Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water,rice biomass and temperature,and its modelling

To attempt to develop physicochemical and physiological modelling for methane transport from the rhizosphere to the atmosphere through rice plants, methane flux, methane concentration in the soil water, and the biomass of rice were measured in lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire growing season in 1992 at Tsukuba, Japan. The addition of exogenous organic matter (rice straw) or soil amendments with the presence or absence of vegetation were also examined for their influence on methane emissions. The total methane emission over the growing season varied from 3.2 g CH₄ m^-2 y^-1 without the addition of rice straw to 49.7 g CH₄ m^-2 y^-1 with rice straw and microbiological amendment. In the unvegetated plot with the addition of rice straw, there was much ebullition of gas bubbles, particularly in the summer. The annual methane emission due to the ebullition of gas bubbles,from the unvegetated plot with the addition of rice straw was estimated to be almost the same as that from the vegetated site with the addition of rice straw. In the early growth stage, the methane flux can be analyzed by the diffusion model (Flux=Methane concentration × Conductance of rice body) using parameters for methane concentration in the soil water as a difference in concentration between the atmosphere and the rhizosphere, and for the biomass of rice as a conductance of rice body. On the other hand, although the diffusion model was inapplicable to a large extent from the middle to late growth stage, methane flux could be estimated by air temperature and concentration in the soil water. Thus, methane transport from the rhizosphere to the atmosphere through rice plants consisted of two phases: one was an explainable small part by diffusion in rice body; the other was a large part strongly, governed by air temperature. The existence of gas bubbles in the soil may be related to the transition between the two phases     

Methane emission by plant communities in an alpine meadow on the Qinghai-Tibetan Plateau: a new experimental study of alpine meadows and oat pasture

Recently, plant-derived methane (CH₄) emission has been questioned because limited evidence of the chemical mechanism has been identified to account for the process. We conducted an experiment with four treatments (i.e. winter-grazed, natural alpine meadow; naturally restored alpine meadow eight years after cultivation; oat pasture and bare soil without roots) during the growing seasons of 2007 and 2008 to examine the question of CH₄ emission by plant communities in the alpine meadow. Each treatment consumed CH₄ in closed, opaque chambers in the field, but two types of alpine meadow vegetation reduced CH₄ consumption compared with bare soil, whereas oat pasture increased consumption. This result could imply that meadow vegetation produces CH₄. However, measurements of soil temperature and water content showed significant differences between vegetated and bare soil and appeared to explain differences in CH₄ production between treatments. Our study strongly suggests that the apparent CH₄ production by vegetation, when compared with bare soil in some previous studies, might represent differences in soil temperature and water-filled pore space and not the true vegetation sources of CH₄.     

Methane oxidation in contrasting soil types: responses to experimental warming with implication for landscape‐integrated CH4 budget

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH₄) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH₄. Here, we present results of in situ CH₄ flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH₄ budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open‐top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH₄‐C m^?2 (8.1 ± 1.2 g CO₂‐eq m^?2) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH₄‐C m^?2 (3.9 ± 0.6 g CO₂‐eq m^?2). These uptake rates were subsequently scaled to the entire study area of 0.15 km², a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH₄‐C m^?2 (3.7 ± 1.2 g CO₂‐eq m^?2). The result was a net landscape sink of 12.71 kg CH₄‐C (0.48 tonne CO₂‐eq) during the growing season. A nonsignificant trend was noticed in seasonal CH₄ uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH₄ fluxes. Overall, the net landscape sink of CH₄ tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH₄ budget of the Arctic due to their potential role in counterbalancing CH₄ emissions from wetlands – not the least when taking the future climatic scenarios of the Arctic into account.     

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.     

Urbanization can accelerate climate change by increasing soil N2O emission while reducing CH4 uptake

Urban land-use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to (1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N₂O) and methane (CH₄) fluxes, (2) quantify direct N₂O emission factors (EF_d) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N₂O emissions by 153%, to 3.0 kg N ha⁻¹ year⁻¹, while rates of soil CH₄ uptake are reduced by 50%, to 2.0 kg C ha⁻¹ year⁻¹. The global mean annual N₂O EF_d of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N₂O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N₂O emission by 0.46 Tg N₂O-N year⁻¹ and decreased soil CH₄ uptake by 0.58 Tg CH₄-C year⁻¹. Urbanization driven changes in soil N₂O emission and CH₄ uptake are associated with changes in soil properties (bulk density, pH, total N content, and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N₂O emissions and reduces the role of soils as a sink for atmospheric CH₄. These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.     

Controls on Soil Carbon Dioxide and Methane Fluxes in a Variety of Taiga Forest Stands in Interior Alaska

CO₂ and CH₄ fluxes were monitored over 4 years in a range of taiga forests along the Tanana River in interior Alaska. Floodplain alder and white spruce sites and upland birch/aspen and white spruce sites were examined. Each site had control, fertilized, and sawdust amended plots; flux measurements began during the second treatment year. CO₂ emissions decreased with successional age across the sites (alder, birch/aspen, and white spruce, in order of succession) regardless of landscape position. Although CO₂ fluxes showed an exponential relationship with soil temperature, the response of CO₂ production to moisture fit an asymptotic model. Of the manipulations, only N fertilization had an effect on CO₂ flux, decreasing flux in the floodplain sites but increasing it in the birch/aspen site. Landscape position was the best predictor of CH₄ flux. The two upland sites consumed CH₄ at similar rates (approximately 0.5 mg C m⁻² d⁻¹), whereas the floodplain sites had lower consumption rates (0–0.3 mg C m⁻² d⁻¹). N fertilization and sawdust both inhibited CH₄ consumption in the upland birch/aspen and floodplain spruce sites but not in the upland spruce site. The biological processes driving CO₂ fluxes were sensitive to temperature, moisture, and vegetation, whereas CH₄ fluxes were sensitive primarily to landscape position and biogeochemical disturbances. Hence, climate change effects on C-gas flux in taiga forest soils will depend on the relationship between soil temperature and moisture and the concomitant changes in soil nutrient pools and cycles. Received 10 March 1998; accepted 29 December 1999.     

A synthesis of methane emissions from shallow vegetated coastal ecosystems

Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cycling, storing 10× more C than temperate forests. Methane (CH₄), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH₄ emissions are a missing component of VCE C budgets. This review summarizes 97 studies describing CH₄ fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH₄ flux in these systems. CH₄ fluxes from these ecosystems were highly variable yet they all act as net methane sources (median, range; mangrove: 279.17, ?67.33 to 72,867.83; salt marsh: 224.44, ?92.60 to 94,129.68; seagrass: 64.80, 1.25–401.50 µmol CH₄ m^?2 day^?1). Together CH₄ emissions from mangrove, salt marsh, and seagrass ecosystems are about 0.33–0.39 Tmol CH₄‐C/year—an addition that increases the current global marine CH₄ budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH₄ fluxes. While organic matter content and quality were commonly reported in individual studies as the most important environmental factors driving CH₄ flux, they were not significant predictors of CH₄ flux when data were combined across studies. Salinity was negatively correlated with CH₄ emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH₄ emissions in vegetated coastal systems. Finally, we examine stressor effects on CH₄ emissions from VCEs and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH₄ emissions from these ecosystems. Overall, this review highlights the current and growing importance of VCEs in the global marine CH₄ budget.     

Warming of subarctic tundra increases emissions of all three important greenhouse gases – carbon dioxide,methane, and nitrous oxide

Rapidly rising temperatures in the Arctic might cause a greater release of greenhouse gases (GHGs) to the atmosphere. To study the effect of warming on GHG dynamics, we deployed open‐top chambers in a subarctic tundra site in Northeast European Russia. We determined carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes as well as the concentration of those gases, inorganic nitrogen (N) and dissolved organic carbon (DOC) along the soil profile. Studied tundra surfaces ranged from mineral to organic soils and from vegetated to unvegetated areas. As a result of air warming, the seasonal GHG budget of the vegetated tundra surfaces shifted from a GHG sink of ?300 to ?198 g CO₂–eq m^?2 to a source of 105 to 144 g CO₂–eq m^?2. At bare peat surfaces, we observed increased release of all three GHGs. While the positive warming response was dominated by CO₂, we provide here the first in situ evidence of increasing N₂O emissions from tundra soils with warming. Warming promoted N₂O release not only from bare peat, previously identified as a strong N₂O source, but also from the abundant, vegetated peat surfaces that do not emit N₂O under present climate. At these surfaces, elevated temperatures had an adverse effect on plant growth, resulting in lower plant N uptake and, consequently, better N availability for soil microbes. Although the warming was limited to the soil surface and did not alter thaw depth, it increased concentrations of DOC, CO_2, and CH₄ in the soil down to the permafrost table. This can be attributed to downward DOC leaching, fueling microbial activity at depth. Taken together, our results emphasize the tight linkages between plant and soil processes, and different soil layers, which need to be taken into account when predicting the climate change feedback of the Arctic.     

Methanogen Communities in a Drained Bog: Effect of Ash Fertilization

Forestry practises such has drainage have been shown to decrease emissions of the greenhouse gas methane (CH₄) from peatlands. The aim of the study was to examine the methanogen populations in a drained bog in northern Finland, and to assess the possible effect of ash fertilization on potential methane production and methanogen communities. Peat samples were collected from control and ash fertilized (15,000 kg/ha) plots 5 years after ash application, and potential CH₄ production was measured. The methanogen community structure was studied by DNA isolation, PCR amplification of the methyl coenzyme-M reductase (mcr) gene, denaturing gradient gel electrophoresis (DGGE), and restriction fragment length polymorphism (RFLP) analysis. The drained peatland showed low potential methane production and methanogen diversity in both control and ash-fertilized plots. Samples from both upper and deeper layers of peat were dominated by three groups of sequences related to Rice cluster-I hydrogenotroph methanogens. Even though pH was marginally greater in the ash-treated site, the occurrence of those sequences was not affected by ash fertilization. Interestingly, a less common group of sequences, related to the Fen cluster, were found only in the fertilized plots. The study confirmed the depth related change of methanogen populations in peatland.     

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH₄ and CO₂ flux from a small shallow pond during a period of 4 months. Mean CH₄ flux and surface water CH₄ concentration were 8.0 [3.3–15.1] ± 3.1 mmol m^?2 day^?1 (mean [range] ± 1 SD) and 1.3 [0.3–3.5] ± 0.9 µM respectively. Mean CO₂ flux was 1.1 [?9.8 to 16.0] ± 6.9 mmol m^?2 day^?1. Substantial diel changes in CO₂ flux and surface water CH₄ concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH₄ emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO₂ flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO₂ consumption by plants was higher relative to CO₂ production by respiration during warm sunny days. Interestingly, CH₄ fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH₄ and CO₂ summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH₄ emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons.     

The role of vegetation in methane flux to the atmosphere: should vegetation be included as a distinct category in the global methane budget?

Currently, the global annual flux of methane (CH₄) to the atmosphere is fairly well constrained at ca. 645 Tg CH₄ year^?1. However, the relative magnitudes of the fluxes generated from different natural (e.g. wetlands, deep seepage, hydrates, ocean sediments) and anthropogenic sources remain poorly resolved. Of the identified natural sources, the contribution of vegetation to the global methane budget is arguably the least well understood. Historically, reviews of the contribution of vegetation to the global methane flux have focused on the role of plants as conduits for soil-borne methane emissions from wetlands, or the aerobic production of methane within plant tissues. Many recent global budgets only include the latter pathway (aerobic methane production) in estimating the importance of terrestrial vegetation to atmospheric CH₄ flux. However, recent experimental evidence suggests several novel pathways through which vegetation can contribute to the flux of this globally important, trace greenhouse gas (GHG), such as plant cisterns that act as cryptic wetlands, heartwood rot in trees, the degradation of coarse woody debris and litter, or methane transport through herbaceous and woody plants. Herein, we synthesize the existing literature to provide a comprehensive estimate of the role of modern vegetation in the global methane budget. This first, albeit uncertain, estimate indicates that vegetation may represent up to 22 % of the annual flux of methane to the atmosphere, contributing ca. 32–143 Tg CH₄ year^?1 to the global flux of this important trace GHG. Overall, our findings emphasize the need to better resolve the role of vegetation in the biogeochemical cycling of methane as an important component of closing the gap in the global methane budget.     

Seasonal variation of methane emissions from a temperate swamp

Methane flux measurements were made at four sites in a freshwater temperate swamp over the 13 month period of April 1985 through May 1986. Emissions were highly variable both between sites and over time at any one site. Ebullition from sediments was an important component of methane release. Although release of methane through bubbling occurred in only 19% of the measurements made between April and June 1985, when instrumentation allowed us to separate diffusive and bubble fluxes, ebullition accounted for 34% of the total flux during this period. Methane release rates showed a strong seasonal variation, with highest emission rates observed in early spring and again in late summer, which was associated with changes in plant growth and physiology. Emission rates were partially correlated with sediment temperature, but the relationship was not straightforward, and resembled a step function. Emissions responded strongly to temperature change through the range of 10–16°C. At winter sediment temperatures between 4–9°C, CH₄ flux continued at low rates (0–28 mg CH₄ m^–2d^–1; average = 7.9 mg CH₄m^–2d^–1) and appeared insensitive to changes in sediment temperature. Annual methane emission from three constantly flooded sites (mean water depth = 35 cm) was 43.7 +/- 7.8 gm^–2 (standard error); annual flux from a bank site was 41.4 +/- 20.5 gm^–2. A comparison of flux measurements from fresh and saline wetlands in the immediate area of Newport News Swamp emphasizes the importance of edaphic factors in controlling flux.     

Emission of methane from chalk streams has potential implications for agricultural practices

1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function. 2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (<2000 μm) dominated by the sand‐sized fraction (63–1000 μm). 3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH₄ transport through Ranunculus stems and an increase in water column CH₄ concentration, while the sediment CH₄ concentration increased 100‐fold between February and April. A marked seasonal enrichment in the δ¹⁵N of N₂ dissolved in the pore water correlated with CH₄ flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin. 4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (>90%) of the methane flux is transported to the atmosphere through the Ranunculus stems. 5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 × 10⁻⁶ Tg CH₄ year⁻¹), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.     

Methane emission from Arctic tundra

Concerns about a possible feedback effect on global warming following possible increased emissions of methane from tundra environments have lead to series of methane flux studies of northern wetland/tundra environments. Most of these studies have been carried out in boreal sub-Arctic regions using different techniques and means of assessing representativeness of the tundra. Here are reported a time series of CH₄ flux measurements from a true Arctic tundra site. A total of 528 independent observations were made at 22 fixed sites during the summers of 1991 and 1992. The data are fully comparable to the most extensive dataset yet produced on methane emissions from sub-Arctic tundra-like environments. Based on the data presented, from a thaw-season with approximately 55% of normal precipitation, a global tundra CH₄ source of 18–30 Tg CH₄ yr⁻¹ is estimated. This is within the range of 42±26 Tg CH₄ yr⁻¹ found in a similar sub-Arctic tundra environment. No single-parameter relationship between one environmental factor and CH₄ flux covering all sites was found. This is also in line with conclusions drawn in the sub-Arctic. However, inter-season variations in CH₄ flux at dry sites were largely controlled by the position of the water table, while flux from wetter sites seemed mainly to be controlled by soil temperature.     

Revising a process-based biogeochemistry model (DNDC) to simulate methane emission from rice paddy fields under various residue management and fertilizer regimes

A comprehensive biogeochemistry model, DNDC, was revised to simulate crop growth and soil processes more explicitly and improve its ability to estimate methane (CH₄) emission from rice paddy fields under a wide range of climatic and agronomic conditions. The revised model simulates rice growth by tracking photosynthesis, respiration, C allocation, tillering, and release of organic C and O₂ from roots. For anaerobic soil processes, it quantifies the production of electron donors [H₂ and dissolved organic carbon (DOC)] by decomposition and rice root exudation, and simulates CH₄ production and other reductive reactions based on the availability of electron donors and acceptors (NO₃^?, Mn⁴⁺, Fe³⁺, and SO₄^2?). Methane emission through rice is simulated by a diffusion routine based on the conductance of tillers and the CH₄ concentration in soil water. The revised DNDC was tested against observations at three rice paddy sites in Japan and China with varying rice residue management and fertilization, and produced estimates consistent with observations for the variation in CH₄ emission as a function of residue management. It also successfully predicted the negative effect of (NH₄)₂SO₄ on CH₄ emission, which the current model missed. Predicted CH₄ emission was highly sensitive to the content of reducible soil Fe³⁺, which is the dominant electron acceptor in anaerobic soils. The revised DNDC generally gave acceptable predictions of seasonal CH₄ emission, but not of daily CH₄ fluxes, suggesting the model's immaturity in describing soil heterogeneity or rice cultivar‐specific characteristics of CH₄ transport. It also overestimated CH₄ emission at one site in a year with low temperatures, suggesting uncertainty in root biomass estimates due to the model's failure to consider the temperature dependence of leaf area development. Nevertheless, the revised DNDC explicitly reflects the effects of soil electron donors and acceptors, and can be used to quantitatively estimate CH₄ emissions from rice fields under a range of conditions.     

Influence of rice cultivar on methane emission from paddy fields

Influence of rice cultivars on CH₄ emissions from a paddy field was studied using four Japonica types, two Indica types, and two Japonica/Indica F₁ hybrids. In addition, the suppression of CH₄ emission by interrupting irrigation at the flowering stage was investigated. Patterns of seasonal variation in CH₄ emission rates were similar among the eight cultivars. Two of the Japonica types showed the maximum and minimum CH₄ emissions among the cultivars investigated. Neither the number of tillers, shoot length, shoot weight, and root weight correlated with the CH₄ emission rates at the tillering and reproductive growth stages. Following temporary interruption of irrigation at the flowering stage, CH₄ emission rates decreased drastically and remained at very low levels until the harvesting stage, indicating its great effectiveness for the suppression of CH₄ emission from rice paddies.     

Elevated concentrations of CO2 may double methane emissions from mires

The potential impact of an increase in methane emissions from natural wetlands on climate change models could be very large. We report a profound increase in methane emissions from cores of mire peat and vegetation as a direct result of increasing the CO₂ concentration from 355 to 550 μol mol^?1 (a 60% increase). Increased CH₄ fluxes were observed throughout the four month period of study. Seasonal variation in CH₄ flux, consistent with that seen in the field, was observed under both ambient and elevated CO₂. Under ambient CO₂, methane fluxes rose from 0.02 μol m_-2 s^?1 in May to 0.11 μol m^?2 s^?3 in July before declining again in August. Under elevated CO₂ methane fluxes were at least 100% greater throughout the experiment, rising from 0.05 μol m_-2 s^?1 in May to a peak of 0.27 μol m^?2 s^?1 in July. The stimulation of CO₄ emissions was accompanied by a 100% increase in rates of photosynthesis from 4.6 (± 0.3) under ambient CO₂ to 9.3 (± 0.7) μol m^?2 s^?1. Root and shoot biomass were unaffected.     

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.     

土壤湿度对中国南部热带森林土壤甲烷吸收的影响

森林土壤是一个重要的大气甲烷的汇。然而,相较于寒带和温带,在热带尤其是东南亚地区,森林土壤甲烷通量的观测较少,这限制了目前对热带森林土壤甲烷通量与环境因子之间关系的认识,也给热带森林土壤甲烷汇的估算带来了一定的不确定性。在中国海南省吊罗山国家森林公园的热带森林土壤,采用激光光谱法测量了2016年9月至2018年9月逐月的土壤甲烷通量,并分析了其与周围环境因子的关系。结果表明：研究区土壤是甲烷的汇,山顶样地的年平均吸收量为0.95 kg CH₄-C hm^-2 a^-1,山脚样地的年平均吸收量为1.93 kg CH₄-C hm^-2 a^-1。干季(11月—次年4月)的甲烷吸收通量明显高于湿季(5—10月),占到全年甲烷吸收的68%。山顶样地年平均土壤湿度为19.2%,年内的波动较小(2.8%)。而山脚样地的年平均湿度相对较低,为12.7%,且年内波动大(5.4%)。土壤湿度是控制甲烷吸收最主要的环境因子,可以解释月际甲烷吸收变化的76%,甲烷吸收通量与土壤温度的相...     

Seasonal variations of methane fluxes from an unvegetated tidal freshwater mudflat (Hammersmith Creek, GA)

Wetlands are estimated to contribute nearly 40 % of global annual methane (CH₄) emissions to the atmosphere. However, because CH₄ fluxes from these systems vary spatially, seasonally, and by wetland type, there is a large uncertainty associated with scaling up the CH₄ flux from these environments. We monitored seasonal patterns of CH₄ cycling from tidal mudflat wetland sediments adjacent to a vegetated freshwater wetland in coastal Georgia between 2008 and 2009. CH₄ emissions were significantly correlated with CH₄ production and sediment saturation state with respect to CH₄ but not with temperature. CH₄ cycling displayed distinct seasonal patterns. Winter months were characterized by low CH₄ production and emissions. During the spring, summer and fall, CH₄ fluxes exceeded CH₄ production in the top 40 cm. Comparison of CH₄ sources and sinks in conjunction with the interpretation of CH₄ concentration profiles using a 1D reactive transport model indicated that CH₄ delivered via lateral tidal pumping likely provided additional CH₄ to the upper sediment column. Seasonally high CH₄ ebullition rates reflected increased CH₄ production and decreased CH₄ solubility. The annual CH₄ flux was estimated to be on the order of 10 mol CH₄ m^?2 y^?1 which is 2–4 times the global average for wetland CH₄ emissions. Thus, even though tidal freshwater mudflats are of limited spatial extent, these environments may serve as globally significant sources of CH₄ to the atmosphere. This study highlights the importance of these dynamic environments to the global CH₄ cycle and their relevance to climate change.