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.     

Carbon emission flux and storage in the degraded peatlands of the Zoige alpine area in the Qinghai–Tibetan Plateau

The Zoige alpine peatlands cover approximately 4,605 km² of the Qinghai–Tibetan Plateau and are considered to constitute the largest plateau peatland on the Eurasian continent. However, the Zoige alpine peatlands are undergoing major degradation because of human activities and climate change, which would cause uncertainty in the budget of greenhouse gases (CH₄ and CO₂) and carbon (C) storage in global peatlands. This study simultaneously investigates the CH₄ and CO₂ emission fluxes and C storage at three typical sites with respect to the peatland degradation gradient: peatland, wet meadow and dry meadow. Results show that peatland degradation would increase the CO₂ emission and decrease the CH₄ emission. Moreover, the average C emission fluxes were 66.05, 165.78 and 326.56 mg C m^?2 hr^?1 for the peatland, wet meadow and dry meadow, respectively. The C storage of the vegetation does not considerably differ among the three sampling sites. However, when compared with the peatland (1,088.17 t C ha^?1), the soil organic C storage decreases by 420 and 570 t C ha^?1 in case of wet and dry meadows, respectively. Although the C storage in the degraded peatlands decreases considerably, it can still represent a large capacity of C sink. Therefore, the degraded peatlands in the Zoige alpine area must be protected and restored to mitigate regional climate change.     

The contribution of plant roots to CO2 fluxes from organic soils

The CO₂ released in soil respiration is formed from organic matter which differs in age and stability, ranging from soluble root exudates to more persistent plant remains. The contribution of roots, a relatively fast component of soil cycling, was studied in three experiments. (1) Willows were grown in a greenhouse and CO₂ fluxes from the substrate soil (milled peat) and from control peat were measured. (2) CO₂ fluxes from various peatland sites were measured at control points and points where the roots were severed from the plants. (3) CO₂ fluxes in cultivated grassland established on peatland were measured in grassy subsites and in subsites where the growth of grass was prevented by regular tilling. The root-derived respiration followed the typical annual phenology of the vegetation, being at its maximum in the middle and late summer. All the experiments gave similar results, root-derived respiration accounting for 35–45% of total soil respiration in the middle and late summer at sites with an abundant vegetation. The root-derived respiration from the virgin peatland sites correlated well with the tree biomass, and also partly with the understorey vegetation, but in the drained sites the root effect was greater, even in the presence of less understorey vegetation than at virgin subsites.     

Water-table management in lowland UK peat soils and its potential impact on CO2 emission

The rate of oxidation of peat soils is highly seasonal and varies with temperature and soil moisture content. Large variations in soil moisture content result in wet–dry cycles that can enhance peat degradation. Water‐table management plays a crucial role in controlling and damping the effect of these environmental factors. However, maintaining high ditch water levels in fields bounded by ditches does not guarantee a high field groundwater level. The effect of installing subsurface irrigation at different spacings on water table elevation was studied in a low‐lying peat grassland. The water table elevation data were compared against values predicted with a water balance model. In addition, greenhouse experiments were carried out on undisturbed soil core samples collected from the peat grassland as well as a low‐lying peatland under intensive arable faming to measure CO₂ evolution under different water regimes. The field data from the peat grassland suggest that sub‐irrigation spacing as low as 10 m is necessary during summer periods to maintain groundwater levels similar to those in the ditches. Over the same period of observation, the difference in water level between the ditches and the non‐irrigated fields is as high as 0.7 m. Modelled outputs are in good correlation with the field observations, and demonstrate that simple water balance models can provide an effective tool to study the effect of water management practices and potential changes in subsurface conditions, climate and land use on water‐table levels. The measurement of CO₂ emission from undisturbed peat soil columns shows that the rate of oxidation of soil organic matter from peat soils is highly seasonal and that drainage exacerbates the rate of peat mineralization.     

Influence of water depth and soil amelioration on greenhouse gas emissions from peat soil columns

Recently, large areas of tropical peatland have been converted into agricultural fields. To be used for agricultural activities, peat soils need to be drained, limed and fertilized due to excess water, low nutrient content and high acidity. Water depth and amelioration have significant effects on greenhouse gas (GHG) production. Twenty-seven soil samples were collected from Jabiren, Central Kalimantan, Indonesia, in 2014 to examine the effect of water depth and amelioration on GHG emissions. Soil columns were formed in the peatland using polyvinyl chloride (PVC) pipe with a diameter of 21 cm and a length of 100 cm. The PVC pipe was inserted vertically into the soil to a depth of 100 cm and carefully pulled up with the soil inside after sealing the bottom. The treatments consisting of three static water depths (15, 35 and 55 cm from the soil surface) and three ameliorants (without ameliorant/control, biochar+compost and steel slag+compost) were arranged using a randomized block design with two factors and three replications. Fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) from the soil columns were measured weekly. There was a linear relationship between water depth and CO₂ emissions. No significant difference was observed in the CH₄ emissions in response to water depth and amelioration. The ameliorations influenced the CO₂ and N₂O emissions from the peat soil. The application of biochar+compost enhanced the CO₂ and N₂O emissions but reduced the CH₄ emission. Moreover, the application of steel slag+compost increased the emissions of all three gases. The highest CO₂ and N₂O emissions occurred in response to the biochar+compost treatment followed by the steel slag-compost treatment and without ameliorant. Soil pH, redox potential (Eh) and temperature influenced the CO₂, CH₄ and N₂O fluxes. Experiments for monitoring water depth and amelioration should be developed using peat soil as well as peat soil–crop systems.     

不同耕作条件下豆麦双序列轮作农田土壤温室气体的排放及影响因素研究

为了研究耕作措施对双序列轮作农田土壤温室气体的排放及影响, 采用CO₂分析仪、静态箱 气相色谱法在陇中黄土高原半干旱区对传统耕作不覆盖、免耕不覆盖、免耕秸秆覆盖和传统耕作+秸秆还田4种耕作措施下豆麦双序列轮作农田土壤温室气体(CO₂、N₂O和CH₄)的排放及影响因素进行了连续测定和分析。结果表明: 测定期内4种耕作措施下农田土壤均表现为CO₂源、N₂O源和CH₄净吸收汇; 除传统耕作不覆盖措施, 其他3种耕作措施不同程度地减少了2种轮作序列土壤的N₂O排放通量, 并显著增加了土壤对CH4的吸收。CO₂和N₂O的排放通量分别与地表、地下5 cm处、地下10 cm处的土壤温度呈极显著和显著正相关关系, 相关系数分别为0.92^**和0.89^**、0.95^**和0.91^**、0.77^*和0.62^*; 而CH₄吸收通量与不同地层的温度之间无明显的相关关系; CO₂和CH₄的通量与0~5 cm、5~10 cm的土壤含水量均呈显著正相关关系, 相关系数分别为0.69^*和0.72^*、0.77^*和0.64^*, 而与10~30 cm土壤含水量无明显相关关系; N₂O排放通量与各层次的土壤含水量之间均呈不显著负相关关系。对2种轮作序列各处理下土壤中排放的3种温室气体的增温潜势计算综合得出: 4种耕作措施中, 免耕不覆盖处理可相对减少土壤温室气体的排放量, 进而降低温室效应。     

Landscape Scale Variation in Nitrous Oxide Flux Along a Typical Northeastern US Topographic Gradient in the Early Summer

Most previous studies investigating controls on nitrous oxide (N₂O) emissions have relied on plot-scale experiments and focused on relative homogeneous biotic and abiotic factors such as soil, vegetation, and moisture. We studied soil N₂O flux at 11 chamber sites along a 620 m topographic gradient in upstate New York, USA, aiming at identifying patterns of N₂O flux and correlating them to hydrological factors and soil substrate properties along the gradient. The topographic gradient is a complex slope with an overall gradient of 8%, covering plant communities of pasture, forest, alfalfa field, and riparian area from the top to the bottom. Mean fluxes of N₂O measured from late March to May ranged from 4.45 to 343 μg N m^?2 h^?1, and these fluxes were not significantly different among chamber sites located in different communities. With the descending of the slope, N₂O fluxes increased with the increase of soil water content, except for the riparian site. Statistically, N₂O fluxes were not strongly correlated with soil temperature, soil bulk density, and water filled pore space (p?>?0.05). Instead, strong correlations (p?2O fluxes and soil C and N content including NO ₃ ^? , NH ₄ ⁺ , total organic carbon, and C/N ratio. Multiple linear regression analyses including both soil physical and substrate properties highlighted the significance of soil NO ₃ ^? content and C/N ratio in regulating N₂O fluxes along the gradient.     

CO2 fluxes and drivers as affected by soil type,tillage and fertilization

Abstract

Importance of agricultural practices for greenhouse gases mitigation is examined worldwide. However, there is no consensus on CO₂ emissions as affected by soil management practices. Deeper understanding of soil CO₂ fluxes and drivers under different management practices are needed. The investigation of net CO₂ exchange rate as dependent variable and drivers (soil water and temperature, air temperature) as affected by soil type (loam and sandy loam), tillage (conservation and no-tillage) and fertilization are presented.

Soil management practices and weather conditions affected the CO₂ flux through effects on soil water and temperature regime. Mean net CO₂ exchange rate on sandy loam was 8% higher than on loam. No-tillage, as a moisture-conserving tool, could be an appropriate tool for CO₂ emissions mitigation in any weather conditions on sandy loam; however, the advantage of no-tillage on loam was negligible. Mineral NPK fertilizers promoted significantly higher net CO₂ exchange rate in both soils, but suppressed it by 15% on sandy loam during a normal year. Effect of soil water content on net CO₂ exchange rate was direct in all tillage and fertilization treatments in both loam and sandy loam, whereas this effect was positive only in dry and normal weather conditions. In wet weather conditions, the direct effect of soil water content on net CO₂ exchange rate was negative. Soil and air temperature acted indirectly on net CO₂ exchange rate. The increase in temperature markedly suppressed the positive direct impact of soil water content on net CO₂ exchange rate in dry weather conditions, but did not reduce the direct effect of soil water content in normal weather conditions. In a wet year the negative indirect effect of increased temperature enhanced the negative direct impact of soil water surplus on net CO₂ exchange rate.     

Long‐term carbon loss and CO2‐C release of drained peatland soils in northeast Germany

Peatlands are common in many parts of the world. Draining and other changes in the use of peatlands increase atmospheric CO₂ concentration. If we are to make reliable quantitative predictions of that effect, we need good information on the CO₂ emission rates from peatlands. The present study uses two different methods for predicting CO₂‐C release of peatland soils: (i) a 40‐year field investigation of balancing organic carbon stocks and (ii) short‐term CO₂‐C release rates from laboratory experiments. To estimate long‐term losses of peat, and its resulting C input to the atmosphere, we combined highly detailed maps of surface topography and its changes, and the organic C contents and bulk densities of a drained peatland from different years. Short‐term CO₂‐C release rates were measured in the laboratory by incubating soil samples from several soil horizons at various temperatures and soil moistures. We then derived nonlinear CO₂‐C production functions, which we incorporated into a numerical simulation model (HYDRUS). Using HYDRUS, we calculated daily soil water components and CO₂‐release for (i) real‐climate data from 1950 to 2003 and (ii) a climate scenario extending to 2050, including an increase in temperature of 2°C and 20% less rainfall during the summer half year, i.e. from April to September inclusive. From our field measurements, we found a mean annual decrease of 0.7 cm in the thickness of the peat. Large losses (> 1.5 cm year^?1) occurred only during periods when groundwater levels were low (i.e. a deep water‐table). The annual CO₂‐C release results in a mean loss from the peat of about 700 g CO₂‐C m^?2, mostly as a direct contribution to the atmosphere. Both methods produced very similar results. The model scenarios demonstrated that CO₂‐C loss is mainly controlled by the groundwater (i.e. water‐table) depth, which controls subsurface aeration. A local climate scenario estimated a c. 5% increase of CO₂‐C losses within the next 50 years.     

The effect of soil moisture,soil particle size,litter layer and carbonic anhydrase on the oxygen isotopic composition of soil‐released CO2

Soil respiration and photosynthesis are the two largest carbon dioxide (CO₂) fluxes between terrestrial ecosystems and the atmosphere and, therefore, the dominant processes influencing the oxygen isotopic composition of atmospheric CO₂. The characterization of temporal and spatial variations of plant and soil‐related fluxes of different oxygen isotopologues of CO₂ (¹²C¹⁶O₂; ¹²C¹⁶O¹⁸O) is relevant to constraining the global carbon budget. The oxygen isotopic composition of soil‐respired CO₂ is controlled by its release rate, the degree of isotopic equilibrium with soil water and the diffusional transport of CO₂. The hypothesis of this study was that, as well as soil moisture, the soil particle size, the presence of an organic litter layer and the enzyme carbonic anhydrase (CA) would have a significant impact on the oxygen isotopic composition of soil‐released CO₂. We tested this hypothesis with soil microcosm experiments on columns of medium and fine sand. Soil water content and soil texture influenced the isotopic composition of soil‐released CO₂ significantly. A litter layer had a significant effect on the isotopic composition of water vapour but not on CO₂ released from soil. In the absence of CA, oxygen isotope equilibration between the CO₂ invasion flux and soil water was insignificant, whereas in the presence of CA about 55% of the CO₂ invading the soil exchanged oxygen isotopes with soil water. Our findings highlight the importance of small‐scale variability of soil attributes for the oxygen isotopic composition of soil‐released CO₂ as well as the strong impact of CA activity in soils.     

The effect of water table depth on emissions of N2O from a grassland soil

Nitrous oxide (N₂O) emissions were measured by the closed chamber technique from five plots along a transect in a nitrogen‐fertilised grassland, together with soil water content, soil temperature and water table depth, to investigate the effect of water table depth on N₂O emissions. N₂O fluxes varied from <1 g N₂O‐N ha^?1 day^?1 to peaks of around 500–1200 g N₂O‐N ha^?1 day^?1 after N fertiliser applications. There was no significant difference in overall average water table depth between four of the five plots, but significant short‐term temporal variations in water table depth did occur. Rises in the water table were accompanied by exponential increases in N₂O emissions, through the associated increases in the water‐filled pore space of the topsoil. Modelling predicted that if the water table could be managed such that it was kept to no less than 35 cm below the ground surface, fluxes during the growing season would be reduced by 50%, while lowering to 45 cm would reduce them by over 80%. The strong implication of these results is that draining grasslands, so that the water tables are only rarely nearer to the surface than 35 cm when N is available for denitrification, would substantially reduce N₂O emissions.     

The impact of logging residue on soil GHG fluxes in a drained peatland forest

Northern peatlands contain substantial reservoirs of carbon (C). Forestry activities endanger the C storages in some of these areas. While the initial impacts of forestry drainage on peatland greenhouse gas (GHG) balance have been studied, the impacts of other silvicultural practices, e.g. logging residue (LR) retention or removal, are not known. We measured the CH₄, N₂O and CO₂ fluxes between peat soil and atmosphere with and without decomposing LR over three (2002–2004) seasons (May–Oct) following clearfelling in a drained peatland forest, along with the mass loss of LR. Seasonal average CO₂ efflux from plots with LR (3070 g CO₂ m⁻² season⁻¹) was twice as high as that from plots without LR (1447 g CO₂ m⁻² season⁻¹). Less than 40% of this difference was accounted for by the decay of logging residues (530 g CO₂ m⁻² season⁻¹), so the majority of the increased CO₂ efflux was caused by increased soil organic matter decomposition under the LR. Furthermore LR increased soil N₂O fluxes over 3-fold (0.70 g N₂O m⁻² season⁻¹), compared to plots without LR (0.19 g N₂O m⁻² season⁻¹), while no change in CH₄ emissions was observed. Our results indicate that LR retention in clearfelled peatland sites may significantly increase GHG emissions and C release from the soil organic matter C storage. This would make the harvesting of LR for biofuel more beneficial, in the form of avoided emissions. Further investigations of the sources of CO₂ under logging residues are, however, needed to confirm this finding.     

Soil heterogeneity effects on O2 distribution and CH4 emissions from wetlands: In situ and mesocosm studies with planar O2 optodes and membrane inlet mass spectrometry

The importance of soil heterogeneity for methane emission from a wetland soil is assessed by in situ point measurements of depth-specific O₂ and CH₄ concentrations and simultaneous soil CH₄ fluxes at contrasting water levels. Profile measurements, and associated assumptions in their interpretation, were validated in a controlled mesocosm drainage and saturation experiment applying planar O₂ optodes and membrane inlet mass spectrometry. Results show that peat soil is heterogeneous containing dynamic macropore systems created by both macrofauna and flora, which facilitate preferential flow of water, O₂ and CH₄ and vary temporally with changes in the moisture regime. The O₂ content above the water table after drainage varied horizontally from 0 to 100% air saturation within few mm. Oxic zones were observed below the water level and anoxic zones were observed in layers above the water level in periods up to days after changes in the water level. This study shows that although water table position is a competent proxy of soil CH₄ fluxes at larger spatio-temporal scales, it becomes inadequate at higher spatial resolution, i.e. at the scale of the soil pedon and below. High resolution O₂ measurements using planar O₂ optodes have great potential to enhance our understanding of the effect of the water table position on O₂ dynamics on scales of several cm to mm in wetland soils.     

Effects of nitrogen and sulfur deposition on CH4 and N2O fluxes in high-altitude peatland soil under different water tables in the Tibetan Plateau

Abstract

The effects of nitrogen (N) and sulfur (S) deposition on methane (CH₄) and nitrous oxide (N₂O) emissions under low (10 cm below soil surface) and high (at soil surface) water tables were investigated in the laboratory. Undisturbed soil columns from the alpine peatland of the Tibetan Plateau were analyzed. CH₄ emission was higher and N₂O emission was lower at the high water table than those at the low water table regardless of nutrient application. Addition of N (NH₄NO₃ (ammonium nitrate), 5 g N m^?2) decreased CH₄ emission up to 57% and 50% at low and high water tables, respectively, but correspondingly increased N₂O emission by 2.5 and 10.4 times. Addition of S (Na₂SO₄ (sodium sulfate), 2.5 g S m^?2) decreased CH₄ and N₂O emission by 64% and 79% at the low water table, respectively, but had a slightly positive effect at the high water table. These results indicated that the responses of CH₄ and N₂O emissions to the S deposition depend on the water table condition in the high-altitude peatland.     

Abiotic processes dominate CO2 fluxes in Antarctic soils

Ecosystems within the McMurdo Dry Valleys of Antarctica are highly sensitive to environmental change. Increases in soil temperature and/or moisture content may dramatically change rates of soil respiration and soil carbon (C) turnover. Present estimates of soil respiration rates and C turnover times are based on surface carbon dioxide (CO₂) fluxes and soil organic C content. However, the assumption that surface CO₂ fluxes are purely biological in origin has not been rigorously tested. We use concentration and, for the first time, the stable C isotopic composition of surface soil CO₂ fluxes and subsurface CO₂ profiles to: 1) examine mechanisms of soil CO₂ uptake and release, 2) identify the location of potential CO₂ sources and sinks within the soil profile, and 3) discriminate between biotic and abiotic contributions to CO₂ fluxes in soils of Taylor Valley. Surface CO₂ fluxes and subsurface CO₂ profiles confirm that these soils take up and release CO₂ on a daily basis (during the austral summer), associated with small changes in soil temperature. Shifts in the C isotopic composition of soil CO₂ are inconsistent with biological mechanisms of CO₂ production and consumption. Instead, the isotopic shifts can be accounted for by Henry's Law dissolution and exsolution of CO₂ into a solution of high pH, driven by changes in soil temperature. Our results constrain the biological component of soil CO₂ fluxes in Taylor Valley to less than 25% (and likely to be significantly less). This finding implies that previous measurements of surface soil CO₂ fluxes are overestimates of soil respiration, thus C turnover times calculated from them are underestimates. Discriminating between biotic and abiotic contributions to CO₂ fluxes in Antarctic dry valley soils is essential if the effects of climate change on these sensitive ecosystems are to be accurately identified.     

CO2 and water vapor fluxes above a subtropical mountain cloud forest—The effect of light conditions and fog

From the 4 August through 22 September 2006, the turbulent vertical fluxes of CO₂ and water vapor were measured above a cypress tree (Chamaecyparis species) plantation within the subtropical mountain cloud forest range of NE Taiwan. The study site is characterized by a high frequency of orographic fog. During the day, humid air is transported uphill and causes condensation and fog as it cools. CO₂ and water vapor fluxes were measured on two instrumented micrometeorological towers aligned along the valley axis. Despite the large reduction of solar radiation during foggy conditions by 64% as compared to clear situations, the reduction of the CO₂ uptake is rather small at tower 1, for which Chamaecyparis is the dominating species within the footprint area. The Chamaecyparis species perform well under these extreme conditions of reduced light. A large section of the footprint area of tower 2 is vegetated with Cryptomeria japonica. CO₂ flux comparison at both towers during foggy and non-foggy conditions, and direct tower-totower comparisons showed a significant difference statistically in the CO₂ fluxes. While the photosynthesis rate of Chamaecyparis was hardly reduced during fog that of Cryptomeria japonica decreased strongly during foggy conditions. Cryptomeria japonica is less well adapted to the low light conditions. The net water vapor flux was dominated by evaporation of intercepted fog. Transpiration contributed only little to the water vapor flux during the 7-week experimental field study.     

Applicability of the soil gradient method for estimating soil–atmosphere CO2, CH4, and N2O fluxes for steppe soils in Inner Mongolia

For evaluating the applicability of the soil gradient method as a substitute for CO₂‐, CH₄‐, and N₂O‐flux measurements in steppe, we carried out chamber measurements and determined soil gas concentration at an ungrazed (UG99) and a grazed (WG) site in Inner Mongolia, China. The agreement of the concentration‐based flux estimates with measured chamber‐based fluxes varied largely depending on the respective GHG in the sequence CO₂ > CH₄ >> N₂O. A calibration of the gas‐transport parameter used to calculate fluxes based on soil gas concentrations improved the results considerably for CO₂ and CH₄. After calibration, the average deviation from the chamber‐based annual cumulative flux for both sites was 11.5%, 10.5%, and 59% for CO₂, CH₄, and N₂O. The gradient method did not constitute an adequate stand‐alone substitute for greenhouse‐gas flux estimation since a calibration using chamber‐based measurements was necessary and vigorous production processes were confined to the uppermost, almost water‐saturated soil layer.     

Nitrous oxide emission derived from soil organic matter decomposition from tropical agricultural peat soil in central Kalimantan,Indonesia

Our previous research showed large amounts of nitrous oxide (N₂O) emission (>200?kg?N?ha^?1?year^?1) from agricultural peat soil. In this study, we investigated the factors influencing relatively large N₂O fluxes and the source of nitrogen (N) substrate for N₂O in a tropical peatland in central Kalimantan, Indonesia. Using a static chamber method, N₂O and carbon dioxide (CO₂) fluxes were measured in three conventionally cultivated croplands (conventional), an unplanted and unfertilized bare treatment (bare) in each cropland, and unfertilized grassland over a three-year period. Based on the difference in N₂O emission from two treatments, contribution of the N source for N₂O was calculated. Nitrous oxide concentrations at five depths (5–80?cm) were also measured for calculating net N₂O production in soil. Annual N fertilizer application rates in the croplands ranged from 472 to 1607?kg?N?ha^?1?year^?1. There were no significant differences in between N₂O fluxes in the two treatments at each site. Annual N₂O emission in conventional and bare treatments varied from 10.9 to 698 and 6.55 to 858?kg?N?ha^?1?year^?1, respectively. However, there was also no significant difference between annual N₂O emissions in the two treatments at each site. This suggests most of the emitted N₂O was derived from the decomposition of peat. There were significant positive correlations between N₂O and CO₂ fluxes in bare treatment in two croplands where N₂O flux was higher than at another cropland. Nitrous oxide concentration distribution in soil measured in the conventional treatment showed that N₂O was mainly produced in the surface soil down to 15?cm in the soil. The logarithmic value of the ratio of N₂O flux and nitrate concentration was positively correlated with water filled pore space (WEPS). These results suggest that large N₂O emission in agricultural tropical peatland was caused by denitrification with high decomposition of peat. In addition, N₂O was mainly produced by denitrification at high range of WFPS in surface soil.     

Influence of water table levels on CO2 emissions in a Colorado subalpine fen: an in situ microcosm study

We quantified the relationship between water table position and CO₂ emissions by manipulating water table levels for two summers in microcosms installed in a Colorado subalpine fen. Water levels were manipulated in the microcosms by either adding water or removing water and ranged from +10 cm above the soil surface to 40 cm below the soil surface, with ambient water levels in the fen averaging +3 and +2 cm above the soil surface during 1998 and 1999, respectively. Microcosm installation had no significant effect on CO₂ efflux; the 2 year means of natural and reference CO₂ efflux were 205.4 and 213.9 mg CO₂-C m⁻² h⁻¹, respectively (p=0.80). Mean CO₂ emissions were lowest at the highest water tables (water +6 to +10 cm above the soil surface), averaging 133.8 mg CO₂-C m⁻² h⁻¹, increased to 231.3 mg CO₂-C m⁻² h⁻¹ when the water table was +1 to +5 cm above the soil surface and doubled to 453.7 mg CO₂-C m⁻² h⁻¹, when the water table was 0-5 cm below the soil surface. However, further lowering of the water table had little additional effect on CO₂ emissions, which averaged 470.3 and 401.1 mg CO₂-C m⁻² h⁻¹ when the water table was 6-10 cm, and 11-40 cm beneath the soil surface, respectively. The large increase in CO₂ emissions as we experimentally lowered the water table beneath the soil surface, coupled with no increase in CO₂ emissions as we furthered lowered water tables beneath the soil surface, suggest the presence of an easily oxidized labile carbon pool near the soil surface.