Water-table height and microtopography control biogeochemical cycling in an Arctic coastal tundra ecosystem

Abstract

Drained thaw lake basins (DTLB's) are the dominant land form of the Arctic Coastal Plain in northern Alaska. The presence of continuous permafrost prevents drainage and so water tables generally remain close to the soil surface, creating saturated, suboxic soil conditions. However, ice wedge polygons produce microtopographic variation in these landscapes, with raised areas such as polygon rims creating more oxic microenvironments. The peat soils in this ecosystem store large amounts of organic carbon which is vulnerable to loss as arctic regions continue to rapidly warm, and so there is great motivation to understand the controls over microbial activity in these complex landscapes. Here we report the effects of experimental flooding, along with seasonal and spatial variation in soil chemistry and microbial activity in a DTLB. The flooding treatment generally mirrored the effects of natural landscape variation in water-table height due to microtopography. The flooded portion of the basin had lower dissolved oxygen, lower oxidation-reduction potential (ORP) and higher pH, as did lower elevation areas throughout the entire basin. Similarly, soil pore water concentrations of organic carbon and aromatic compounds were higher in flooded and low elevation areas. Dissolved ferric iron (Fe(III)) concentrations were higher in low elevation areas and responded to the flooding treatment in low areas, only. The high concentrations of soluble Fe(III) in soil pore water were explained by the presence of siderophores, which were much more concentrated in low elevation areas. All the aforementioned variables were correlated, showing that Fe(III) is solubilized in response to anoxic conditions. Dissolved carbon dioxide (CO₂ ) and methane (CH₄ ) concentrations were higher in low elevation areas, but showed only subtle and/or seasonally dependent effects of flooding. In anaerobic laboratory incubations, more CH₄ was produced by soils from low and flooded areas, whereas anaerobic CO₂ production only responded to flooding in high elevation areas. Seasonal changes in the oxidation state of solid phase Fe minerals showed that net Fe reduction occurred, especially in topographically low areas. The effects of Fe reduction were also seen in the topographic patterns of pH, as protons were consumed where this process was prevalent. This suite of results can all be attributed to the effect of water table on oxygen availability: flooded conditions promote anoxia, stimulating dissolution and reduction of Fe(III), and to some extent, methanogenesis. However, two lines of evidence indicated the inhibition of methanogenesis by alternative e- acceptors such as Fe(III) and humic substances: (1) ratios of CO₂ :CH₄ evolved from anaerobic soil incubations and dissolved in soil pore water were high; (2) CH₄ concentrations were negatively correlated with the oxidation state of the soluble Fe pool in both topographically high and low areas. A second set of results could be explained by increased soil temperature in the flooding treatment, which presumably arose from the increased thermal conductivity of the soil surface: higher N mineralization rates and dissolved P concentrations were observed in flooded areas. Overall, these results could have implications for C and nutrient cycling in high Arctic areas where warming and flooding are likely consequences of climate change.

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