Carbon dioxide,methane, and nitrous oxide exchanges in an age‐sequence of temperate pine forests

We investigated soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) exchanges in an age‐sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid‐April to mid‐December in 2006 and 2007. For both CH₄ and N₂O, we observed uptake and emission ranging from ?160 to 245 μg CH₄ m^?2 h^?1 and ?52 to 21 μg N₂O m^?2 h^?1, respectively (negative values indicate uptake). Mean fluxes from mid‐April to mid‐December across the 4, 17, 32, 67 years old stands were similar for CO₂ fluxes (259, 246, 220, and 250 mg CO₂ m^?2 h^?1, respectively), without pattern for N₂O fluxes (?3.7, 1.5, ?2.2, and ?7.6 μg N₂O m^?2 h^?1, respectively), whereas the uptake rates of CH₄ increased with stand age (6.4, ?7.9, ?10.8, and ?23.3 μg CH₄ m^?2 h^?1, respectively). For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated soil exchanges of CH₄ and N₂O was on average 4%, <1%, <1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO₂ fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH₄ and N₂O fluxes, but CH₄ emission was observed following summer rainfall events. LFH layer removal reduced CO₂ emissions by 43%, increased CH₄ uptake during dry and warm soil conditions by more than twofold, but did not affect N₂O flux. We suggest that significant alternating sink and source potentials for both CH₄ and N₂O may occur in N‐ and soil water‐limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.     

Carbon sequestration and ecosystem respiration for 4 years in a Scots pine forest

The net exchange of CO₂ (NEE) between a Scots pine (Pinus sylvestris L.) forest ecosystem in eastern Finland and the atmosphere was measured continuously by the eddy covariance (EC) technique over 4 years (1999–2002). The annual temperature coefficient (Q₁₀) of ecosystem respiration (R) for these years, respectively, was 2.32, 2.66, 2.73 and 2.69. The light‐saturated rate of photosynthesis (A_max) was highest in July or August, with an annual average A_max of 10.9, 14.6, 15.3 and 17.1 μmol m^?2 s^?1 in the 4 years, respectively. There was obvious seasonality in NEE, R and gross primary production (GPP), exhibiting a similar pattern to photosynthetically active radiation (PAR) and air temperature. The integrated daily NEE ranged from 2.59 to ?4.97 g C m^?2 day^?1 in 1999, from 2.70 to ?4.72 in 2000, from 2.61 to ?4.71 in 2001 and from 5.27 to ?4.88 in 2002. The maximum net C uptake occurred in July, with the exception of 2000, when it was in June. The interannual variation in ecosystem C flux was pronounced. The length of the growing season, based on net C uptake, was 179, 170, 175 and 176 days in 1999–2002, respectively, and annual net C sequestration was 152, 101, 172 and 205 g C m^?2 yr^?1. It is estimated that ecosystem respiration contributed 615, 591, 752 and 879 g C m^?2 yr^?1 to the NEE in these years, leading to an annual GPP of ?768, ?692, ?924 and ?1084 g C m^?2 yr^?1. It is concluded that temperature and PAR were the main determinants of the ecosystem CO₂ flux. Interannual variations in net C sequestration are predominantly controlled by average air temperature and integrated radiation in spring and summer. Four years of EC data indicate that boreal Scots pine forest ecosystem in eastern Finland acts as a relatively powerful carbon sink. Carbon sequestration may benefit from warmer climatic conditions.     

The annual cycles of CO2 and H2O exchange over a northern mixed forest as observed from a very tall tower

We present the annual patterns of net ecosystem‐atmosphere exchange (NEE) of CO₂ and H2O observed from a 447 m tall tower sited within a mixed forest in northern Wisconsin, USA. The methodology for determining NEE from eddy‐covariance flux measurements at 30, 122 and 396 m above the ground, and from CO₂ mixing ratio measurements at 11, 30, 76, 122, 244 and 396 m is described. The annual cycle of CO₂ mixing ratio in the atmospheric boundary layer (ABL) is also discussed, and the influences of local NEE and large‐scale advection are estimated. During 1997 gross ecosystem productivity (947?18 g C m^?2 yr^?1), approximately balanced total ecosystem respiration (963±19 g C m^?2 yr^?1), and NEE of CO₂ was close to zero (16±19 g C m^?2 yr^?1 emitted into the atmosphere). The error bars represent the standard error of the cumulative daily NEE values. Systematic errors are also assessed. The identified systematic uncertainties in NEE of CO₂ are less than 60 g C m^?2 yr^?1. The seasonal pattern of NEE of CO₂ was highly correlated with leaf‐out and leaf‐fall, and soil thaw and freeze, and was similar to purely deciduous forest sites. The mean daily NEE of CO₂ during the growing season (June through August) was ?1.3 g C m^?2 day^?1, smaller than has been reported for other deciduous forest sites. NEE of water vapor largely followed the seasonal pattern of NEE of CO₂, with a lag in the spring when water vapor fluxes increased before CO₂ uptake. In general, the Bowen ratios were high during the dormant seasons and low during the growing season. Evapotranspiration normalized by potential evapotranspiration showed the opposite pattern. The seasonal course of the CO₂ mixing ratio in the ABL at the tower led the seasonal pattern of NEE of CO₂ in time: in spring, CO₂ mixing ratios began to decrease prior to the onset of daily net uptake of CO₂ by the forest, and in fall mixing ratios began to increase before the forest became a net source for CO₂ to the atmosphere. Transport as well as local NEE of CO₂ are shown to be important components of the ABL CO₂ budget at all times of the year.     

Carbon balance of different aged Scots pine forests in Southern Finland

We estimated annual net ecosystem exchange (NEE) of a chronosequence of four Scots pine stands in southern Finland during years 2000–2002 using eddy covariance (EC). Net ecosystem productivity (NEP) was estimated using growth measurements and modelled mass losses of woody debris. The stands were 4, 12, 40 and 75 years old. The 4‐year‐old clearcut was a source of carbon throughout the year combining a low gross primary productivity (GPP) with a total ecosystem respiration (TER) similar to the forest stands. The annual NEE of the clearcut, measured by EC, was 386 g C m^?2. Tree growth was negligible and the estimated NEP was ?262 g C m^?2 a^?1. The annual GPPs at the other sites were close to each other (928?1072 g C m^?2 a^?1), but TER differed markedly, being greatest at the 12‐year‐old site (905 g C m^?2 a^?1) and smallest in the 75‐year‐old stand (616 g C m^?2 a^?1). Measurements of soil CO₂ efflux showed that different rates of soil respiration largely explained the differences in TER. The NEE and NEP of the 12‐year‐old stand were close to zero. The forested stands were sinks of carbon. They had similar annual patterns of carbon exchange and half‐hourly eddy fluxes were highly correlated, indicating similar responses to the environment. The NEE in the 40‐year‐old stand varied between ?179 and –192 g C m^?2 a^?1, while NEP was between 214 and 242 g C m^?2 a^?1. The annual NEE of the 75‐year‐old stand was 323 g C m^?2 and NEP was 252 g C m^?2. This indicates that there was no reduction in carbon sink strength with stand age.     

Consequences of wildfire on ecosystem CO2 and water vapour fluxes in the Great Basin

Increased fire frequency in the Great Basin of North America's intermountain West has led to large‐scale conversion of native sagebrush (Artemisia tridentata Nutt.) communities to postfire successional communities dominated by native and non‐native annual species during the last century. The consequences of this conversion for basic ecosystem functions, however, are poorly understood. We measured net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET) during the first two dry years after wildfire using a 4‐m diameter (16.4 m³) translucent static chamber (dome), and found that both NEE and ET were higher in a postfire successional ecosystem (?0.9–2.6 µ mol CO₂ m^?2 s^?1 and 0.0–1.0 mmol H₂O m^?2 s^?2, respectively) than in an adjacent intact sagebrush ecosystem (?1.2–2.3 µ mol CO₂ m^?2 s^?1 and ?0.1–0.8 mmol H₂O m^?2 s^?2, respectively) during relatively moist periods. Higher NEE in the postfire ecosystem appears to be due to lower rates of above‐ground plant respiration while higher ET appears to be caused by higher surface soil temperatures and increased soil water recharge after rains. These patterns disappeared or were reversed, however, when the conditions were drier. Daily net ecosystem productivity (NEP; g C m^?2 d^?1), derived from multiple linear regressions of measured fluxes with continuously measured climate variables, was very small (close to zero) throughout most of the year. The wintertime was an exception in the intact sagebrush ecosystem with C losses exceeding C gains leading to negative NEP while C balance of the postfire ecosystem remained near zero. Taken together, our results indicate that wildfire‐induced conversion of native sagebrush steppe to ecosystems dominated by herbaceous annual species may have little effect on C balance during relatively dry years (except in winter months) but may stimulate water loss immediately following fires.     

Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand

Understanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO₂ exchange (NEE) and its components – gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) – in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit (VPD > 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic light–response curve (NEE partitioning algorithm) consistently failed to provide good fits at high VPD. Modified rectangular hyperbolic light–VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to ?33.02 ± 1.96 μmol CO₂ m^?2 s^?1 and the highest daily integrated NEE was ?35.89 g CO₂ m^?2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP. The ecosystem was a net sink of CO₂ during 5–6 months and total seasonal uptakes were ?1128 ± 130 and ?1796 ± 217 g CO₂ m^?2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year‐round measurements over several years are required to assess a long‐term source‐sink status of the ecosystem.     

Diurnal and seasonal variation in methane emissions in a northern Canadian peatland measured by eddy covariance

Eddy covariance measurements of methane (CH₄) net flux were made in a boreal fen, typical of the most abundant peatlands in western Canada during May–September 2007. The objectives of this study were to determine: (i) the magnitude of diurnal and seasonal variation in CH₄ net flux, (ii) the relationship between the temporally varying flux rates and associated changes in controlling biotic and abiotic factors, and (iii) the contribution of CH₄ emission to the ecosystem growing season carbon budget. There was significant diurnal variation in CH₄ emission during the peak of the growing season that was strongly correlated with associated changes in solar radiation, latent heat flux, air temperature and ecosystem conductance to water vapor. During days 181–215, nighttime average CH₄ efflux was only 47% of the average midday values. The peak value for daily average CH₄ emission rate was approximately 80 nmol m^?2 s^?1 (4.6 mg CH₄ m^?2 h^?1), and seasonal variation in CH₄ flux was strongly correlated with changes in soil temperature. Integrated over the entire measurement period [days 144–269 (late May–late September)], the total CH₄ emission was 3.2 g CH₄ m^?2, which was quite low relative to other wetland ecosystems and to the simultaneous high rate of ecosystem net CO₂ sequestration that was measured (18.1 mol CO₂ m^?2 or 217 g C m^?2). We estimate that the negative radiative forcing (cooling) associated with net carbon storage over the life of the peatland (approximately 2200 years) was at least twice the value of positive radiative forcing (warming) caused by net CH₄ emission over the last 50 years.     

Methane release from stands of water horsetail ( Equisetum fluviatile ) in a boreal lake

1. Annual and diel variations in methane (CH₄) release in stands of Equisetum fluviatile were measured from June to November in Lake Pääjärvi, southern Finland, where E. fluviatile is the dominant emergent macrophyte. An estimate of total annual release of CH₄ from stands of E. fluviatile in this lake was also made. Diel variation was measured twice (June and August), whereas measurements for annual variation were performed monthly. The hypothesis that a relationship exists between the productivity of stands and CH₄ release was also tested, whereupon net ecosystem exchange (NEE) of CO₂ as well as standing stock of E. fluviatile were determined, in addition to simultaneous recordings of air temperature and solar radiation. 2. Seasonal variations in CH₄ release were pronounced, with the highest release rate of 813 mg m^–2 day^–1 measured in July and the lowest 6.5 mg m^–2 day^–1 in November, when the shoreline was already frozen. 3. Methane release rates were strongly correlated with mean air temperature in the measuring chambers and with total solar radiation. There was no significant correlation between the instantaneous radiation and CH₄ release rates. 4. The seasonal patterns of CH₄ release and NEE of CO₂ resembled each other, except in July when NEE suddenly dropped. The decrease in NEE coincided with the highest CH₄ release rate measured and the highest temperature during the measuring period, i.e. 32 °C outside and 37 °C inside the chamber. Excluding this date, daily CH₄ release was strongly correlated with NEE (r² = 0.971). 5. No diel changes in CH₄ release rates were detected. In June and August the maximum release rates were 11.4 and 16.8 mg CH₄ m^–2 h^–1, respectively. 6. The standing stock of E. fluviatile at different times of the growing season was not correlated with CH₄ efflux; the CH₄ release rates could be related neither to the number of shoots, i.e. sufficient conduits for gas transport were always present, nor to the shoot biomass in the measuring chambers. 7. For an estimate of the annual release, the monthly values measured at noon were integrated over the entire growing season; this resulted in 43.7 g CH₄ m^–2 for the annual emission. The total annual emission of CH₄ from the area covered with E. fluviatile in Lake Pääjärvi was calculated to be ≈ 5000 kg. 8. Significant amounts of CH₄ are released from stands of E. fluviatile in boreal lakes. The CH₄ release rate follows a seasonal pattern but there is no diel pattern. Methane release rate can be related to temperature, solar radiation and NEE of CO₂, but not to the standing stock of E. fluviatile or the number of shoots.     

Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO₂]. Net ecosystem CO₂ exchange (NEE) was measured in the eighth year of CO₂ enrichment at the Nevada Desert Free‐Air CO₂ Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO₂ m^?2 s^?1) of ecosystems exposed to elevated atmospheric CO₂ were similar to those maintained at current ambient CO₂ levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO₂] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO₂]. Mean daily NEE varied seasonally across both CO₂ treatments, increasing from about 0.1 μmol CO₂ m^?2 s^?1 in December to a maximum of 0.5–0.6 μmol CO₂ m^?2 s^?1 in early spring. Maximum NEE in ecosystems exposed to elevated CO₂ occurred 1 month earlier than it did in ecosystems exposed to ambient CO₂, with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO₂ m^?2 s^?1), but then increasing to near peak levels in late October (0.36±0.08 μmol CO₂ m^?2 s^?1), November (0.28±0.03 μmol CO₂ m^?2 s^?1), and December (0.54±0.06 μmol CO₂ m^?2 s^?1). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO₂ uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO₂ treatments were net CO₂ sinks in 2004, but exposure to elevated CO₂ reduced CO₂ sink strength by 30% (positive net ecosystem productivity=127±17 g C m^?2 yr^?1 ambient CO₂ and 90±11 g C m^?2 yr^?1 elevated CO₂, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO₂– along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.     

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.     

Year-round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie

An Ameriflux site was established in mid 1996 to study the exchange of CO₂ in a native tallgrass prairie of north‐central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid‐July–early August 1996), moisture‐stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid‐day net ecosystem CO₂ exchange (NEE), under well‐watered conditions, reached a maximum magnitude of 1.4 mg CO₂ m^?2 s^?1 (flux toward the surface is positive) during peak growth (mid‐July 1997), with green leaf area index of 2.8. In contrast, under moisture‐stress conditions in the same growth stage in 1996, mid‐day NEE was reduced to near‐zero. Average night NEE ranged from near‐zero, during winter dormancy, to ? 0.50 mg CO₂ m^?2 s^?1, during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y‐intercept, so obtained, can be potentially used as an estimate of night‐time CO₂ emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO₂ m^?2 d^?1 (8.4 g C m^?2 d^?1) in mid‐July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m^?2 y^?1. After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near‐zero in 1998.     

Effects of nutrient addition on vegetation and carbon cycling in an ombrotrophic bog

We measured net ecosystem CO₂ exchange (NEE), plant biomass and growth, species composition, peat microclimate, and litter decomposition in a fertilization experiment at Mer Bleue Bog, Ottawa, Ontario. The bog is located in the zone with the highest atmospheric nitrogen deposition for Canada, estimated at 0.8–1.2 g N m⁻² yr⁻¹ (wet deposition as NH₄ and NO₃). To establish the effect of nutrient addition on this ecosystem, we fertilized the bog with six treatments involving the application of 1.6–6 g N m⁻² yr⁻¹ (as NH₄NO₃), with and without P and K, in triplicate 3 m × 3 m plots. The initial 5–6 years have shown a loss of first Sphagnum, then Polytrichum mosses, and an increase in vascular plant biomass and leaf area index. Analyses of NEE, measured in situ with climate‐controlled chambers, indicate that contrary to expectations, the treatments with the highest levels of nutrient addition showed lower rates of maximum NEE and gross photosynthesis, but little change in ecosystem respiration after 5 years. Although shrub biomass and leaf area increased in the high nutrient plots, loss of moss photosynthesis owing to nutrient toxicity, increased vascular plant shading and greater litter accumulation contributed to the lower levels of CO₂ uptake. Our study highlights the importance of long‐term experiments as we did not observe lower NEE until the fifth year of the experiment. However, this may be a transient response as the treatment plots continue to change. Higher levels of nutrients may cause changes in plant composition and productivity and decrease the ability of peatlands to sequester CO₂ from the atmosphere.     

Multi‐year carbon budget of a mature commercial short rotation coppice willow plantation

Energy derived from second generation perennial energy crops is projected to play an increasingly important role in the decarbonization of the energy sector. Such energy crops are expected to deliver net greenhouse gas emissions reductions through fossil fuel displacement and have potential for increasing soil carbon (C) storage. Despite this, few empirical studies have quantified the ecosystem‐level C balance of energy crops and the evidence base to inform energy policy remains limited. Here, the temporal dynamics and magnitude of net ecosystem carbon dioxide (CO₂) exchange (NEE) were quantified at a mature short rotation coppice (SRC) willow plantation in Lincolnshire, United Kingdom, under commercial growing conditions. Eddy covariance flux observations of NEE were performed over a four‐year production cycle and combined with biomass yield data to estimate the net ecosystem carbon balance (NECB) of the SRC. The magnitude of annual NEE ranged from ?147 ± 70 to ?502 ± 84 g CO₂‐C m^?2 year^?1 with the magnitude of annual CO₂ capture increasing over the production cycle. Defoliation during an unexpected outbreak of willow leaf beetle impacted gross ecosystem production, ecosystem respiration, and net ecosystem exchange during the second growth season. The NECB was ?87 ± 303 g CO₂‐C m^?2 for the complete production cycle after accounting for C export at harvest (1,183 g C m^?2), and was approximately CO₂‐C neutral (?21 g CO₂‐C m^?2 year^?1) when annualized. The results of this study are consistent with studies of soil organic C which have shown limited changes following conversion to SRC willow. In the context of global decarbonization, the study indicates that the primary benefit of SRC willow production at the site is through displacement of fossil fuel emissions.     

Seasonal nitrous oxide and methane emissions across a subtropical estuarine salinity gradient

Currently, there is a lack of knowledge about GHG emissions, specifically N₂O and CH₄, in subtropical coastal freshwater wetland and mangroves in the southern hemisphere. In this study, we quantified the gas fluxes and substrate availability in a subtropical coastal wetland off the coast of southeast Queensland, Australia over a complete wet-dry seasonal cycle. Sites were selected along a salinity gradient ranging from marine (34 psu) in a mangrove forest to freshwater (0.05 psu) wetland, encompassing the range of tidal influence. Fluxes were quantified for CH₄ (range ?0.4–483 mg C–CH₄ h^?1 m^?2) and N₂O (?5.5–126.4 μg N–N₂O h^?1 m^?2), with the system acting as an overall source for CH₄ and N₂O (mean N₂O and CH₄ fluxes: 52.8 μg N–N₂O h^?1 m^?2 and 48.7 mg C–CH₄ h^?1 m^?2, respectively). Significantly higher N₂O fluxes were measured during the summer months (summer mean 64.2 ± 22.2 μg N–N₂O h^?1 m^?2; winter mean 33.1 ± 24.4 µg N–N₂O h^–1 m^?2) but not CH₄ fluxes (summer mean 30.2 ± 81.1 mg C–CH₄ h^?1 m^?2; winter mean 37.4 ± 79.6 mg C–CH₄ h^?1 m^?2). The changes with season are primarily driven by temperature and precipitation controls on the dissolved inorganic nitrogen (DIN) concentration. A significant spatial pattern was observed based on location within the study site, with highest fluxes observed in the freshwater tidal wetland and decreasing through the mangrove forest. The dissolved organic carbon (DOC) varied throughout the landscape and was correlated with higher CH₄ fluxes, but this was a nonlinear trend. DIN availability was dominated by N–NH₄ and correlated to changes in N₂O fluxes throughout the landscape. Overall, we did not observe linear relationships between CH₄ and N₂O fluxes and salinity, oxygen or substrate availability along the fresh-marine continuum, suggesting that this ecosystem is a mosaic of processes and responses to environmental changes.     

Dissolved carbon leaching from soil is a crucial component of the net ecosystem carbon balance

Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its δ¹³C signature. Leaching of biogenic DIC was 8.3±4.9 g m^?2 yr^?1 for forests, 24.1±7.2 g m^?2 yr^?1 for grasslands, and 14.6±4.8 g m^?2 yr^?1 for croplands. DOC leaching equalled 3.5±1.3 g m^?2 yr^?1 for forests, 5.3±2.0 g m^?2 yr^?1 for grasslands, and 4.1±1.3 g m^?2 yr^?1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4±4.0 g C m^?2 yr^?1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5–98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24–105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems.     

Plant-mediated CH4 transport and C gas dynamics quantified in-situ in a Phalaris arundinacea-dominant wetland

Northern peatland methane (CH₄) budgets are important for global CH₄ emissions. This study aims to determine the ecosystem CH₄ budget and specifically to quantify the importance of Phalaris arundinacea by using different chamber techniques in a temperate wetland. Annually, roughly 70?±?35% of ecosystem CH₄ emissions were plant-mediated, but data show no evidence of significant diurnal variations related to convective gas flow regardless of season or plant growth stages. Therefore, despite a high percentage of arenchyma, P. arundinacea-mediated CH₄ transport is interpreted to be predominantly passive. Thus, diurnal variations are less important in contrast to wetland vascular plants facilitating convective gas flow. Despite of plant-dominant CH₄ transport, net CH₄ fluxes were low (–?0.005–0.016 μmol m^?2 s^?1) and annually less than 1% of the annual C-CO₂ assimilation. This is considered a result of an effective root zone oxygenation resulting in increased CH₄ oxidation in the rhizosphere at high water levels. This study shows that although CH₄, having a global warming potential 25 times greater than CO₂, is emitted from this P. arundinacea wetland, less than 9% of the C sequestered counterbalances the CH₄ emissions to the atmosphere. It is concluded that P. arundinacea-dominant wetlands are an attractive C-sequestration ecosystem.     

Measurement and modelling of CO2 flux from a drained fen peatland cultivated with reed canary grass and spring barley

Cultivation of bioenergy crops has been suggested as a promising option for reduction of greenhouse gas (GHG) emissions from arable organic soils (Histosols). Here, we report the annual net ecosystem exchange (NEE) fluxes of CO₂ as measured with a dynamic closed chamber method at a drained fen peatland grown with reed canary grass (RCG) and spring barley (SB) in a plot experiment (n = 3 for each cropping system). The CO₂ flux was partitioned into gross photosynthesis (GP) and ecosystem respiration (R_E). For the data analysis, simple yet useful GP and R_E models were developed which introduce plot‐scale ratio vegetation index as an active vegetation proxy. The GP model captures the effect of temperature and vegetation status, and the R_E model estimates the proportion of foliar biomass dependent respiration (R_fb) in the total R_E. Annual R_E was 1887 ± 7 (mean ± standard error, n = 3) and 1288 ± 19 g CO₂‐C m^?2 in RCG and SB plots, respectively, with R_fb accounting for 32 and 22% respectively. Total estimated annual GP was ?1818 ± 42 and ?1329 ± 66 g CO₂‐C m^?2 in RCG and SB plots leading to a NEE of 69 ± 36 g CO₂‐C m^?2 yr^?1 in RCG plots (i.e., a weak net source) and ?41 ± 47 g CO₂‐C m^?2 yr^?1 in SB plots (i.e., a weak net sink). Standard errors related to spatial variation were small (as shown above), but more significant uncertainties were related to the modelling approach for establishment of annual budgets. In conclusion, the bioenergy cropping system was not more favourable than the food cropping system when looking at the atmospheric CO₂ emissions during cultivation. However, in a broader GHG life‐cycle perspective, the lower fertilizer N input and the higher biomass yield in bioenergy cropping systems could be beneficial.     

Species-specific Effects of Vascular Plants on Carbon Turnover and Methane Emissions from Wetlands

Species composition affects the carbon turnover and the formation and emission of the greenhouse gas methane (CH₄) in wetlands. Here we investigate the individual effects of vascular plant species on the carbon cycling in a wetland ecosystem. We used a novel combination of laboratory methods and controlled environment facilities and studied three different vascular plant species (Eriophorum vaginatum, Carex rostrata and Juncus effusus) collected from the same wetland in southern Sweden. We found distinct differences in the functioning of these wetland sedges in terms of their effects on carbon dioxide (CO₂) and CH₄ fluxes, bubble emission of CH₄, decomposition of ¹⁴C-labelled acetate into ¹⁴CH₄ and ¹⁴CO₂, rhizospheric oxidation of CH₄ to CO₂ and stimulation of methanogenesis through root exudation of substrate (e.g., acetate). The results show that the emission of CH₄ from peat–plant monoliths was highest when the vegetation was dominated by Carex (6.76 mg CH₄ m⁻² h⁻¹) than when it was dominated by Eriophorum (2.38 mg CH₄ m⁻² h⁻¹) or Juncus (2.68 mg CH₄ m⁻² h⁻¹). Furthermore, the CH₄ emission seemed controlled primarily by the degree of rhizospheric CH₄ oxidation which was between 20 and 40% for Carex but >90% for both the other species. Our results point toward a direct and very important linkage between the plant species composition and the functioning of wetland ecosystems and indicate that changes in the species composition may alter important processes relating to controls of and interactions between greenhouse gas fluxes with significant implications for feedback mechanisms in a changing climate as a result.     

Carbon balance of rewetted and drained peat soils used for biomass production: a mesocosm study

Rewetting of drained peatlands has been recommended to reduce CO₂ emissions and to restore the carbon sink function of peatlands. Recently, the combination of rewetting and biomass production (paludiculture) has gained interest as a possible land use option in peatlands for obtaining such benefits of lower CO₂ emissions without losing agricultural land. This study quantified the carbon balance (CO₂, CH₄ and harvested biomass C) of rewetted and drained peat soils under intensively managed reed canary grass (RCG) cultivation. Mesocosms were maintained at five different groundwater levels (GWLs), that is 0, 10, 20 cm below the soil surface, representing rewetted peat soils, and 30 and 40 cm below the soil surface, representing drained peat soils. Net ecosystem exchange (NEE) of CO₂ and CH₄ emissions was measured during the growing period of RCG (May to September) using transparent and opaque closed chamber methods. The average dry biomass yield was significantly lower from rewetted peat soils (12 Mg ha^?1) than drained peat soils (15 Mg ha^?1). Also, CO₂ fluxes of gross primary production (GPP) and ecosystem respiration (ER) from rewetted peat soils were significantly lower than from drained peat soils, but net uptake of CO₂ was higher from rewetted peat soils. Cumulative CH₄ emissions were negligible (0.01 g CH₄ m^?2) from drained peat soils but were significantly higher (4.9 g CH₄ m^?2) from rewetted peat soils during measurement period (01 May–15 September 2013). The extrapolated annual C balance was 0.03 and 0.68 kg C m^?2 from rewetted and drained peat soils, respectively, indicating that rewetting and paludiculture can reduce the loss of carbon from peatlands.     

Environmental controls on net ecosystem-level carbon exchange and productivity in a Central American tropical wet forest

Difficulty in balancing the global carbon budget has lead to increased attention on tropical forests, which have been estimated to account for up to one third of global gross primary production. Whether tropical forests are sources, sinks, or neutral with respect to their carbon balance with the atmosphere remains unclear. To address this issue, estimates of net ecosystem exchange of carbon (NEE) were made for 3 years (1998–2000) using the eddy‐covariance technique in a tropical wet forest in Costa Rica. Measurements were made from a 42 m tower centred in an old‐growth forest. Under unstable conditions, the measurement height was at least twice the estimated zeroplane height from the ground. The canopy at the site is extremely rough; under unstable conditions the median aerodynamic roughness length ranged from 2.4 to 3.6 m. No relationship between NEE and friction velocity (u*) was found using all of the 30‐min averages. However, there was a linear relationship between the nighttime NEE and averaged u* (R² = 0.98). The diurnal pattern of flux was similar to that found in other tropical forests, with mean daytime NEE ca. ? 18 μ mol CO₂ m^?2 s^?1 and mean nighttime NEE 4.6 μ mol CO₂ m^?2 s^?1. However, because ～ 80% of the nighttime data in this forest were collected during low u* conditions ( < 0.2 m s^?1), nighttime NEE was likely underestimated. Using an alternative analysis, mean nighttime NEE increased to 7.05 μ mol CO₂ m^?2 s^?1. There were interannual differences in NEE, but seasonal differences were not apparent. Irradiance accounted for ～ 51% of the variation in the daytime fluxes, with temperature and vapour pressure deficit together accounting for another ～ 20%. Light compensation points ranged from 100 to 207 μ mol PPFD m^?2 s^?1. No was relationship was found between 30‐min nighttime NEE and tower‐top air temperature. A weak relationship was found between hourly nighttime NEE and canopy air temperature using data averaged hourly over the entire sampling period (Q₁₀ = 1.79, R² = 0.17). The contribution of below‐sensor storage was fairly constant from day to day. Our data indicate that this forest was a slight carbon source in 1998 (0.05 to ?1.33 t C ha^?1 yr^?1), a moderate sink in 1999 (?1.53 to ?3.14 t C ha^?1 yr^?1), and a strong sink in 2000 (?5.97 to ?7.92 t C ha^?1 yr^?1). This trend is interpreted as relating to the dissipation of warm‐phase El Niño effects over the course of this study.