Surrounding pressure controlled by water table alters CO2 and CH4 fluxes in the littoral zone of a brackish-water lake

We studied the effect of water table on CO₂ and CH₄ fluxes at different time scales in the littoral zone of Lake Obuchi, a brackish lake in northern Japan. The vegetation formed three distinct zones along the water table gradient, two dominated by emergent aquatic macrophytes (the Phragmites australis-dominated zone and the Juncus yokoscensis-dominated zone) and one dominated by terrestrial macrophytes (Miscanthus sinensis and Cirsium inundatum-dominated zone). To clarify the impact of variations in water table on monthly and yearly summed CO₂ and CH₄ fluxes, we examined the relationship between water table and the ratio of observed flux to calculated flux, whereby the calculated flux was based solely on the exponential relationship between flux and soil temperature for each gas. This study revealed that the impact of variations in water table on monthly and yearly summed CO₂ and CH₄ fluxes differed markedly between the vegetation zones. By taking the temporal change in water table into account in the estimation of both the CO₂ and CH₄ fluxes, the monthly summed CO₂ and CH₄ fluxes in the Phragmites-zone were markedly greater in every month of the year compared to estimation based on temperature alone. In the Juncus-zone, the effect of water table on monthly summed CO₂ and CH₄ fluxes differed between months. In addition, the magnitude of water-table effects controlling monthly summed CO₂ and CH₄ fluxes differed with atmospheric conditions, i.e., between the pressure-falling and low-pressure phase on the one hand and other pressure phases on the other hand. After weighting all the impacts of temporal changes in water table on fluxes, the yearly summed CO₂ and CH₄ fluxes showed a 1.26–6.64-fold increase compared with not taking water table effects into account, and the increase differed among the three vegetation zones.     

Emissions of CO2, CH4 and N2O from Southern European peatlands

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

Heterogeneity of O2 dynamics in soil amended with animal manure and implications for greenhouse gas emissions

Soil oxygen (O₂) availability influences nitrification and denitrification, the major biological processes responsible for nitrous oxide (N₂O) production and emissions from soil. In this study O₂-specific planar optodes were used to visualise O₂ distribution with high spatial and temporal resolution in soils in which the same amount of solid fraction of pig manure had been distributed in three different ways (mixed, layered, single patch) and which were maintained at a water potential of −5 kPa (corresponding to 91% of water-filled pore space). In parallel, the greenhouse gas emissions (N₂O, CO₂ and CH₄) from soil at high temporal resolution were monitored. At the end of the incubations, vertical profiles of mineral nitrogen (ammonium and nitrate) in the soil matrix were quantified. The optode results revealed that anoxia rapidly developed in zones with manure addition and gradually expanded to the entire soil during the 66-h experimental period. The anoxia in the soil developed more quickly as the heterogeneity of manure distribution decreased (from single patch to layered to mixed). The single patch distribution of manure solids delayed peak emission rates of both N₂O and CO₂, but stimulated the cumulative N₂O emissions and reduced the cumulative CO₂ fluxes. The faster the anoxia developed, the less the nitrification process appeared to contribute to N₂O emissions. No treatment effects on CH₄ emissions were observed. Combined high resolution imaging of O₂ dynamics and measurements of N₂O emission rates are essential to get a detailed understanding of how O₂ availability regulates the distribution and coupling of denitrification and nitrification activity in soil. Such unique information on soil O₂ dynamics could be used for further modelling and quantification of processes producing greenhouse gases from soil ecosystems.     

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

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

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

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

Elevated atmospheric CO2 reduces CH4 and N2O emissions under two contrasting rice cultivars from a subtropical paddy field in China

Elevated CO₂(eCO₂) and rice cultivars can strongly alter CH₄ and N₂ O emissions from paddy fields.However,detailed information on how their interaction affects greenhouse gas fluxes in the field is still lacking.In this study,we investigated CH4 and N₂ O emissions and rice growth under two contrasting rice cultivars(the strongly and weakly responsive cultivars) in response to eCO₂,200 μmol mol^-1 higher than the ambient ...     

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

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

水分类型对土壤排放的温室气体组成和综合温室效应的影响

实验室研究表明,土壤排放出的温室气体（ＣＯ２、ＣＨ４和Ｎ２Ｏ）组成及总理显著地受土壤水分类型和施用秸秆的影响。连续淹水条件下,土壤仅排放微理的Ｎ２Ｏ,但排放出大量的Ｃ睡Ｃ敢条件下,土壤不排放Ｃ上键合的但排放出大量的Ｎ２Ｏ;虽然淹水的土壤排水促进Ｎ２Ｏ排放,但显著抑制ＣＨ４的排放,淹水好气交替处理的土壤其排放的ＣＯ２、ＣＨ４和Ｎ２Ｏ均在好气和连续淹水之间。根据各种温室产生温室效应的相对潜力,计算土壤     

Fluxes of CO2, CH4 and N2O from drained organic soils in deciduous forests

We examined net greenhouse gas exchange at the soil surface in deciduous forests on soils with high organic contents. Fluxes of CO₂, CH₄ and N₂O were measured using dark static chambers for two consecutive years in three different forest types; (i) a drained and medium productivity site dominated by birch, (ii) a drained and highly productive site dominated by alder and (iii) an undrained and highly productive site dominated by alder. Although the drained sites had shallow mean groundwater tables (15 and 18 cm, respectively) their average annual rates of forest floor CO₂ release were almost twice as high compared to the undrained site (1.9±0.4 and 1.7±0.3, compared to 1.0±0.2 kg CO₂ m⁻² yr⁻¹). The average annual CH₄ emission was almost 10 times larger at the undrained site (7.6±3.1 compared to 0.9±0.5 g CH₄ m⁻² yr⁻¹ for the two drained sites). The average annual N₂O emissions at the undrained site (0.1±0.05 g N₂O m⁻² yr⁻¹) were lower than at the drained sites, and the emissions were almost five times higher at the drained alder site than at the drained birch site (0.9±0.35 compared to 0.2±0.11 g N₂O m⁻² yr⁻¹). The temporal variation in forest floor CO₂ release could be explained to a large extent by differences in groundwater table and air temperature, but little of the variation in the CH₄ and N₂O fluxes could be explained by these variables. The measured soil variables were only significant to explain for the within-site spatial variation in CH₄ and N₂O fluxes at the undrained swamp, and dark forest floor CO₂ release was not explained by these variables at any site. The between-site spatial variation was attributed to variations in drainage, groundwater level position, productivity and tree species for all three gases. The results indicate that N₂O emissions are of greater importance for the net greenhouse gas exchange at deciduous drained forest sites than at coniferous drained forest sites.     

Effects of soil moisture and temperature on CO2 and CH4 soil-atmosphere exchange of various land use/cover types in a semi-arid grassland in Inner Mongolia, China

The aim of this study was to investigate the combined effects of soil moisture and temperature as well as drying/re-wetting and freezing/thawing on soil-atmosphere exchange of CO₂ and CH₄ of the four dominant land use/cover types (typical steppe, TS; sand dune, SD; mountain meadow, MM; marshland, ML) in the Xilin River catchment, China. For this purpose, intact soil cores were incubated in the laboratory under varying soil moisture and temperature levels according to field conditions in the Xilin River catchment. CO₂ and CH₄ fluxes were determined approximately daily, while soil CH₄ gas profile measurements at four soil depths (5 cm, 10 cm, 20 cm and 30 cm) were measured at least twice per week. Land use/cover generally had a substantial influence on CO₂ and CH₄ fluxes, with the order of CH₄ uptake and CO₂ emission rates of the different land use/cover types being TS ≥ MM ≥ SD > ML and MM > TS ≥ SD > ML, respectively. Significant negative soil moisture and positive temperature effects on CH₄ uptake were found for most soils, except for ML soils. As for CO₂ flux, both significant positive soil moisture and temperature effects were observed for all the soils. The combination of soil moisture and temperature could explain a large part of the variation in CO₂ (up to 87%) and CH₄ (up to 68%) fluxes for most soils. Drying/re-wetting showed a pronounced stimulation of CO₂ emissions for all the soils —with maximum fluxes of 28.4 ± 2.6, 50.0 ± 5.7, 81.9 ± 2.7 and 10.6 ± 1.2 mg C m⁻² h⁻¹ for TS, SD, MM and ML soils, respectively—but had a negligible effect on CH₄ fluxes (TS: −3.6 ± 0.2; SD: 1.0 ± 0.9; MM: −4.1 ± 1.3; ML: −5.6 ± 0.8; all fluxes in μg C m⁻² h⁻¹). Enhanced CO₂ emission and CH₄ oxidation were observed for all soils during thawing periods. In addition, a very distinct vertical gradient of soil air CH₄ concentrations was observed for all land use/cover types, with gradually decreasing CH₄ concentrations down to 30 cm soil depth. The changes in soil air CH₄ concentration gradients were in accordance with the changes of CH₄ fluxes during the entire incubation experiment for all soils.     

Net fluxes of CO2, but not N2O or CH4, are affected following agronomic-scale additions of urea to prairie and arable soils

While experimental addition of nitrogen (N) tends to enhance soil fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), it is not known if lower and agronomic-scale additions of urea-N applied also enhance trace gas fluxes, particularly for semi-arid agricultural lands in the northern plains. We aimed to test if this were true at agronomic rates [low (11 kg N ha⁻¹), moderate (56 kg N ha⁻¹), and high (112 kg N ha⁻¹)] for central North Dakota arable and prairie soils using intact soil cores to minimize disturbance and simulate field conditions. Additions of urea to cores incubated at 21 °C and 57% water-filled pore space enhanced fluxes of CO₂ but not CH₄ and N₂O. At low, moderate, and high urea-N, CO₂ fluxes were significantly greater than control but not fluxes of CH₄ and N₂O. The increases in CO₂ emission with rate of urea-N application indicate that agronomic-scale N inputs may stimulate microbial carbon cycling in these soils, and that the contribution of CO₂ to net greenhouse gas source strength following fertilization of semi-arid agroecosystems may at times be greater than contributions by N₂O and CH₄.     

意大利中部地区粘土稻田干湿交替减少CH4排放但诱发N2O排放高峰

Reducing CH₄ and N₂O emissions from rice cropping systems while sustaining production levels with less water requires a better understanding of the key processes involved. Alternate wetting and drying (AWD) irrigation is one promising practice that has been shown to reduce CH₄ emissions. However, little is known about the impact of this practice on N₂O emissions, in particular under Mediterranean climate. To close this knowledge gap, we assessed how AWD influenced grain yield, fluxes and annual budgets of CH₄ and N₂O emissions, and global warming potential (GWP) in Italian rice systems over a 2-year period. Overall, a larger GWP was observed under AWD, as a result of high N₂O emissions which offset reductions in CH₄ emissions. In the first year, with 70% water reduction, the yields were reduced by 33%, CH₄ emissions decreased by 97%,while N₂O emissions increased by more than 5-foldunder AWD as compared to PF;in the second year, with a 40% watersaving,the reductions of rice yields and CH₄ emissions (13% and 11%, respectively) were not significant, but N₂O fluxes more than doubled. The transition from anaerobic to aerobic soil conditions resulted in the highest N₂O fluxes under AWD. The duration of flooding, transition to aerobic conditions, water levelabove the soil surface, and the relative timing between fertilization and flooding were the main drivers affecting greenhouse gas mitigation potential under AWD and should be carefully planned through site-specific management options.     

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

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

Spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils during a relatively dry season in Indonesia

We investigated spatial structures of N₂O, CO₂, and CH₄ fluxes during a relatively dry season in an Acacia mangium plantation stand in Sumatra, Indonesia. The fluxes and soil properties were measured at 1-m intervals in a 1 × 30-m plot (62 grid points) and at 10-m intervals in a 40 × 100-m plot (55 grid points) at different topographical positions of the upper plateau, slope, and valley bottom in the plantation. Spatial structures of each gas flux and soil property were identified using geostatistical analysis. The means (±SD) of N₂O, CO₂, and CH₄ fluxes in the 10-m grids were 0.54 (±0.33) mg N m⁻² d⁻¹, 2.81 (±0.71) g C m⁻² d⁻¹, and −0.84 (±0.33) mg C m⁻² d⁻¹, respectively. This suggests that A. mangium soils function as a larger source of N₂O than natural forest soils in the adjacent province on Sumatra during the relatively dry season, while CO₂ and CH₄ emissions from the A. mangium soils were less than or consistent with those in the natural forest soils. Multiple spatial dependence of N₂O fluxes within 3.2 m (1-m grids) and 35.0 m (10-m grids), and CO₂ fluxes within 1.8 m (1-m grids) and over 65 m (10-m grids) was detected. From the relationship among N₂O and CO₂ gas fluxes, soil properties, and topographic elements, we suggest that the multiple spatial structures of N₂O and CO₂ fluxes are mainly associated with soil resources such as readily mineralizable carbon and nitrogen in a relatively dry season. The soil resource distributions were probably controlled by the meso- and microtopography. Meanwhile, CH₄ fluxes were spatially independent in the A. mangium soils, and the water-filled pore space appeared to mainly control the spatial distribution of these fluxes.     

Effects of elevated CO2 and O3 on soil respiration under ponderosa pine

Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system. The study used a 2×2 factorial design with two levels of CO₂ and two levels of O₃ and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO₂ and O₃, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO₂ will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O₃ will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO₂ and O₃ have opposing effects on C assimilation and allocation, elevated CO₂ will eliminate or reduce the negative effects of elevated O₃ on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO₂ and O₃ on soil respiration. The covariance analysis showed that elevated CO₂ significantly reduced the soil respiration while elevated O₃ had no significant effect. Despite the lack of a direct CO₂ stimulation of soil respiration, there were significant interactions between CO₂ and soil temperature, soil moisture and days from planting indicating that elevated CO₂ altered soil respiration indirectly. In elevated CO₂, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO₂. Soil respiration increased more with days from planting in elevated than in ambient CO₂. Elevated CO₂ had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO₂ had no significant effect on microarthropod and nematode abundance. Elevated O₃ had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.     

Methane emissions from different vegetation zones in a Qinghai-Tibetan Plateau wetland

We measured methane (CH₄) emissions in the Luanhaizi wetland, a typical alpine wetland on the Qinghai-Tibetan Plateau, China, during the plant growth season (early July to mid-September) in 2002. Our aim was to quantify the spatial and temporal variation of CH₄ flux and to elucidate key factors in this variation. Static chamber measurements of CH₄ flux were made in four vegetation zones along a gradient of water depth. There were three emergent-plant zones (Hippuris-dominated; Scirpus-dominated; and Carex-dominated) and one submerged-plant zone (Potamogeton-dominated). The smallest CH₄ flux (seasonal mean=33.1 mg CH₄ m⁻² d⁻¹) was observed in the Potamogeton-dominated zone, which occupied about 74% of the total area of the wetland. The greatest CH₄ flux (seasonal mean=214 mg CH₄ m⁻² d⁻¹) was observed in the Hippuris-dominated zone, in the second-deepest water area. CH₄ flux from three zones (excluding the Carex-dominated zone) showed a marked diurnal change and decreased dramatically under dark conditions. Light intensity had a major influence on the temporal variation in CH₄ flux, at least in three of the zones. Methane fluxes from all zones increased during the growing season with increasing aboveground biomass. CH₄ flux from the Scirpus-dominated zone was significantly lower than in the other emergent-plant zones despite the large biomass, because the root and rhizome intake ports for CH₄ transport in the dominant species were distributed in shallower and more oxidative soil than occupied in the other zones. Spatial and temporal variation in CH₄ flux from the alpine wetland was determined by the vegetation zone. Among the dominant species in each zone, there were variations in the density and biomass of shoots, gas-transport system, and root-rhizome architecture. The CH₄ flux from a typical alpine wetland on the Qinghai-Tibetan Plateau was as high as those of other boreal and alpine wetlands.     

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

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

The inhibitory effect of difluoromethane on CH4 oxidation in reconstructed peat columns and side-effects on CO2 and N2O emissions at two water levels

Methane emissions from soils are the net result of two processes: methane (CH₄) production and CH₄ oxidation. In order to understand how both processes respond to environmental changes, it is necessary to distinguish between CH₄ production and oxidation. In bacterial cultures and small soil samples, difluoromethane (CH₂F₂) was found to inhibit CH₄ oxidation reversibly, without affecting CH₄ production. Hence, CH₂F₂ allows the study of CH₄ production directly and of CH₄ oxidation indirectly. To our knowledge, however, the inhibitory effect of CH₂F₂ within soil columns has not yet been evaluated. We therefore tested which CH₂F₂ concentration is needed for complete inhibition of CH₄ oxidation in reconstructed 28 cm high peat soil columns under different water levels (WL). We found that soil columns require considerably higher headspace CH₂F₂ concentrations for complete inhibition of CH₄ oxidation than small soil samples. Inhibition remained complete until ca. 24 h after CH₂F₂ exposure. Then, the inhibitory effect diminished. The time needed for the inhibitory effect to disappear depended on WL; at a low WL of −15 cm, the inhibitory effect declined slowly and oxidation rates recovered by 90% only after 12 days. At WL = −5 cm, CH₄ oxidation recovered much faster (90% recovery after ca. 3 days). Last, CH₂F₂ addition significantly decreased the N₂O emissions, whereas CO₂ emissions remained unaltered.     

The dynamics of N2O near the groundwater table and the transfer of N2O into the unsaturated zone: A case study from a sandy aquifer in Germany

The research area was the Fuhrberger Feld aquifer (FFA) in northern Germany. It is situated about 30km northeast of the city of Hannover and covers about 300km². Six multilevel sampling wells along a representative strip under predominantly arable land along a groundwater flow-line were sampled from the groundwater table down to a depth of 10m below the soil surface. We measured N₂O, CO₂, NO₃⁻, SO₄²⁻, DOC, pH, redox potentials and O₂ concentrations.N₂O accumulated at four out of six wells close to the groundwater table. About 20% of N₂O that occurred between the groundwater table and 7–8m below it resided in the top 0.4m. An exchange zone for N₂O at the interface between the saturated and the unsaturated zone extended 0.55 ± 0.22m below the groundwater table and acted as a source and sink for N₂O. N₂O below the exchange zone cannot be transferred from the groundwater to the atmosphere. The upward fluxes from the exchange zone into the unsaturated zone at the six wells ranged between 0.0009 and 0.3kg N₂O ha^{− 1} year^{− 1}. The yearly downward fluxes into the exchange zones had about the same order of magnitude as the upward fluxes. The upward and downward fluxes of N₂O at the (fluctuating) water table did cancel out each other, but this does not yet imply, that the N₂O fluxes at the soil surface also cancel out each other.N₂O–N:NO₃–N ratios were highly variable ranging from 0.0002 to 0.0417.A multiple regression for the monthly N₂O amounts in the exchange zone could explain 66% of the yearly variation. The significant variables were NO₃⁻, CO₂, pH, and O₂. Therefore, a combination of the land use (NO₃⁻), the geochemical boundary conditions (pH) and the type of denitrification reaction (O₂ and CO₂ indicate the importance of a heterotrophic denitrification process) governed the N₂O dynamics in the surface groundwater of the FFA and its transfer into the unsaturated zone.     

The role of hydropedologic vegetation zones in greenhouse gas emissions for agricultural wetland landscapes

Net greenhouse gas (GHG) source strength for agricultural wetland ecosystems in the Prairie Pothole Region (PPR) is currently unknown. In particular, information is lacking to constrain spatial variability associated with GHG emissions (CH₄, CO₂, and N₂O). GHG fluxes typically vary with edaphic, hydrologic, biologic, and climatic factors. In the PPR, characteristic wetland plant communities integrate hydropedologic factors and may explain some variability associated with trace gas fluxes at ecosystem and landscape scales. We addressed this question for replicate wetland basins located in central North Dakota stratified by hydropedologic vegetation zone on Jul 12 and Aug 3, 2003. Data were collected at the soil-atmosphere interface for six plant zones: deep marsh, shallow marsh, wet meadow, low prairie, pasture, and cropland. Controlling for soil moisture and temperature, CH₄ fluxes varied significantly with zone (p < 0.05). Highest CH₄ emissions were found near the water in the deep marsh (277,800 μg m^− 2 d^− 1 CH₄), which declined with distance from water to − 730 μg m^− 2 d^− 1 CH₄ in the pasture. Carbon dioxide fluxes also varied significantly with zone. Nitrous oxide variability was greater within zones than between zones, with no significant effects of zone, moisture, or temperature. Data were extrapolated for a 205.6 km² landscape using a previously developed synoptic classification for PPR plant communities. For this landscape, we found croplands contributed the greatest proportion to the net GHG source strength on Jul 12 (45,700 kg d^− 1 GHG-C equivalents) while deep marsh zones contributed the greatest proportion on Aug 3 (26,145 kg d^− 1 GHG-C equivalents). This was driven by a 30-fold reduction in cropland N₂O–N emissions between dates. The overall landscape average for each date, weighted by zone, was 462.4 kg km^− 2 d^− 1 GHG-C equivalents on Jul 12 and 314.3 kg km^− 2 d^− 1 GHG-C equivalents on Aug 3. Results suggest GHG fluxes vary with hydropedologic soil zone, particularly for CH₄, and provide initial estimates of net GHG emissions for heterogeneous agricultural wetland landscapes.