Dissolved organic matter is a key compartment for biogeochemical cycles in the Arctic and Subarctic terrestrial environments. With changing vegetation ecosystems, the chemical composition of organic matter is expected to shift and thus, the most labile part of it, namely the extractable fraction. To this date, few studies have focused on the fingerprinting of DOM fraction from different primary sources, and even less on its potential repercussions on the environment. In this study, we jointly characterized the chemical composition of bulk and water-extractable organic matter (WEOM) from different vegetation species typical of Subarctic ecosystems. Through a multi-analyses approach, including elementary analysis, solid state 13C nuclear magnetic resonance, UV and 3D fluorescence spectroscopy, and high-resolution mass spectrometry, our results highlighted that the quantity and composition of produced WEOM significantly differed between vegetation sources and specifically between plant functional types (PFT, e.g., lichens, graminoids, and trees and shrubs). The relevance of optical indices was questioned, and the use of several of them was discarded for unprocessed WEOM study. However, the DOM proxies (optical indices, molecular composition, and stoichiometry) enabled to conclude that the lichen WEOM was likely less degradable than vascular plants WEOM, and among the latter group, graminoids produced more degradable WEOM than trees and shrubs. This work reported specific organic fingerprints for the different PFT. Consequently, the ongoing changes of vegetation in Arctic and Subarctic regions may greatly affect the composition of DOM that enters the soil and the hydrosystems, as well as the biogeochemical processes it is involved in.

Wetlands are significant sources of the important greenhouse gas CH4. Here we explore the use of an experimental system developed for the determination of continuous fluxes of CO2 and CH4 in closed ecosystem monoliths including the capture of (CO2)-C-14 and (CH4)-C-14 following pulse labelling with (CO2)-C-14. We show that, in the ecosystem studied, ebullition (bubble emission) may account for 18 to 50% of the total CH4 emission, representing fluxes that have been difficult to estimate accurately in the past. Furthermore, using plant removal and C-14 labelling techniques, we use the system to detail the direct influence of vascular plants on CH4 emission. This influence is observed to be dependent on the amount of vascular plants present. The results that may be produced using the presented experimental set-up have implications for an improved understanding of wetland ecosystem/atmosphere interactions, including possible feedback effects on climate change. In recent years much attention has been devoted to ascertaining and subsequently using the relationship between net ecosystem productivity and CH4 emission as a basis for extrapolation of fluxes across large areas. The experimental system presented may be used to study the complex relationship between vascular plants and CH4 emission and here we show examples of how this may vary considerably in nature between and even within ecosystems.

A major challenge in peatland carbon cycle modeling is the estimation of subsurface methane (CH4) and carbon dioxide (CO2) production and consumption rates and pathways. The most common methods for modeling these processes are soil incubations and stable isotope modeling, both of which may involve departures from field conditions. To explore the impacts of these departures, we measured CH4/CO2 concentration ratios and 13C fractionation factors (αC, indicating CH4 production pathways) in field pore water from a thawing subarctic peatland, and compared these values to those observed in incubations of corresponding peat samples. Incubation CH4/CO2 production ratios were significantly and positively correlated with observed field CH4/CO2 concentration ratios, though observed field ratios were ~20 % of those in incubations due to CH4’s lower solubility in pore water. After correcting the field ratios for CH4 loss with an isotope mass balance model, the incubation CH4/CO2 ratios and αC were both significantly positively correlated with field ratios and αC (respectively), both with slopes indistinguishable from 1. Although CH4/CO2 ratios and αC were slightly higher in the incubations, these shifts were consistent along the thaw progression, indicating that ex situ incubations can replicate trends in in situ CH4 production.

The aim of our study was to estimate emissions of natural chloroform from soil in arctic and subarctic ecosystems. We therefore determined the seasonal and spatial variation in soil-to-air fluxes of chloroform at 11 sites representing typical vegetation types in Greenland (Narsarsuaq, Kangerlussuaq and Disko Island) and northern Scandinavia (Abisko). Fluxes of chloroform showed a large variation, ranging from 4 to 2850 ng m(-2) h(-1). The local variation within a 12-m transect at each site was frequently five to tenfold, which emphasizes the need for multiple measurements even within field plots that seem homogenous. At one site, the transect was extended to 58 m and 40 measurements and a large number of environmental parameters were recorded as well. In this transect, collars separated by 60 cm distances were in most cases similar but at 3 m distance variation was as big as between collars with greater separation. CO2 flux was the parameter that showed the most correlation to the chloroform flux in the extended transect. Chloroform fluxes also varied over the year, but this variation was smaller than the variation between the five collars of each site and much smaller than the variation between sites. All arctic sites except a non-tussock sedge wetland showed low fluxes. A subarctic pine forest had by far the highest fluxes. Subarctic and boreal coniferous forests generally seem to be important global sources of biogenic chloroform to the troposphere. The future spatial extent of coniferous forest in the subarctic to arctic region, in response to climate change, may be the key driver of future chloroform emissions from these areas.

Warming may increase the extent and intensity of insect defoliations within Arctic ecosystems. A thorough understanding of the implications of this for litter decomposition is essential to make predictions of soil-atmosphere carbon (C) feedbacks. Soil nitrogen (N) and C cycles naturally are interlinked, but we lack a detailed understanding of how insect herbivores impact these cycles. In a laboratory microcosm study, we investigated the growth responses of heterotrophic soil fungi and bacteria as well as C and N mineralisation to simulated defoliator outbreaks (frass addition), long-term increased insect herbivory (litter addition at higher background N-level) and non-outbreak conditions (litter addition only) in soils from a Subarctic birch forest. Larger amounts of the added organic matter were mineralised in the outbreak simulations compared to a normal year; yet, the fungal and bacterial growth rates and biomass were not significantly different. In the simulation of long-term increased herbivory, less litter C was respired per unit mineralised N (C:N of mineralisation decreased to 20 ± 1 from 38 ± 3 for pure litter), which suggests a directed microbial mining for N-rich substrates. This was accompanied by higher fungal dominance relative to bacteria and lower total microbial biomass. In conclusion, while a higher fraction of foliar C will be respired by insects and microbes during outbreak years, predicted long-term increases in herbivory linked to climate change may facilitate soil C-accumulation, as less foliar C is respired per unit mineralised N. Further work elucidating animal-plant-soil interactions is needed to improve model predictions of C-sink capacity in high latitude forest ecosystems.

Climate change currently manifests in upward and northward shifting treelines, which encompasses changes to the carbon (C) and nitrogen (N) composition of organic inputs to soils. Whether these changed inputs will increase or decrease microbial mineralisation of native soil organic matter remains unknown, making it difficult to estimate how treeline shifts will affect the C balance. Aiming to improve mechanistic understanding of C cycling in regions experiencing treeline shifts, we quantified priming effects in soils of high altitudes (Peruvian Andes) and high latitudes (subarctic Sweden), differentiating landcover types (boreal forest, tropical forest, tundra heath, Puna grassland) and soil horizons (organic, mineral). In a controlled laboratory incubation, soils were amended with substrates of different C:N, composed of an organic C source at a constant ratio of 30% substrate-C to microbial biomass C, combined with different levels of a nutrient solution neutral in pH. Substrate additions elicited both positive and negative priming effects in both ecosystems, independent from substrate C:N. Positive priming prevailed above the treeline in high altitudes and in mineral soils in high latitudes, where consequently climate change-induced treeline shifts and deeper rooting plants may enhance SOM-mineralisation and soil C emissions. However, such C loss may be compensated by negative priming, which dominated in the other soil types and was of larger magnitude than positive priming. In line with other studies, these results indicate a consistent mechanism linking decreased SOM-mineralisation (negative priming) to increased microbial substrate utilisation, suggesting preferential substrate use as a potential tool to support soil C storage.

Net methane (CH4) flux across the ecosystem-atmosphere boundary is governed by two counteracting processes, CH4 oxidation and production. Recent research on CH4 cycling has focused on net CH4 fluxes, however, the separate processes of CH4 oxidation and production may vary at local scales and respond differently to environmental change. Here, we separate CH4 oxidation and production, measured as emission, in situ using CH4 oxidation inhibition combined with a novel in situ 13CH4 labeling experiment to determine the rate of soil oxidation of atmospheric CH4. The study was conducted in a subarctic heath ecosystem with three characteristic plant community types: moist mixed species heath, dry Carex-dominated heath, and wet Eriophorum-dominated fen. We further explored the projected climate change effects of increased temperature and enhanced leaf litter input. The CH4 oxidation inhibition experiment revealed significant potential CH4 emission despite net CH4 uptake. Total CH4 oxidation and potential CH4 emission rates differed significantly between plant communities, demonstrating high local-scale variation in CH4 fluxes. Climate treatments did not affect CH4 oxidation rates, however, warming tended to increase potential CH4 emission, indicating that climate change may affect oxidation and production rates asymmetrically. Near-surface soil oxidation of atmospheric CH4 was successfully traced using 13C stable isotope labeling in situ. CH4 oxidation rates ranged widely, yet preliminarily suggested some degree of substrate limitation. Accounting for the local-scale variation in CH4 fluxes and the relative importance of the separate processes of CH4 oxidation and production will contribute importantly to predicting changes in landscape-scale CH4 budgets and climate feedbacks.

High-latitude boreal and arctic surface/inland waters contain sizeable reservoirs of dissolved organic matter (DOM) and trace elements (TE), which are subject to seasonal freezing. Specifically, shallow ponds and lakes in the permafrost zone often freeze solid, which can lead to transformations in the colloidal and dissolved fractions of DOM and TE. Here, we present results from experimental freeze-thaw cycles using iron (Fe)- and DOM-rich water from thaw ponds situated in Stordalen and Storflaket palsa mires in northern Sweden. After ten cycles of freezing, 85% of Fe and 25% of dissolved organic carbon (DOC) were removed from solution in circumneutral fen water (pH 6.9) but a much smaller removal of Fe and DOC (< 7%) was found in acidic bog water (pH 3.6). This removal pattern was consistent with initial supersaturation of fen water with respect to Fe hydroxide and a lack of supersaturation with any secondary mineral phase in the bog water. There was a nearly two- to threefold increase in the low-molecular-weight (LMW) fraction of organic carbon (OC) and several TEs caused by the repeated freeze-thaw cycles. Future increases in the freeze-thaw frequency of surface waters with climate warming may remove up to 25% of DOC in circumneutral organic-rich waters. Furthermore, an increase of LMW OC may result in enhanced carbon dioxide losses from aquatic ecosystems since this fraction is potentially more susceptible to biodegradation.

The aim of this study is to infer past changes in total organic carbon (TOC) content of lake water during the Holocene in eight boreal forest, tree-limit and alpine lakes using a new technique - near-infrared spectroscopy (NIRS). A training set of 100 lakes from northern Sweden covering a TOC gradient from 0.7 to 14.9 mg l(-1) was used to establish a relationship between the NIRS signal from surface sediments ( 0 - 1 cm) and the TOC content of the water mass. The NIRS model for TOC has a root mean squared error (RMSECV) of calibration of 1.6 mg l(-1) (11% of the gradient) assessed by internal cross-validation (CV), which yields an R-cv(2) of 0.61. The results show that the most dramatic change among the studied lakes occurs in both tree-line lakes around 1000 yrs BP when the TOC content decreases from ca. 7 to 3 mg l(-1) at the present, which is probably due to a descending tree-limit. The TOC content in the alpine lakes shows a declining trend throughout most of the Holocene indicating that TOC may be more directly correlated to climate in alpine lakes than forest lakes. All boreal forest lakes show a declining trend in TOC during the past 3000 yrs with the largest amplitude of change occurring in the lake with a connected mire. The results indicate that a change to a warmer and more humid climate can increase the TOC levels in lakes, which in turn may increase the saturation of CO2 in lake waters and the emission of CO2 to the atmosphere.

Frost-patterned grounds, such as mostly barren frost boils surrounded by denser vegetation, are typical habitat mosaics in tundra. Plant and microbial processes in these habitats may be susceptible to short-term warming outside the growing season, while the areal cover of barren frost boils has decreased during the past decades due to climate warming-induced shrub expansion. The relative importance of such short-term and long-term climate impacts on carbon (C) dynamics remains unknown. We measured ecosystem CO2 uptake and release (in the field), microbial respiration (in the laboratory), as well as microbial biomass N and soil extractable N in frost boils and the directly adjacent heath in late spring and late summer. These habitats had been experimentally warmed with insulating fleeces from late September until late May for three consecutive years, which allowed us to investigate the direct short-term effects of warming and longer-term, indirect climate effects via vegetation establishment into frost boils. Non-growing season warming increased C uptake at the frost boils in late spring and decreased it in late summer, while the timing and direction of responses was opposite for the heath. Experimental warming had no effects on microbial or ecosystem C release or soil N at either of the habitats. However, C cycling was manifold higher at the heath compared to the frost boils, likely because of a higher SOM stock in the soil. Short-term climate change can thus directly alter ecosystem C uptake at frost-patterned grounds but will most likely not affect microbial C release. We conclude that the C dynamics at frost-patterned grounds under a changing climate depend most strongly on the potential of vegetation to encroach into frost boils in the long-term.

The nature of linkages between soil C and N cycling is important in the context of terrestrial ecosystem responses to global environmental change. Extracellular enzymes produced by soil microorganisms drive organic matter decomposition, and are considered sensitive indicators of soil responses to environmental variation. We investigated the response of eight hydrolytic soil enzymes (four peptidases and four glycosidases) to experimental warming in a long-term climate manipulation experiment in a sub-arctic peat bog, to determine to what extent the response of these two functional groups are similar. We found no significant effect of experimental spring and summer warming and/or winter snow addition on either the potential activity or the temperature sensitivity (of Vmax) of any of the enzymes. However, strong and contrasting seasonal patterns in both variables were observed. All of the peptidases, as well as alpha-glucosidase, had lower potential activity at the end of summer (August) compared to the beginning (June). Conversely, beta-glucosidase had significantly higher potential activity in August. Peptidases had consistently higher temperature sensitivities in June compared to August, while all four glycosidases showed the opposite pattern. Our results suggest that warming effects on soil enzymes are small compared to seasonal differences, which are most likely mediated by the seasonality of substrate supply and microbial nutrient demand. Furthermore the contrasting seasonal patterns for glycosidases and peptidases suggest that enzyme-based models of soil processes need to allow for potential divergence between the production and activity of these two enzyme functional groups.