ECOSYSTEM CARBON DYNAMICS IN RESPONSE TO FIVE WINTERS OF EXPERIMENTAL SOIL WARMING AND PERMAFROST DEGRADATION

Abstract:

Arctic permafrost soils store 1700 Pg carbon (C), almost half the global soil C. For millennia permafrost soil C has been protected from decomposition by cold, waterlogged conditions. Warming temperatures will likely thaw permafrost, however the impact on arctic C balance is uncertain. Nutrient availability is predicted to increase with thaw depth and promote plant growth, potentially creating an ecosystem C sink. However, deeper thaw could also increase microbial respiration and eventually exceed C gains.

Using data from a warming experiment in sub-arctic moist acidic tundra, designed to insulate soils in winter and stimulate permafrost degradation, we investigated spatial and temporal drivers of ecosystem C balance. Net ecosystem exchange (NEE) was measured continuously from May-September 2009-2013 using clear automated chambers; ecosystem respiration (Reco) was extrapolated from low light NEE and gross primary productivity (GPP) was derived (GPP = NEE-Reco).

Five years of warming led to progressive increases in active layer depth. Active layer depth was positively correlated with cumulative growing season NEE, GPP and Reco. Although warming increased Reco the ecosystem remained a C sink during the growing season because high Reco was offset by increased plant growth and GPP. Eriophorum vaginatum growth accounted for most of the increased plant biomass, and was correlated with cumulative growing season GPP and Reco. NEE, GPP and Reco all peaked mid-season, and the mid-season amplitudes increased annually leading to higher cumulative NEE, GPP and Reco. In the shoulder seasons NEE and GPP were similar among years. In contrast, Reco increased at the end of the growing season each year, and high mid-season GPP was positively correlated with end season Reco. Thus, conditions that promoted plant growth also promoted C loss.

These results suggest plant responses to permafrost thaw are an important driver of C dynamics. Reco associated with high biomass may result from greater autotrophic respiration as well as enhanced microbial soil decomposition. Focusing only on the growing season may overestimate the C sink of the ecosystem because high uptake appears to enhance C losses at the end of the growing season, and C loss is likely to extend beyond the observation period.