Assessing the effects of chamber placement, manual sampling and headspace mixing on CH4 fluxes in a laboratory experiment

A laboratory experiment was conducted with two types of closed static chambers to estimate the effects of chamber placement, manual headspace sampling and headspace mixing on methane (CH₄) fluxes. Chamber fluxes were compared to a known reference flux in a chamber calibration system. The measurements were conducted with three types of soils (coarse dry, fine dry and fine wet quarts sand) at five flux levels ranging from 60 to 2000 ??g CH₄ m^?2 h^?1. We found that the placement of a non-vented chamber disturbed the initial CH₄ concentration development within the chamber headspace for 10 to 30 s. Excluding this short period from the flux calculation resulted in a lower flux estimate (mean±SE) of 126?±?26 ??g CH₄ m^?2 h^?1 compared to 134?±?26 ??g CH₄ m^?2 h^?1 if data from time zero of the enclosure were included. We also found that in non-mixed chambers (no fan mixing) the gas sampling by syringes or gas bottles disturbed the development of CH₄ concentration during the enclosure. Furthermore, flux estimates in non-mixed chambers were significantly underestimated (on average 36%) compared to the measured reference fluxes. However, the use of fans to constantly mix the chamber headspace during enclosure significantly improved the goodness-of-fit of the regression analysis used to calculate the flux and further eliminated the disturbance of the manual sampling on the concentration development. We recommend that chambers should be vented during the placement of the chamber, and that fans are used as an integrated part of static chambers while headspace mixing with syringes should be avoided.     

A cost-effective method for reducing soil disturbance-induced errors in static chamber measurement of wetland methane emissions

Static chambers used for sampling methane (CH₄) in wetlands are highly sensitive to soil disturbance. Temporary compression around chambers during sampling can inflate the initial chamber CH₄ headspace concentration and/or lead to generation of non-linear, unreliable flux estimates that must be discarded. In this study, we tested an often-used rubber gasket (RG)-sealed static chamber against a water-filled gutter (WFG) seal design that could be set up and sampled from a distance of 2 m with a newly designed remote rod sampling system to reduce soil disturbance. Compared to conventional RG design, our remotely sampled static chambers reduced the chance of detecting inflated initial CH₄ concentrations (>3.6 ppm) from 66 to 6 % and nearly doubled the proportion of robust linear regressions (r ² > 0.9) from 45 to 86 %. Importantly, the remote rod sampling system allows for more accurate and reliable CH₄ sampling without costly boardwalk construction. This paper presents results demonstrating that the remote rod sampling system combined with WFG static chambers improves CH₄ data reliability by reducing initial gas measurement variability due to chamber disturbance when tested on a mineral soil-restored wetland in Charles City County, Virginia, USA.     

Impact of Grazing Intensity and Seasons on Greenhouse Gas Emissions in Tropical Grassland

Greenhouse gases (GHG) can be affected by grazing intensity, soil, and climate variables. This study aimed at assessing GHG emissions from a tropical pasture of Brazil to evaluate (i) how the grazing intensity affects the magnitude of GHG emissions; (ii) how season influences GHG production and consumption; and (iii) what are the key driving variables associated with GHG emissions. We measured under field conditions, during two years in a palisade-grass pasture managed with 3 grazing intensities: heavy (15 cm height), moderate (25 cm height), and light (35 cm height) N₂O, CH₄ and CO₂ fluxes using static closed chambers and chromatographic quantification. The greater emissions occurred in the summer and the lower in the winter. N₂O, CH₄, and CO₂ fluxes varied according to the season and were correlated with pasture grazing intensity, temperature, precipitation, % WFPS (water-filled pores space), and soil inorganic N. The explanatory variables differ according to the gas and season. Grazing intensity had a negative linear effect on annual cumulative N₂O emissions and a positive linear effect on annual cumulative CO₂ emissions. Grazing intensity, season, and year affected N₂O, CH₄, and CO₂ emissions. Tropical grassland can be a large sink of N₂O and CH₄. GHG emissions were explained for different key driving variables according to the season.     

Effects of organic and mineral fertilizer nitrogen on greenhouse gas emissions and plant-captured carbon under maize cropping in Zimbabwe

Optimizing a three-way pact comprising crop yields, fertility inputs and greenhouse gases may minimize the contribution of croplands to global warming. Fluxes of N₂O, CO₂ and CH₄ from soil were measured under maize (Zea mays L.) grown using 0, 60 and 120 kg N hm^-2 as NH₄NO₃-N and composted manure-N in three seasons on clay (Chromic luvisol) and sandy loam (Haplic lixisol) soils in Zimbabwe. The fluxes were measured using the static chamber methodology involving gas chromatography for ample air analysis. Over an average of 122 days we estimated emissions of 0.1 to 0.5 kg N₂O-N hm^?2, 711 to 1574 kg CO₂-C hm^?2 and?2.6 to 5.8 kg CH₄-C hm^?2 from six treatments during season II with the highest fluxes. The posed hypothesis that composted manure-N may be better placed as a mitigation option against soil emissions of GHG than mineral fertilizer-N was largely supported by N₂O fluxes during the wet period of the year, but with high level of uncertainty. Nitrogen addition might have stimulated both emissions and consumption of CH₄ but the sink or source strength depended highly on soil water content. We concluded that the application of mineral-N and manure input may play an important role with reference to global warming provided the season can support substantial crop productivity that may reduce the amount of N₂O loss per unit yield. Confidence in fluxes response to agricultural management is still low due to sporadic measurements and limited observations from the southern African region.     

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.     

Methane and carbon dioxide dynamics within four vernal pools in Maine,USA

Vernal pools are small, seasonal wetlands that are a common landscape feature contributing to biodiversity in northeastern North American forests. Basic information about their biogeochemical functions, such as carbon cycling, is limited. Concentrations of dissolved methane (CH₄) and carbon dioxide (CO₂) and other water chemistry parameters were monitored weekly at the bottom and surface of four vernal pools in central and eastern Maine, USA, from April to August 2016. The vernal pools were supersaturated with respect to CH₄ and CO₂ at all sampling dates and locations. Concentrations of dissolved CH₄ and CO₂ ranged from 0.4 to 210 μmol L^?1 and 72–2300 μmol L^?1, respectively. Diffusive fluxes of CH₄ and CO₂ into the atmosphere ranged from 0.2 to 73 mmol m^?2 d^?1, and 30–590 mmol m^?2 d^?1, respectively. During the study period, the four vernal pools emitted 0.1–5.8 kg C m^?2 and 9.6–120 kg C m^?2 as CH₄ and CO₂, respectively. The production fluxes (production rates normalized to surface area) of CH₄ and CO₂ ranged from ? 0.02 to 0.66 and 0.40–4.6 g C m^?2 d^?1, respectively, and increased significantly over the season. Methane concentrations were best predicted by alkalinity, ortho-phosphate and depth, while CO₂ concentrations were best predicted with only alkalinity. Alkalinity as a predictor variable highlights the importance of anaerobic respiration in production of both gases. Our study pools had large concentrations and effluxes of CH₄ and CO₂ compared to permanently inundated wetlands, indicating vernal pools are metabolically active sites and may be important contributors to the global carbon budget.     

The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

Ephemeral streams and wetlands are characterized by complex cycles of submersion and emersion, which influence the greenhouse gas flux rates. In this study we quantify the spatiotemporal variability in CO₂ and CH₄ concentrations and fluxes of an intermittent first-order stream over three consecutive wet and dry cycles spanning 56 days, to assess how hydrologic phase transitions influence greenhouse gas evasion. Water column excess CO₂ ranged from ?11 to 1600 μM, and excess CH₄ from 1 to 15 μM. After accounting for temporal changes in the ratio of wet versus dry streambed hydraulic radius, total CO₂–C fluxes ranged from 12 to 156 mmol m^?2 day^?1, with an integrated daily mean of 61 ± 25 mmol m^?2 day^?1. Soil–air evasion rates were approximately equal to those of water–air evasion. Rainfall increased background water–air CO₂–C fluxes by up to 780% due to an increase in gas transfer velocity in the otherwise still waters. CH₄–C fluxes increased 19-fold over the duration of the initial, longer wet-cycle from 0.1 to 1.9 mmol m^?2 day^?1. Temporal shifts in water depth and site-specific ephemerality were key drivers of carbon dynamics in the upper Jamison Creek watercourse. Based on these findings, we hypothesise that the cyclic periodicity of fluxes of biogenic gases from frequently intermittent streams (wet and dry cycles ranging from days to weeks) and seasonally ephemeral watercourses (dry for months at a time) are likely to differ, and therefore these differences should be considered when integrating transient systems into regional carbon budgets and models of global change.     

Annual emissions of greenhouse gases from sheepfolds in Inner Mongolia

Sheepfolds represent significant hot spot sources of greenhouse gases (GHG) in semi-arid grassland regions, such as Inner Mongolia in China. However, the annual contribution of sheepfolds to regional GHG emissions is still unknown. In order to quantify its annual contribution, we conducted measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes at two sheepfold sites in the Baiyinxile administrative region of Inner Mongolia for 1 year, using static opaque chamber and gas chromatography methods. Our data show that, at an annual scale, both sheepfolds functioned as net sources of CO₂, CH₄ and N₂O. Temperatures primarily determined the seasonal pattern of CO₂ emission; 60–84% of the CO₂ flux variation could be explained by temperature changes. High rates of net CH₄ emissions from sheepfold soils were only observed when animals (sheep and goats) were present. While nitrous oxide emissions were also stimulated by the presence of animals, pulses of N₂O emissions were also be related to rainfall and spring-thaw events. The total annual cumulative GHG emissions in CO₂ equivalents (CO₂: 1; CH₄: 25; and N₂O: 298) were quantified as 87.4?±?18.4 t ha^?1 for the sheepfold that was used during the non-grazing period (i.e., winter sheepfold) and 136.7?±?15.9 t ha^?1 used during the grazing period (i.e., summer sheepfold). Of the annual total GHG emissions, CH₄ release accounted for approximately 1% of emissions, while CO₂ and N₂O emissions contributed to approximately 59% and 40%, respectively. The total GHG emission factor (CO₂?+?CH₄?+?N₂O) per animal for the sheepfolds investigated in this study was 30.3 kg CO₂ eq yr^?1 head^?1, which translates to 0.3, 18.8 and 11.2 kg CO₂ eq yr^?1 head^?1 for CH₄, CO₂ and N₂O, respectively. Sheepfolds accounted for approximately 34% of overall N₂O emissions in the Baiyinxile administrative region, a typical steppe region within Inner Mongolia. The contribution of sheepfolds to the regional CO₂ or CH₄ exchange is marginal.     

Release of CO2 and CH4 from lakes and drainage ditches in temperate wetlands

Shallow fresh water bodies in peat areas are important contributors to greenhouse gas fluxes to the atmosphere. In this study we determined the magnitude of CH₄ and CO₂ fluxes from 12 water bodies in Dutch wetlands during the summer season and studied the factors that might regulate emissions of CH₄ and CO₂ from these lakes and ditches. The lakes and ditches acted as CO₂ and CH₄ sources of emissions to the atmosphere; the fluxes from the ditches were significantly larger than the fluxes from the lakes. The mean greenhouse gas flux from ditches and lakes amounted to 129.1 ± 8.2 (mean ± SE) and 61.5 ± 7.1 mg m^?2 h^?1 for CO₂ and 33.7 ± 9.3 and 3.9 ± 1.6 mg m^?2 h^?1 for CH₄, respectively. In most water bodies CH₄ was the dominant greenhouse gas in terms of warming potential. Trophic status of the water and the sediment was an important factor regulating emissions. By using multiple linear regression 87% of the variation in CH₄ could be explained by PO₄ ^3? concentration in the sediment and Fe²⁺ concentration in the water, and 89% of the CO₂ flux could be explained by depth, EC and pH of the water. Decreasing the nutrient loads and input of organic substrates to ditches and lakes by for example reducing application of fertilizers and manure within the catchments and decreasing upward seepage of nutrient rich water from the surrounding area will likely reduce summer emissions of CO₂ and CH₄ from these water bodies.     

华南地区尾巨桉和马占相思人工林地表温室气体通量

为探索华南地区尾巨桉人工林和马占相思人工林地表温室气体的季节排放规律、排放通量和主控因子,采用静态箱-气相色谱法,对两种林型地表3种温室气体(CO_2、CH_4、N_2O)通量进行为期1年的逐月测定。结果表明:(1)尾巨桉人工林和马占相思人工林均为CO_2和N_2O的排放源,CH_4的吸收汇。马占相思林地表N_2O通量显著(P0.01)高于尾巨桉林,CO_2通量和CH_4通量无明显差异。(2)两种林型3种温室气体通量有着相似季节变化规律,地表CO_2通量均呈现雨季高旱季低的单峰规律;地表CH_4吸收通量表现为旱季高雨季低的单峰趋势;地表N_2O通量呈现雨季高旱季低且雨季内有两个峰值的排放规律。(3)地表CO_2、N_2O通量和土壤5 cm温度呈极显著(P0.01)正相关,3种温室气体地表通量同土壤含水量呈极显著(P0.01)或显著相关(P0.05)。(4)尾巨桉林和马占相思林温室气体年温室气体排放总量为31.014 t/hm~2和28.782 t/hm~2,均以CO_2排放占绝对优势(98.46%—99.15%),CH_4和N_2O处于次要地位。     

Effect of water table on greenhouse gas emissions from peatland mesocosms

Peatland landscapes typically exhibit large variations in greenhouse gas (GHG) emissions due to microtopographic and vegetation heterogeneity. As many peatland budgets are extrapolated from small-scale chamber measurements it is important to both quantify and understand the processes underlying this spatial variability. Here we carried out a mesocosm study which allowed a comparison to be made between different microtopographic features and vegetation communities, in response to conditions of both static and changing water table. Three mesocosm types (hummocks?+?Juncus effusus, hummocks?+?Eriophorum vaginatum, and hollows dominated by moss) were subjected to two water table treatments (0–5 cm and 30–35 cm depth). Measurements were made of soil-atmosphere GHG exchange, GHG concentration within the peat profile and soil water solute concentrations. After 14 weeks the high water table group was drained and the low water table group flooded. Measurement intensity was then increased to examine the immediate response to change in water table position. Mean CO₂, CH₄ and N₂O exchange across all chambers was 39.8 μg m^?2 s^?1, 54.7 μg m^?2 h^?1 and ?2.9 μg m^?2 h^?1, respectively. Hence the GHG budget was dominated in this case by CO₂ exchange. CO₂ and N₂O emissions were highest in the low water table treatment group; CH₄ emissions were highest in the saturated mesocosms. We observed a strong interaction between mesocosm type and water table for CH₄ emissions. In contrast to many previous studies, we found that the presence of aerenchyma-containing vegetation reduced CH₄ emissions. A significant pulse in both CH₄ and N₂O emissions occurred within 1–2 days of switching the water table treatments. This pulsing could potentially lead to significant underestimation of landscape annual GHG budgets when widely spaced chamber measurements are upscaled.     

Carbon Dioxide and Methane Fluxes From Tree Stems,Coarse Woody Debris,and Soils in an Upland Temperate Forest

Forest soils and canopies are major components of ecosystem CO₂ and CH₄ fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO₂ and CH₄ fluxes. We measured CO₂ and CH₄ fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO₂ sources (4.58 ± 2.46 µmol m^?2 s^?1, mean ± 1 SD) and net sinks of CH₄ (?2.17 ± 1.60 nmol m^?2 s^?1). Coarse woody debris was a CO₂ source (4.23 ± 3.42 µmol m^?2 s^?1) and net CH₄ sink, but with large uncertainty (?0.27 ± 1.04 nmol m^?2 s^?1) and with substantial differences depending on wood decay status. Stems were CO₂ sources (1.93 ± 1.63 µmol m^?2 s^?1), but also net CH₄ sources (up to 0.98 nmol m^?2 s^?1), with a mean of 0.11 ± 0.21 nmol m^?2 s^?1 and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, and L. tulipifera consistently emitted CH₄, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO₂ fluxes, whereas CH₄ emissions from living stems offset net soil and CWD CH₄ uptake by 3.5%. Our results demonstrate the importance of CH₄ emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO₂ and CH₄ dynamics.     

Spatial variability of N2O, CH4 and CO2 fluxes within the Xilin River catchment of Inner Mongolia, China: a soil core study

In order to identify the effects of land-use/cover types, soil types and soil properties on the soil-atmosphere exchange of greenhouse gases (GHG) in semiarid grasslands as well as provide a reliable estimate of the midsummer GHG budget, nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂) fluxes of soil cores from 30 representative sites were determined in the upper Xilin River catchment in Inner Mongolia. The soil N₂O emissions across all of the investigated sites ranged from 0.18 to 21.8 μg N m^-2 h^-1, with a mean of 3.4 μg N m^-2 h^-1 and a coefficient of variation (CV, which is given as a percentage ratio of one standard deviation to the mean) as large as 130%. CH₄ fluxes ranged from -88.6 to 2,782.8 μg C m^-2 h^-1 (with a CV of 849%). Net CH₄ emissions were only observed from cores taken from a marshland site, whereas all of the other 29 investigated sites showed net CH₄ uptake (mean: -33.3 μg C m^-2 h^-1). CO₂ emissions from all sites ranged from 3.6 to 109.3 mg C m^-2 h^-1, with a mean value of 37.4 mg C m^-2 h^-1 and a CV of 66%. Soil moisture primarily and positively regulated the spatial variability in N₂O and CO₂ emissions (R²?=?0.15–0.28, P?<?0.05). The spatial variation of N₂O emissions was also influenced by soil inorganic N contents (P?<?0.05). By simply up-scaling the site measurements by the various land-use/cover types to the entire catchment area (3,900 km²), the fluxes of N₂O, CH₄ and CO₂ at the time of sampling (mid-summer 2007) were estimated at 29 t CO₂-C-eq d^-1, -26 t CO₂-C-eq d^-1 and 3,223 t C d^-1, respectively. This suggests that, in terms of assessing the spatial variability of total GHG fluxes from the soils at a semiarid catchment/region, intensive studies may focus on CO₂ exchange, which is dominating the global warming potential of midsummer soil-atmosphere GHG fluxes. In addition, average GHG fluxes in midsummer, weighted by the areal extent of these land-use/cover types in the region, were approximately -30.0 μg C m^-2 h^-1 for CH₄, 2.4 μg N m^-2 h^-1 for N₂O and 34.5 mg C m^-2 h^-1 for CO₂.     

Soil–atmosphere exchange of N2O, CO2 and CH4 along a slope of an evergreen broad-leaved forest in southern China

At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) was estimated based on a few chambers located in a limited area. Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N₂O and CO₂, but take up less atmospheric CH₄ than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N₂O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N₂O ranged from 2.0 to 4.4 kg N ha^?1 year^?1, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N₂O emissions in the laboratory incubated soils varied with slope positions. Soil CO₂ release and CH₄ uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N₂O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling.     

Biogeochemistry of organic carbon, CO2, CH4, and trace elements in thermokarst water bodies in discontinuous permafrost zones of Western Siberia

Active processes of permafrost thaw in Western Siberia increase the number of soil subsidencies, thermokarst lakes and thaw ponds. In continuous permafrost zones, this process promotes soil carbon mobilisation to water reservoirs, as well as organic matter (OM) biodegradation, which produces a permanent flux of carbon dioxide (CO₂) to the atmosphere. At the same time, the biogeochemical evolution of aquatic ecosystems situated in the transition zone between continuous permafrost and permafrost-free terrain remains poorly known. In order to better understand the biogeochemical processes that occur in thaw ponds and lakes located in discontinuous permafrost zones, we studied ~30 small (1–100,000 m²) shallow (<1 m depth) lakes and ponds formed as a result of permafrost subsidence and thaw of the palsa bog located in the transition zone between the tundra and forest-tundra (central part of Western Siberia). There is a significant increase in dissolved CO₂ and methane (CH₄) concentration with decreasing water body surface area, with the largest supersaturation with respect to atmospheric CO₂ and CH₄ in small (<100 m²) permafrost depressions filled with thaw water. Dissolved organic carbon (DOC), conductivity, and metal concentrations also progressively increase from large lakes to thaw ponds and depressions. As such, small water bodies with surface areas of 1–100 m² that are not accounted for in the existing lake and pond databases may significantly contribute to CO₂ and CH₄ fluxes to the atmosphere, as well as to the stocks of dissolved trace elements and organic carbon. In situ lake water incubation experiments yielded negligible primary productivity but significant oxygen consumption linked to the mineralisation rate of dissolved OM by heterotrophic bacterioplankton, which produce a net CO₂ flux to the atmosphere of 5 ± 2.5 mol C m² year^?1. The most significant result of this study, which has long-term consequences on our prediction of aquatic ecosystem development in the course of permafrost degradation is CO₂, CH₄, and DOC concentrations increase with decreasing lake age and size. As a consequence, upon future permafrost thaw, the increase in the number of small water bodies, accompanied by the drainage of large thermokarst lakes to the hydrological network, will likely favour (i) the increase of DOC and colloidal metal stocks in surface aquatic systems, and (ii) the enhancement of CO₂ and CH₄ fluxes from the water surface to the atmosphere. According to a conservative estimation that considers that the total area occupied by water bodies in Western Siberia will not change, this increase in stocks and fluxes could be as high as a factor of ten.     

Polymers of intrinsic microporosity for gas permeation: a molecular simulation study

We report a molecular simulation study for gas permeation in two membranes constructed from polymers of intrinsic microporosity (PIM-1 and PIM-7). With rigid ladder polymer chains, the membranes posses approximately 47.7 and 46.6% fractional free volumes (FFVs) in PIM-1 and PIM-7, respectively. The voids in the membranes have a diameter up to 9 Å and are largely interconnected. The sorption and diffusion of four gases (H₂, O₂, CH₄ and CO₂) were calculated by Monte Carlo and molecular dynamics simulations. The solubility coefficients increase in the order of H₂ < O₂ < CH₄ < CO₂, while the diffusion coefficients increase in the following order: CH₄ < CO₂ < O₂ < H₂. The simulation results agree well with experimental data, particularly for the solubility coefficients. The solubility and diffusion coefficients correlate well separately with the critical temperatures and effective diameters of gases. These molecular-based correlations can be used in the prediction for other gases. As attributed to the microporous structure, PIM-1 and PIM-7 outperform most glassy polymeric membranes in sorption and diffusion. PIM-1 has larger solubility and diffusion coefficients than PIM-7 because the cyano groups in PIM-1 lead to a stronger affinity and a larger FFV. The simulated solubility, diffusivity and permeation selectivities of CO₂/H₂, CO₂/O₂ and CO₂/CH₄ are consistent with experimental data. The quantitative microscopic understanding of gas permeation in the PIM membranes is useful for the new development of high-performance membranes.     

Effects of short-term drying and irrigation on CO2 and CH4 production and emission from mesocosms of a northern bog and an alpine fen

Atmospheric CO₂ and CH₄ exchange in peatlands is controlled by water table levels and soil moisture, but impacts of short periods of dryness and rainfall are poorly known. We conducted drying-rewetting experiments with mesocosms from an ombrotrophic northern bog and an alpine, minerotrophic fen. Efflux of CO₂ and CH₄ was measured using static chambers and turnover and diffusion rates were calculated from depth profiles of gas concentrations. Due to a much lower macroporosity in the fen compared to the bog peat, water table fluctuated more strongly when irrigation was stopped and resumed, about 11 cm in the fen and 5 cm in the bog peat. Small changes in air filled porosity caused CO₂ and CH₄ concentrations in the fen peat to be insensitive to changes in water table position. CO₂ emission was by a factor of 5 higher in the fen than in the bog mesocosms and changed little with water table position in both peats. This was probably caused by the importance of the uppermost, permanently unsaturated zone for auto- and heterotrophic CO₂ production, and a decoupling of air filled porosity from water table position. CH₄ emission was <0.4 mmol m^?2 day^?1 in the bog peat, and up to >12.6 mmol m^?2 day^?1 in the fen peat, where it was lowered by water table fluctuations. CH₄ production was limited to the saturated zone in the bog peat but proceeded in the capillary fringe of the fen peat. Water table drawdown partly led to inhibition of methanogenesis in the newly unsaturated zone, but CH₄ production appeared to continue after irrigation without time-lag. The identified effects of irrigation on soil moisture and respiration highlight the importance of peat physical properties for respiratory dynamics; but the atmospheric carbon exchange was fairly insensitive to the small-scale fluctuations induced.     

Soil Greenhouse Gas Emissions in Response to Corn Stover Removal and Tillage Management Across the US Corn Belt

In-field measurements of direct soil greenhouse gas (GHG) emissions provide critical data for quantifying the net energy efficiency and economic feasibility of crop residue-based bioenergy production systems. A major challenge to such assessments has been the paucity of field studies addressing the effects of crop residue removal and associated best practices for soil management (i.e., conservation tillage) on soil emissions of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This regional survey summarizes soil GHG emissions from nine maize production systems evaluating different levels of corn stover removal under conventional or conservation tillage management across the US Corn Belt. Cumulative growing season soil emissions of CO₂, N₂O, and/or CH₄ were measured for 2–5 years (2008–2012) at these various sites using a standardized static vented chamber technique as part of the USDA-ARS’s Resilient Economic Agricultural Practices (REAP) regional partnership. Cumulative soil GHG emissions during the growing season varied widely across sites, by management, and by year. Overall, corn stover removal decreased soil total CO₂ and N₂O emissions by -4 and -7 %, respectively, relative to no removal. No management treatments affected soil CH₄ fluxes. When aggregated to total GHG emissions (Mg CO₂?eq ha^?1) across all sites and years, corn stover removal decreased growing season soil emissions by ?5?±?1 % (mean?±?se) and ranged from -36 % to 54 % (n?=?50). Lower GHG emissions in stover removal treatments were attributed to decreased C and N inputs into soils, as well as possible microclimatic differences associated with changes in soil cover. High levels of spatial and temporal variabilities in direct GHG emissions highlighted the importance of site-specific management and environmental conditions on the dynamics of GHG emissions from agricultural soils.     

Transitional slopes act as hotspots of both soil CO<Subscript>2</Subscript> emission and CH<Subscript>4</Subscript> uptake in a temperate forest landscape

Forest soils are an important component of CO₂ and CH₄ fluxes at the global scale, but the magnitude of these fluxes varies greatly in space and time within a landscape. Understanding the spatial and temporal distributions of these fluxes across complex landscapes remains a major challenge for researchers and land managers alike. We investigated the spatiotemporal variability of soil-atmosphere CO₂ and CH₄ fluxes and the relationships of these fluxes to chemical and physical soil properties distributed across a topographically-heterogeneous landscape. Soil CO₂ and CH₄ fluxes were measured along with soil temperature, moisture, bulk density, texture, carbon, sorption capacity, and dissolved organic matter quality over 2 years along hillslope transects spanning valley bottom, transition zone, and upland landscape positions in a temperate forest watershed. Transition zone soil CO₂ efflux was 54–160% higher than low-lying valley bottoms, and 15–54% higher than uplands. Net seasonal CH₄ uptake was 58–150% higher in transition zone soils than in uplands, while valley bottoms were occasionally large net sources (up to 19 nmol CH₄ m^?2 s^?1). Soil CO₂ efflux and net CH₄ uptake were both positively associated with seasonal temperature, and were highest in soils with relatively high carbon and clay content, and relatively low bulk density, moisture, and sorption capacity. We concluded that: (1) transition zone soils act as landscape hotspots for net CH₄ uptake in addition to CO₂ efflux, and (2) that this spatial distribution is more consistent across seasons for net CH₄ uptake than for CO₂ efflux.     

Increased night temperature reduces the stimulatory effect of elevated carbon dioxide concentration on methane emission from rice paddy soil

To determine how elevated night temperature interacts with carbon dioxide concentration ([CO₂]) to affect methane (CH₄) emission from rice paddy soil, we conducted a pot experiment using four controlled‐environment chambers and imposed a combination of two [CO₂] levels (ambient: 380 ppm; elevated: 680 ppm) and two night temperatures (22 and 32 °C). The day temperature was maintained at 32 °C. Rice (cv. IR72) plants were grown outside until the early‐reproductive growth stage and then transferred to the chambers. After onset of the treatment, day and night CH₄ fluxes were measured every week. The CH₄ fluxes changed significantly with the growth stage, with the largest fluxes occurring around the heading stage in all treatments. The total CH₄ emission during the treatment period was significantly increased by both elevated [CO₂] (P=0.03) and elevated night temperature (P<0.01). Elevated [CO₂] increased CH₄ emission by 3.5% and 32.2% under high and low night temperature conditions, respectively. Elevated [CO₂] increased the net dry weight of rice plants by 12.7% and 38.4% under high and low night temperature conditions, respectively. These results imply that increasing night temperature reduces the stimulatory effect of elevated [CO₂] on both CH₄ emission and rice growth. The CH₄ emission during the day was larger than at night even under the high‐night‐temperature treatment (i.e. a constant temperature all day). This difference became larger after the heading stage. We observed significant correlations between the night respiration and daily CH₄ flux (P<0.01). These results suggest that net plant photosynthesis contributes greatly to CH₄ emission and that increasing night temperature reduces the stimulatory effect of elevated [CO₂] on CH₄ emission from rice paddy soil.