Variation in CO 2 and CH 4 fluxes among land cover types in heterogeneous Arctic tundra in northeastern Siberia

[摘要] Arctic tundra is facing unprecedented warming, resulting in shifts in thevegetation, thaw regimes, and potentially in the ecosystem–atmosphereexchange of carbon (C). However, the estimates of regional carbon dioxide(CO 2 ) and methane (CH 4 ) budgets are highly uncertain. We measuredCO 2 and CH 4 fluxes, vegetation composition and leaf area index(LAI), thaw depth, and soil wetness in Tiksi (71 ∘  N,128 ∘  E), a heterogeneous site located within the prostratedwarf-shrub tundra zone in northeastern Siberia. Using the closed chambermethod, we determined the net ecosystem exchange (NEE) of CO 2 ,ecosystem respiration in the dark (ER), ecosystem gross photosynthesis (Pg),and CH 4 flux during the growing season. We applied a previouslydeveloped high-spatial-resolution land cover map over an area of 35.8 km 2 for spatial extrapolation. Among the land cover types varying frombarren to dwarf-shrub tundra and tundra wetlands, the NEE and Pg at thephotosynthetically active photon flux density of 800  µ mol m −2  h −1 (NEE 800 and Pg 800 ) were greatest in thegraminoid-dominated habitats, i.e., streamside meadow and fens, withNEE 800 and Pg 800 of up to − 21 (uptake) and 28 mmol m −2  h −1 , respectively. Vascular LAI was a robust predictor of bothNEE 800 and Pg 800 and, on a landscape scale, the fens weredisproportionately important for the summertime CO 2 sequestration. Drytundra, including the dwarf-shrub and lichen tundra, had smaller CO 2 exchange rates. The fens were the largest source of CH 4 , while the drymineral soil tundra consumed atmospheric CH 4 , which on a landscapescale amounted to − 9 % of the total CH 4 balance during the growingseason. The largest seasonal mean CH 4 consumption rate of 0.02 mmol m −2  h −1 occurred in sand- and stone-covered barren areas. The highconsumption rate agrees with the estimate based on the eddy covariancemeasurements at the same site. We acknowledge the uncertainty involved inspatial extrapolations due to a small number of replicates per land covertype. This study highlights the need to distinguish different land covertypes including the dry tundra habitats to account for their differentCO 2 and CH 4 flux patterns, especially the consumption ofatmospheric CH 4 , when estimating tundra C exchange on a larger spatialscale.

[学科分类] 大气科学