The interaction of iron and humic substances in the Arctic environment

Background/Question/Methods

Reduction of humic substances and iron appears to play a large role in carbon cycling in Arctic peat soils and may directly compete with methanogenesis. Increased availability of these substances, as could result from increased thaw depth and exposure of buried mineral horizons, may lead to altered CO₂ and CH₄ fluxes of the ecosystem. This may prove critical since these are powerful greenhouse gases and Arctic soils are a substantial pool of carbon. Drained thaw lake basins in northern Alaska are an excellent site to study the role of humic substances and iron in redox behavior as soils age and develop. These basins are classified into four age groups, ranging from young (<50 years) to ancient (<5500 years). We expected the availability of humic substances as electron acceptors to increase as soils age and the surface organic layer thickens. Additionally we sought to clarify the interactions between iron and humic substances in redox cycling. The potential electron accepting capacities of humic samples were compared from soils across the age gradient. We assessed the redox properties of humic substances using cyclic voltammetry (CV) and measured the interaction between humic samples and iron with potentiometric redox titrations.

Results/Conclusions

Contrary to our prediction, the second oldest basin showed the maximum potential of humic acids to accept electrons. Titration data clearly shows that humic substances exchange electrons with iron. As iron(III) was added to the humic sample, the potential increased revealing that the humic substance was oxidized by iron. Reduction with iron(II) showed the opposite occurring. Voltammograms created by CV on multiple soil samples from each age basin showed differences changing with basin age. Samples from young and medium basins were more varied between samples than humic samples extracted from older basins which were more consistent. However, comparison of voltammograms between the youngest basin with samples from older basins indicated maturation of the humic compounds leading to more electroactive behavior as basins age. As a basin ages and the organic layer containing humic substances expands, the iron rich mineral layer tends to recede underground. The observation that the second oldest basin had the highest potential humic electron accepting capacity could have resulted from optimal levels of both organic material and iron minerals. Thus, a phase shift may occur in the oldest basins in which microbial communities switch from inorganic electron acceptors to humic substances to fulfill their energetic needs.