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Determining the temperature sensitivity of terrestrial carbon (C) stores is an urgent priority for predicting future climate feedbacks. A key aspect to solve this long‐standing research gap is to determine whether warmer temperatures will increase autotrophic activities leading to greater C storage or promote heterotrophic activities that will drive these systems to become C sources. We experimentally addressed this critical question by subjecting intact plant‐soil systems in a UK upland ecosystem to simulated climate warming under natural field conditions. We report the results of a 13‐year field‐based climate manipulation experiment combining in situ respiration measurements with radiocarbon (14C) analyses of respired CO2, dissolved organic carbon (DOC), soil and the tissue contents of the dominant soil fauna (enchytraeids). We found that warming during the growing season produced the largely expected increases in ecosystem respiration (63%) and leaching of DOC (19%) with no evidence for thermal acclimation or substrate exhaustion over the whole 13‐year experimental period. Contrary to expectations, we found no evidence to support an increased release of old soil C after more than a decade of simulated climatic change, and indeed, 14C analyses indicated that warming caused a significant shift towards mineralisation of more recent plant‐derived C inputs. Further support came from the radiocarbon analyses of the enchytraeid tissues, which showed a greater assimilation of the more recent (plant‐derived) C sources following warming. Therefore, in contrast to subarctic ecosystems, our results suggest that changes in C storage in this UK upland soil are strongly coupled to plant activities and that increasing temperatures will drive the turnover of organic material fixed only within recent years, without resulting in the loss of existing old carbon stores.

Determining the temperature sensitivity of terrestrial carbon (C) stores is an urgent priority for predicting future climate feedbacks. A key aspect to solve this long‐standing research gap is to determine whether warmer temperatures will increase autotrophic activities leading to greater C storage or promote heterotrophic activities that will drive these systems to become C sources. We experimentally addressed this critical question by subjecting intact plant‐soil systems in a UK upland ecosystem to simulated climate warming under natural field conditions. We report the results of a 13‐year field‐based climate manipulation experiment combining in situ respiration measurements with radiocarbon (14C) analyses of respired CO2, dissolved organic carbon (DOC), soil and the tissue contents of the dominant soil fauna (enchytraeids). We found that warming during the growing season produced the largely expected increases in ecosystem respiration (63%) and leaching of DOC (19%) with no evidence for thermal acclimation or substrate exhaustion over the whole 13‐year experimental period. Contrary to expectations, we found no evidence to support an increased release of old soil C after more than a decade of simulated climatic change, and indeed, 14C analyses indicated that warming caused a significant shift towards mineralisation of more recent plant‐derived C inputs. Further support came from the radiocarbon analyses of the enchytraeid tissues, which showed a greater assimilation of the more recent (plant‐derived) C sources following warming. Therefore, in contrast to subarctic ecosystems, our results suggest that changes in C storage in this UK upland soil are strongly coupled to plant activities and that increasing temperatures will drive the turnover of organic material fixed only within recent years, without resulting in the loss of existing old carbon stores.

Miscanthus x giganteus's efficacy as an energy crop relies on maintaining low greenhouse gas (GHG) emissions. As demand for Miscanthus is expected to rise to meet bioenergy targets, fertilizers and composts may be employed to increase yields, but will also increase GHG emissions. Manipulation experiments are vital to investigate the consequences of any fertilizer additions, but there is currently no way to measure whole‐plant GHG fluxes from crops taller than 2.5 m, such as Miscanthus, at the experimental plot scale. We employed a unique combination of eddy covariance (EC), soil chambers and an entirely new automated chamber system, SkyBeam, to measure high frequency (ca. hourly) fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from a Miscanthus crop amended with green compost. Untreated controls were also monitored in a fully replicated experimental design. Net ecosystem exchange (NEE) of CO2 was partitioned into soil respiration (Rs), gross primary productivity (GPP) and ecosystem respiration, and the crop was harvested to determine the effect of compost on crop productivity. Compost increased NEE emissions by 100% (p < .05), which was the result of a 20% increase of Rs (p < .06) and a 32% reduction in GPP (p < .05) and biomass of 37% (p < .06). Methane fluxes were small and unaffected by compost addition. N2O emissions increased 34% under compost during an emission event; otherwise, fluxes were low and often negative, even under dry conditions. Diurnal variation in N2O fluxes, with uptake during the day and emission at night was observed. These fluxes displayed a negative relationship with soil temperature and a hitherto undescribed diurnal temperature hysteresis. We conclude that compost addition negatively affected the productivity and environmental effects of Miscanthus cultivation during the first year following application.

Miscanthus x giganteus's efficacy as an energy crop relies on maintaining low greenhouse gas (GHG) emissions. As demand for Miscanthus is expected to rise to meet bioenergy targets, fertilizers and composts may be employed to increase yields, but will also increase GHG emissions. Manipulation experiments are vital to investigate the consequences of any fertilizer additions, but there is currently no way to measure whole‐plant GHG fluxes from crops taller than 2.5 m, such as Miscanthus, at the experimental plot scale. We employed a unique combination of eddy covariance (EC), soil chambers and an entirely new automated chamber system, SkyBeam, to measure high frequency (ca. hourly) fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from a Miscanthus crop amended with green compost. Untreated controls were also monitored in a fully replicated experimental design. Net ecosystem exchange (NEE) of CO2 was partitioned into soil respiration (Rs), gross primary productivity (GPP) and ecosystem respiration, and the crop was harvested to determine the effect of compost on crop productivity. Compost increased NEE emissions by 100% (p < .05), which was the result of a 20% increase of Rs (p < .06) and a 32% reduction in GPP (p < .05) and biomass of 37% (p < .06). Methane fluxes were small and unaffected by compost addition. N2O emissions increased 34% under compost during an emission event; otherwise, fluxes were low and often negative, even under dry conditions. Diurnal variation in N2O fluxes, with uptake during the day and emission at night was observed. These fluxes displayed a negative relationship with soil temperature and a hitherto undescribed diurnal temperature hysteresis. We conclude that compost addition negatively affected the productivity and environmental effects of Miscanthus cultivation during the first year following application.

A fundamental challenge in experimental ecology is to capture nonlinearities of ecological responses to interacting environmental drivers. Here, we demonstrate that gradient designs outperform replicated designs for detecting and quantifying nonlinear responses. We report the results of (1) multiple computer simulations and (2) two purpose‐designed empirical experiments. The findings consistently revealed that unreplicated sampling at a maximum number of sampling locations maximised prediction success (i.e. the R² to the known truth) irrespective of the amount of stochasticity and the underlying response surfaces, including combinations of two linear, unimodal or saturating drivers. For the two empirical experiments, the same pattern was found, with gradient designs outperforming replicated designs in revealing the response surfaces of underlying drivers. Our findings suggest that a move to gradient designs in ecological experiments could be a major step towards unravelling underlying response patterns to continuous and interacting environmental drivers in a feasible and statistically powerful way.

Oilseed rape (OSR, Brassica napus L.) is an important feedstock for biodiesel; hence, carbon dioxide (CO2), methane (CH4) and particularly fertilizer-derived nitrous oxide (N2O) emissions during cultivation must be quantified to assess putative greenhouse gas (GHG) savings, thus creating an urgent and increasing need for such data. Substrates of nitrification [ammonium (NH4)] and denitrification [nitrate (NO3)], the predominant N2O production pathways, were supplied separately and in combination to OSR in a UK field trial aiming to: (i) produce an accurate GHG budget of fertilizer application; (ii) characterize short- to medium-term variation in GHG fluxes; (iii) establish the processes driving N2O emission. Three treatments were applied twice, 1 week apart: ammonium nitrate fertilizer (NH4NO3, 69 kg-N ha−1) mimicking the farm management, ammonium chloride (NH4Cl, 34.4 kg-N ha−1) and sodium nitrate (NaNO3, 34.6 kg-N ha−1). We deployed SkyLine2D for the very first time, a novel automated chamber system to measure CO2, CH4 and N2O fluxes at unprecedented high temporal and spatial resolution from OSR. During 3 weeks following the fertilizer application, CH4 fluxes were negligible, but all treatments were a net sink for CO2 (ca. 100 g CO2 m−2). Cumulative N2O emissions (ca. 120 g CO2-eq m−2) from NH4NO3 were significantly greater (P < 0.04) than from NaNO3 (ca. 80 g CO2-eq m−2), but did not differ from NH4Cl (ca. 100 g CO2-eq m−2) and reduced the carbon sink of photosynthesis so that OSR was a net GHG source in the fertilizer treatment. Diurnal variation in N2O emissions, peaking in the afternoon, was more strongly associated with photosynthetically active radiation (PAR) than temperature. This suggests that the supply of carbon (C) from photosynthate may have been the key driver of the observed diurnal pattern in N2O emission and thus should be considered in future process-based models of GHG emissions.

Oilseed rape (OSR, Brassica napus L.) is an important feedstock for biodiesel; hence, carbon dioxide (CO2), methane (CH4) and particularly fertilizer-derived nitrous oxide (N2O) emissions during cultivation must be quantified to assess putative greenhouse gas (GHG) savings, thus creating an urgent and increasing need for such data. Substrates of nitrification [ammonium (NH4)] and denitrification [nitrate (NO3)], the predominant N2O production pathways, were supplied separately and in combination to OSR in a UK field trial aiming to: (i) produce an accurate GHG budget of fertilizer application; (ii) characterize short- to medium-term variation in GHG fluxes; (iii) establish the processes driving N2O emission. Three treatments were applied twice, 1 week apart: ammonium nitrate fertilizer (NH4NO3, 69 kg-N ha−1) mimicking the farm management, ammonium chloride (NH4Cl, 34.4 kg-N ha−1) and sodium nitrate (NaNO3, 34.6 kg-N ha−1). We deployed SkyLine2D for the very first time, a novel automated chamber system to measure CO2, CH4 and N2O fluxes at unprecedented high temporal and spatial resolution from OSR. During 3 weeks following the fertilizer application, CH4 fluxes were negligible, but all treatments were a net sink for CO2 (ca. 100 g CO2 m−2). Cumulative N2O emissions (ca. 120 g CO2-eq m−2) from NH4NO3 were significantly greater (P < 0.04) than from NaNO3 (ca. 80 g CO2-eq m−2), but did not differ from NH4Cl (ca. 100 g CO2-eq m−2) and reduced the carbon sink of photosynthesis so that OSR was a net GHG source in the fertilizer treatment. Diurnal variation in N2O emissions, peaking in the afternoon, was more strongly associated with photosynthetically active radiation (PAR) than temperature. This suggests that the supply of carbon (C) from photosynthate may have been the key driver of the observed diurnal pattern in N2O emission and thus should be considered in future process-based models of GHG emissions.

The effects of increased tropospheric ozone (O3) pollution levels on methane (CH4) emissions from peatlands, and their underlying mechanisms, remain unclear. In this study, we exposed peatland mesocosms from a temperate wet heath dominated by the sedge Schoenus nigricans and Sphagnum papillosum to four O3 treatments in open-top chambers for 2.5 years, to investigate the O3 impacts on CH4 emissions and the processes that underpin these responses. Summer CH4 emissions, were significantly reduced, by 27% over the experiment, due to summer daytime (8 h day− 1) O3 exposure to non-filtered air (NFA) plus 35 ppb O3, but were not significantly affected by year-round, 24 h day− 1, exposure to NFA plus 10 ppb or NFA plus 25 ppb O3. There was no evidence that the reduced CH4 emissions in response to elevated summer O3 exposure were caused by reduced plant-derived carbon availability below-ground, because we found no significant effect of high summer O3 exposure on root biomass, pore water dissolved organic carbon concentrations or the contribution of recent photosynthate to CH4 emissions. Our CH4 production potential and CH4 oxidation potential measurements in the different O3 treatments could also not explain the observed CH4 emission responses to O3. However, pore water ammonium concentrations at 20 cm depth were consistently reduced during the experiment by elevated summer O3 exposure, and strong positive correlations were observed between CH4 emission and pore water ammonium concentration at three peat depths over the 2.5-year study. Our results therefore imply that elevated regional O3 exposures in summer, but not the small increases in northern hemisphere annual mean background O3 concentrations predicted over this century, may lead to reduced CH4 emissions from temperate peatlands as a consequence of reductions in soil inorganic nitrogen affecting methanogenic and/or methanotrophic activity.

The effects of increased tropospheric ozone (O3) pollution levels on methane (CH4) emissions from peatlands, and their underlying mechanisms, remain unclear. In this study, we exposed peatland mesocosms from a temperate wet heath dominated by the sedge Schoenus nigricans and Sphagnum papillosum to four O3 treatments in open-top chambers for 2.5 years, to investigate the O3 impacts on CH4 emissions and the processes that underpin these responses. Summer CH4 emissions, were significantly reduced, by 27% over the experiment, due to summer daytime (8 h day− 1) O3 exposure to non-filtered air (NFA) plus 35 ppb O3, but were not significantly affected by year-round, 24 h day− 1, exposure to NFA plus 10 ppb or NFA plus 25 ppb O3. There was no evidence that the reduced CH4 emissions in response to elevated summer O3 exposure were caused by reduced plant-derived carbon availability below-ground, because we found no significant effect of high summer O3 exposure on root biomass, pore water dissolved organic carbon concentrations or the contribution of recent photosynthate to CH4 emissions. Our CH4 production potential and CH4 oxidation potential measurements in the different O3 treatments could also not explain the observed CH4 emission responses to O3. However, pore water ammonium concentrations at 20 cm depth were consistently reduced during the experiment by elevated summer O3 exposure, and strong positive correlations were observed between CH4 emission and pore water ammonium concentration at three peat depths over the 2.5-year study. Our results therefore imply that elevated regional O3 exposures in summer, but not the small increases in northern hemisphere annual mean background O3 concentrations predicted over this century, may lead to reduced CH4 emissions from temperate peatlands as a consequence of reductions in soil inorganic nitrogen affecting methanogenic and/or methanotrophic activity.

There is increasing interest in the use of nonfood second-generation lignocellulosic feedstocks and a move away from food crops for bioenergy applications, but questions still remain on sustainability. Empirical data are needed to quantify the GHG balance of land-use transition to lignocellulosic bioenergy cropping systems, to inform lifecycle analyses and aid model validation. The aim of this project ‘Ecosystem Land Use Modeling and Soil Carbon GHG Flux Trial’ is to produce a framework for predicting the sustainability of bioenergy deployment across the UK. This GB£4m consortium project is commissioned and funded by the Energy Technologies Institute, UK.

Unravelling the role of structural and environmental drivers of gross primary productivity (GPP) and ecosystem respiration (Reco) in highly heterogeneous tundra is a major challenge for the upscaling of chamberbased CO2 fluxes in Arctic landscapes. In a mountain birch woodland-mire ecotone, we investigated the role of LAI (and NDVI), environmental factors (microclimate, soil moisture), and microsite type across tundra shrub plots (wet hummocks, dry hummocks, dry hollows) and lichen hummocks, in controlling net ecosystem CO2 exchange (NEE). During a growing season, we measured NEE fluxes continuously, with closed dynamic chambers,andperformedmultiple fits (onefor each 3-day period) of a simple light and temperature response model to hourlyNEE data. Tundra shrub plots were largely CO2 sinks, as opposed to lichen plots, although fluxes were highly variable within microsite type. For tundra shrub plots, microsite type did not influence photosynthetic parameters but it affected basal (that is, temperature-normalized) ecosystem respiration (R0). PAR-normalized photosynthesis (P600) increased with air temperature and declined with increasing vapor pressure deficit. R0 declined with soil moisture and showed an apparent increase with temperature, which may underlie a tight link between GPP and Reco. NDVI was a good proxy for LAI, maximum P600 and maximum R0 of shrub plots. Cumulative CO2 fluxes were strongly correlated with LAI (NDVI) but we observed a comparatively low GPP/LAI in dry hummocks. Our results broadly agree with the reported functional convergence across tundra vegetation, but here we show that the role of decreased productivity in transition zones and the influence of temperature and water balance on seasonal CO2 fluxes in sub-Arctic forest–mire ecotones cannot be overlooked.

There is increasing interest in the use of nonfood second-generation lignocellulosic feedstocks and a move away from food crops for bioenergy applications, but questions still remain on sustainability. Empirical data are needed to quantify the GHG balance of land-use transition to lignocellulosic bioenergy cropping systems, to inform lifecycle analyses and aid model validation. The aim of this project ‘Ecosystem Land Use Modeling and Soil Carbon GHG Flux Trial’ is to produce a framework for predicting the sustainability of bioenergy deployment across the UK. This GB£4m consortium project is commissioned and funded by the Energy Technologies Institute, UK.

Unravelling the role of structural and environmental drivers of gross primary productivity (GPP) and ecosystem respiration (Reco) in highly heterogeneous tundra is a major challenge for the upscaling of chamberbased CO2 fluxes in Arctic landscapes. In a mountain birch woodland-mire ecotone, we investigated the role of LAI (and NDVI), environmental factors (microclimate, soil moisture), and microsite type across tundra shrub plots (wet hummocks, dry hummocks, dry hollows) and lichen hummocks, in controlling net ecosystem CO2 exchange (NEE). During a growing season, we measured NEE fluxes continuously, with closed dynamic chambers,andperformedmultiple fits (onefor each 3-day period) of a simple light and temperature response model to hourlyNEE data. Tundra shrub plots were largely CO2 sinks, as opposed to lichen plots, although fluxes were highly variable within microsite type. For tundra shrub plots, microsite type did not influence photosynthetic parameters but it affected basal (that is, temperature-normalized) ecosystem respiration (R0). PAR-normalized photosynthesis (P600) increased with air temperature and declined with increasing vapor pressure deficit. R0 declined with soil moisture and showed an apparent increase with temperature, which may underlie a tight link between GPP and Reco. NDVI was a good proxy for LAI, maximum P600 and maximum R0 of shrub plots. Cumulative CO2 fluxes were strongly correlated with LAI (NDVI) but we observed a comparatively low GPP/LAI in dry hummocks. Our results broadly agree with the reported functional convergence across tundra vegetation, but here we show that the role of decreased productivity in transition zones and the influence of temperature and water balance on seasonal CO2 fluxes in sub-Arctic forest–mire ecotones cannot be overlooked.

This report describes the second year of Work Package 3 (WP3) activities within the ETI’s Ecosystem Land Use Modelling Project (“ELUM”). It expands upon information reported in the first year and provides a forward look to WP3 activities for the remainder of the project. The soil C (carbon) and GHG (Greenhouse Gas) measurements recorded as part of WP3 are required to help reduce the uncertainty associated with the sustainability of bioenergy crop deployment across the UK. This data will be used to parameterise and test the underlying process models in the WP4 modelling work, as part of the development of the over-arching meta-model. A full review of all the data collected across the WP3 network sites will be reported in the D3.5 deliverable due in May 2014. Progress with the development and testing of novel methods for GHG measurement is also included in this report; these could offer means of improving monitoring resolution, thereby enhancing the collection of GHG flux data. A complete review of this work will follow in May 2014 with the D3.4 deliverable.

This report describes the second year of Work Package 3 (WP3) activities within the ETI’s Ecosystem Land Use Modelling Project (“ELUM”). It expands upon information reported in the first year and provides a forward look to WP3 activities for the remainder of the project. The soil C (carbon) and GHG (Greenhouse Gas) measurements recorded as part of WP3 are required to help reduce the uncertainty associated with the sustainability of bioenergy crop deployment across the UK. This data will be used to parameterise and test the underlying process models in the WP4 modelling work, as part of the development of the over-arching meta-model. A full review of all the data collected across the WP3 network sites will be reported in the D3.5 deliverable due in May 2014. Progress with the development and testing of novel methods for GHG measurement is also included in this report; these could offer means of improving monitoring resolution, thereby enhancing the collection of GHG flux data. A complete review of this work will follow in May 2014 with the D3.4 deliverable.

A radiocarbon approach was used to investigate the roles of temperature and soil fauna activity in the turnover of ‘old’ non-labile carbon in a peatland ecosystem. We investigated the impacts of enchytraeids on carbon turnover in two different soil layers, with different incorporation of the ‘bomb’ peak, when incubated at two different temperatures. Results showed that, in agreement with previous studies, warmer temperatures promoted reproduction rates of enchytraeids, with the top layer supporting higher animal densities and biomass. With independence of the animal treatment, soil respiration in the top 5 cm was four times higher than in the deeper layer suggesting that decomposition was greater in the upper layer, with the response being greater at the highest temperature treatment. Furthermore, independent of temperature, the presence of enchytraeids in the top layer significantly enhanced the release of non-labile C as DOC. Similarly, at the bottom layer, ‘older’ C sources were mobilised in response to warming and a greater amount of pre-bomb carbon was released into the soil solution at 20 °C when the worms were present. A strong positive link between the ages of the C assimilated by the animals and released through mineralization suggests an important role of soil biology in the mobilisation of the older C pools in soils and should be taken into account in developing global C models to predict the response of soil C dynamics to climate change.

A radiocarbon approach was used to investigate the roles of temperature and soil fauna activity in the turnover of ‘old’ non-labile carbon in a peatland ecosystem. We investigated the impacts of enchytraeids on carbon turnover in two different soil layers, with different incorporation of the ‘bomb’ peak, when incubated at two different temperatures. Results showed that, in agreement with previous studies, warmer temperatures promoted reproduction rates of enchytraeids, with the top layer supporting higher animal densities and biomass. With independence of the animal treatment, soil respiration in the top 5 cm was four times higher than in the deeper layer suggesting that decomposition was greater in the upper layer, with the response being greater at the highest temperature treatment. Furthermore, independent of temperature, the presence of enchytraeids in the top layer significantly enhanced the release of non-labile C as DOC. Similarly, at the bottom layer, ‘older’ C sources were mobilised in response to warming and a greater amount of pre-bomb carbon was released into the soil solution at 20 °C when the worms were present. A strong positive link between the ages of the C assimilated by the animals and released through mineralization suggests an important role of soil biology in the mobilisation of the older C pools in soils and should be taken into account in developing global C models to predict the response of soil C dynamics to climate change.

We investigated turnover of methane (CH4) in soils from a poorly drained UK forest. In situ, this forest exhibited a negligible soil-atmosphere CH4 flux, whereas adjacent grassland plots were sources of CH4. We hypothesised that the forest plots exhibited reduced anaerobic CH4 production through water-table draw down. Consequently, we exposed soil cores from under oak to high and low water-table conditions in the laboratory. Methane fluxes increased significantly in the high water-table (1925±1702 μg CH4 m-2 h-1) compared to the low one (-3.5±6.8 μg CH4 m-2 h-1). Natural abundance δ13C values of CH4 showed a strong depletion in high water-table cores (-56.7±2.9 ‰) compared to methane in ambient air (-46.0 ‰) indicative of methanogenic processes. The δ13C values of CH4 from low water-table cores (δ13C-46.8±0.2 ‰) was similar to ambient air and suggested little alteration of headspace CH4 by the soil microbial community. In order to assess the CH4 oxidizing activity of the two treatments conclusively, a 13CH4 spike was added to the cores and 13CO2 production was measured as the by-product of CH4 oxidation. 13CH4 oxidation rates were 57.5 (±12.7) and 0.5 (±0.1) μg CH4 m-2 h-1 for high and low water-tables, respectively. These data show that the lower water-table hydrology treatment impacted methanogenic processes without stimulating methanotrophy.

Passive and active ammonia (NH3) sampling devices have been tested for their nitrogen (N) capture potential and 15N fractionation effects. Several sampling techniques produced significantly different 15NH3 signals when sampling the same NH3 source released from field site fumigation campaigns. Conventional passive NH3-monitoring systems have shown to provide insufficient N for isotope-ratio mass spectrometry and various modified devices have been developed, based on existing diffusion tube designs, to overcome this problem. The final sampler design was then tested in a wind tunnel to verify that sampling NH3 in different environmental conditions did not significantly fractionate the 15N signal.

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

Passive and active ammonia (NH3) sampling devices have been tested for their nitrogen (N) capture potential and 15N fractionation effects. Several sampling techniques produced significantly different 15NH3 signals when sampling the same NH3 source released from field site fumigation campaigns. Conventional passive NH3-monitoring systems have shown to provide insufficient N for isotope-ratio mass spectrometry and various modified devices have been developed, based on existing diffusion tube designs, to overcome this problem. The final sampler design was then tested in a wind tunnel to verify that sampling NH3 in different environmental conditions did not significantly fractionate the 15N signal.

Rhizosphere microorganisms play an important role in soil carbon flow, through turnover of root exudates, but there is little information on which organisms are actively involved or on the influence of environmental conditions on active communities. In this study, a 13CO2 pulse labelling field experiment was performed in an upland grassland soil, followed by RNA-stable isotope probing (SIP) analysis, to determine the effect of liming on the structure of the rhizosphere microbial community metabolizing root exudates. The lower limit of detection for SIP was determined in soil samples inoculated with a range of concentrations of 13C-labelled Pseudomonas fluorescens and was found to lie between 105 and 106 cells per gram of soil. The technique was capable of detecting microbial communities actively assimilating root exudates derived from recent photo-assimilate in the field. Denaturing gradient gel electrophoresis (DGGE) profiles of bacteria, archaea and fungi derived from fractions obtained from caesium trifluoroacetate (CsTFA) density gradient ultracentrifugation indicated that active communities in limed soils were more complex than those in unlimed soils and were more active in utilization of recently exuded 13C compounds. In limed soils, the majority of the community detected by standard RNA-DGGE analysis appeared to be utilizing root exudates. In unlimed soils, DGGE profiles from 12C and 13C RNA fractions differed, suggesting that a proportion of the active community was utilizing other sources of organic carbon. These differences may reflect differences in the amount of root exudation under the different conditions

Forests have a key role as carbon sinks, which could potentially mitigate the continuing increase in atmospheric carbon dioxide concentration and associated climate change. We show that carbon dioxide enrichment, although causing short-term growth stimulation in a range of European tree species, also leads to an increase in soil microbial respiration and a marked decline in sequestration of root-derived carbon in the soil. These findings indicate that, should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration.

Forests have a key role as carbon sinks, which could potentially mitigate the continuing increase in atmospheric carbon dioxide concentration and associated climate change. We show that carbon dioxide enrichment, although causing short-term growth stimulation in a range of European tree species, also leads to an increase in soil microbial respiration and a marked decline in sequestration of root-derived carbon in the soil. These findings indicate that, should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration.

Rhizosphere microorganisms play an important role in soil carbon flow, through turnover of root exudates, but there is little information on which organisms are actively involved or on the influence of environmental conditions on active communities. In this study, a 13CO2 pulse labelling field experiment was performed in an upland grassland soil, followed by RNA-stable isotope probing (SIP) analysis, to determine the effect of liming on the structure of the rhizosphere microbial community metabolizing root exudates. The lower limit of detection for SIP was determined in soil samples inoculated with a range of concentrations of 13C-labelled Pseudomonas fluorescens and was found to lie between 105 and 106 cells per gram of soil. The technique was capable of detecting microbial communities actively assimilating root exudates derived from recent photo-assimilate in the field. Denaturing gradient gel electrophoresis (DGGE) profiles of bacteria, archaea and fungi derived from fractions obtained from caesium trifluoroacetate (CsTFA) density gradient ultracentrifugation indicated that active communities in limed soils were more complex than those in unlimed soils and were more active in utilization of recently exuded 13C compounds. In limed soils, the majority of the community detected by standard RNA-DGGE analysis appeared to be utilizing root exudates. In unlimed soils, DGGE profiles from 12C and 13C RNA fractions differed, suggesting that a proportion of the active community was utilizing other sources of organic carbon. These differences may reflect differences in the amount of root exudation under the different conditions

The effect of liming on the flow of recently photosynthesized carbon to rhizosphere soil was studied using 13CO2 pulse labelling, in an upland grassland ecosystem in Scotland. The use of 13C enabled detection, in the field, of the effect of a 4-year liming period of selected soil plots on C allocation from plant biomass to soil, in comparison with unlimed plots. Photosynthetic rates and carbon turnover were higher in plants grown in limed soils than in those from unlimed plots. Higher δ13C‰ values were detected in shoots from limed plants than in those from unlimed plants in samples clipped within 15 days of the end of pulse labelling. Analysis of the aboveground plant production corresponding to the 4-year period of liming indicated that the standing biomass was higher in plots that received lime. Lower δ13C‰ values in limed roots compared with unlimed roots were found, whereas no significant difference was detected between soil samples. Extrapolation of our results indicated that more C has been lost through the soil than has been gained via photosynthetic assimilation because of pasture liming in Scotland during the period 1990–1998. However, the uncertainty associated with such extrapolation based on this single study is high and these estimates are provided only to set our findings in the broader context of national soil carbon emissions.

Climate change due to greenhouse gas emissions is predicted to raise the mean global temperature by 1.0-3.5°C in the next 50-100 years. The direct and indirect effects of this potential increase in temperature on terrestrial ecosystems and ecosystem processes are likely to be complex and highly varied in time and space. The Global Change and Terrestrial Ecosystems core project of the International Geosphere-Biosphere Programme has recently launched a Network of Ecosystem Warming Studies, the goals of which are to integrate and foster research on ecosystem-level effects of rising temperature. In this paper, we use meta-analysis to synthesize data on the response of soil respiration, net N mineralization, and aboveground plant productivity to experimental ecosystem warming at 32 research sites representing four broadly defined biomes, including high (latitude or altitude) tundra, low tundra, grassland, and forest. Warming methods included electrical heat-resistance ground cables, greenhouses, vented and unvented field chambers, overhead infrared lamps, and passive night-time warming. Although results from individual sites showed considerable variation in response to warming, results from the meta-analysis showed that, across all sites and years, 2-9 years of experimental warming in the range 0.3-6.0°C significantly increased soil respiration rates by 20% (with a 95% confidence interval of 18-22%), net N mineralization rates by 46% (with a 95% confidence interval of 30-64%), and plant productivity by 19% (with a 95% confidence interval of 15-23%). The response of soil respiration to warming was generally larger in forested ecosystems compared to low tundra and grassland ecosystems, and the response of plant productivity was generally larger in low tundra ecosystems than in forest and grassland ecosystems. With the exception of aboveground plant productivity, which showed a greater positive response to warming in colder ecosystems, the magnitude of the response of th

Climate change due to greenhouse gas emissions is predicted to raise the mean global temperature by 1.0-3.5°C in the next 50-100 years. The direct and indirect effects of this potential increase in temperature on terrestrial ecosystems and ecosystem processes are likely to be complex and highly varied in time and space. The Global Change and Terrestrial Ecosystems core project of the International Geosphere-Biosphere Programme has recently launched a Network of Ecosystem Warming Studies, the goals of which are to integrate and foster research on ecosystem-level effects of rising temperature. In this paper, we use meta-analysis to synthesize data on the response of soil respiration, net N mineralization, and aboveground plant productivity to experimental ecosystem warming at 32 research sites representing four broadly defined biomes, including high (latitude or altitude) tundra, low tundra, grassland, and forest. Warming methods included electrical heat-resistance ground cables, greenhouses, vented and unvented field chambers, overhead infrared lamps, and passive night-time warming. Although results from individual sites showed considerable variation in response to warming, results from the meta-analysis showed that, across all sites and years, 2-9 years of experimental warming in the range 0.3-6.0°C significantly increased soil respiration rates by 20% (with a 95% confidence interval of 18-22%), net N mineralization rates by 46% (with a 95% confidence interval of 30-64%), and plant productivity by 19% (with a 95% confidence interval of 15-23%). The response of soil respiration to warming was generally larger in forested ecosystems compared to low tundra and grassland ecosystems, and the response of plant productivity was generally larger in low tundra ecosystems than in forest and grassland ecosystems. With the exception of aboveground plant productivity, which showed a greater positive response to warming in colder ecosystems, the magnitude of the response of th

Recently, a link has been suggested between volcanic deposition of SO4 and the suppression of CH4 emissions in northern peatlands (Gauci et al., 2008). This link stems from the widely accepted idea that acid rain SO4 additions to peatlands can cause a shift in microbial communities as SO4 reducing bacteria out-compete methanogens for substrates, which results in a suppression of CH4 emission. However, volcanic emissions contain besides S other chemically reactive species that are potentially harmful to the environment. In particular, gaseous and particulate F emissions from volcanoes constitute a steady or intermittent source of F emission and deposition into the environment both close to the source and within fallout range of large eruptions. The objective of this study was to investigate the effect of volcanic depositions of SO4, both alone and in combination with F, on CH4 emission in peatlands. Peat mesocosms collected from Pennine uplands in the UK were treated with weekly pulses of Na2SO4 and NaF over 20 weeks in doses of 74 kg SO4/ ha and 13.5 and 135 kg F /ha. CH4 emissions were measured at regular intervals by taking headspace samples, which were analysed by GC-FID. CO2 fluxes were also measured using a portable Infra Red Gas Analyser (IRGA). No significant differences in CH4 and CO2 emissions were observed for any of the treatments when compared to the controls, which had only received deionised water. These findings are in contrast with previous studies where SO4 reduces CH4 emission in peatlands. The reason for this is unclear but may be due to the heterogeneous nature of peat soils. An alternative explanation relates to the previous history of the soils used in the mesocosms which are known to have been previously exposed to large volumes of anthropogenic S pollution. This may have caused microbial communities to evolve and become acclimatised to high levels of S addition. In either case, the assumption that CH4 suppression in peatlands occurs upon expo

Recently, a link has been suggested between volcanic deposition of SO4 and the suppression of CH4 emissions in northern peatlands (Gauci et al., 2008). This link stems from the widely accepted idea that acid rain SO4 additions to peatlands can cause a shift in microbial communities as SO4 reducing bacteria out-compete methanogens for substrates, which results in a suppression of CH4 emission. However, volcanic emissions contain besides S other chemically reactive species that are potentially harmful to the environment. In particular, gaseous and particulate F emissions from volcanoes constitute a steady or intermittent source of F emission and deposition into the environment both close to the source and within fallout range of large eruptions. The objective of this study was to investigate the effect of volcanic depositions of SO4, both alone and in combination with F, on CH4 emission in peatlands. Peat mesocosms collected from Pennine uplands in the UK were treated with weekly pulses of Na2SO4 and NaF over 20 weeks in doses of 74 kg SO4/ ha and 13.5 and 135 kg F /ha. CH4 emissions were measured at regular intervals by taking headspace samples, which were analysed by GC-FID. CO2 fluxes were also measured using a portable Infra Red Gas Analyser (IRGA). No significant differences in CH4 and CO2 emissions were observed for any of the treatments when compared to the controls, which had only received deionised water. These findings are in contrast with previous studies where SO4 reduces CH4 emission in peatlands. The reason for this is unclear but may be due to the heterogeneous nature of peat soils. An alternative explanation relates to the previous history of the soils used in the mesocosms which are known to have been previously exposed to large volumes of anthropogenic S pollution. This may have caused microbial communities to evolve and become acclimatised to high levels of S addition. In either case, the assumption that CH4 suppression in peatlands occurs upon expo