Emissions of CO2, CH4 and N2O from Southern European peatlands

Peatlands play an important role in emissions of the greenhouse gases CO₂, CH₄ and N₂O, which are produced during mineralization of the peat organic matter. To examine the influence of soil type (fen, bog soil) and environmental factors (temperature, groundwater level), emission of CO₂, CH₄ and N₂O and soil temperature and groundwater level were measured weekly or biweekly in loco over a one-year period at four sites located in Ljubljana Marsh, Slovenia using the static chamber technique. The study involved two fen and two bog soils differing in organic carbon and nitrogen content, pH, bulk density, water holding capacity and groundwater level. The lowest CO₂ fluxes occurred during the winter, fluxes of N₂O were highest during summer and early spring (February, March) and fluxes of CH₄ were highest during autumn. The temporal variation in CO₂ fluxes could be explained by seasonal temperature variations, whereas CH₄ and N₂O fluxes could be correlated to groundwater level and soil carbon content. The experimental sites were net sources of measured greenhouse gases except for the drained bog site, which was a net sink of CH₄. The mean fluxes of CO₂ ranged between 139 mg m⁻² h⁻¹ in the undrained bog and 206 mg m⁻² h⁻¹ in the drained fen; mean fluxes of CH₄ were between −0.04 mg m⁻² h⁻¹ in the drained bog and 0.05 mg m⁻² h⁻¹ in the drained fen; and mean fluxes of N₂O were between 0.43 mg m⁻² h⁻¹ in the drained fen and 1.03 mg m⁻² h⁻¹ in the drained bog. These results indicate that the examined peatlands emit similar amounts of CO₂ and CH₄ to peatlands in Central and Northern Europe and significantly higher amounts of N₂O.     

Short-term effects of clearfelling on soil CO2, CH4, and N2O fluxes in a Sitka spruce plantation

We examined the effects of forest clearfelling on the fluxes of soil CO₂, CH₄, and N₂O in a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation on an organic-rich peaty gley soil, in Northern England. Soil CO₂, CH₄, N₂O as well as environmental factors such as soil temperature, soil water content, and depth to the water table were recorded in two mature stands for one growing season, at the end of which one of the two stands was felled and one was left as control. Monitoring of the same parameters continued thereafter for a second growing season. For the first 10 months after clearfelling, there was a significant decrease in soil CO₂ efflux, with an average efflux rate of 4.0 g m⁻² d⁻¹ in the mature stand (40-year) and 2.7 g m⁻² d⁻¹ in clearfelled site (CF). Clearfelling turned the soil from a sink (−0.37 mg m⁻² d⁻¹) for CH₄ to a net source (2.01 mg m⁻² d⁻¹). For the same period, soil N₂O fluxes averaged 0.57 mg m⁻² d⁻¹ in the CF and 0.23 mg m⁻² d⁻¹ in the 40-year stand. Clearfelling affected environmental factors and lead to higher daily soil temperatures during the summer period, while it caused an increase in the soil water content and a rise in the water table depth. Despite clearfelling, CO₂ remained the dominant greenhouse gas in terms of its greenhouse warming potential.     

NO, N2O, CH4 and CO2 fluxes in winter barley field of Japanese Andisol as affected by N fertilizer management

The study was carried out at the experimental station of the Japan International Research Center for Agricultural Sciences to investigate gas fluxes from a Japanese Andisol under different N fertilizer managements: CD, a deep application (8 cm) of the controlled release urea; UD, a deep application (8 cm) of the conventional urea; US, a surface application of the conventional urea; and a control, without any N application. NO, N₂O, CH₄ and CO₂ fluxes were measured simultaneously in a winter barley field under the maize/barley rotation. The fluxes of NO and N₂O from the control were very low, and N fertilization increased the emissions of NO and N₂O. NO and N₂O from N fertilization treatments showed different emission patterns: significant NO emissions but low N₂O emissions in the winter season, and low NO emissions but significant N₂O emissions during the short period of barley growth in the spring season. The controlled release of the N fertilizer decreased the total NO emissions, while a deep application increased the total N₂O emissions. Fertilizer-derived NO-N and N₂O-N from the treatments CD, UD and US accounted for 0.20±0.07%, 0.71±0.15%, 0.62±0.04%, and 0.52±0.04%, 0.50±0.09%, 0.35±0.03%, of the applied N, respectively, during the barley season. CH₄ fluxes from the control were negative on most sampling dates, and its net soil uptake was 33±7.1 mg m⁻² during the barley season. The application of the N fertilizer decreased the uptake of atmospheric CH₄ and resulted in positive emissions from the soil. CO₂ fluxes were very low in the early period of crop growth while higher emissions were observed in the spring season. The N fertilization generally increased the direct CO₂ emissions from the soil. N₂O, CH₄ and CO₂ fluxes were positively correlated (P<0.01) with each other, whereas NO and CO₂ fluxes were negatively correlated (P<0.05). The N fertilization increased soil-derived global warming potential (GWP) significantly in the barley season. The net GWP was calculated by subtracting the plant-fixed atmospheric CO₂ stored in its aboveground parts from the soil-derived GWP in CO₂ equivalent. The net GWP from the CD, UD, US and the control were all negative at −243±30.7, −257±28.4, −227±6.6 and −143±9.7 g C m⁻² in CO₂ equivalent, respectively, in the barley season.     

Carbon dioxide, nitrous oxide and methane dynamics in boreal organic agricultural soils with different soil characteristics

The annual carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) dynamics were measured with static chambers on two organic agricultural soils with different soil characteristics. Site 1 had a peat layer of 30 cm, with an organic matter (OM) content of 74% in the top 20 cm. Site 2 had a peat layer of 70 cm but an OM content of only 40% in the top 20 cm. On both sites there were plots under barley and grass and also plots where the vegetation was removed. All soils were net sources of CO₂ and N₂O, but they consumed atmospheric CH₄. Soils under barley had higher net CO₂ emissions (830 g CO₂-C m⁻² yr⁻¹) and N₂O emissions (848 mg N₂O-N m⁻² yr⁻¹) than those under grass (395 g CO₂-C m⁻³ yr⁻¹ and 275 mg N₂O-N m⁻² yr⁻¹). Bare soils had the highest N₂O emissions, mean 2350 mg N₂O-N m⁻² yr⁻¹. The mean CH₄ uptake rate from vegetated soils was 100 mg CH₄-C m⁻³ yr⁻¹ and from bare soils 55 mg CH₄-C m⁻² yr⁻¹. The net CO₂ emissions were higher from Site 2, which had a high peat bulk density and a low OM content derived from the addition of mineral soil to the peat during the cultivation history of that site. Despite the differences in soil characteristics, the mean N₂O emissions were similar from vegetated peat soils from both sites. However, bare soils from Site 2 with mineral soil addition had N₂O emissions of 2-9 times greater than those from Site 1. Site 1 consumed atmospheric CH₄ at a higher rate than Site 2 with additional mineral soil. N₂O emissions during winter were an important component of the N₂O budget even though they varied greatly, ranging from 2 to 99% (mean 26%) of the annual emission.     

Seasonal changes in the spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils in Indonesia

We evaluated the spatial structures of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) fluxes in an Acacia mangium plantation stand in Sumatra, Indonesia, in drier (August) and wetter (March) seasons. A 60 × 100-m plot was established in an A. mangium plantation that included different topographical elements of the upper plateau, lower plateau, upper slope and foot slope. The plot was divided into 10 × 10-m grids and gas fluxes and soil properties were measured at 77 grid points at 10-m intervals within the plot. Spatial structures of the gas fluxes and soil properties were identified using geostatistical analyses. Averaged N₂O and CO₂ fluxes in the wetter season (1.85 mg N m⁻² d⁻¹ and 4.29 g C m⁻² d⁻¹, respectively) were significantly higher than those in the drier season (0.55 mg N m⁻² d⁻¹ and 2.73 g C m⁻² d⁻¹, respectively) and averaged CH₄ uptake rates in the drier season (−0.62 mg C m⁻² d⁻¹) were higher than those in the wetter season (−0.24 mg C m⁻² d⁻¹). These values of N₂O fluxes in A. mangium soils were higher than those reported for natural forest soils in Sumatra, while CO₂ and CH₄ fluxes were in the range of fluxes reported for natural forest soils. Seasonal differences in these gas fluxes appears to be controlled by soil water content and substrate availability due to differing precipitation and mineralization of litter between seasons. N₂O fluxes had strong spatial dependence with a range of about 18 m in both the drier and wetter seasons. Topography was associated with the N₂O fluxes in the wetter season with higher and lower fluxes on the foot slope and on the upper plateau, respectively, via controlling the anaerobic-aerobic conditions in the soils. In the drier season, however, we could not find obvious topographic influences on the spatial patterns of N₂O fluxes and they may have depended on litter amount distribution. CO₂ fluxes had no spatial dependence in both seasons, but the topographic influence was significant in the drier season with lowest fluxes on the foot slope, while there was no significant difference between topographic positions in the wetter season. The distributions of litter amount and soil organic matter were possibly associated with CO₂ fluxes through their effects on microbial activities and fine root distribution in this A. mangium plantation.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Fluxes of CO2, CH4, and N2O in an alpine meadow affected by yak excreta on the Qinghai-Tibetan plateau during summer grazing periods

To assess the impacts of yak excreta patches on greenhouse gas (GHG) fluxes in the alpine meadow of the Qinghai-Tibetan plateau, methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) fluxes were measured for the first time from experimental excreta patches placed on the meadow during the summer grazing seasons in 2005 and 2006. Dung patches were CH₄ sources (average 586 μg m⁻² h⁻¹ in 2005 and 199 μg m⁻² h⁻¹ in 2006) during the investigation period of two years, while urine patches (average −31 μg m⁻² h⁻¹ in 2005 and −33 μg m⁻² h⁻¹ in 2006) and control plots (average −28 μg m⁻² h⁻¹ in 2005 and −30 μg m⁻² h⁻¹ in 2006) consumed CH₄. The cumulative CO₂ emission for dung patches was about 36-50% higher than control plots during the experimental period in 2005 and 2006. The cumulative N₂O emissions for both urine and dung patches were 2.1-3.7 and 1.8-3.5 times greater than control plots in 2005 and 2006, respectively. Soil water-filled pore space (WFPS) explained 35% and 36% of CH₄ flux variation for urine patches and control plots, respectively. Soil temperature explained 40-75% of temporal variation of CO₂ emissions for all treatments. Temporal N₂O flux variation in urine patches (34%), dung patches (48%), and control (56%) plots was mainly driven by the simultaneous effect of soil temperature and WFPS. Although yak excreta patches significantly affected GHG fluxes, their contributions to the whole grazing alpine meadow in terms of CO₂ equivalents are limited under the moderate grazing intensity (1.45 yak ha⁻¹). However, the contributions of excreta patches to N₂O emissions are not negligible when estimating N₂O emissions in the grazing meadow. In this study, the N₂O emission factor of yak excreta patches varied with year (about 0.9-1.0%, and 0.1-0.2% in 2005 and 2006, respectively), which was lower than IPCC default value of 2%.     

Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia

Tropical savanna ecosystems are a major contributor to global CO₂, CH₄ and N₂O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N₂O and CH₄ exchange. We measured soil CO₂, CH₄ and N₂O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1-2 years). There was no seasonal change and no fire effect upon soil N₂O exchange. Soil N₂O fluxes were very low, generally between −1.0 and 1.0 μg N m⁻² h⁻¹, and often below the minimum detection limit. There was an increase in soil NH₄⁺ in the months after the 2008 fire event, but no change in soil NO₃⁻. There was considerable nitrification in the early wet season but minimal nitrification at all other times.Savanna soil was generally a net CH₄ sink that equated to between −2.0 and −1.6 kg CH₄ ha⁻¹ y⁻¹ with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH₄ uptake. There were short periods of soil CH₄ emission, up to 20 μg C m⁻² h⁻¹, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO₂ fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH₄ fluxes, but a much stronger relationship with soil CO₂ fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N₂O source, and possibly even a sink. Annual soil CH₄ flux measurements suggest that the 1.9 million km² of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO₂-e per year. This sink estimate would offset potentially 10% of Australian transport related CO₂-e emissions. This CH₄ sink estimate does not include concurrent CH₄ emissions from termite mounds or ephemeral wetlands in Australian savannas.     

CH4 oxidation and emissions of CH4 and N2O from Lolium perenne swards under elevated atmospheric CO2

Emissions of N₂O and CH₄ and CH₄ oxidation rates were measured from Lolium perenne swards in a short-term study under ambient (36 Pa) and elevated (60 Pa) atmospheric CO₂ at the Free Air Carbon dioxide Enrichment experiment, Eschikon, Switzerland. Elevated pCO₂ increased (P<0.05) N₂O emissions from high N fertilised (11.2 g N m⁻²) swards by 69%, but had no significant effect on net emissions of CH₄. Application of ¹³C-CH₄ (11 μl l⁻¹; 11 at.% excess ¹³C) to closed chamber headspaces in microplots enabled determination of rates of ¹³C-CH₄ oxidation even when net CH₄ fluxes from main plots were positive. We found a significant interaction between fertiliser application rate and atmospheric pCO₂ on ¹³C-CH₄ oxidation rates that was attributed to differences in gross nitrification rates and C and N availability. CH₄ oxidation was slower and thought to be temporarily inhibited in the high N ambient pCO₂ sward. The most rapid CH₄ oxidation of 14.6 μg ¹³C-CH₄ m⁻² h⁻¹ was measured in the high fertilised elevated pCO₂ sward, and we concluded that either elevated pCO₂ had a stimulatory effect on CH₄ oxidation or inhibition of oxidation following fertiliser application was lowered under elevated pCO₂. Application of ¹⁴NH₄¹⁵NO₃ and ¹⁵NH₄¹⁵NO₃ (10 at.% excess ¹⁵N) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N₂O emissions. Inhibition of CH₄ oxidation in the high fertilised ambient pCO₂ sward, due to competition between NH₃ and CH₄ for methane monooxygenase enzymes or toxic effects of NH₂OH or NO₂⁻ produced during nitrification, was hypothesised to increase gross nitrification (12.0 mg N kg dry soil⁻¹) and N₂O emissions during nitrification (327 mg ¹⁵N-N₂O m⁻² over 11 d). Our results indicate that increasing atmospheric concentrations of CO₂ may increase emissions of N₂O by denitrification, lower nitrification rates and either increase or decrease the ability of soil to act as a sink for atmospheric CH₄ depending on fertiliser management.     

Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO₂, CH₄ and N₂O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH₄ uptake rates (<1 kg C ha⁻¹ yr⁻¹) and, coinciding with WFPS values >70%, episodes of CH₄ emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH₄ (4.7 kg C ha⁻¹ yr⁻¹). The N₂O releases at the cropland and pasture sites (2.7 and 4.8 kg N₂O-N ha⁻¹ yr⁻¹) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N₂O-N ha⁻¹ yr⁻¹). The highest N₂O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH₄ and N₂O. On the contrary, afforestation resulted in increases in SOM content and CH₄ uptake, as well as decreases in N₂O emissions.     

Emission of CO2, CH4 and N2O from lakeshore soils in an Antarctic dry valley

We measured soil profile concentrations and emission of CO₂, CH₄ and N₂O from soils along a lakeshore in Garwood Valley, Antarctica, to assess the extent and biogeochemical significance of biogenic gas emission to C and N cycling processes. Simultaneous emission of all three gases from the same site indicated that aerobic and anaerobic processes occurred in different layers or different parts of each soil profile. The day and location of high gas concentrations in the soil profile corresponded to those having high gas emission, but the pattern of concentration with depth in the soil profile was not consistent across sites. That the highest gas concentrations were not always in the deepest soil layer suggests either limited production or gas diffusion in the deeper layers. Emission of CO₂ was as high as 47 μmol m⁻² min⁻¹ and was strongly related to soil temperature. Soil respiration differed significantly according to location on the lakeshore, suggesting that factors other than environmental variables, such as the amount and availability of O₂ and nutrients, play an important role in C mineralization processes in these soils. High surface emission (maximum: 15 μmol m⁻² min⁻¹) and profile gas concentration (maximum: 5780 μL L⁻¹) of CH₄ were at levels comparable to those in resource-rich temperate ecosystems, indicating an active indigenous population of methanogenic organisms. Emission of N₂O was low and highly variable, but the presence of this gas and NO₃ in some of the soils suggest that denitrification and nitrification occur there. No significant relationships between N₂O emission and environmental variables were found. It appears that considerable C and N turnover occurs in the lakeshore soils, and accurate accounting will require measurements of aerobic and anaerobic mineralization. The production and emission of biogenic gases confirm the importance of these soils as hotspots of biological activity in the dry valleys and probable reservoirs of biological diversity.     

Soil CO2 efflux in a temperate deciduous forest: Environmental drivers and component contributions

Soil CO₂ efflux is a large component of total respiration in many ecosystems. It is important to understand the environmental controls on soil CO₂ efflux, in order to evaluate potential responses of ecosystems to climate change. This study investigated the relationship between total soil CO₂ efflux and soil temperature, soil moisture and solar radiation on an interannual basis for a plot of temperate deciduous ancient semi-natural woodland at Wytham Woods in central southern England. We also aimed to quantify the contribution of soil organic matter decomposition (SOM), root-and-rhizosphere respiration, and mycorrhizal respiration components to total soil CO₂ efflux, and determine their environmental correlates. Total soil CO₂ efflux was measured regularly from April 2006 to December 2008 and found to average 4.1 Mg C ha⁻¹ yr⁻¹ in both 2007 and 2008. In addition, we applied a recently developed approach to partition the efflux into SOM, root-and-rhizosphere, and mycorrhizal components in situ using mesh bags. SOM decomposition, root-and-rhizosphere, and mycorrhizal respiration were estimated to contribute 70 ± 6%, 22 ± 6% and 8 ± 3% of total soil CO₂ efflux respectively, equating to 3.0 ± 0.3, 0.9 ± 0.2 and 0.3 ± 0.1 Mg C ha⁻¹ yr⁻¹. In order to avoid the effect of temporal correlation between variables caused by seasonality, we investigated interannual variability by examining the relationship between CO₂ flux anomalies and anomalies in environmental variables. Variation in soil temperature explained 50% of the interannual variance in soil CO₂ efflux, and soil moisture a further 18% of the residual variance. Solar radiation, as a proxy for plant photosynthesis, had no significant effect on total soil CO₂ efflux, but was positively correlated with root-and-rhizosphere respiration, and mycorrhizal respiration. The relationship between anomalies in soil CO₂ efflux and soil temperature was highly significant, with a sensitivity of 0.164 ± 0.023 μmol CO₂ m⁻² s⁻¹ °C⁻¹. For mean peak summer efflux rates (2.03 μmol CO₂ m² s⁻¹), this is equivalent to 8% per °C, or a Q₁₀ temperature sensitivity of 2.2 ± 0.2. We demonstrate the utility of an anomaly analysis approach and conclude that soil temperature is the key driver of total soil CO₂ efflux primarily through its positive relationship with SOM-decomposition rate.     

Increasing net CO2 uptake by a Danish beech forest during the period from 1996 to 2009

The exchange of CO₂ between the atmosphere and a beech forest near Sorø, Denmark, was measured continuously over 14 years (1996-2009). The simultaneous measurement of many parameters that influence CO₂ uptake makes it possible to relate the CO₂ exchange to recent changes in e.g. temperature and atmospheric CO₂ concentration. The net CO₂ exchange (NEE) was measured by the eddy covariance method. Ecosystem respiration (RE) was estimated from nighttime values and gross ecosystem exchange (GEE) was calculated as the sum of RE and NEE. Over the years the beech forest acted as a sink of on average of 157 g C m⁻² yr⁻¹. In one of the years only, the forest acted as a small source. During 1996-2009 a significant increase in annual NEE was observed. A significant increase in GEE and a smaller and not significant increase in RE was also found. Thus the increased NEE was mainly attributed to an increase in GEE. The overall trend in NEE was significant with an average increase in uptake of 23 g C m⁻² yr⁻². The carbon uptake period (i.e. the period with daily net CO₂ gain) increased by 1.9 days per year, whereas there was a non significant tendency of increase of the leafed period. This means that the leaves stayed active longer. The analysis of CO₂ uptake by the forest by use of light response curves, revealed that the maximum rate of photosynthetic assimilation increased by 15% during the 14-year period. We conclude that the increase in the overall CO₂ uptake of the forest is due to a combination of increased growing season length and increased uptake capacity. We also conclude that long time series of flux measurements are necessary to reveal trends in the data because of the substantial inter-annual variation in the flux.     

Can a mixed stand of N2-fixing and non-fixing plants restrict N2O emissions with increasing CO2 concentration?

Initial effects of elevated atmospheric CO₂ concentration on N₂O fluxes and biomass production of timothy/red clover were studied in the laboratory. The experimental design consisted of two levels of atmospheric CO₂ (ca. 360 and 720 μmol CO₂ mol⁻¹) and two N fertilisation levels (5 and 10 g N m⁻²). There was a total of 36 mesocosms comprising sandy loam soil, which were equally distributed in four thermo-controlled greenhouses. In two of the greenhouses, the CO₂ concentration was kept at ambient concentration and in the other two at doubled concentration. Forage was harvested and the plants fertilised three times during the basic experiment, followed by harvest, a fertilisation with the double amount of nitrogen and rise of water level. Under elevated CO₂, harvestable and total aboveground dry biomass production of a mixed Trifolium/Phleum stand was increased at both N treatments compared to ambient CO₂. The N₂O flux rates under ambient CO₂ were significantly higher at both N treatments during the early growth of mixed Phleum/Trifolium mesocosms compared to the N₂O flux rate under elevated CO₂. However, when the conditions were favourable for denitrification at the end of the experiment, i.e. N availability and soil moisture were high enough, the elevated CO₂ concentration enhanced the N₂O efflux.     

Spatial and temporal variation of nitrous oxide and methane flux between subtropical mangrove sediments and the atmosphere

We quantified spatial and temporal variations of the fluxes of nitrous oxide (N₂O) and methane (CH₄) and associated abiotic sediment parameters across a subtropical river estuary sediment dominated by grey mangrove (Avicennia marina). N₂O and CH₄ fluxes from sediment were measured adjacent to the river (“fringe”) and in the mangrove forest (“forest”) at 3-h intervals throughout the day during autumn, winter and summer. N₂O fluxes from sediment ranged from an average of −4 μg to 65 μg N₂O m⁻² h⁻¹ representing N₂O sink and emission. CH₄ emissions varied by several orders of magnitude from 3 μg to 17.4 mg CH₄ m⁻² h⁻¹. Fluxes of N₂O and CH₄ differed significantly between sampling seasons, as well as between fringe and forest positions. In addition, N₂O flux differed significantly between time of day of sampling. Higher bulk density and total carbon content in sediment were significant contributors towards decreasing N₂O emission; rates of N₂O emission increased with less negative sediment redox potential (E_h). Porewater profiles of nitrate plus nitrite (NO_x⁻) suggest that denitrification was the major process of nitrogen transformation in the sediment and possible contributor to N₂O production. A significant decrease in CH₄ emission was observed with increasing E_h, but higher sediment temperature was the most significant variable contributing to CH₄ emission. From April 2004 to July 2005, sediment levels of dissolved ammonium, nitrate, and total carbon content declined, most likely from decreased input of diffuse nutrient and carbon sources upstream from the study site; concomitantly average CH₄ emissions decreased significantly. On the basis of their global warming potentials, N₂O and CH₄ fluxes, expressed as CO₂-equivalent (CO₂-e) emissions, showed that CH₄ emissions dominated in summer and autumn seasons (82-98% CO₂-e emissions), whereas N₂O emissions dominated in winter (67-95% of CO₂-e emissions) when overall CO₂-e emissions were low. Our study highlights the importance of seasonal N₂O contributions, particularly when conditions driving CH₄ emissions may be less favourable. For the accurate upscaling of N₂O and CH₄ flux to annual rates, we need to assess relative contributions of individual trace gases to net CO₂-e emissions, and the influence of elevated nutrient inputs and mitigation options across a number of mangrove sites or across regional scales. This requires a careful sampling design at site-level that captures the potentially considerable temporal and spatial variation of N₂O and CH₄ emissions.     

Spatial and temporal variability in CH4 and N2O fluxes from a Scottish ombrotrophic peatland: Implications for modelling and up-scaling

Peatlands typically exhibit significant spatial heterogeneity which can lead to large uncertainties when catchment scale greenhouse gas fluxes are extrapolated from chamber measurements (generally <1 m²). Here we examined the underlying environmental and vegetation characteristics which led to within-site variability in both CH₄ and N₂O emissions and the importance of such variability in up-scaling. We also consider within-site variation in the controls of temporal dynamics. Net annual emissions (and coefficients of variation) for CH₄ and N₂O were 1.06 kg ha⁻¹ y⁻¹ (300%) and 0.02 kg ha⁻¹ y⁻¹ (410%), respectively. The riparian zone was a significant CH₄ hotspot contributing ∼12% of the total catchment emissions whilst covering only ∼0.5% of the catchment area. In contrast to many other studies we found smaller CH₄ emissions and greater uptake in chambers containing either sedges or rushes. We also found clear differences in the drivers of temporal CH₄ dynamics across the site, e.g. water table was important only in chambers which did not contain aerenchymous plants. We suggest that depending on the heterogeneity of the site, flux models could be improved by incorporating a number of spatially distinct sub-models, rather than a single model parameterized using whole-catchment averages.     

Denitrification in grass swards is increased under elevated atmospheric CO2

Emissions of N₂O and N₂ were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO₂ at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m⁻² as ¹⁵NH₄¹⁵NO₃ or ¹⁴NH₄¹⁵NO₃ (1 at.% excess ¹⁵N). Total denitrification (N₂O+N₂) was increased under elevated pCO₂ with emissions of 6.2 and 19.5 mg ¹⁵N m⁻² measured over 22 d from ambient and elevated pCO₂ swards, respectively, supporting the hypothesis that increased belowground C allocation under elevated pCO₂ provides the energy for denitrification. Nitrification was the predominant N₂O producing process under ambient pCO₂ whereas denitrification was predominant under elevated pCO₂. The N₂-to-N₂O ratio was often higher under elevated pCO₂ suggesting that previous estimates of gaseous N losses based only on N₂O emissions have greatly underestimated the loss of N by denitrification.     

Influence of warming and enchytraeid activities on soil CO2 and CH4 fluxes

To determine the sum of ‘direct’ and ‘indirect’ effects of climatic change on enchytraeid activity and C fluxes from an organic soil we assessed the influence of temperature (4, 10 and 15 °C incubations) on enchytraeid populations and soil CO₂ and CH₄ fluxes over 116 days. Moisture was maintained at 60% of soil dry weight during the experimental period and measurements of enchytraeid biomass and numbers, and CO₂ and CH₄ fluxes were made after 3, 16, 33, 44, 65, 86 and 116 days. Enchytraeid population numbers and biomass increased in all temperature treatments with the greatest increase produced at 15 °C (to over threefold initial values by day 86). Results also showed that enchytraeid activity increased CO₂ fluxes by 10.7±4.5, 3.4±4.0 and 26.8±2.6% in 4, 10 and 15 °C treatments, respectively, with the greatest CO₂ production observed at 15 °C for the entire 116 day incubation period (P<0.05). The soil respiratory quotient analyses at lower temperatures (i.e. 4-10 °C) gave a Q₁₀ of 1.7 and 1.9 with and without enchytraeids, respectively. At temperatures above 10 °C (i.e. 10-15 °C) Q₁₀ significantly increased (P<0.01) and was 25% greater in the presence of enchytraeids (Q₁₀=3.4) than without (Q₁₀=2.6). In contrast to CO₂ production, no significant relationships were observed between net CH₄ fluxes and temperature and only time showed a significant effect on CH₄ production (P<0.01).Total soil CO₂ production was positively linked with enchytraeid biomass and mean soil CO₂-C production was 77.01±6.05 CO₂-C μg mg enchytraeid tissue⁻¹ day⁻¹ irrespective of temperature treatment. This positive relationship was used to build a two step regression model to estimate the effects of temperature on enchytraeid biomass and soil CO₂ respiration in the field. Predictions of potential CO₂ production were made using enchytraeid biomass data obtained in the field from two upland grassland sites (Sourhope and Great Dun Fell at the Moor House Nature Reserve, both in the UK). The findings of this work suggest that a 5 °C increase in atmospheric temperature above mean ambient temperature could have the potential to produce a significant increase in enchytraeid biomass resulting in a near twofold increase in soil CO₂ release from both soil types. The interaction between temperature and soil biology will clearly be an important determinant of soil respiration responses to global warming.     

Effects of elevated atmospheric CO2, prolonged summer drought and temperature increase on N2O and CH4 fluxes in a temperate heathland

In temperate regions, climate change is predicted to increase annual mean temperature and intensify the duration and frequency of summer droughts, which together with elevated atmospheric carbon dioxide (CO₂) concentrations, may affect the exchange of nitrous oxide (N₂O) and methane (CH₄) between terrestrial ecosystems and the atmosphere. We report results from the CLIMAITE experiment, where the effects of these three climate change parameters were investigated solely and in all combinations in a temperate heathland. Field measurements of N₂O and CH₄ fluxes took place 1-2 years after the climate change manipulations were initiated. The soil was generally a net sink for atmospheric CH₄. Elevated temperature (T) increased the CH₄ uptake by on average 10 μg C m⁻² h⁻¹, corresponding to a rise in the uptake rate of about 20%. However, during winter elevated CO₂ (CO₂) reduced the CH₄ uptake, which outweighed the positive effect of warming when analyzed across the study period. Emissions of N₂O were generally low (<10 μg N m⁻² h⁻¹). As single experimental factors, elevated CO₂, temperature and summer drought (D) had no major effect on the N₂O fluxes, but the combination of CO₂ and warming (TCO₂) stimulated N₂O emission, whereas the N₂O emission ceased when CO₂ was combined with drought (DCO₂). We suggest that these N₂O responses are related to increased rhizodeposition under elevated CO₂ combined with increased and reduced nitrogen turnover rates caused by warming and drought, respectively. The N₂O flux in the multifactor treatment TDCO₂ was not different from the ambient control treatment. Overall, our study suggests that in the future, CH₄ uptake may increase slightly, while N₂O emission will remain unchanged in temperate ecosystems on well-aerated soils. However, we propose that continued exposure to altered climate could potentially change the greenhouse gas flux pattern in the investigated heathland.     

Explaining temporal variation in soil CO2 efflux in a mature spruce forest in Southern Germany

An open dynamic chamber system was used to measure the soil CO₂ efflux intensively and continuously throughout a growing season in a mature spruce forest (Picea abies) in Southern Germany. The resulting data set contained a large amount of temporally highly resolved information on the variation in soil CO₂ efflux together with environmental variables. Based on this background, the dependencies of the soil CO₂ efflux rate on the controlling environmental factors were analysed in-depth. Of the abiotic factors, soil temperature alone explained 72% of the variation in the efflux rate, and including soil water content (SWC) as an additional variable increased the explained variance to about 83%. Between April and December, average rates ranged from 0.43 to 5.15 μmol CO₂ m⁻² s⁻¹ (in November and July, respectively) with diurnal variations of up to 50% throughout the experiment. The variability in wind speed above the forest floor influenced the CO₂ efflux rates for measuring locations with a litter layer of relatively low bulk density (and hence relatively high proportions of pore spaces). For the temporal integration of flux rates for time scales of hours to days, however, wind velocities were of no effect, reflecting the fact that wind forcing acts on the transport, but not the production of CO₂ in the soil. The variation in both the magnitude of the basal respiration rate and the temperature sensitivity throughout the growing season was only moderate (coefficient of variation of 15 and 25%, respectively). Soil water limitation of the CO₂ production in the soil could be best explained by a reduction in the temperature-insensitive basal respiration rate, with no discernible effect on the temperature sensitivity. Using a soil CO₂ efflux model with soil temperature and SWC as driving variables, it was possible to calculate the annual soil CO₂ efflux for four consecutive years for which meteorological data were available. These simulations indicate an average efflux sum of 560 g C m⁻² yr⁻¹ (SE=22 g C m⁻² yr⁻¹). An alternative model derived from the same data but using temperature alone as a driver over-estimated the annual flux sum by about 7% and showed less inter-annual variability. Given a likely shift in precipitation patterns alongside temperature changes under projected global change scenarios, these results demonstrate the necessity to include soil moisture in models that calculate the evolution of CO₂ from temperate forest soils.