Key publications
Hartley IP, Hill TC, Wade TJ, Clement RJ, Moncrieff JB, Prieto-Blanco A, Disney MI, Huntley B, Williams M, Howden NJK, et al (2015). Quantifying landscape-level methane fluxes in subarctic Finland using a multiscale approach.
Global Change Biology,
21(10), 3712-3725.
Abstract:
Quantifying landscape-level methane fluxes in subarctic Finland using a multiscale approach
© 2015 John Wiley. &. Sons Ltd. Quantifying 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.
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Karhu K, Auffret MD, Dungait JAJ, Hopkins DW, Prosser JI, Singh BK, Subke J-A, Wookey PA, Agren GI, Sebastià M-T, et al (2014). Temperature sensitivity of soil respiration rates enhanced by microbial community response.
Nature,
513(7516), 81-84.
Abstract:
Temperature sensitivity of soil respiration rates enhanced by microbial community response.
Soils store about four times as much carbon as plant biomass, and soil microbial respiration releases about 60 petagrams of carbon per year to the atmosphere as carbon dioxide. Short-term experiments have shown that soil microbial respiration increases exponentially with temperature. This information has been incorporated into soil carbon and Earth-system models, which suggest that warming-induced increases in carbon dioxide release from soils represent an important positive feedback loop that could influence twenty-first-century climate change. The magnitude of this feedback remains uncertain, however, not least because the response of soil microbial communities to changing temperatures has the potential to either decrease or increase warming-induced carbon losses substantially. Here we collect soils from different ecosystems along a climate gradient from the Arctic to the Amazon and investigate how microbial community-level responses control the temperature sensitivity of soil respiration. We find that the microbial community-level response more often enhances than reduces the mid- to long-term (90 days) temperature sensitivity of respiration. Furthermore, the strongest enhancing responses were observed in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions. After 90 days, microbial community responses increased the temperature sensitivity of respiration in high-latitude soils by a factor of 1.4 compared to the instantaneous temperature response. This suggests that the substantial carbon stores in Arctic and boreal soils could be more vulnerable to climate warming than currently predicted.
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Author URL.
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Hartley I, Garnett MH, Sommerkorn M, Hopkins DW, Fletcher BJ, Sloan VL, Phoenix GK, Wookey PA (2013). A potential loss of carbon associated with greater
plant growth in the European Arctic. Nature Climate Change, 2, 875-879.
Hartley IP, Hopkins DW, Garnett MH, Sommerkorn M, Wookey PA (2008). Soil microbial respiration in arctic soil does not acclimate to temperature.
Ecol Lett,
11(10), 1092-1100.
Abstract:
Soil microbial respiration in arctic soil does not acclimate to temperature.
Warming-induced release of CO2 from the large carbon (C) stores in arctic soils could accelerate climate change. However, declines in the response of soil respiration to warming in long-term experiments suggest that microbial activity acclimates to temperature, greatly reducing the potential for enhanced C losses. As reduced respiration rates with time could be equally caused by substrate depletion, evidence for thermal acclimation remains controversial. To overcome this problem, we carried out a cooling experiment with soils from arctic Sweden. If acclimation causes the reduction in soil respiration observed after experimental warming, then it should subsequently lead to an increase in respiration rates after cooling. We demonstrate that thermal acclimation did not occur following cooling. Rather, during the 90 days after cooling, a further reduction in the soil respiration rate was observed, which was only reversed by extended re-exposure to warmer temperatures. We conclude that over the time scale of a few weeks to months, warming-induced changes in the microbial community in arctic soils will amplify the instantaneous increase in the rates of CO2 production and thus enhance C losses potentially accelerating the rate of 21st century climate change.
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Author URL.
Hartley IP, Ineson P (2008). Substrate quality and the temperature sensitivity of soil organic matter decomposition.
Soil Biology and Biochemistry,
40(7), 1567-1574.
Abstract:
Substrate quality and the temperature sensitivity of soil organic matter decomposition
Determining the relative temperature sensitivities of the decomposition of the different soil organic matter (SOM) pools is critical for predicting the long-term impacts of climate change on soil carbon (C) storage. Although kinetic theory suggests that the temperature sensitivity of SOM decomposition should increase with substrate recalcitrance, there remains little empirical evidence to support this hypothesis. In the study presented here, sub-samples from a single bulk soil sample were frozen and sequentially defrosted to produce samples of the same soil that had been incubated for different lengths of time, up to a maximum of 124 days. These samples were then placed into an incubation system which allowed CO2production to be monitored constantly and the response of soil respiration to short-term temperature manipulations to be investigated. The temperature sensitivity of soil CO2production increased significantly with incubation time suggesting that, as the most labile SOM pool was depleted the temperature sensitivity of SOM decomposition increased. This study is therefore one of the first to provide empirical support for kinetic theory. Further, using a modelling approach, we demonstrate that it is the temperature sensitivity of the decomposition of the more recalcitrant SOM pools that will determine long-term soil-C losses. Therefore, the magnitude of the positive feedback to global warming may have been underestimated in previous modelling studies. © 2008 Elsevier Ltd. All rights reserved.
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Heinemeyer A, Hartley IP, Evans SP, Carreira De La Fuente JA, Ineson P (2007). Forest soil CO<inf>2</inf>flux: Uncovering the contribution and environmental responses of ectomycorrhizas.
Global Change Biology,
13(8), 1786-1797.
Abstract:
Forest soil CO2flux: Uncovering the contribution and environmental responses of ectomycorrhizas
Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO2efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO2efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the 'autotrophic' root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO2efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO2flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO2flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO2flux component represents an overflow 'CO2tap' through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO2. © 2007 Blackwell Publishing Ltd.
Abstract.
Publications by year
In Press
Gatis N, Anderson K, Grand-Clement E, Luscombe D, Hartley I, Smith D, Brazier R (In Press). Evaluating MODIS vegetation products using digital images for quantifying local peatland CO2 gas fluxes.
Remote Sensing in Ecology and Conservation Full text.
Gatis N, Luscombe D, Grand-Clement E, Hartley I, Anderson K, Smith DM, Brazier RE (In Press). The effect of drainage ditches on vegetation diversity and CO2 fluxes in a Molinia caerulea dominated peatland.
Ecohydrology Full text.
Fisher JP, Estop-Aragonés C, Thierry A, Charman DJ, Wolfe SA, Hartley IP, Murton JB, Williams M, Phoenix GK (In Press). The influence of vegetation and soil characteristics on active-layer thickness of permafrost soils in boreal forest.
Glob Chang Biol,
22(9), 3127-3140.
Abstract:
The influence of vegetation and soil characteristics on active-layer thickness of permafrost soils in boreal forest.
Carbon release from thawing permafrost soils could significantly exacerbate global warming as the active-layer deepens, exposing more carbon to decay. Plant community and soil properties provide a major control on this by influencing the maximum depth of thaw each summer (active-layer thickness; ALT), but a quantitative understanding of the relative importance of plant and soil characteristics, and their interactions in determine ALTs, is currently lacking. To address this, we undertook an extensive survey of multiple vegetation and edaphic characteristics and ALTs across multiple plots in four field sites within boreal forest in the discontinuous permafrost zone (NWT, Canada). Our sites included mature black spruce, burned black spruce and paper birch, allowing us to determine vegetation and edaphic drivers that emerge as the most important and broadly applicable across these key vegetation and disturbance gradients, as well as providing insight into site-specific differences. Across sites, the most important vegetation characteristics limiting thaw (shallower ALTs) were tree leaf area index (LAI), moss layer thickness and understory LAI in that order. Thicker soil organic layers also reduced ALTs, though were less influential than moss thickness. Surface moisture (0-6 cm) promoted increased ALTs, whereas deeper soil moisture (11-16 cm) acted to modify the impact of the vegetation, in particular increasing the importance of understory or tree canopy shading in reducing thaw. These direct and indirect effects of moisture indicate that future changes in precipitation and evapotranspiration may have large influences on ALTs. Our work also suggests that forest fires cause greater ALTs by simultaneously decreasing multiple ecosystem characteristics which otherwise protect permafrost. Given that vegetation and edaphic characteristics have such clear and large influences on ALTs, our data provide a key benchmark against which to evaluate process models used to predict future impacts of climate warming on permafrost degradation and subsequent feedback to climate.
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2018
Lopez-Sangil L, Hartley IP, Rovira P, Casals P, Sayer EJ (2018). Drying and rewetting conditions differentially affect the mineralization of fresh plant litter and extant soil organic matter.
Soil Biology and Biochemistry,
124, 81-89.
Abstract:
Drying and rewetting conditions differentially affect the mineralization of fresh plant litter and extant soil organic matter
© 2018 Elsevier Ltd Drought is becoming more common globally and has the potential to alter patterns of soil carbon (C) storage in terrestrial ecosystems. After an extended dry period, a pulse of soil CO2 release is commonly observed upon rewetting (the so-called ‘Birch effect’), the magnitude of which depends on soil rewetting frequency. But the source and implications of this CO2 efflux are unclear. We used a mesocosm field experiment to subject agricultural topsoil to two distinct drying and rewetting frequencies, measuring Birch effects (as 3-day cumulative CO2 efflux upon rewetting) and the overall CO2 efflux over the entire drying-rewetting cycle. We used 14C-labelled wheat straw to determine the contribution of fresh (recently incorporated) plant litter or extant soil organic matter (SOM) to these fluxes, and assessed the extent to which the amount of soil microbial biomass + K2SO4-extractable organic C (fumigated-extracted C, FEC) before rewetting determined the magnitude of Birch effect CO2 pulses. Our results showed a gradual increase in SOM-derived organic solutes within the FEC fraction, and a decrease in soil microbial biomass, under more extreme drying and rewetting conditions. But, contrary to our hypothesis, pre-wetting levels of FEC were not related to the magnitude of the Birch effects. In the longer term, rewetting frequency and temperature influenced the overall (31-day cumulative) amount of CO2–C released from SOM upon rewetting, but the overall 14CO2–C respired from fresh straw was only influenced by the rewetting frequency, with no effect of seasonal temperature differences of ∼15 °C. We conclude that the mineralization of fresh plant litter in soils is more sensitive to water limitations than extant SOM in soils under drying-rewetting conditions. Moreover, we found little evidence to support the hypothesis that the availability of microbial and soluble organic C before rewetting determined the magnitude of the Birch effects, and suggest that future work should investigate whether these short-term CO2 pulses are predominantly derived from substrate-supply mechanisms resulting from the disruption of the soil organo-mineral matrix.
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Estop-Aragonés C, Cooper MDA, Fisher JP, Thierry A, Garnett MH, Charman DJ, Murton JB, Phoenix GK, Treharne R, Sanderson NK, et al (2018). Limited release of previously-frozen C and increased new peat formation after thaw in permafrost peatlands.
Soil Biology and Biochemistry,
118, 115-129.
Abstract:
Limited release of previously-frozen C and increased new peat formation after thaw in permafrost peatlands
© 2017 the Authors Permafrost stores globally significant amounts of carbon (C) which may start to decompose and be released to the atmosphere in form of carbon dioxide (CO2) and methane (CH4) as global warming promotes extensive thaw. This permafrost carbon feedback to climate is currently considered to be the most important carbon-cycle feedback missing from climate models. Predicting the magnitude of the feedback requires a better understanding of how differences in environmental conditions post-thaw, particularly hydrological conditions, control the rate at which C is released to the atmosphere. In the sporadic and discontinuous permafrost regions of north-west Canada, we measured the rates and sources of C released from relatively undisturbed ecosystems, and compared these with forests experiencing thaw following wildfire (well-drained, oxic conditions) and collapsing peat plateau sites (water-logged, anoxic conditions). Using radiocarbon analyses, we detected substantial contributions of deep soil layers and/or previously-frozen sources in our well-drained sites. In contrast, no loss of previously-frozen C as CO2 was detected on average from collapsed peat plateaus regardless of time since thaw and despite the much larger stores of available C that were exposed. Furthermore, greater rates of new peat formation resulted in these soils becoming stronger C sinks and this greater rate of uptake appeared to compensate for a large proportion of the increase in CH4 emissions from the collapse wetlands. We conclude that in the ecosystems we studied, changes in soil moisture and oxygen availability may be even more important than previously predicted in determining the effect of permafrost thaw on ecosystem C balance and, thus, it is essential to monitor, and simulate accurately, regional changes in surface wetness.
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2017
Cooper MDA, Estop-Aragones C, Fisher JP, Thierry A, Garnett MH, Charman DJ, Murton JB, Phoenix GK, Treharne R, Kokelj SV, et al (2017). Limited contribution of permafrost carbon to
methane release from thawing peatlands.
Nature Climate Change,
7, 507-511.
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2016
De Baets S, Van de Weg MJ, Lewis R, Steinberg N, Meersmans J, Quine TA, Shaver GR, Hartley IP (2016). Investigating the controls on soil organic matter decomposition in tussock tundra soil and permafrost after fire.
Soil Biology and Biochemistry,
99, 108-116.
Abstract:
Investigating the controls on soil organic matter decomposition in tussock tundra soil and permafrost after fire
© 2016 Elsevier Ltd. Rapid warming in Arctic ecosystems is resulting in increased frequency of disturbances such as fires, changes in the distribution and productivity of different plant communities, increasing thaw depths in permafrost soils and greater nutrient availability, especially nitrogen. Individually and collectively, these factors have the potential to strongly affect soil C decomposition rates, with implications for the globally significant stores of carbon in this region. However, considerable uncertainty remains regarding how C decomposition rates are controlled in Arctic soils. In this study we investigated how temperature, nitrogen availability and labile C addition affected rates of CO2production in short (10-day for labile C) and long-term (1.5 year for temperature and N) incubations of samples collected from burned and unburned sites in the Anaktuvuk river burn on the North Slope of Alaska from different depths (organic horizon, mineral horizon and upper permafrost). The fire in this region resulted in the loss of several cms of the organic horizon and also increased active layer depth allowing the impacts of four years of thaw on deeper soil layers to be investigated. Respiration rates did not decline substantially during the long-term incubation, although decomposition rates per unit organic matter were greater in the organic horizon. In the mineral and upper permafrost soil horizons, CO2production was more temperature sensitive, while N addition inhibited respiration in the mineral and upper permafrost layers, especially at low temperatures. In the short-term incubations, labile C additions promoted the decomposition of soil organic matter in the mineral and upper permafrost samples, but not in the organic samples, with this effect being lost following N addition in the deeper layers. These results highlight that (i) there are substantial amounts of labile organic matter in these soils (ii), the organic matter stored in mineral and upper permafrost in the tussock tundra is less readily decomposable than in the organic horizon, but that (iii) its decomposition is more sensitive to changes in temperature and that (iv) microbial activity in deeper soil layers is limited by labile C availability rather than N. Collectively, these results indicate that in addition to the loss of C by combustion of organic matter, increasing fire frequency also has the potential to indirectly promote the release of soil C to the atmosphere in the years following the disturbance.
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Schädel C, Bader MK-F, Schuur EAG, Biasi C, Bracho R, Čapek P, De Baets S, Diáková K, Ernakovich J, Estop-Aragones C, et al (2016). Potential carbon emissions dominated by carbon dioxide from thawed permafrost soils.
Nature Climate Change,
6(10), 950-953.
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Auffret MD, Karhu K, Khachane A, Dungait JAJ, Fraser F, Hopkins DW, Wookey PA, Singh BK, Freitag TE, Hartley IP, et al (2016). The Role of Microbial Community Composition in Controlling Soil Respiration Responses to Temperature.
PLoS One,
11(10).
Abstract:
The Role of Microbial Community Composition in Controlling Soil Respiration Responses to Temperature.
Rising global temperatures may increase the rates of soil organic matter decomposition by heterotrophic microorganisms, potentially accelerating climate change further by releasing additional carbon dioxide (CO2) to the atmosphere. However, the possibility that microbial community responses to prolonged warming may modify the temperature sensitivity of soil respiration creates large uncertainty in the strength of this positive feedback. Both compensatory responses (decreasing temperature sensitivity of soil respiration in the long-term) and enhancing responses (increasing temperature sensitivity) have been reported, but the mechanisms underlying these responses are poorly understood. In this study, microbial biomass, community structure and the activities of dehydrogenase and β-glucosidase enzymes were determined for 18 soils that had previously demonstrated either no response or varying magnitude of enhancing or compensatory responses of temperature sensitivity of heterotrophic microbial respiration to prolonged cooling. The soil cooling approach, in contrast to warming experiments, discriminates between microbial community responses and the consequences of substrate depletion, by minimising changes in substrate availability. The initial microbial community composition, determined by molecular analysis of soils showing contrasting respiration responses to cooling, provided evidence that the magnitude of enhancing responses was partly related to microbial community composition. There was also evidence that higher relative abundance of saprophytic Basidiomycota may explain the compensatory response observed in one soil, but neither microbial biomass nor enzymatic capacity were significantly affected by cooling. Our findings emphasise the key importance of soil microbial community responses for feedbacks to global change, but also highlight important areas where our understanding remains limited.
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2015
Hartley IP, Hill TC, Wade TJ, Clement RJ, Moncrieff JB, Prieto-Blanco A, Disney MI, Huntley B, Williams M, Howden NJK, et al (2015). Quantifying landscape-level methane fluxes in subarctic Finland using a multiscale approach.
Global Change Biology,
21(10), 3712-3725.
Abstract:
Quantifying landscape-level methane fluxes in subarctic Finland using a multiscale approach
© 2015 John Wiley. &. Sons Ltd. Quantifying 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.
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Laudicina VA, Benhua S, Dennis PG, Badalucco L, Rushton SP, Newsham KK, O Donnell AG, Hartley IP, Hopkins DW (2015). Responses to increases in temperature of heterotrophic micro-organisms in soils from the maritime Antarctic.
Polar Biology,
38(8), 1153-1160.
Abstract:
Responses to increases in temperature of heterotrophic micro-organisms in soils from the maritime Antarctic
© 2015, Springer-Verlag Berlin Heidelberg. Understanding relationships between environmental changes and soil microbial respiration is critical for predicting changes in soil organic carbon (SOC) fluxes and content. The maritime Antarctic is experiencing one of the fastest rates of warming in the world and is therefore a key location to examine the effect of temperature on SOC mineralization by the respiration of soil micro-organisms. However, depletion of the labile substrates at higher temperatures relative to the total SOC and greater temperature sensitivity of recalcitrant components of the SOC confound simple interpretations of the effects of warming. We have addressed these issues by testing the hypothesis that respiration by heterotrophic soil micro-organisms is not down-regulated with increasing temperature by comparing the increase in biomass-specific respiration rate with temperature to the increase in respiration rate per unit SOC. We used five soils from the maritime Antarctic ranging in latitude and SOC content and measured the soil respiratory responses to temperatures ranging from 2 to 50 °C in laboratory incubations lasting up to 31 days. In all cases, soil respiration increased with temperature up to 50 °C, even though this exceeds the temperatures normally be experienced, indicating that the community contained sufficient physiological diversity to be able to respire over large temperature ranges. Both the biomass-specific respiration rate and the overall rate of SOC mineralization increased with temperature, which we interpret as respiration by soil micro-organisms not down-regulating relative to temperature.
Abstract.
2014
Hartley IP (2014). Soil carbon: Resisting climate change. Nature Climate Change, 4(9), 760-761.
Karhu K, Auffret MD, Dungait JAJ, Hopkins DW, Prosser JI, Singh BK, Subke J-A, Wookey PA, Agren GI, Sebastià M-T, et al (2014). Temperature sensitivity of soil respiration rates enhanced by microbial community response.
Nature,
513(7516), 81-84.
Abstract:
Temperature sensitivity of soil respiration rates enhanced by microbial community response.
Soils store about four times as much carbon as plant biomass, and soil microbial respiration releases about 60 petagrams of carbon per year to the atmosphere as carbon dioxide. Short-term experiments have shown that soil microbial respiration increases exponentially with temperature. This information has been incorporated into soil carbon and Earth-system models, which suggest that warming-induced increases in carbon dioxide release from soils represent an important positive feedback loop that could influence twenty-first-century climate change. The magnitude of this feedback remains uncertain, however, not least because the response of soil microbial communities to changing temperatures has the potential to either decrease or increase warming-induced carbon losses substantially. Here we collect soils from different ecosystems along a climate gradient from the Arctic to the Amazon and investigate how microbial community-level responses control the temperature sensitivity of soil respiration. We find that the microbial community-level response more often enhances than reduces the mid- to long-term (90 days) temperature sensitivity of respiration. Furthermore, the strongest enhancing responses were observed in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions. After 90 days, microbial community responses increased the temperature sensitivity of respiration in high-latitude soils by a factor of 1.4 compared to the instantaneous temperature response. This suggests that the substantial carbon stores in Arctic and boreal soils could be more vulnerable to climate warming than currently predicted.
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2013
Hartley I, Garnett MH, Sommerkorn M, Hopkins DW, Fletcher BJ, Sloan VL, Phoenix GK, Wookey PA (2013). A potential loss of carbon associated with greater
plant growth in the European Arctic. Nature Climate Change, 2, 875-879.
Fraser FC, Hallett PD, Wookey PA, Hartley IP, Hopkins DW (2013). How do enzymes catalysing soil nitrogen transformations respond to changing temperatures?.
Biology and Fertility of Soils,
49(1), 99-103.
Abstract:
How do enzymes catalysing soil nitrogen transformations respond to changing temperatures?
Biological processes in soils are regulated in part by soil temperature, and there is currently considerable interest in obtaining robust information on the temperature sensitivity of carbon cycling process. However, very little comparable information exists on the temperature regulation of specific nitrogen cycling processes. This paper addresses this problem by measuring the temperature sensitivity of nitrogen cycling enzymes in soil. A grassland soil was incubated over a range of temperatures (-2 to 21 °C) reflecting 99 % of the soil temperature range during the previous 50 years at the site. After 7 and 14 days of incubation, potential activities of protease, amidase and urease were determined. Activities of protease and urease were positively related to temperature (activation energy; Ea = 49. 7 and 73. 4 kJ mol-1, respectively, and Q10 = 2. 97 and 2. 78, respectively). By contrast, amidase activity was relatively insensitive to temperature, but the activity was significantly increased after the addition of glucose. This indicated that there was a stoichiometric imbalance with amidase activity only being triggered when there was a supply of exogenous carbon. Thus, carbon supply was a greater constraint to amidase activity than temperature was in this particular soil. © 2012 Springer-Verlag.
Abstract.
Hartley IP, Garnett MH, Sommerkorn M, Hopkins DW, Wookey PA (2013). The age of CO<inf>2</inf>released from soils in contrasting ecosystems during the arctic winter.
Soil Biology and Biochemistry,
63, 1-4.
Abstract:
The age of CO2released from soils in contrasting ecosystems during the arctic winter
In arctic ecosystems, winter soil respiration can contribute substantially to annual CO2release, yet the source of this C is not clear. We analysed the14C content of C released from plant-free plots in mountain birch forest and tundra-heath. Winter-respired CO2was 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. copy; 2013 Elsevier Ltd.
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2012
Burke EJ, Hartley I, Jones CD (2012). Uncertainties in the global temperature change caused by carbon
release from permafrost thawing. The Cryosphere, 6, 1063-1076.
2010
Garnett MH, Hartley IP (2010). A passive sampling method for radiocarbon analysis of atmospheric CO2 using molecular sieve.
Atmospheric Environment,
44(7), 877-883.
Abstract:
A passive sampling method for radiocarbon analysis of atmospheric CO2 using molecular sieve
Radiocarbon (14C) analysis of atmospheric CO2can provide information on CO2sources and is potentially valuable for validating inventories of fossil fuel-derived CO2emissions to the atmosphere. We tested zeolite molecular sieve cartridges, in both field and laboratory experiments, for passively collecting atmospheric CO2. Cartridges were exposed to the free atmosphere in two configurations which controlled CO2trapping rate, allowing collection of sufficient CO2in between 1.5 and 10 months at current levels.14C results for passive samples were within measurement uncertainty of samples collected using a pump-based system, showing that the method collected samples with14C contents representative of the atmosphere. δ13C analysis confirmed that the cartridges collected representative CO2samples, however, fractionation during passive trapping means that δ13C values need to be adjusted by an amount which we have quantified. Trapping rate was proportional to atmospheric CO2concentration, and was not affected by exposure time unless this exceeded a threshold. Passive sampling using molecular sieve cartridges provides an easy and reliable method to collect atmospheric CO2for14C analysis. © 2009 Elsevier Ltd. All rights reserved.
Abstract.
Hartley IP, Hopkins DW, Sommerkorn M, Wookey PA (2010). The response of organic matter mineralisation to nutrient and substrate additions in sub-arctic soils.
Soil Biology and Biochemistry,
42(1), 92-100.
Abstract:
The response of organic matter mineralisation to nutrient and substrate additions in sub-arctic soils
Global warming in the Arctic may alter decomposition rates in Arctic soils and therefore nutrient availability. In addition, changes in the length of the growing season may increase plant productivity and the rate of labile C input below ground. We carried out an experiment in which inorganic nutrients (NH4NO3and NaPO4) and organic substrates (glucose and glycine) were added to soils sampled from across the mountain birch forest-tundra heath ecotone in northern Sweden (organic and mineral soils from the forest, and organic soil only from the heath). Carbon dioxide production was then monitored continuously over the following 19 days. Neither inorganic N nor P additions substantially affected soil respiration rates when added separately. However, combined N and P additions stimulated microbial activity, with the response being greatest in the birch forest mineral soil (57% increase in CO2production compared with 26% in the heath soil and 8% in the birch forest organic soil). Therefore, mineralisation rates in these soils may be stimulated if the overall nutrient availability to microbes increases in response to global change, but N deposition alone is unlikely to enhance decomposition. Adding either, or both, glucose and glycine increased microbial respiration. Isotopic separation indicated that the mineralisation of native soil organic matter (SOM) was stimulated by glucose addition in the heath soil and the forest mineral soil, but not in the forest organic soil. These positive 'priming' effects were lost following N addition in forest mineral soil, and following both N and P additions in the heath soil. In order to meet enhanced microbial nutrient demand, increased inputs of labile C from plants could stimulate the mineralisation of SOM, with the soil C stocks in the tundra-heath potentially most vulnerable. © 2009 Elsevier Ltd. All rights reserved.
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2009
Garnett MH, Hartley IP, Hopkins DW, Sommerkorn M, Wookey PA (2009). A passive sampling method for radiocarbon analysis of soil respiration using molecular sieve.
Soil Biology and Biochemistry,
41(7), 1450-1456.
Abstract:
A passive sampling method for radiocarbon analysis of soil respiration using molecular sieve
Radiocarbon analysis of soil CO2can provide information on the age, source and turnover rate of soil organic C. We developed a new method for passively trapping respired CO2on molecular sieve, allowing it to be returned to the laboratory and recovered for C isotope analysis. We tested the method on a soil at a grassland site, and using a synthetic soil created to provide a contrasting isotopic signature. As with other passive sampling techniques, a small amount of fractionation of the13C isotope occurs during sampling, which we have quantified, otherwise the results show that the molecular sieve traps a sufficiently large and representative sample of CO2for C isotope analysis. Since14C results are routinely corrected for mass-dependent fractionation, our results show that passive sampling of soil respiration using molecular sieve offers a reliable method to collect soil-respired CO2for14C analysis. © 2009 Elsevier Ltd. All rights reserved.
Abstract.
Wookey PA, Aerts R, Bardgett RD, Baptist F, Bråthen K, Cornelissen JHC, Gough L, Hartley IP, Hopkins DW, Lavorel S, et al (2009). Ecosystem feedbacks and cascade processes: Understanding their role in the responses of Arctic and alpine ecosystems to environmental change.
Global Change Biology,
15(5), 1153-1172.
Abstract:
Ecosystem feedbacks and cascade processes: Understanding their role in the responses of Arctic and alpine ecosystems to environmental change
Global environmental change, related to climate change and the deposition of airborne N-containing contaminants, has already resulted in shifts in plant community composition among plant functional types in Arctic and temperate alpine regions. In this paper, we review how key ecosystem processes will be altered by these transformations, the complex biological cascades and feedbacks that might result, and some of the potential broader consequences for the earth system. Firstly, we consider how patterns of growth and allocation, and nutrient uptake, will be altered by the shifts in plant dominance. The ways in which these changes may disproportionately affect the consumer communities, and rates of decomposition, are then discussed. We show that the occurrence of a broad spectrum of plant growth forms in these regions (from cryptogams to deciduous and evergreen dwarf shrubs, graminoids and forbs), together with hypothesized low functional redundancy, will mean that shifts in plant dominance result in a complex series of biotic cascades, couplings and feedbacks which are supplemental to the direct responses of ecosystem components to the primary global change drivers. The nature of these complex interactions is highlighted using the example of the climate-driven increase in shrub cover in low-Arctic tundra, and the contrasting transformations in plant functional composition in mid-latitude alpine systems. Finally, the potential effects of the transformations on ecosystem properties and processes that link with the earth system are reviewed. We conclude that the effects of global change on these ecosystems, and potential climate-change feedbacks, cannot be predicted from simple empirical relationships between processes and driving variables. Rather, the effects of changes in species distributions and dominances on key ecosystem processes and properties must also be considered, based upon best estimates of the trajectories of key transformations, their magnitude and rates of change. © Journal compilation © 2009 Blackwell Publishing.
Abstract.
Hartley IP, Hopkins DW, Garnett MH, Sommerkorn M, Wookey PA (2009). No evidence for compensatory thermal adaptation of soil microbial respiration in the study of Bradford et al. (2008).
Ecol Lett,
12(7), E12-E14.
Abstract:
No evidence for compensatory thermal adaptation of soil microbial respiration in the study of Bradford et al. (2008).
Bradford et al. (2008) conclude that thermal adaptation will reduce the response of soil microbial respiration to rising global temperatures. However, we question both the methods used to calculate mass-specific respiration rates and the interpretation of the results. No clear evidence of thermal adaptation reducing soil microbial activity was produced.
Abstract.
Author URL.
2008
Zaragoza-Castells J, Sánchez-Gómez D, Hartley IP, Matesanz S, Valladares F, Lloyd J, Atkin OK (2008). Climate-dependent variations in leaf respiration in a dry-land, low productivity Mediterranean forest: the importance of acclimation in both high-light and shaded habitats.
Functional Ecology,
22(1), 172-184.
Abstract:
Climate-dependent variations in leaf respiration in a dry-land, low productivity Mediterranean forest: the importance of acclimation in both high-light and shaded habitats
1. Climate-driven changes in leaf respiration (R) in darkness have the potential to determine whether low productivity ecosystems exhibit positive or negative carbon balances. 2. We investigated whether sustained exposure to full sunlight, shade and seasonal drought alters the temperature response of leaf R of field-grown Quercus ilex subsp. ballota in a dry-land continental Mediterranean ecosystem. The plants studied, experience large diurnal and seasonal variations in temperature. 3. Whilst growth irradiance impacted on photosynthesis, it had little effect on the short-term temperature dependence of leaf R. Moreover, although basal rates of leaf R (i.e. rates of R at a common measuring temperature) were higher in sun-exposed than shade-exposed leaves, growth irradiance had little impact on the degree of acclimation to seasonal changes in temperature and/or moisture. Basal rates of leaf R were higher in winter than summer in both sun-exposed and shaded plants. Estimated Q 10 values (i.e. proportional increase in R per 10 °C rise in temperature) for leaf R were greater in winter than summer; however, no seasonal variation was found in the apparent activation energy (E0) of leaf R. These observations were used to construct a simple Arrhenius model that fully accounted for both daily and seasonal variations in the temperature dependence of R in both sun-exposed and shaded plants. Crucial to the model was accounting for the seasonal and irradiance-dependent shifts in the basal rate of leaf R. 4. Although the balance between daily R and photosynthesis increased markedly in summer (particularly under full sun), the increase in this ratio was markedly less than would have been the case if leaf R had not acclimated to the high average day time temperatures in summer. 5. It is concluded that seasonal acclimation of leaf R plays a crucial role in determining the viability of tree growth in dry-land, low productivity forest ecosystems. © 2007 the Authors.
Abstract.
Hartley IP, Hopkins DW, Garnett MH, Sommerkorn M, Wookey PA (2008). Soil microbial respiration in arctic soil does not acclimate to temperature.
Ecol Lett,
11(10), 1092-1100.
Abstract:
Soil microbial respiration in arctic soil does not acclimate to temperature.
Warming-induced release of CO2 from the large carbon (C) stores in arctic soils could accelerate climate change. However, declines in the response of soil respiration to warming in long-term experiments suggest that microbial activity acclimates to temperature, greatly reducing the potential for enhanced C losses. As reduced respiration rates with time could be equally caused by substrate depletion, evidence for thermal acclimation remains controversial. To overcome this problem, we carried out a cooling experiment with soils from arctic Sweden. If acclimation causes the reduction in soil respiration observed after experimental warming, then it should subsequently lead to an increase in respiration rates after cooling. We demonstrate that thermal acclimation did not occur following cooling. Rather, during the 90 days after cooling, a further reduction in the soil respiration rate was observed, which was only reversed by extended re-exposure to warmer temperatures. We conclude that over the time scale of a few weeks to months, warming-induced changes in the microbial community in arctic soils will amplify the instantaneous increase in the rates of CO2 production and thus enhance C losses potentially accelerating the rate of 21st century climate change.
Abstract.
Author URL.
Hawkes CV, Hartley IP, Ineson P, Fitter AH (2008). Soil temperature affects carbon allocation within arbuscular mycorrhizal networks and carbon transport from plant to fungus.
Global Change Biology,
14(5), 1181-1190.
Abstract:
Soil temperature affects carbon allocation within arbuscular mycorrhizal networks and carbon transport from plant to fungus
How soil carbon balance will be affected by plant-mycorrhizal interactions under future climate scenarios remains a significant unknown in our ability to forecast ecosystem carbon storage and fluxes. We examined the effects of soil temperature (14, 20, 26 °C) on the structure and extent of a multispecies community of arbuscular mycorrhizal (AM) fungi associated with Plantago lanceolata. To isolate fungi from roots, we used a mesh-divided pot system with separate hyphal compartments near and away from the plant. A 13C pulse label was then used to trace the flow of recently fixed photosynthate from plants into belowground pools and respiration. Temperature significantly altered the structure and allocation of the AM hyphal network, with a switch from more vesicles (storage) in cooled soils to more extensive extraradical hyphal networks (growth) in warmed soils. As soil temperature increased, we also observed an increase in the speed at which plant photosynthate was transferred to and respired by roots and AM fungi coupled with an increase in the amount of carbon respired per unit hyphal length. These differences were largely independent of plant size and rates of photosynthesis. In a warmer world, we would therefore expect more carbon losses to the atmosphere from AM fungal respiration, which are unlikely to be balanced by increased growth of AM fungal hyphae. © 2008 the Authors Journal compilation © 2008 Blackwell Publishing Ltd.
Abstract.
Hartley IP, Ineson P (2008). Substrate quality and the temperature sensitivity of soil organic matter decomposition.
Soil Biology and Biochemistry,
40(7), 1567-1574.
Abstract:
Substrate quality and the temperature sensitivity of soil organic matter decomposition
Determining the relative temperature sensitivities of the decomposition of the different soil organic matter (SOM) pools is critical for predicting the long-term impacts of climate change on soil carbon (C) storage. Although kinetic theory suggests that the temperature sensitivity of SOM decomposition should increase with substrate recalcitrance, there remains little empirical evidence to support this hypothesis. In the study presented here, sub-samples from a single bulk soil sample were frozen and sequentially defrosted to produce samples of the same soil that had been incubated for different lengths of time, up to a maximum of 124 days. These samples were then placed into an incubation system which allowed CO2production to be monitored constantly and the response of soil respiration to short-term temperature manipulations to be investigated. The temperature sensitivity of soil CO2production increased significantly with incubation time suggesting that, as the most labile SOM pool was depleted the temperature sensitivity of SOM decomposition increased. This study is therefore one of the first to provide empirical support for kinetic theory. Further, using a modelling approach, we demonstrate that it is the temperature sensitivity of the decomposition of the more recalcitrant SOM pools that will determine long-term soil-C losses. Therefore, the magnitude of the positive feedback to global warming may have been underestimated in previous modelling studies. © 2008 Elsevier Ltd. All rights reserved.
Abstract.
Full text.
2007
Hartley IP, Heinemeyer A, Ineson P (2007). Effects of three years of soil warming and shading on the rate of soil respiration: Substrate availability and not thermal acclimation mediates observed response.
Global Change Biology,
13(8), 1761-1770.
Abstract:
Effects of three years of soil warming and shading on the rate of soil respiration: Substrate availability and not thermal acclimation mediates observed response
In a number of recent field studies, the positive response of soil respiration to warming has been shown to decline over time. The two main differing hypotheses proposed to explain these results are: (1) soil microbial respiration acclimates to the increased temperature, and (2) substrate availability within the soil decreases with warming so reducing the rate of soil respiration. To investigate the relative merits of these two hypotheses, soil samples (both intact cores and sieved samples) from a 3-year grassland soil-warming and shading experiment were incubated for 4 weeks at three different temperatures under constant laboratory conditions. We tested the hypothesis that sieving the soils would reduce differences in substrate availability between warmed and control plot samples and would therefore result in similar respiration rates if microbial activity had not acclimated to soil warming. In addition, to further test the effect of substrate availability, we compared the respiration rates of soils taken from shaded and unshaded plots. Both soil warming and shading significantly reduced respiration rates in the intact cores, especially under higher incubation temperatures. However, sieving the soil greatly reduced these differences suggesting that substrate availability, and not microbial acclimation to the higher temperatures, played the dominant role in determining the response of heterotrophic soil respiration to warming. The effect of shading appeared to be mediated by reduced plant productivity affecting substrate availability within the soil and hence microbial activity. Given the lack of evidence for thermal acclimation of microbial respiration, there remains the potential for prolonged carbon losses from soils in response to warming. © 2007 Blackwell Publishing Ltd.
Abstract.
Heinemeyer A, Hartley IP, Evans SP, Carreira De La Fuente JA, Ineson P (2007). Forest soil CO<inf>2</inf>flux: Uncovering the contribution and environmental responses of ectomycorrhizas.
Global Change Biology,
13(8), 1786-1797.
Abstract:
Forest soil CO2flux: Uncovering the contribution and environmental responses of ectomycorrhizas
Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO2efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO2efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the 'autotrophic' root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO2efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO2flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO2flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO2flux component represents an overflow 'CO2tap' through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO2. © 2007 Blackwell Publishing Ltd.
Abstract.
Hartley IP, Heinemeyer A, Evans SP, Ineson P (2007). The effect of soil warming on bulk soil vs. rhizosphere respiration.
Global Change Biology,
13(12), 2654-2667.
Abstract:
The effect of soil warming on bulk soil vs. rhizosphere respiration
There has been considerable debate on whether root/rhizosphere respiration or bulk soil respiration is more sensitive to long-term temperature changes. We investigated the response of belowground respiration to soil warming by 3 °C above ambient in bare soil plots and plots planted with wheat and maize. Initially, belowground respiration responded more to the soil warming in bare soil plots than in planted plots. However, as the growing season progressed, a greater soil-warming response developed in the planted plots as the contribution of root/rhizosphere respiration to belowground respiration declined. A negative correlation was observed between the contribution of root/ rhizosphere respiration to total belowground respiration and the magnitude of the soil-warming response indicating that bulk soil respiration is more temperature sensitive than root/rhizosphere respiration. The dependence of root/rhizosphere respiration on substrate provision from photosynthesis is the most probable explanation for the observed lower temperature sensitivity of root/rhizosphere respiration. At harvest in late September, final crop biomass did not differ between the two soil temperature treatments in either the maize or wheat plots. Postharvest, flux measurements during the winter months indicated that the response of belowground respiration to the soil-warming treatment increased in magnitude (response equated to a Q10value of 5.7 compared with ∼2.3 during the growing season). However, it appeared that this response was partly caused by a strong indirect effect of soil warming. When measurements were made at a common temperature, belowground respiration remained higher in the warmed subplots suggesting soil warming had maintained a more active microbial community through the winter months. It is proposed that any changes in winter temperatures, resulting from global warming, could alter the sink strength of terrestrial ecosystems considerably. © 2007 Blackwell Publishing Ltd.
Abstract.
2006
Hartley IP, Armstrong AF, Murthy R, Barron-Gafford G, Ineson P, Atkin OK (2006). The dependence of respiration on photosynthetic substrate supply and temperature: Integrating leaf, soil and ecosystem measurements.
Global Change Biology,
12(10), 1954-1968.
Abstract:
The dependence of respiration on photosynthetic substrate supply and temperature: Integrating leaf, soil and ecosystem measurements
Interactions between photosynthetic substrate supply and temperature in determining the rate of three respiration components (leaf, belowground and ecosystem respiration) were investigated within three environmentally controlled, Populus deltoides forest bays at Biosphere 2, Arizona. Over 2 months, the atmospheric CO2 concentration and air temperature were manipulated to test the following hypotheses: (1) the responses of the three respiration components to changes in the rate of photosynthesis would differ both in speed and magnitude; (2) the temperature sensitivity of leaf and belowground respiration would increase in response to a rise in substrate availability; and, (3) at the ecosystem level, the ratio of respiration to photosynthesis would be conserved despite week-to-week changes in temperature. All three respiration rates responded to the CO2 concentration-induced changes in photosynthesis. However, the proportional change in the rate of leaf respiration was more than twice that of belowground respiration and, when photosynthesis was reduced, was also more rapid. The results suggest that aboveground respiration plays a key role in the overall response of ecosystem respiration to short-term changes in canopy photosynthesis. The short-term temperature sensitivity of leaf respiration, measured within a single night, was found to be affected more by developmental conditions than photosynthetic substrate availability, as the Q10 was lower in leaves that developed at high CO2, irrespective of substrate availability. However, the temperature sensitivity of belowground respiration, calculated between periods of differing air temperature, appeared to be positively correlated with photosynthetic substrate availability. At the ecosystem level, respiration and photosynthesis were positively correlated but the relationship was affected by temperature; for a given rate of daytime photosynthesis, the rate of respiration the following night was greater at 25 than 20°C. This result suggests that net ecosystem exchange did not acclimate to temperature changes lasting up to 3 weeks. Overall, the results of this study demonstrate that the three respiration terms differ in their dependence on photosynthesis and that, short- and medium-term changes in temperature may affect net carbon storage in terrestrial ecosystems. © 2006 Blackwell Publishing Ltd.
Abstract.
