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Abstract. Arctic warming and permafrost degradation are modifying northernecosystems through changes in microtopography, soil water dynamics, nutrientavailability, and vegetation succession. Upon permafrost degradation, therelease of deep stores of nutrients, such as nitrogen and phosphorus, fromnewly thawed permafrost stimulates Arctic vegetation production. Morespecifically, wetter lowlands show an increase in sedges (as part ofgraminoids), whereas drier uplands favor shrub expansion. These shifts inthe composition of vegetation may influence local mineral element cyclingthrough litter production. In this study, we evaluate the influence ofpermafrost degradation on mineral element foliar stocks and potential annualfluxes upon litterfall. We measured the foliar elemental composition (Al,Ca, Fe, K, Mn, P, S, Si, and Zn) of ∼ 500 samples of typicaltundra plant species from two contrasting Alaskan tundra sites, i.e., anexperimental sedge-dominated site (Carbon in Permafrost Experimental Heating Research, CiPEHR) and natural shrub-dominated site(Gradient). The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration andshrub leaves having relatively high Ca and Mn concentrations. Therefore,changes in the species biomass composition of the Arctic tundra in responseto permafrost thaw are expected to be the main factors that dictate changesin elemental composition of foliar stocks and maximum potential foliarfluxes upon litterfall. We observed an increase in the mineral elementfoliar stocks and potential annual litterfall fluxes, with Si increasingwith sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing withshrub expansion in drier sites (Gradient). Consequently, we expect thatsedge and shrub expansion upon permafrost thaw will lead to changes inlitter elemental composition and therefore affect nutrient cycling acrossthe sub-Arctic tundra with potential implications for further vegetationsuccession. 

Understanding the processes that influence and control carbon cycling in Arctic tundra ecosystems is essential for making accurate predictions about what role these ecosystems will play in potential future climate change scenarios. Particularly, air–surface fluxes of methane and carbon dioxide are of interest as recent observations suggest that the vast stores of soil carbon found in the Arctic tundra are becoming more available to release to the atmosphere in the form of these greenhouse gases. Further, harsh wintertime conditions and complex logistics have limited the number of year-round and cold season studies and hence too our understanding of carbon cycle processes during these periods. We present here a two-year micrometeorological data set of methane and carbon dioxide fluxes that provides near-continuous data throughout the active summer and cold winter seasons. Net emission of methane and carbon dioxide in one of the study years totalled 3.7 and 89 g C m−2 a−1 respectively, with cold season methane emission representing 54% of the annual total. In the other year, net emission totals of methane and carbon dioxide were 4.9 and 485 g C m−2 a−1 respectively, with cold season methane emission here representing 82% of the annual total – a larger proportion than has been previously reported in the Arctic tundra. Regression tree analysis suggests that, due to relatively warmer air temperatures and deeper snow depths, deeper soil horizons – where most microbial methanogenic activity takes place – remained warm enough to maintain efficient methane production whilst surface soil temperatures were simultaneously cold enough to limit microbial methanotrophic activity. These results provide valuable insight into how a changing Arctic climate may impact methane emission, and highlight a need to focus on soil temperatures throughout the entire active soil profile, rather than rely on air temperature as a proxy for modelling temperature–methane flux dynamics. 

Abstract. The tundra plays a pivotal role in the Arctic mercury(Hg) cycle by storing atmospheric Hg deposition and shuttling it to theArctic Ocean. A recent study revealed that 70 % of the atmospheric Hgdeposition to the tundra occurs through gaseous elemental mercury (GEM or Hg(0))uptake by vegetation and soils. Processes controlling land–atmosphereexchange of Hg(0) in the Arctic tundra are central, but remainunderstudied. Here, we combine Hg stable isotope analysis of Hg(0) in theatmosphere, interstitial snow air, and soil pore air, with Hg(0) fluxmeasurements in a tundra ecosystem at Toolik Field Station in northernAlaska (USA). In the dark winter months, planetary boundary layer (PBL)conditions and Hg(0) concentrations were generally stable throughout the dayand small Hg(0) net deposition occurred. In spring, halogen-inducedatmospheric mercury depletion events (AMDEs) occurred, with the fastre-emission of Hg(0) after AMDEs resulting in net emission fluxes of Hg(0).During the short snow-free growing season in summer, vegetation uptake ofatmospheric Hg(0) enhanced atmospheric Hg(0) net deposition to the Arctictundra. At night, when PBL conditions were stable, ecosystem uptake ofatmospheric Hg(0) led to a depletion of atmospheric Hg(0). The night-timedecline of atmospheric Hg(0) was concomitant with a depletion of lighterHg(0) isotopes in the atmospheric Hg pool. The enrichment factor,ε202Hgvegetationuptake=-4.2 ‰ (±1.0 ‰) was consistentwith the preferential uptake of light Hg(0) isotopes by vegetation. Hg(0)flux measurements indicated a partial re-emission of Hg(0) during daytime,when solar radiation was strongest. Hg(0) concentrations in soil pore airwere depleted relative to atmospheric Hg(0) concentrations, concomitant withan enrichment of lighter Hg(0) isotopes in the soil pore air, ε202Hgsoilair-atmosphere=-1.00 ‰(±0.25 ‰) and E199Hgsoilair-atmosphere=0.07 ‰ (±0.04 ‰). Thesefirst Hg stable isotope measurements of Hg(0) in soil pore air areconsistent with the fractionation previously observed during Hg(0) oxidationby natural humic acids, suggesting abiotic oxidation as a cause for observedsoil Hg(0) uptake. The combination of Hg stable isotope fingerprints withHg(0) flux measurements and PBL stability assessment confirmed a dominantrole of Hg(0) uptake by vegetation in the terrestrial–atmosphere exchange ofHg(0) in the Arctic tundra.