Carbon Balance and Greenhouse Gas Fluxes in a Thermokarst Bog in Interior Alaska: Positive and Negative Feedbacks from Permafrost Thaw

Climate change in northern latitudes is expected to cause widespread permafrost thaw in Interior Alaska over the 21st century. One result of permafrost thaw is land subsidence and the formation of thermokarst bogs. The net result of permafrost thaw on carbon (C) balance depends on the difference between forest floor carbon loss and Sphagnum productivity in the bog. However, greenhouse gas feedbacks including methane (CH4) and nitrous oxide (N2O) can be significant from a thawed saturated permafrost environment, strongly modifying the net climate forcing caused by CO2 exchange. We hypothesized that the saturated conditions in thermokarst bogs would decrease respiration compared to an intact permafrost forest, potentially promoting net CO2 uptake. However, CH4 and N2O production in the thermokarst bog may reduce any potential negative climate feedback. Our field sites are located at the Alaska Peatland Experiment (APEX), part of the Bonanza Creek LTER outside Fairbanks, Alaska. We examined net changes in C storage, greenhouse gas fluxes, and soil nutrients in a lowland black spruce forest with intact permafrost and an adjacent young thermokarst bog that developed 20-40 years ago. Using combined flux towers and autochambers (0.36 m2), we quantified net ecosystem exchange (NEE), ecosystem respiration (ER), and gross primary productivity (GPP). We also quantified semi-continuous CH4 fluxes using an isotopic CH4 analyzer (Picarro Inc) connected in-line to the autochambers, and N2O was measured using static chambers. Chamber measurements suggest that in mid-summer of 2012 the thermokarst bog was a net sink of CO2, while the understory black spruce was a net source. Furthermore, preliminary chamber measurements from 2012 indicate that thermokarst conditions have decreased respiration compared to the black spruce forest, potentially promoting net CO2 uptake in the bog. However, eddy covariance measurements of CO2 in 2011 indicate that the thermokarst bog was a source of CO2 while the black spruce forest was a sink. These contrasting patterns will be reconciled with environmental data and 2012 flux tower estimates. Carbon flux data will also be compared to decadal scale changes in ecosystem C balance through analysis of the loss rate of forest floor C and the accretion rate of bog C in soil cores from the thermokarst site. We were not able to detect significant N2O fluxes in the bog, but CH4 fluxes were significant. These data will be used to quantify the extent to which CH4 production in the bog may reduce any potential negative climate feedback. The data and understanding from these measurements will be incorporated into a peatland biogeochemistry model to assess the response of lowland black spruce in Interior Alaska to thawing permafrost caused by climate change.