Effects of permafrost thaw on nitrogen availability and plant soil interaction in a boreal Alaska lowland

Abstract

Permafrost thaw in boreal peatlands causes an ecosystem state change as black spruce forests with permafrost transition into saturated, non-forested collapse scar bogs or fens. Previous studies have suggested that permafrost thaw can increase soil nitrogen (N) bioavailability. However, it is unclear whether these changes in N availability are directly related to changes in N pools or mineralization rates in surface soils (active layer dynamics), or whether increasing N can be attributed to N mobilization from thawing permafrost soil organic matter at depth. I examined plant species composition and aspects of N cycling along triplicate permafrost-thaw gradients in interior Alaskan peatlands. Each gradient comprised plots situated in a 1) forested plateau with intact permafrost (hereafter called permafrost forest) 2) forest experiencing active thaw (drunken forest), 3) moat representing initial stages of complete thaw (moat), and 4) collapse scar bog representing post-thaw succession following complete permafrost thaw (collapse bog). I found that both organic and inorganic N concentrations in soil solution increased with thaw. Drunken forests with active permafrost thaw had the greatest mean concentrations of total dissolved N relative to the other gradient plots. Elevated levels of dissolved N in the drunken forest plots were due primarily to high concentrations of large molecular DON. The moat and collapse scar bog plots had greater inorganic N concentrations on average than the permafrost forest or the drunken forest, suggesting that changes in plantavailable N do not occur immediately upon thaw, but are influenced by vegetation or soil succession occurring decades to centuries after thaw is initiated. However, across all of the community types, I found that deeper soil horizons corresponded to greater concentrations of DIN and DON when thaw was deepest (September), suggesting that permafrost soil horizons are playing a role in changing N availability post-thaw. Vegetation responses to permafrost thaw included changes in plant community composition, deeper rooting profiles, and changes in foliar N and δ¹⁵N values. Plant foliar and litterfall N concentrations increased with collapse bog succession and showed relationships with concentrations of DIN, suggesting that plants are utilizing additional mineralized N. Together, my results suggest that the conversion of lowland permafrost forests to collapse scar bogs increases N availability both by increasing turnover of the permafrost organic matter pool as well as through longer-term successional processes. At least some plants are able to capitalize on "new" sources of N available post-thaw, but it is unclear from my results whether plants are able to acquire N mobilized by thawing permafrost organic matter deeper in soil profiles.