The age of CO2 released from soils in contrasting ecosystems during the arctic winter

Copyright © 2013 Elsevier. NOTICE: This is the author’s version of a work accepted for publication by Elsevier. Changes resulting from the publishing process, including peer review, editing, corrections, structural formatting and other quality control mechanisms, may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Soil Biology and Biochemistry, Vol. 63, pp. 1 – 4 DOI: http://dx.doi.org/10.1016/j.soilbio.2013.03.011In arctic ecosystems, winter soil respiration can contribute substantially to annual CO2 release, yet the source of this C is not clear. We analysed the 14C content of C released from plant-free plots in mountain birch forest and tundra-heath. Winter-respired CO2 was found to be a similar age (tundra) or older (forest) than C released during the previous autumn. Overall, our study demonstrates that the decomposition of older C can continue during the winter, in these two contrasting arctic ecosystems

Trees out‐forage understorey shrubs for nitrogen patches in a subarctic mountain birch forest

This is the final version. Available on open access from Wiley via the DOI in this record. Data availability statement: Data are available from the Dryad Digital Repository: https:// doi.org/10.5061/dryad.j0zpc86j6 (Friggens et al. 2022).Nitrogen (N), acquired by roots and mycorrhizal fungi and supplied to plant foliage, is a growth-limiting nutrient at the subarctic treeline. Due to this limitation, interspecific competition and acquisition of N is an important control on plant community composition and distribution. The ability of trees and shrubs to access N shapes community dynamics at this ecotone undergoing species range shifts and changes in primary productivity driven by climate change. Using 15N soil labelling we investigate the fate of soil inorganic N, and spatial distances over which trees and understorey shrubs access soil N, in a treeline forest. 15N was injected into soil rooting zones in discrete 1 m2 patches and foliar samples were collected from trees between 1 and 50 m away, and understorey shrubs between 0.5 and 11 m away from labelled soil. The 15N label was found in mountain birch trees up to 5 m, and in understorey shrubs up to 2 m, away from labelled soil. We estimate that 1.27% of pulse-derived N was found in foliage of birch trees, compared to 1.16% in the understorey. However, mountain birch trees contributed only 31% of ecosystem leaf area index (LAI), thus there was a disproportionate allocation of added label to the birch canopy compared with its contribution to ecosystem LAI. The difference in root and mycorrhizal exploration distances and community N partitioning between mountain birch trees and understorey shrubs may confer competitive advantage to trees with respect to nitrogen and nutrient patches, which may alter plant community structures within these forests. This is particularly important considering predicted climate-driven tree and tall shrub expansion in subarctic regions, with likely consequences for ecosystem N and carbon (C) cycling, as well as for community composition and biodiversity.European Union Horizon 2020Natural Environment Research Council (NERC

Quantifying landscape-level methane fluxes in subarctic Finland using a multiscale approach

Journal ArticleQuantifying landscape-scale methane (CH4) fluxes from boreal and arctic regions, and determining how they are controlled, is critical for predicting the magnitude of any CH4 emission feedback to climate change. Furthermore, there remains uncertainty regarding the relative importance of small areas of strong methanogenic activity, vs. larger areas with net CH4 uptake, in controlling landscape-level fluxes. We measured CH4 fluxes from multiple microtopographical subunits (sedge-dominated lawns, interhummocks and hummocks) within an aapa mire in subarctic Finland, as well as in drier ecosystems present in the wider landscape, lichen heath and mountain birch forest. An intercomparison was carried out between fluxes measured using static chambers, up-scaled using a high-resolution landcover map derived from aerial photography and eddy covariance. Strong agreement was observed between the two methodologies, with emission rates greatest in lawns. CH4 fluxes from lawns were strongly related to seasonal fluctuations in temperature, but their floating nature meant that water-table depth was not a key factor in controlling CH4 release. In contrast, chamber measurements identified net CH4 uptake in birch forest soils. An intercomparison between the aerial photography and satellite remote sensing demonstrated that quantifying the distribution of the key CH4 emitting and consuming plant communities was possible from satellite, allowing fluxes to be scaled up to a 100 km2 area. For the full growing season (May to October), ~ 1.1-1.4 g CH4 m-2 was released across the 100 km2 area. This was based on up-scaled lawn emissions of 1.2-1.5 g CH4 m-2, vs. an up-scaled uptake of 0.07-0.15 g CH4 m-2 by the wider landscape. Given the strong temperature sensitivity of the dominant lawn fluxes, and the fact that lawns are unlikely to dry out, climate warming may substantially increase CH4 emissions in northern Finland, and in aapa mire regions in general.This work was carried out within the Natural Environment Research Council (NERC) funded Arctic Biosphere Atmosphere Coupling at Multiple Scales (ABACUS) project (a contribution to International Polar Year 2007_2008) plus NERC small grant NE/F010222/1 awarded to RB and BH. We are grateful for the support of the staff at the Kevo Subarctic Research Institute, to David Sayer for operation and maintenance of the eddy covariance apparatus, and to Lorna English for helping with the analysis of the CH4 samples. We also thank the NERC Field Spectroscopy Facility for support in ground data collection for the remote sensing analysis. Finally, we wish to express our gratitude to two anonymous reviewers whose comments and suggestions substantially improved the manuscript

Twenty-Two Years of Warming, Fertilisation and Shading of Subarctic Heath Shrubs Promote Secondary Growth and Plasticity but Not Primary Growth

Most manipulation experiments simulating global change in tundra were short-term or did not measure plant growth directly. Here, we assessed the growth of three shrubs (Cassiope tetragona, Empetrum hermaphroditum and Betula nana) at a subarctic heath in Abisko (Northern Sweden) after 22 years of warming (passive greenhouses), fertilisation (nutrients addition) and shading (hessian fabric), and compare this to observations from the first decade of treatment. We assessed the growth rate of current-year leaves and apical stem (primary growth) and cambial growth (secondary growth), and integrated growth rates with morphological measurements and species coverage. Primary- and total growth of Cassiope and Empetrum were unaffected by manipulations, whereas growth was substantially reduced under fertilisation and shading (but not warming) for Betula. Overall, shrub height and length tended to increase under fertilisation and warming, whereas branching increased mostly in shaded Cassiope. Morphological changes were coupled to increased secondary growth under fertilisation. The species coverage showed a remarkable increase in graminoids in fertilised plots. Shrub response to fertilisation was positive in the short-term but changed over time, likely because of an increased competition with graminoids. More erected postures and large, canopies (requiring enhanced secondary growth for stem reinforcement) likely compensated for the increased light competition in Empetrum and Cassiope but did not avoid growth reduction in the shade intolerant Betula. The impact of warming and shading on shrub growth was more conservative. The lack of growth enhancement under warming suggests the absence of long-term acclimation for processes limiting biomass production. The lack of negative effects of shading on Cassiope was linked to morphological changes increasing the photosynthetic surface. Overall, tundra shrubs showed developmental plasticity over the longer term. However, such plasticity was associated clearly with growth rate trends only in fertilised plots

Potential macro-detritivore range expansion into the subarctic stimulates litter decomposition: a new positive feedback mechanism to climate change?

As a result of low decomposition rates, high-latitude ecosystems store large amounts of carbon. Litter decomposition in these ecosystems is constrained by harsh abiotic conditions, but also by the absence of macro-detritivores. We have studied the potential effects of their climate change-driven northward range expansion on the decomposition of two contrasting subarctic litter types. Litter of Alnus incana and Betula pubescens was incubated in microcosms together with monocultures and all possible combinations of three functionally different macro-detritivores (the earthworm Lumbricus rubellus, isopod Oniscus asellus, and millipede Julus scandinavius). Our results show that these macro-detritivores stimulated decomposition, especially of the high-quality A. incana litter and that the macro-detritivores tested differed in their decomposition-stimulating effects, with earthworms having the largest influence. Decomposition processes increased with increasing number of macro-detritivore species, and positive net diveristy effects occurred in several macro-detritivore treatments. However, after correction for macro-detritivore biomass, all interspecific differences in macro-detritivore effects, as well as the positive effects of species number on subarctic litter decomposition disappeared. The net diversity effects also appeared to be driven by variation in biomass, with a possible exception of net diversity effects in mass loss. Based on these results, we conclude that the expected climate change-induced range expansion of macro-detritivores into subarctic regions is likely to result in accelerated decomposition rates. Our results also indicate that the magnitude of macro-detritivore effects on subarctic decomposition will mainly depend on macro-detritivore biomass, rather than on macro-detritivore species number or identity

Climatic and resource quality controls on soil respiration across a forest-tundra ecotone in Swedish Lapland

We studied resource quality and climatic constraints on soil respiration over a mountain birch forest-tundra ecotone in northern Swedish Lapland during 1999-2001 by means of both a field-based soil transplant experiment and a laboratory incubation experiment. Average carbon dioxide fluxes over the 2000 thaw season were 0.62 and 0.48 g CO2 m-2h-11, at forest and tundra control plots, respectively. We attribute the higher respiration rate at the forest site mainly to more favourable microclimate but also to higher resource quality. Temperature-respiration relationships described using Arrhenius equations explained 37% of the variation in the tundra soil respiration and 42% for the forest soils on a season-wide basis in 2000. Q10 values (exponential temperature-response) were generally high (except in August in the field experiment) compared to the global average (2.4) and varied over time, with increased temperature-dependency at low soil temperatures. In the laboratory, higher activation energy was found in soils incubated at higher temperatures (12 and 17°C; in the range 133-109 kJ mol-1) compared with lower temperatures (2 and 7°C; in the range 98-92 kJ mol-1) suggesting an adaptation of the decomposer community toward more psychrophilic organisms or metabolism in low-temperature environments. Soil moisture, however, could also play an important role in modifying any temperature response of soil respiration in this sub-arctic ecotone area. These mesic soils have a relatively rapid turnover time of carbon and should be compared to boreal forest and temperate woodland in carbon dynamics. We conclude that a shift from tundra to birch forest would give an initial pulse of carbon released from soil to the atmosphere as labile carbon stored in tundra soils is metabolised by decomposer organisms. © 2002 Elsevier Science Ltd. All rights reserved

Role of CH(4) oxidation, production and transport in forest soil CH(4) flux

Forest soils are an important sink for atmospheric CH4 but the contribution of CH4 oxidation, production and transport to the overall CH4 flux is difficult to quantify. It is important to understand the role these processes play in CH4 dynamics of forest soils, to enable prediction of how the size of this sink will respond to future environmental change. Methane oxidation, production and transport were investigated for a temperate forest soil, previously shown to be a net CH4 consumer, to determine the extent to which physical and biological processes contributed to the net flux. The sum of oxidation rates for soil layers were significantly greater (P < 0.05) than for the intact soil cores from which the layers were taken. Combined with the immediate inhibition of CH4 uptake on waterlogging soils, the findings suggested that soil CH4 diffusion was an important regulator of CH4 uptake. In support of this, a subsurface maximum for CH4 oxidation was observed, but the exact depth of the maximum differed when rates were calculated on a mass or on an areal basis. Markedly varying potential CH4 uptake activities between soil cores were masked in intact core rates. Potential CH4 oxidation conformed well to Michaelis-Menten kinetics but Vmax, Kt and aAO values varied with depth, suggesting different functional methanotrophic communities were active in the profile. The presence of monophasic kinetics in fresh soil could not be used to infer that the soil was exposed only to CH4 mixing ratios ≤ atmospheric, as challenging soils with 20% CH4 in air did not induce low-affinity oxidation kinetics. Atmospheric CH4 oxidation potentials exceeded production potentials by 10-220 times. The results show that the forest soil CH4 flux was dominated by CH4 oxidation and transport, methanogenesis played only a minor role

Soil CH4 oxidation: response to forest clearcutting and thinning

First paragraph: We measured CH4 flux rates for temperate forest soils, with adjacent intact and recently felled areas, to test the hypothesis that net soil CH4 consumption would be reduced after felling. The results showed that while clearcutting reduced net CH4 consumption, thinning actually increased the rate of net soil CH4 consumption. The e ffects on CH4 consumption appeared to be linked to changes in soil N cycling or pH following felling. In well-drained soils, such as the ones studied, the soil CH4 flux will be the resultant of CH4 oxidation and CH4 production within the soil profile. As the soils were net CH4 consumers over the course of this experiment, CH4 oxidation dominated production and this is typical for such well-drained forest soils (Conrad, 1995)

Controlling factors and effects of chronic nitrogen and sulphur deposition on methane oxidation in a temperate forest soil

Soil CH4 flux rates were determined on 28 occasions between June 1996 and July 1997 in a temperate deciduous woodland in south-west England. The effects of environmental and edaphic factors on flux rates and the effects of chronic deposition of sulphuric acid, nitric acid and ammonium sulphate were investigated. The soil was a consistent net CH4 oxidiser, with mean (n = 10) oxidation rates for plots exposed to ambient throughfall ranging from 44.3 to 110.6 μg CH4 m-2 h-1 between samplings; net CH4 production was not observed. The annual mean uptake rate differed by only 6% from the annual mean flux calculated from the literature for other studies of >364 d duration in temperate and boreal deciduous woodlands. The CH4 uptake rates were correlated with soil water potential (square-root transformed), temperature and depth of organic horizon (r2 = 0.78, 0.30 and 0.41, respectively). Soil water potential was the best predictor of net CH4 oxidation rates and when temperature was added to the regression model no improvement in the r2 was observed. The chronic deposition of sulphuric acid stimulated net methane oxidation (