctic C cycle. Methanogenesis has been studied in arctic ecosystems, but little is known about other anaerobic processes in these soils. This study attempts to understand the nature and importance of anaerobic respiration in an arctic ecosystem. The study was conducted in a Sphagnum-dominated bog within the Barrow Environmental Observatory (BEO) in Barrow, Alaska. The saturated soils were found to be highly reduced and low in oxygen. Oxidation-reduction potentials measured from 2-10 cm in depth, and the lack of other alternative electron acceptors such as sulfate or nitrate in the pore water, indicated that anaerobic respiration may be generally dominated by iron reduction. Analysis of soil pore water collected in microlysimeters confirmed the presence of soluble forms of both Fe(III) and Fe(II). Laboratory experiments revealed high, but spatially variable, potential Fe(III) reduction activity in soils throughout the summer.  These experiments also showed that some component of the soil is required for Fe(III) reduction, possibly indicating an important role for humic acid reduction in anaerobic respiration in this ecosystem. Molecular analysis of microbial communities present in the soil samples confirmed the presence of Rhodoferrax ferrireducens, a known iron-reducing microorganism, also capable of using humic substances as terminal electron acceptors. The mechanisms of iron and humic acid reduction in anaerobic respiration is currently under investigation

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