Abstract

Tundra soils contain large amounts of organic carbon (C) that might become available to microbial decomposition as soils warm. To elucidate the C sources currently sustaining CO2 emissions from striped tundra soils (soil respiration) in Northwest Greenland, we studied the seasonal pattern and radiocarbon (14C) signature of soil respiration and of CO2 within the pore space, respired from roots and non-root–associated microbes, and of bulk soil organic matter. Old C pools are present in the topsoil of both barren ridges (1000–5000 yrs) and vegetated troughs (modern to 600 yrs). Before leaf-out, soil respiration was depleted in 14C relative to atmospheric CO2, root and microbial respiration within the topsoil, demonstrating a substantial contribution of C fixed before 1950. As the growing season progressed, the contribution of older C pools decreased, but remained apparent in the soil respiration from ridges and in pore space CO2. Soil respiration from troughs became dominated by recently fixed C. As the active layer deepens with permafrost thaw, buried C may become an increasingly larger component of soil respiration. Detecting microbial decomposition of older C pools requires continuous monitoring of soil and microbial respiration and better constraints on soil C pools.

Highlights

  • Soils of the arctic permafrost region contain an estimated 496 Pg organic carbon (C) within the top meter and 1024 Pg C within the top three meters (Tarnocai et al, 2009)

  • Our study focused on two questions: (1) How much do root and microbial respiration contribute to the overall soil respiration flux in high arctic polar semi-deserts? (2) Are soils currently losing old C that had been unavailable to microorganisms under the climatic conditions of the last few centuries?

  • We studied two high arctic prostrate dwarf shrub tundra systems (‘‘South Mountain’’ and ‘‘Polar Desert’’) in Northwest Greenland near Thule Air Base (76u329N, 68u509W; 200–350 m a.s.l.) (Table 1) as part of the U.S National Science Foundation Biocomplexity Program (Walker et al, 2008)

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Summary

Introduction

Soils of the arctic permafrost region contain an estimated 496 Pg organic carbon (C) within the top meter and 1024 Pg C within the top three meters (Tarnocai et al, 2009). A recent study (Horwath, 2007; Horwath et al, 2008) concluded that high arctic soils (1.984 3 106 km2) contain 12.06 Pg C—an order of magnitude more C than previously estimated (Bliss and Matveyeva, 1992). Given that permafrost thawing will expose deeper-lying pools of soil C, the lack of data on C depth distributions and its exchange with the modern atmosphere constrains our ability to model accurately the panarctic C cycle and its climate feedback potential (Sitch et al, 2007)

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