Potential Decomposability of Permafrost Carbon Stocks in Ice-Wedge Polygons of the Alaskan Arctic Coastal Plain

Wednesday, December 12, 2018 - 08:00
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In the lowland permafrost soils of the Alaskan Arctic Coastal Plain, much of the organic matter exists in a poorly degraded state and is often weakly associated with soil minerals due to the cold, wet environment and cryoturbation. Thus, the impact of future increases in active layer thickness on mineralization rates are expected to be correlated, at least initially, to the level of organic matter decomposition and mineral association of existing permafrost carbon stocks. We are investigating particle size fractionation as an indicator of organic matter decomposition state for permafrost-region soils. Samples representing the soil horizons of flat-, low-, and high-centered ice-wedge polygons near Barrow (Utqiaġvik), Alaska, were size-fractionated to isolate fibric (coarse; >250 μm) from more degraded (fine; 53−250 μm) particulate organic matter (POM) and to separate mineral-associated organic matter into silt- and clay-sized fractions. Data from over 160 samples were used to develop calibration models that can predict the amount of carbon associated with each size fraction from the mid-infrared (MIR) spectra of unfractionated bulk soils (organic carbon concentrations ranging from 0.8 to 47%). The MIR calibration models were then used to supplement measured data to estimate the size distribution of carbon stocks throughout entire 3-m deep profiles of the sampled ice-wedge polygons. We found that organic matter in both active layer and permafrost was relatively undecomposed, even in mineral horizons. High-centered polygons had greater carbon stocks than other polygon types, with most of this difference attributed to POM rather than mineral-associated organic matter. However, the carbon stocks that might be de-stabilized by model-projected increases in active layer thickness under future climate scenarios did not vary significantly among polygon types. On average, about 70% of these carbon stocks were composed of un-complexed POM, with about half of this POM characterized as lightly decomposed fibric materials. Our findings suggest that upper permafrost carbon stocks in ice-wedge polygons of the Alaskan Arctic Coastal Plain are likely to have intrinsically high mineralization potentials upon thawing.

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