Constraining Soil C Loss upon Thaw: Comparing Soils with and without Permafrost

Permafrost thaw, with its state change and increased temperature, clearly results in increased decomposition, but constraining directions and amounts of net C exchange is confounded by feedbacks among dynamic vegetation and soil layers, nutrients, and microbial communities. One way to constrain potential loss is to compare soils with and without permafrost. We compared three sets of soil profiles developed in late Pleistocene loess from various slope positions in western Iowa (no permafrost for >10ka), south-central Alaska (no permafrost for > 3550 y), and interior Alaska (current permafrost). In Iowa C where deep C was protected by loess burial, % soil C declined most precipitously with depth (down to < 0.6 %C at 1m). Alaska soils with and without permafrost were similar in %C at 1m depths (up to 2% C). However soils with permafrost had 2X to 4X more C than non-permafrost soils at 1.5 m and maintained high and highly variable (0.8 to 11% C) C contents below 150 cm. Data provide an additional line of evidence that carbon in deep permafrost is highly susceptible to loss upon thawing. Meanwhile modeling and forecasting C fate requires more insight into C protection and stabilization by burial.