Landscape controls of CH4 fluxes in a catchment of the forest tundra ecotone in northern Siberia

Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH₄) with the atmosphere. Climate warming could have a great impact on CH₄ exchange, in particular in regions where degradation of permafrost is induced. In order to improve the understanding of the present and future methane dynamics in permafrost regions, we studied CH₄ fluxes of typical landscape structures in a small catchment in the forest tundra ecotone in northern Siberia. Gas fluxes were measured using a closed‐chamber technique from August to November 2003 and from August 2006 to July 2007 on tree‐covered mineral soils with and without permafrost, on a frozen bog plateau, and on a thermokarst pond. For areal integration of the CH₄ fluxes, we combined field observations and classification of functional landscape structures based on a high‐resolution Quickbird satellite image. All mineral soils were net sinks of atmospheric CH₄. The magnitude of annual CH₄ uptake was higher for soils without permafrost (1.19 kg CH₄ ha⁻¹ yr⁻¹) than for soils with permafrost (0.37 kg CH₄ ha⁻¹ yr⁻¹). In well‐drained soils, significant CH₄ uptake occurred even after the onset of ground frost. Bog plateaux, which stored large amounts of frozen organic carbon, were also a net sink of atmospheric CH₄ (0.38 kg CH₄ ha⁻¹ yr⁻¹). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH₄ emission (approximately 200 kg CH₄ ha⁻¹ yr⁻¹). Despite the low area coverage of thermokarst ponds (only 2.1% of the total catchment area), emissions from these sites resulted in a mean catchment CH₄ emission of 3.8 kg CH₄ ha⁻¹ yr⁻¹. Export of dissolved CH₄ with stream water was insignificant. The results suggest that mineral soils and bog plateaux in this region will respond differently to increasing temperatures and associated permafrost degradation. Net uptake of atmospheric CH₄ in mineral soils is expected to gradually increase with increasing active layer depth and soil drainage. Changes in bog plateaux will probably be much more rapid and drastic. Permafrost collapse in frozen bog plateaux would result in high CH₄ emissions that act as positive feedback to climate warming.     

Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field

Methane (CH₄) is a particularly potent greenhouse gas with a radiative forcing 23 times that of CO₂ on a per mass basis. Flooded rice paddies are a major source of CH₄ emissions to the Earth's atmosphere. A free‐air CO₂ enrichment (FACE) experiment was conducted to evaluate changes in crop productivity and the crop ecosystem under enriched CO₂ conditions during three rice growth seasons from 1998 to 2000 in a rice paddy at Shizukuishi, Iwate, Japan. To understand the influence of elevated atmospheric CO₂ concentrations on CH₄ emission, we measured methane flux from FACE rice fields and rice fields with ambient levels of CO₂ during the 1999 and 2000 growing seasons. Methane production and oxidation potentials of soil samples collected when the rice was at the tillering and flowering stages in 2000 were measured in the laboratory by the anaerobic incubation and alternative propylene substrates methods, respectively. The average tiller number and root dry biomass were clearly larger in the plots with elevated CO₂ during all rice growth stages. No difference in methane oxidation potential between FACE and ambient treatments was found, but the methane production potential of soils during the flowering stage was significantly greater under FACE than under ambient conditions. When free‐air CO₂ was enriched to 550 ppmv, the CH₄ emissions from the rice paddy field increased significantly, by 38% in 1999 and 51% in 2000. The increased CH₄ emissions were attributed to accelerated CH₄ production potential as a result of more root exudates and root autolysis products and to increased plant‐mediated CH₄ emissions because of the larger rice tiller numbers under FACE conditions.     

Experimental soil warming effects on CO2 and CH4 flux from a low elevation spruce–fir forest soil in Maine, USA

The effect of soil warming on CO₂ and CH₄ flux from a spruce–fir forest soil was evaluated at the Howland Integrated Forest Study site in Maine, USA from 1993 to 1995. Elevated soil temperatures (～5 °C) were maintained during the snow-free season (May – November) in replicated 15 × 15-m plots using electric cables buried 1–2 cm below the soil surface; replicated unheated plots served as the control. CO₂ evolution from the soil surface and soil air CO₂ concentrations both showed clear seasonal trends and significant (P < 0.0001) positive exponential relationships with soil temperature. Soil warming caused a 25–40% increase in CO₂ flux from the heated plots compared to the controls. No significant differences were observed between heated and control plot soil air CO₂ concentrations which we attribute to rapid equilibration with the atmosphere in the O horizon and minimal treatment effects in the B horizon. Methane fluxes were highly variable and showed no consistent trends with treatment.     

Soil CO2 efflux in a tropical forest in the central Amazon

This study investigated the spatial and temporal variation in soil carbon dioxide (CO₂) efflux and its relationship with soil temperature, soil moisture and rainfall in a forest near Manaus, Amazonas, Brazil. The mean rate of efflux was 6.45±0.25 SE μmol CO₂ m^?2s^?1 at 25.6±0.22 SE°C (5 cm depth) ranging from 4.35 to 9.76 μmol CO₂ m^?2s^?1; diel changes in efflux were correlated with soil temperature (r²=0.60). However, the efflux response to the diel cycle in temperature was not always a clear exponential function. During period of low soil water content, temperature in deeper layers had a better relationship with CO₂ efflux than with the temperature nearer the soil surface. Soil water content may limit CO₂ production during the drying‐down period that appeared to be an important factor controlling the efflux rate (r²=0.39). On the other hand, during the rewetting period microbial activity may be the main controlling factor, which may quickly induce very high rates of efflux. The CO₂ flux chamber was adapted to mimic the effects of rainfall on soil CO₂ efflux and the results showed that efflux rates reduced 30% immediately after a rainfall event. Measurements of the CO₂ concentration gradient in the soil profile showed a buildup in the concentration of CO₂ after rain on the top soil. This higher CO₂ concentration developed shortly after rainfall when the soil pores in the upper layers were filled with water, which created a barrier for gas exchange between the soil and the atmosphere.     

Effects of raised CO2 on potential CH4 production and oxidation in, and CH4 emission from, a boreal mire

1 In a glasshouse experiment we studied the effect of raised CO₂ concentration (720 p.p.m.) on CH₄ emission at natural boreal peat temperatures using intact cores of boreal peat with living vascular plants and Sphagnum mosses. After the end of the growing season half of the cores were kept unnaturally warm (17–20 °C). The potential for CH₄ production and oxidation was measured at the end of the emission experiment.
2 The vascular cores ('Sedge') consisted of a moss layer with sedges, and the moss cores (' Sphagnum ') of Sphagnum mosses (some sedge seedlings were removed by cutting). Methane efflux was 6–12 times higher from the Sedge cores than from the Sphagnum cores. The release of CH ₄ from Sedge cores increased with increasing temperature of the peat and decreased with decreasing temperature. Methane efflux from Sphagnum cores was quite stable independent of the peat temperatures.
3 In both Sedge and Sphagnum samples, CO₂ treatment doubled the potential CH₄ production but had no effect on the potential CH₄ oxidation. A raised concentration of CO₂ increased CH₄ efflux weakly and only at the highest peat temperatures (17–20 °C).
4 The results suggest that in cool regions, such as boreal wetlands, temperature would restrict decomposition of the extra substrates probably derived from enhanced primary production of mire vegetation under raised CO₂ concentrations, and would thus retard any consequent increase in CH₄ emission.     

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

Because biological and physical processes alter the stable isotopic composition of atmospheric CO₂, variations in isotopic content can be used to investigate those processes. Isotopic flux measurements of ¹³CO₂ above terrestrial ecosystems can potentially be used to separate net ecosystem CO₂ exchange (NEE) into its component fluxes, net photosynthetic assimilation (F_A) and ecosystem respiration (F_R). In this paper theory is developed to partition measured NEE into F_A and F_R, using measurements of fluxes of CO₂ and ¹³CO₂, and isotopic composition of respired CO₂ and forest air. The theory is then applied to fluxes measured (or estimated, for ¹³CO₂) in a temperate deciduous forest in eastern Tennessee (Walker Branch Watershed). It appears that there is indeed enough additional information in ¹³CO₂ fluxes to partition NEE into its photosynthetic and respiratory components. Diurnal patterns in F_A and F_R were obtained, which are consistent in magnitude and shape with patterns obtained from NEE measurements and an exponential regression between night‐time NEE and temperature (a standard technique which provides alternate estimates of F_R and F_A). The light response curve for photosynthesis (F_A vs. PAR) was weakly nonlinear, indicating potential for saturation at high light intensities. Assimilation‐weighted discrimination against ¹³CO₂ for this forest during July 1999 was 16.8–17.1‰, depending on canopy conductance. The greatest uncertainties in this approach lie in the evaluation of canopy conductance and its effect on whole‐canopy photosynthetic discrimination, and thus the indirect methods used to estimate isotopic fluxes. Direct eddy covariance measurements of ¹³CO₂ flux are needed to assess the validity of the assumptions used and provide defensible isotope‐based estimates of the component fluxes of net ecosystem exchange.     

Seasonal variation in pathways of CH4 production and in CH4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors

Flooded rice fields, which are an important source of the atmospheric methane, have become a model system for the study of interactions between various microbial processes. We used a combination of stable carbon isotope measurements and application of specific inhibitors in order to investigate the importance of various methanogenic pathways and of CH₄ oxidation for controlling CH₄ emission. The fraction of CH₄ produced from acetate and H₂/CO₂ was calculated from the isotopic signatures of acetate, carbon dioxide (CO₂) and methane (CH₄) measured in porewater, gas bubbles, in the aerenchyma of the plants and/or in incubation experiments. The calculated ratio between both pathways reflected well the ratio determined by application of methyl fluoride (CH₃F) as specific inhibitor of acetate‐dependent methanogenesis. Only at the end of the season, the theoretical ratio of acetate: H₂ = 2 : 1 was reached, whereas at the beginning H₂/CO₂‐dependent methanogenesis dominated. The isotope discrimination was different between rooted surface soil and unrooted deep soil. Root‐associated CH₄ production was mainly driven by H₂/CO₂. Porewater CH₄ was found to be a poor proxy for produced CH₄. The fraction of CH₄ oxidised was calculated from the isotopic signature of CH₄ produced in vitro compared to CH₄ emitted in situ, corrected for the fractionation during the passage from the aerenchyma to the atmosphere. Isotope mass balances and in situ inhibition experiments with difluoromethane (CH₂F₂) as specific inhibitor of methanotrophic bacteria agreed that CH₄ oxidation was quantitatively important at the beginning of the season, but decreased later. The seasonal pattern was consistent with the change of potential CH₄ oxidation rates measured in vitro. At the end of the season, isotope techniques detected an increase of oxidation activity that was too small to be measured with the flux‐based inhibitor technique. If porewater CH₄ was used as a proxy of produced CH₄, neither magnitude nor seasonal pattern of in situ CH₄ oxidation could be reproduced. An oxidation signal was also found in the isotopic signature of CH₄ from gas bubbles that were released by natural ebullition. In contrast, bubbles stirred up from the bulk soil had preserved the isotopic signature of the originally produced CH₄.     

Precipitation pulses and soil CO2 flux in a Sonoran Desert ecosystem

Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water‐limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole‐system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO₂ efflux increased up to 30‐fold, but generally returned to background levels within 48 h. CO₂ production integrated over 48 h ranged from 2.5 to 19.3 g C m⁻² and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO₂ were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short‐term losses of CO₂. In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO₂ efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long‐term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.     

Hierarchical saturation of soil carbon pools near a natural CO2 spring

Soil has been identified as a possible carbon (C) sink to mitigate increasing atmospheric CO₂ concentration. However, several recent studies have suggested that the potential of soil to sequester C is limited and that soil may become saturated with C under increasing CO₂ levels. To test this concept of soil C saturation, we studied a gley and organic soil at a grassland site near a natural CO₂ spring. Total and aggregate‐associated soil organic C (SOC) concentration showed a significant increase with atmospheric CO₂ concentration. An asymptotic function showed a better fit of SOC and aggregation with CO₂ level than a linear model. There was a shift in allocation of total C from smaller size fractions to the largest aggregate fraction with increasing CO₂ concentration. Litter inputs appeared to be positively related to CO₂ concentration. Based on modeled function parameters and the observed shift in the allocation of the soil C from small to large aggregate‐size classes, we postulate that there is a hierarchy in C saturation across different SOC pools. We conclude that the asymptotic response of SOC concentration at higher CO₂ levels indicates saturation of soil C pools, likely because of a limit to physical protection of SOC.     

A moderate increase in the annual CH4 efflux by raised CO2 or NH4NO3 supply in a boreal oligotrophic mire

Natural wetlands release about 20% of global emissions of CH₄, an effective greenhouse gas contributing to the total radiative forcing. Thus, changes in the carbon cycle in wetlands could have significant impacts on climate. The effect of raised supply of CO₂ or NH₄NO₃ on the annual CH₄ efflux from the lawn of a boreal oligotrophic mire was investigated over two years. Ten study plots were enclosed with mini‐FACE rings, five vented with CO₂‐enriched air and the other five with ambient air. In addition, five plots were sprayed with NH₄NO₃ so that the cumulative addition of N was 3 g m^?2 y^?1; and five plots were controls. The CO₂ enrichment (target concentration 560 ppm_v) increased CH₄ efflux about 30–40%, but half of this increase seemed to be caused by the air‐blowing system. The increasing atmospheric concentration of CO₂ would promote CH₄ release in boreal mires, but the increase in CH₄ efflux would be clearly smaller than that reported in studies made in temperate or subtropical temperature conditions. Addition of N enhanced the annual release of CH₄ only slightly. At least over the short‐term, the increase in N deposition would have little effect on CH₄ effluxes. The increase in CH₄ release would probably increase radiative forcing and thus accelerate climate change. However, CH₄ effluxes are only a small part in the whole matter balance in mires and thus further studies are needed to define the net effects of raised supply of CO₂ or N for carbon accumulation, trace gas fluxes and radiative forcing.     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

Ecosystem–atmosphere exchange of CH4 and N2O and ecosystem respiration in wetlands in the Sanjiang Plain, Northeastern China

Natural wetlands are critically important to global change because of their role in modulating atmospheric concentrations of CO₂, CH₄, and N₂O. One 4‐year continuous observation was conducted to examine the exchanges of CH₄ and N₂O between three wetland ecosystems and the atmosphere as well as the ecosystem respiration in the Sanjiang Plain in Northeastern China. From 2002 to 2005, the mean annual budgets of CH₄ and N₂O, and ecosystem respiration were 39.40 ± 6.99 g C m^?2 yr^?1, 0.124 ± 0.05 g N m^?2 yr^?1, and 513.55 ± 8.58 g C m^?2 yr^?1 for permanently inundated wetland; 4.36 ± 1.79 g C m^?2 yr^?1, 0.11 ± 0.12 g N m^?2 yr^?1, and 880.50 ± 71.72 g C m^?2 yr^?1 for seasonally inundated wetland; and 0.21 ± 0.1 g C m^?2 yr^?1, 0.28 ± 0.11 g N m^?2 yr^?1, and 1212.83 ± 191.98 g C m^?2 yr^?1 for shrub swamp. The substantial interannual variation of gas fluxes was due to the significant climatic variability which underscores the importance of long‐term continuous observations. The apparent seasonal pattern of gas emissions associated with a significant relationship of gas fluxes to air temperature implied the potential effect of global warming on greenhouse gas emissions from natural wetlands. The budgets of CH₄ and N₂O fluxes and ecosystem respiration were highly variable among three wetland types, which suggest the uncertainties in previous studies in which all kinds of natural wetlands were treated as one or two functional types. New classification of global natural wetlands in more detailed level is highly expected.     

Responses of net ecosystem CO2 exchange in managed grassland to long-term CO2 enrichment, N fertilization and plant species

The effects of elevated pCO₂ on net ecosystem CO₂ exchange were investigated in managed Lolium perenne (perennial ryegrass) and Trifolium repens (white clover) monocultures that had been exposed continuously to elevated pCO₂ (60 Pa) for nine growing seasons using Free Air CO₂ Enrichment (FACE) technology. Two levels of nitrogen (N) fertilization were applied. Midday net ecosystem CO₂ exchange (mNEE) and night-time ecosystem respiration (NER) were measured in three growing seasons using an open-flow chamber system. The annual net ecosystem carbon (C) input resulting from the net CO₂ fluxes was estimated for one growing season. In both monocultures and at both levels of N supply, elevated pCO₂ stimulated mNEE by up to 32%, the exact amount depending on intercepted PAR. The response of mNEE to elevated pCO₂ was larger than that of harvestable biomass. Elevated pCO₂ increased NER by up to 39% in both species at both levels of N supply. NER, which was affected by mNEE of the preceding day, was higher in T. repens than in L. perenne. High N increased NER compared to low N supply. According to treatment, the annual net ecosystem C input ranged between 210 and 631 g C m⁻² year⁻¹ and was not significantly affected by the level of pCO₂. Low N supply led to a higher net C input than high N supply. We demonstrated that at the ecosystem level, there was a long-term stimulation in the net C assimilation during daytime by elevated pCO₂. However, because NER was also stimulated, net ecosystem C input was not significantly increased at elevated pCO₂. The annual net ecosystem C input was primarily affected by the amount of N supplied.     

Effects of an induced drought on soil carbon dioxide (CO2) efflux and soil CO2 production in an Eastern Amazonian rainforest, Brazil

In the next few decades, climate of the Amazon basin is expected to change, as a result of deforestation and rising temperatures, which may lead to feedback mechanisms in carbon (C) cycling that are presently unknown. Here, we report how a throughfall exclusion (TFE) experiment affected soil carbon dioxide (CO₂) production in a deeply weathered sandy Oxisol of Caxiuanã (Eastern Amazon). Over the course of 2 years, we measured soil CO₂ efflux and soil CO₂ concentrations, soil temperature and moisture in pits down to 3 m depth. Over a period of 2 years, TFE reduced on average soil CO₂ efflux from 4.3±0.1 μmol CO₂ m⁻² s⁻¹ (control) to 3.2±0.1 μmol CO₂ m⁻² s⁻¹ (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO₂ production was higher in the TFE plot (28%) compared with the control plot (17%), and it did not differ between years. We distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO₂ efflux in the TFE plot was related to a reduction of soil and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO₂ production was dominated by a water stress‐induced decrease in total root respiration. Our results contrast to results of a drought experiment on clay Oxisols, which may be related to differences in soil water retention characteristics and depth of rooting zone. These results show that large differences exist in drought sensitivity among Amazonian forest ecosystems, which primarily seem to be affected by the combined effects of texture (affecting water holding capacity) and depth of rooting zone.     

Spring warming and carbon dioxide exchange over low Arctic tundra in central Canada

Tundra‐atmosphere exchanges of carbon dioxide (CO₂) and water vapour were measured near Daring Lake, Northwest Territories in the Canadian Low Arctic for 3 years, 2004–2006. The measurement period spanned late‐winter until the end of the growing period. Mean temperatures during the measurement period varied from about 2 °C less than historical average in 2004 and 2005 to 2 °C greater in 2006. Much of the added warmth in 2006 occurred at the beginning of the study, when snow melt occurred 3 weeks earlier than in the other years. Total precipitation in 2006 (163 mm) was more than double that of the driest year, 2004 (71 mm). The tundra was a net sink for CO₂ carbon in all years. Mid‐summer net ecosystem exchange of CO₂ (NEE) achieved maximum values of ?1.3 g C m^?2 day^?1 (2004) to ?1.8 g C m^?2 day^?1 (2006). Accumulated NEE values over the 109‐day period were ?32,?51 and ?61 g C m^?2 in 2004, 2005 and 2006, respectively. The larger CO₂ uptake in 2006 was attributed to the early spring coupled with warmer air and soil conditions. In 2004, CO₂ uptake was limited by the shorter growing season and mid‐summer dryness, which likely reduced ecosystem productivity. Seasonal total evapotranspiration (ET) ranged from 130 mm (2004) to 181 mm (2006) and varied in accordance with the precipitation received and with the timing of snow melt. Maximum daily ET rates ranged from 2.3 to 2.7 mm day^?1, occurring in mid July. Ecosystem water use efficiency (WUE_eco) varied slightly between years, ranging from 2.2 in the driest year to 2.5 in the year with intermediate rainfall amounts. In the wettest year, increased soil evaporation may have contributed to a lower WUE_eco (2.3). We speculate that most, if not all, of the modest growing season CO₂ sink measured at this site could be lost due to fall and winter respiration leading to the tundra being a net CO₂ source or CO₂ neutral on an annual basis. However, this hypothesis is untested as yet.