Winter CO2 CH4 and N2O fluxes on some natural and drained boreal peatlands

CO₂ and CH₄ fluxes during the winter were measured at natural and drained bog and fen sites in eastern Finland using both the closed chamber method and calculations of gas diffusion along a concentration gradient through the snowpack. The snow diffusion results were compared with those obtained by chamber, but the winter flux estimates were derived from chamber data only. CH₄ emissions from a poor bog were lower than those from an oligotrophic fen, while both CO₂ and CH₄ fluxes were higher in theCarex rostrata- occupied marginal (lagg) area of the fen than in the slightly less fertile centre. Average estimated winter CO₂-C losses from virgin and drained forested peatlands were 41 and 68 g CO₂-C m^–2, respectively, accounting for 23 and 21% of the annual total CO₂ release from the peat. The mean release of CH₄-C was 1.0 g in natural bogs and 3.4 g m^–2 in fens, giving rise to winter emissions averaging to 22% of the annual emission from the bogs and 10% of that from the fens. These wintertime carbon gas losses in Finnish natural peatlands were even greater than reported average long-term annual C accumulation values (less than 25g C m^–2). The narrow range of 10–30% of the proportion of winter CO₂ and CH₄ emissions from annual emissions found in Finnish peatlands suggest that a wider generalization in the boreal zone is possible. Drained forested bogs emitted 0.3 g CH₄-C m^–2 on the average, while the effectively drained fens consumed an average of 0.01 g CH₄-C m^–2. Reason for the low CH₄. efflux or net oxidation in drained peatlands probably lies in low substrate supply and thus low CH₄ production in the anoxic deep peat layers. N₂O release from a fertilized grassland site in November–May was 0.7 g N₂O m^–2, accounting for 38% of the total annual emission, while a forested bog released none and two efficiently drained forested fens 0.09 (28% of annual release) and 0.04 g N₂O m^–2 (27%) during the winter, respectively.     

Fluxes of N2O,CH4 and CO2 on afforested boreal agricultural soils

After drainage of natural boreal peatlands, the decomposition of organic matter increases and peat soil may turn into a net source of CO₂ and N₂O, whereas CH₄ emission is known to decrease. Afforestation is a potential mitigation strategy to reduce greenhouse gas emission from organic agricultural soils. A static chamber technique was used to evaluate the fluxes of CH₄, N₂O and CO₂ from three boreal organic agricultural soils in western Finland, afforested 1, 6 or 23 years before this study. The mean emissions of CH₄ and N₂O during the growing seasons did not correlate with the age of the tree stand. All sites were sources of N₂O. The highest daily N₂O emission during the growing season, measured in the oldest site, was as high as 29 mg N₂O m^–2d^–1. In general, organic agricultural soils are sinks for methane. Here, the oldest site acted as a small sink for methane, whereas the two youngest afforested organic soils were sources for methane with maximum emission rates (up to 154 mg m^–2d^–1) similar to those reported for minerogenous natural peatlands. Soil respiration rates decreased with the age of the forest. The high soil respiration in the younger sites, probably resulted from the high biomass production of herbs, could create soil anaerobiosis and increase methane production. Our results show that afforestation of agricultural peat soils does not abruptly terminate the N₂O emissions during the first two decades, and afforestation can even enhance methane emission for a few years. The carbon accumulation in the developing tree stand can partly compensate the carbon loss from soil.     

Emission of N2O, N2, CH4, and CO2 from constructed wetlands for wastewater treatment and from riparian buffer zones

We measured nitrous oxide (N₂O), dinitrogen (N₂), methane (CH₄), and carbon dioxide (CO₂) fluxes in horizontal and vertical flow constructed wetlands (CW) and in a riparian alder stand in southern Estonia using the closed chamber method in the period from October 2001 to November 2003. The replicates’ average values of N₂O, N₂, CH₄ and CO₂ fluxes from the riparian gray alder stand varied from −0.4 to 58 μg N₂O-N m⁻² h⁻¹, 0.02–17.4 mg N₂-N m⁻² h⁻¹, 0.1–265 μg CH₄-C m⁻² h⁻¹ and 55–61 mg CO₂-C m⁻² h⁻¹, respectively. In horizontal subsurface flow (HSSF) beds of CWs, the average N₂ emission varied from 0.17 to 130 and from 0.33 to 119 mg N₂-N m⁻² h⁻¹ in the vertical subsurface flow (VSSF) beds. The average N₂O-N emission from the microsites above the inflow pipes of the HSSF CWs was 6.4–31 μg N₂O-N m⁻² h⁻¹, whereas the outflow microsites emitted 2.4–8 μg N₂O-N m⁻² h⁻¹. In VSSF beds, the same value was 35.6–44.7 μg N₂O-N m⁻² h⁻¹. The average CH₄ emission from the inflow and outflow microsites in the HSSF CWs differed significantly, ranging from 640 to 9715 and from 30 to 770 μg CH₄-C m⁻² h⁻¹, respectively. The average CO₂ emission was somewhat higher in VSSF beds (140–291 mg CO₂-C m⁻² h⁻¹) and at the inflow microsites of HSSF beds (61–140 mg CO₂-C m⁻² h⁻¹). The global warming potential (GWP) from N₂O and CH₄ was comparatively high in both types of CWs (4.8 ± 9.8 and 6.8 ± 16.2 t CO₂ eq ha⁻¹ a⁻¹ in the HSSF CW 6.5 ± 13.0 and 5.3 ± 24.7 t CO₂ eq ha⁻¹ a⁻¹ in the hybrid CW, respectively). The GWP of the riparian alder forest from both N₂O and CH₄ was relatively low (0.4 ± 1.0 and 0.1 ± 0.30 t CO₂ eq ha⁻¹ a⁻¹, respectively), whereas the CO₂-C flux was remarkable (3.5 ± 3.7 t ha⁻¹ a⁻¹). The global influence of CWs is not significant. Even if all global domestic wastewater were treated by wetlands, their share of the trace gas emission budget would be less than 1%.     

Effect of tree distance on N2O and CH4-fluxes from soils in temperate forest ecosystems

Complete annual cycles of N₂O and CH₄ flux in forest soils at a beech and at a spruce site at the Höglwald Forest were followed in 1997 by use of fully automatic measuring systems. In order to test if on a microsite scale differences in the magnitude of trace gas exchange between e.g. areas in direct vicinity of stems and areas in the interstem region at both sites exist, tree chambers and gradient chambers were installed in addition to the already existing interstem chambers at our sites. N₂O fluxes were in a range of –4.6–473.3 g N₂O-N m^–2 h^–1 at the beech site and in a range of –3.7–167.2 g N₂O-N m^–2 h^–1 at the spruce site, respectively. Highest N₂O emissions were observed during and at the end of a prolonged frost period, thereby further supporting previous findings that frost periods are of crucial importance for controlling annual N₂O losses from temperate forests. Fluxes of CH₄ were in a range of +10.4––194.0 g CH₄ m^–2 h^–1 at the beech site and in a range of –4.4––83.5 g CH₄ m^–2 h^–1 at the spruce site. In general, both N₂O-fluxes as well as CH₄-fluxes were higher at the beech site. On a microsite scale, N₂O and CH₄ fluxes at the beech site were highest within the stem area (annual mean: 49.6±3.3 g N₂O-N m^–2 h^–1; –77.2±3.1 g CH₄ m^–2 h^–1), and significantly lower within interstem areas (18.5±1.4 g N₂O-N m^–2 h^–1; –60.2±1.8 g CH₄ m^–2 h^–1). Significantly higher values of total N, C and pH in the organic layer, as well as increased soil moisture, especially in spring, in the stem areas, are likely to contribute to the higher N₂O fluxes within the stem area of the beech. Also for the spruce site, such differences in trace gas fluxes could be demonstrated to exist (mean annual N₂O emission within (a) stem areas: 9.7±0.9 g N₂O-N m^–2 h^–1 and (b) interstem areas: 6.2±0.6 g N₂O-N m^–2 h^–1; mean annual CH₄ uptake within (a) stem areas: –26.1±0.6 g CH₄ m^–2 h^–1 and (b) interstem areas: –38.4±0.8 g CH₄ m^–2 h^–1), though they were not as pronounced as at the beech site.     

Carbon dioxide efflux and pCO2 in soils of threeQuercus ilex montane forests

Soil CO₂ efflux and pCO₂ in the soil atmosphere were measured during one year at three montane sites of Mediterranean sclerophyllous forests in NE Spain. Two sites were located in the upper and lower slopes of a small catchment in the Prades mountains (mean precipitation 550 mm year^–1), and a third site was located on a lower slope in the Montseny mountains (mean precipitation 900 mm year^–1). The three sites were similar in bedrock and vegetation, but differed in soil characteristics and water availability. Seasonal variation of CO₂ efflux and soil pCO₂ were affected by soil temperature and, to a lesser extent, by soil moisture. Annual mean soil CO₂ efflux (considered as soil respiration) was similar at Montseny and at the comparably located site at Prades (83 ± 18 S.E. vs. 75 ± 9 mg CO₂ m^–2 hour^–1 , respectively), and was highest at the Prades upper slope site (122 ± 22 mg C0₂ m^–2 hour^–1 ). Despite those relatively similar CO₂ effluxes, mean soil pCO₂ was much higher at both Prades sites than at Montseny. Soil pCO₂ always increased with depth at Prades while maxima pCO₂ at Montseny were often at 20–30 cm depth. A model based on gas diffusion theory was able to explain why soil pCO₂ was much higher at Prades than at Montseny, and to reproduce the shape of the vertical profile of pCO₂ at the Prades soils. Nevertheless, the model failed to simulate the soil pCO₂ maximum found at 20–30 cm depth at the Montseny site. Model simulations using a time-variable CO₂ production rate suggested that pCO₂ maxima at intermediate depth could be the result of a transient situation instead of an equilibrium one.     

Dynamics of dissolved O2, CO2, CH4, and N2O in a tropical coastal swamp in southern Thailand

We studied the distribution of dissolved O₂, CO₂, CH₄, and N₂O in a coastal swamp system in Thailand with the goal to characterize the dynamics of these gases within the system. The gas concentrations varied spatially and seasonally in both surface and ground waters. The entire system was a strong sourcefor CO₂ and CH₄, and a possible sink for atmospheric N₂O. Seasonal variation in precipitation primarily regulated the redox conditions in the system. However, distributions of CO₂, CH₄, and N₂O in the river that received swamp waters were not always in agreement with redox conditions indicated by dissolvedO₂ concentrations. Sulfate production through pyriteoxidation occurred in the swamp with thin peat layerunder aerobic conditions and was reflected by elevatedSO ₄ ^2– /Cl^– in the river water. When SO ₄ ^2– /Cl^– was high, CO₂ and CH₄ concentrations decreased, whereas the N₂O concentration increased. The excess SO ₄ ^2– in the river water was thus identified as a potential indicator for gas dynamics in this coastal swamp system.     

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N₂O, CO₂ and CH₄ soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N₂O emission in these regions on a landscape level. The mean N₂O emission at site PF was 6.0 ± 0.1 SE μg N m⁻² h⁻¹. Fluxes of N₂O increased from <5 μg N m⁻² h⁻¹ during dry season conditions to up to 24.5 μg N m⁻² h⁻¹ with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N₂O were measured in the SF and RP sites, where mean N₂O emissions were 7.3 ± 0.7 SE μg N m⁻² h⁻¹ and 4.1 ± 0.5 SE μg N m⁻² h⁻¹, respectively. The dependency of N₂O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N₂O emission peaks, not the total amount of N₂O emitted. For all sites, significant positive correlations existed between N₂O emissions and both soil moisture and soil temperature. Mean CH₄ uptake rates were highest at the PF site (−29.5 ± 0.3 SE μg C m⁻² h⁻¹), slightly lower at the SF site (−25.6 ± 1.3 SE μg C m⁻² h⁻¹) and lowest for the RP site (−5.7 ± 0.5 SE μg C m⁻² h⁻¹). At all sites, CH₄ uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N₂O emissions, CH₄ uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO₂ emissions at the PF site increased significantly from 50 mg C m⁻² h⁻¹ up to 100 mg C m⁻² h⁻¹ (mean value 68.8 ± 0.8 SE mg C m⁻² h⁻¹), whereas CO₂ emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m⁻² h⁻¹ (SF) and 34.9 ± 1.1 SE mg C m⁻² h⁻¹ (RP). A dependency of soil CO₂ emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO₂ emissions as compared to control chambers. Correlation of CO₂ emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.     

CO2 Efflux from Shrimp Ponds in Indonesia

The conversion of mangrove forest to aquaculture ponds has been increasing in recent decades. One of major concerns of this habitat loss is the release of stored ‘blue’ carbon from mangrove soils to the atmosphere. In this study, we assessed carbon dioxide (CO₂) efflux from soil in intensive shrimp ponds in Bali, Indonesia. We measured CO₂ efflux from the floors and walls of shrimp ponds. Rates of CO₂ efflux within shrimp ponds were 4.37 kg CO₂ m⁻² y⁻¹ from the walls and 1.60 kg CO₂ m⁻² y⁻¹ from the floors. Combining our findings with published data of aquaculture land use in Indonesia, we estimated that shrimp ponds in this region result in CO₂ emissions to the atmosphere between 5.76 and 13.95 Tg y⁻¹. The results indicate that conversion of mangrove forests to aquaculture ponds contributes to greenhouse gas emissions that are comparable to peat forest conversion to other land uses in Indonesia. Higher magnitudes of CO₂ emission may be released to atmosphere where ponds are constructed in newly cleared mangrove forests. This study indicates the need for incentives that can meet the target of aquaculture industry without expanding the converted mangrove areas, which will lead to increased CO₂ released to atmosphere.     

Emissions of greenhouse gases from ponds constructed for nitrogen removal

Methane and carbon dioxide emission from three constructed ponds were monitored during an annual cycle. Water temperature was a good predictor of methane emission in all three ponds. In the most intensively studied pond, nitrate concentration in the bottom water could further explain the amount of methane emitted. When water temperature exceeded 15 °C between 1 and 54 mg, CH₄ m⁻² h⁻¹ was emitted on all occasions, while at temperatures below 10 °C, less than 0.6 mg CH₄ m⁻² h⁻¹ was emitted. The flux of carbon dioxide differed between the ponds and no consistent patterns were found. In a laboratory study at 20 °C, we showed that high, but naturally occurring, nitrate concentrations (8 and 16 mg NO₃⁻–N l⁻¹) constrained the production of methane compared to the treatment with no nitrate addition. Nitrous oxide production was positively correlated with nitrate concentration. Carbon dioxide production was highest at the highest nitrate concentration, which indicates that increased nitrate loading on ponds and wetlands will stimulate organic matter decomposition rates. Our conclusion is that these ponds constructed for nitrate removal emit greenhouse gases comparable to lakes in the temperate region.     

Losses of inorganic carbon and nitrous oxide from a temperate freshwater wetland in relation to nitrate loading

We studied the export of inorganic carbon and nitrous oxide (N₂O) from a Danish freshwater wetland. The wetland is situated in an agricultural catchment area and is recharged by groundwater enriched with nitrate (NO₃ ^–) (1000 M). NO₃ ^– in recharging groundwater was reduced (57.5 mol NO₃ ^– m^–2 yr^–) within a narrow zone of the wetland. Congruently, the annual efflux of carbon dioxide (CO₂) from the sediment was 19.1 mol C m^–2 when estimated from monthly in situ measurements. In comparison the CO₂ efflux was 4.8 mol C m^–2 yr^–1 further out in the wetland, where no NO₃ ^– reduction occurred. Annual exports of inorganic carbon in groundwater and surface water was 78.4 mol C m^–2 and 6.1 mol C m^–2 at the two sites, respectively. N₂O efflux from the sedimenst was detectable on five out of twelve sampling dates and was significantly (P < 0.0001) higher in the NO₃ ^– reduction zone (0.35–9.40 mol m^–2 h^–1, range of monthly means) than in the zone without NO₃ ^– reduction (0.21–0.41 mol m^–2 h^–1). No loss of dissolved N₂O could be measured. Total annual export of N₂O was not estimated. The reduction of oxygen (O₂) in groundwater was minor throughout the wetland and did not exceed 0.2 mol 0₂ m^–2yr^–1. Sulfate (SO₄ ^––) was reduced in groundwater (2.1 mol SO₄ ^–– m^–2 yr^–1) in the zone without NO₃ ^– reduction. Although the NO₃ ^– in our wetland can be reduced along several pathways our results strongly suggest that NO₃ ^– loading of freshwater wetlands disturb the carbon balance of such areas, resulting in an accelerated loss of inorganic carbon in gaseous and dissolved forms.     

Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil

The influence of forest stand age in a Picea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO₂, CH₄ and N₂O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2‐year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30‐ and 20‐year‐old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30‐year‐old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO₂ efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH₄ and N₂O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO₂ were lower at the 20‐year‐old (20y) site than at the 30‐year‐old (30y) and CF sites, with soil temperature the dominant control. CH₄ efflux was highest at the CF site, with peak flux 491±54.5 μg m⁻² h⁻¹ and maximum annual flux 18.0±1.1 kg CH₄ ha⁻¹ yr⁻¹. No consistent uptake of CH₄ was noted at any site. A linear relationship was found between log CH₄ flux and the closeness of the water table to the soil surface across all sites. N₂O efflux was highest in the 30y site, reaching 108±38.3 μg N₂O‐N m⁻² h⁻¹ (171 μg N₂O m⁻² h⁻¹) in midsummer and a maximum annual flux of 4.7±1.2 kg N₂O ha⁻¹ yr⁻¹ in 2001. Automatic chamber data showed a positive exponential relationship between N₂O flux and soil temperature at this site. The relationship between N₂O emission and soil volumetric moisture indicated an optimum moisture content for N₂O flux of 40–50% by volume. The relationship between C : N ratio data and integrated N₂O flux was consistent with a pattern previously noted across temperate and boreal forest soils.     

Soil-atmosphere exchange of CH4, CO2, NOx,and N2O in the Colorado shortgrass steppe under elevated CO2

In late March 1997, an open-top-chamber (OTC) CO₂ enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO₂ (720 mol mol^–1) on plant production, photosynthesis, and water use of this mixed C₃/C₄ plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO₂ on trace gas exchange. From this study, we report here our weekly measurements of CO₂, CH₄, NO_x and N₂O fluxes within control (unchambered), ambient CO₂ and elevated CO₂ OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C₃ and C₄ plant biomass increased under elevated CO₂ and soil moisture content was typically higher than under ambient CO₂ conditions, none of the trace gas fluxes were significantly altered by CO₂ enrichment. Over the 43 month period of observation NO_x and N₂O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 g N m^–2 hr ^–1 in elevated CO₂ OTCs, respectively. NO_x flux was negatively correlated to plant biomass production. Methane oxidation rates averaged –31 and –34 g C m^–2 hr^–1 and ecosystem respiration averaged 43 and 44 mg C m^–2 hr^–1 under ambient and elevated CO₂, respectively, over the same time period.     

Greenhouse gas budget (CO2, CH4 and N2O) of intensively managed grassland following restoration

The first full greenhouse gas (GHG) flux budget of an intensively managed grassland in Switzerland (Chamau) is presented. The three major trace gases, carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) were measured with the eddy covariance (EC) technique. For CO₂ concentrations, an open‐path infrared gas analyzer was used, while N₂O and CH₄ concentrations were measured with a recently developed continuous‐wave quantum cascade laser absorption spectrometer (QCLAS). We investigated the magnitude of these trace gas emissions after grassland restoration, including ploughing, harrowing, sowing, and fertilization with inorganic and organic fertilizers in 2012. Large peaks of N₂O fluxes (20–50 nmol m^?2 s^?1 compared with a <5 nmol m^?2 s^?1 background) were observed during thawing of the soil after the winter period and after mineral fertilizer application followed by re‐sowing in the beginning of the summer season. Nitrous oxide (N₂O) fluxes were controlled by nitrogen input, plant productivity, soil water content and temperature. Management activities led to increased variations of N₂O fluxes up to 14 days after the management event as compared with background fluxes measured during periods without management (<5 nmol m^?2 s^?1). Fluxes of CO₂ remained small until full plant development in early summer 2012. In contrast, methane emissions showed only minor variations over time. The annual GHG flux budget was dominated by N₂O (48% contribution) and CO₂ emissions (44%). CH₄ flux contribution to the annual budget was only minor (8%). We conclude that recently developed multi‐species QCLAS in an EC system open new opportunities to determine the temporal variation of N₂O and CH₄ fluxes, which further allow to quantify annual emissions. With respect to grassland restoration, our study emphasizes the key role of N₂O and CO₂ losses after ploughing, changing a permanent grassland from a carbon sink to a significant carbon source.     

Carbon turnover in peatland mesocosms exposed to different water table levels

Changes of water table position influence carbon cycling in peatlands, but effects on the sources and sinks of carbon are difficult to isolate and quantify in field investigations due to seasonal dynamics and covariance of variables. We thus investigated carbon fluxes and dissolved carbon production in peatland mesocosms from two acidic and oligotrophic peatlands under steady state conditions at two different water table positions. Exchange rates and CO₂, CH₄ and DOC production rates were simultaneously determined in the peat from diffusive-advective mass-balances of dissolved CO₂, CH₄ and DOC in the pore water. Incubation experiments were used to quantify potential CO₂, CH₄, and DOC production rates. The carbon turnover in the saturated peat was dominated by the production of DOC (10–15 mmol m^–2 d^–1) with lower rates of DIC (6.1–8.5 mmol m^–2 d^–1) and CH₄ (2.2–4.2 mmol m^–2 d^–1) production. All production rates strongly decreased with depth indicating the importance of fresh plant tissue for dissolved C release. A lower water table decreased area based rates of photosynthesis (24–42%), CH₄ production (factor 2.5–3.5) and emission, increased rates of soil respiration and microbial biomass C, and did not change DOC release. Due to the changes in process rates the C net balance of the mesocosms shifted by 36 mmol m^–2 d^–1. According to our estimates the change in C mineralization contributed most to this change. Anaerobic rates of CO₂ production rates deeper in the peat increased significantly by a factor of 2–3.5 (DOC), 2.9–3.9 (CO₂), and 3–14 (CH₄) when the water table was lowered by 30 cm. This phenomenon might have been caused by easing an inhibiting effect by the accumulation of CO₂ and CH₄ when the water table was at the moss surface.     

Role of the aquatic pathway in the carbon and greenhouse gas budgets of a peatland catchment

Peatland streams have repeatedly been shown to be highly supersaturated in both CO₂ and CH₄ with respect to the atmosphere, and in combination with dissolved (DOC) and particulate organic carbon (POC) represent a potentially important pathway for catchment greenhouse gas (GHG) and carbon (C) losses. The aim of this study was to create a complete C and GHG (CO₂, CH₄, N₂O) budget for Auchencorth Moss, an ombrotrophic peatland in southern Scotland, by combining flux tower, static chamber and aquatic flux measurements from 2 consecutive years. The sink/source strength of the catchment in terms of both C and GHGs was compared to assess the relative importance of the aquatic pathway. During the study period (2007–2008) the catchment functioned as a net sink for GHGs (352 g CO₂‐Eq m^?2 yr^?1) and C (69.5 g C m^?2 yr^?1). The greatest flux in both the GHG and C budget was net ecosystem exchange (NEE). Terrestrial emissions of CH₄ and N₂O combined returned only 4% of CO₂ equivalents captured by NEE to the atmosphere, whereas evasion of GHGs from the stream surface returned 12%. DOC represented a loss of 24% of NEE C uptake, which if processed and evaded downstream, outside of the catchment, may lead to a significant underestimation of the actual catchment‐derived GHG losses. The budgets clearly show the importance of aquatic fluxes at Auchencorth Moss and highlight the need to consider both the C and GHG budgets simultaneously.     

Effects of tree species composition on the CO2 and N2O efflux of a Mediterranean mountain forest soil

Background and Aims

Tree species composition shifts can alter soil CO₂ and N₂O effluxes. We quantified the soil CO₂ and N₂O efflux rates and temperature sensitivity from Pyrenean oak, Scots pine and mixed stands in Central Spain to assess the effects of a potential expansion of oak forests.

Methods

Soil CO₂ and N₂O effluxes were measured from topsoil samples by lab incubation from 5 to 25 °C. Soil microbial biomass and community composition were assessed.

Results

Pine stands showed highest soil CO₂ efflux, followed by mixed and oak forests (up to 277, 245 and 145 mg CO₂-C m^?2 h^?1, respectively). Despite contrasting soil microbial community composition (more fungi and less actinomycetes in pine plots), carbon decomposability and temperature sensitivity of the soil CO₂ efflux remain constant among tree species. Soil N₂O efflux rates and its temperature sensitivity was markedly higher in oak stands than in pine stands (70 vs. 27 μg N₂O-N m^?2 h^?1, Q₁₀, 4.5 vs. 2.5).

Conclusions

Conversion of pine to oak forests in the region will likely decrease soil CO₂ effluxes due to decreasing SOC contents on the long run and will likely enhance soil N₂O effluxes. Our results present only a seasonal snapshot and need to be confirmed in the field.     

Nitrous oxide and methane exchange in two small temperate forest catchments—effects of hydrological gradients and implications for global warming potentials of forest soils

The magnitude of greenhouse gas (GHG) flux rates may be important in wet and intermediate wet forest soils, but published estimates are scarce. We studied the surface exchange of methane (CH₄) and nitrous oxide (N₂O) from soil along toposequences in two temperate deciduous forest catchments: Strødam and Vestskoven. The soil water regime ranged from fully saturated to aerated within the catchments. At Strødam the largest mean flux rates of N₂O (15 μg N₂O-N m^?2 h^?1) were measured at volumetric soil water contents (SWC) between 40 and 60% and associated with low soil pH compared to smaller mean flux rates of 0-5 μg N₂O-N m^?2 h^?1 for drier (SWC < 40%) and wet conditions (SWC > 80%). At Vestskoven the same response of N₂O to soil water content was observed. Average CH₄ flux rates were highly variable along the toposequences (?17 to 536 μg CH₄-C m^?2 h^?1) but emissions were only observed above soil water content of 45%. Scaled flux rates of both GHGs to catchment level resulted in emission of 322 and 211 kg CO₂-equivalents ha^?1 year^?1 for Strødam and Vestskoven, respectively, with N₂O contributing the most at both sites. Although the wet and intermediate wet forest soils occupied less than half the catchment area at both sites, the global warming potential (GWP) derived from N₂O and CH₄ was more than doubled when accounting for these wet areas in the catchments. The results stress the importance of wet soils in assessments of forest soil global warming potentials, as even small proportions of wet soils contributes substantially to the emissions of N₂O and CH₄.     

Greenhouse gas dynamics in boreal, littoral sediments under raised CO2 and nitrogen supply

SUMMARY 1. The effects of increasing CO₂ and nitrogen loading and of a change in water table and temperature on littoral CH₄, N₂O and CO₂ fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at ?15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO₂ for 18 weeks. The experiment included fertilisation with NO₃^– and NH₄⁺ (to a total 3 g N m^?2). 2. Changes in the water table and temperature strongly regulated sediment CH₄ and cCO₂ fluxes (community CO₂ release), but did not affect N₂O emissions. Increase in the water table increased CH₄ emissions but reduced cCO₂ release, while increase in temperature increased emissions of both CO₂ and CH₄. 3. The raised CO₂ increased carbon turnover in the sediments, such that cCO₂ release was increased by 16–26%. However, CH₄ fluxes were not significantly affected by raised CO₂, although CH₄ production potential (at 22 °C) of the sediments incubated at high CO₂ was increased. In the boreal region, littoral CH₄ production is more likely to be limited by temperature than by the availability of carbon. Raised CO₂ did not affect N₂O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO₃^– did severely limit N₂O production. The NO₃^– addition caused up to a 100‐fold increase in the fluxes of N₂O. The NH₄⁺ addition did not increase N₂O fluxes, indicating low nitrification capacity in the sediments.     

The Impact of Nitrogen Placement and Tillage on NO, N2O, CH4 and CO2 Fluxes from a Clay Loam Soil

To evaluate the impact of N placement depth and no-till (NT) practice on the emissions of NO, N₂O, CH₄ and CO₂ from soils, we conducted two N placement experiments in a long-term tillage experiment site in northeastern Colorado in 2004. Trace gas flux measurements were made 2–3 times per week, in zero-N fertilizer plots that were cropped continuously to corn (Zea mays L.) under conventional-till (CT) and NT. Three N placement depths, replicated four times (5, 10 and 15 cm in Exp. 1 and 0, 5 and 10 cm in Exp. 2, respectively) were used. Liquid urea–ammonium nitrate (UAN, 224 kg N ha⁻¹) was injected to the desired depth in the CT- or NT-soils in each experiment. Mean flux rates of NO, N₂O, CH₄ and CO₂ ranged from 3.9 to 5.2 μg N m⁻² h⁻¹, 60.5 to 92.4 μg N m⁻² h⁻¹, −0.8 to 0.5 μg C m⁻² h⁻¹, and 42.1 to 81.7 mg C m⁻² h⁻¹ in both experiments, respectively. Deep N placement (10 and 15 cm) resulted in lower NO and N₂O emissions compared with shallow N placement (0 and 5 cm) while CH₄ and CO₂ emissions were not affected by N placement in either experiment. Compared with N placement at 5 cm, for instance, averaged N₂O emissions from N placement at 10 cm were reduced by more than 50% in both experiments. Generally, NT decreased NO emission and CH₄ oxidation but increased N₂O emissions compared with CT irrespective of N placement depths. Total net global warming potential (GWP) for N₂O, CH₄ and CO₂ was reduced by deep N placement only in Exp. 1 but was increased by NT in both experiments. The study results suggest that deep N placement (e.g., 10 cm) will be an effective option for reducing N oxide emissions and GWP from both fertilized CT- and NT-soils.     

Environmental effects on CO2 efflux from riparian tundra in the northern foothills of the Brooks Range,Alaska, USA

Summary Carbon dioxide efflux and soil microenvironmental factors were measured diurnally in Carex aquatilus-and Eriophorum angustifolium-dominated riparian tundra communities to determine the relative importance of soil environmental factors controlling ecosystem carbon dioxide exchange with the atmosphere. Measurements were made weekly between 18 June and 24 July 1990. Diurnal patterns in carbon dioxide efflux were best explained by changes in soil temperature, while seasonal changes in efflux were correlated with changes in depth to water table, depth to frozen soil and soil moisture. Carbon dioxide efflux rates were lowest early in the growing season when high water tables and low soil temperatures limited microbial and root activity. Individual rainfall events that raised the water table were found to strongly reduce carbon dioxide efflux. As the growing season progressed, rainfall was low and depth to water table and soil temperatures increased. In response, carbon dioxide efflux increased strongly, attaining rates late in the season of approximately 10 g CO₂ m^–2 day^–1. These rates are as high as maxima recorded for other arctic sites. A mathematical model is developed which demonstrates that soil temperature and depth to water table may be used as efficient predictors of ecosystem CO₂ efflux in this habitat. In parallel with the field measurements of CO₂ efflux, microbial respiration was studied in the laboratory as a function of temperature and water content. Estimates of microbial respiration per square meter under field conditions were made by adjusting for potential respiring soil volume as water table changed and using measured soil temperatures. The results indicate that the effect of these factors on microbial respiration may explain a large part of the diurnal and seasonal variation observed in CO₂ efflux. As in coastal tundra sites, environmental changes that alter water table depth in riparian tundra communities will have large effects on ecosystem CO₂ efflux and carbon balance.