Diffusion probe for gas sampling in undisturbed soil

Soil‐atmosphere fluxes of trace gases such as methane (CH₄) and nitrous oxide (N₂O) are determined by complex interactions between biological activity and soil conditions. Soil gas concentration profiles may, in combination with other information about soil conditions, help us to understand emission controls. This paper describes a simple and robust diffusion probe for soil gas sampling as part of flux monitoring programmes. It can be deployed with minimum disturbance of in‐situ conditions, and also at sites with a high or fluctuating water table. Separate probes are used for each sampling depth, in this study ranging from 5 to 100 cm. The probe has a 10‐ml diffusion cell with a 3‐mm diameter opening covered by a 0.5‐mm silicone membrane. At sampling the diffusion cell is flushed with 10 ml N₂ containing 50 µl l^?1 ethylene (C₂H₄) as a tracer; tracer recovery is used to calculate sample concentrations. Ethylene is immediately removed by flushing with unamended N₂. Equations are presented to correct for dead volumes of connecting tubing and valves. Laboratory tests evaluated recovery of CH₄, N₂O and carbon dioxide (CO₂), removal of C₂H₄ and equilibration of CH₄, N₂O and CO₂ in air and water. Field tests on peat soils used for grazing showed soil gas concentrations of CH₄ and N₂O as influenced by topography, site conditions and season. The applicability of the diffusion probe for trace gas monitoring is discussed.     

A re-examination of closed flux chamber methods for the measurement of trace gas emissions from soils to the atmosphere

The most widely used method for measuring the emission of a trace gas such as N₂O from soil to the atmosphere involves the accumulation of the gas under closed chambers followed by sampling and analysis (by gas chromatography or infrared methods). These chambers can affect the gas exchange, and so improved designs have been proposed. We have tested their performance. One design includes a vent tube to allow ambient pressure fluctuations to occur also inside the chamber. We tested it against a sealed version on two different grassland sites during N₂O peak emissions in spring 1997. On a welldrained soil with a fairly large air permeability vented chambers yielded fluxes as much as five times those of sealed chambers, depending on wind speed. By contrast, on a heavier and wetter soil with smaller air permeability vented chambers averaged only 88% of the fluxes observed with sealed chambers. The effects of venting cannot be explained solely on the basis of mean pressure differences inside and outside the chamber. It seems more likely that wind blowing over the vent depressurizes the chamber (Venturi effect), resulting in significant gas flow from the more permeable soil into the interior of the chamber. The opposite trend for the less permeable soil suggests that diffusion losses through the vent tube are greater than the increase in concentration due to soil gas flow. Venting can create larger errors than the ones it is supposed to overcome.     

An improved method for measuring soil N2O fluxes using a quantum cascade laser with a dynamic chamber

A dynamic chamber method was developed to measure fluxes of N₂O from soils with greater accuracy than previously possible, through the use of a quantum cascade laser (QCL). The dynamic method was compared with the conventional static chamber method, where samples are analysed subsequently on a gas chromatograph. Results suggest that the dynamic method is capable of measuring soil N₂O fluxes with an uncertainty of typically less than 1–2 µg N₂O‐N m^?2 hour^?1 (0.24–0.48 g N₂O‐N ha^?1 day^?1), much less than the conventional static chamber method, because of the greater precision and temporal resolution of the QCL. The continuous record of N₂O and CO₂ concentration at 1 Hz during chamber closure provides an insight into the effects that enclosure time and the use of different regression methods may introduce when employed with static chamber systems similar in design. Results suggest that long enclosure times can contribute significantly to uncertainty in chamber flux measurements. Non‐linear models are less influenced by effects of long enclosure time, but even these do not always adequately describe the observed concentrations when enclosure time exceeds 10 minutes, especially with large fluxes.     

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.     

硅胶管气样原位采集技术研究土壤N2O浓度及通量变化

箱法被广泛用于监测土壤N2O排放通量,但在原位采集高浓度土壤N2O、全天候监测N2O通量变化、动态研究土壤剖面N2O的行为等方面存在弊端。本研究通过室内模拟硅胶管对N2O的通透性,探索硅胶管用于原位采集土壤气样的理论可行性。田间试验设施用铵态氮肥（NH+4）、施用硝态氮肥（NO-3）及施用硝态氮肥加葡萄糖（NO-3+C）等3个处理,同时安置硅胶管和采样箱,验证硅胶管法在原位采集高浓度土壤N2O气样、监测土壤N2O浓度以及排放通量的实际效果,并与箱法进行比较。结果表明,硅胶管内外的N2O气体经2.9 h达到95%的平衡,完全能满足大田采样要求; 用硅胶管法原位采集高浓度土壤N2O气样的效果显著优于箱法采样。其浓度变化表现出明显的时间规律,浓度梯度法计算的N2O排放通量与箱法测定结果呈显著正相关,但数值偏低; 偏低的程度取决于采样位置和土壤中N2O产生位置的匹配程度。建议采用埋于土壤表层的硅胶管计算地面N2O排放通量,或在不同土层埋入硅胶管研究土壤剖面N2O行为的时空变异。     

Accumulation of nitrous oxide and depletion of oxygen in seasonally frozen soils in northern Japan - Snow cover manipulation experiments

It has been suggested that soil-thawing and snow-melting are critical triggers for vigorous emissions of nitrous oxide (N₂O) from soils in cold regions. However, because soil freezing is affected by air temperature and snow cover, accurate predictions that estimate subsequent emissions of this important greenhouse gas are difficult to make. In this study, we measured in situ soil gas N₂O and oxygen (O₂) concentrations at two experimental sites in northern Japan over the period of a year, from November 2008 to October 2009, to clarify the factors stimulating N₂O production in soil at low temperatures. The sites were N-fertilized bare arable lands with different soil frost depths and snowmelt rates, according to the snow cover management imposed. Winter-to-spring net N₂O fluxes, ranging from −0.10 to 1.95 kg N₂O-N ha⁻¹, were positively correlated with the annual maximum soil frost depth (ranging from 0.03 to 0.41 m; r = 0.951***). In the plots with deeper maximum soil frost, winter-to-spring N₂O fluxes represented 58% to 85% of the annual values. Soil N₂O production was stimulated when the soil frost depth was greater than 0.15 m or the daily mean soil temperature at 0.05-m depth was below −2.0 °C. In the soil with the greatest frost depth, soil gas N₂O concentrations at the depth of 0.10 m peaked at 46 ppm when soil gas O₂ concentrations fell down to 0.12 m³ m⁻³ under soil temperature below 0.0 °C. Snowmelt acceleration had no stimulating effect on N₂O production in the soil during the winter-to-spring period.     

Optimizing chamber methods for measuring nitrous oxide emissions from plot‐based agricultural experiments

Nitrous oxide emissions (N₂O) from agricultural land are spatially and temporally variable. Most emission measurements are made with small (? 1 m² area) static chambers. We used N₂O chamber data collected from multiple field experiments across different geo‐climatic zones in the UK and from a range of nitrogen treatments to quantify uncertainties associated with flux measurements. Data were analysed to assess the spatial variability of fluxes, the degree of linearity of headspace N₂O accumulation and the robustness of using ambient air N₂O concentrations as a surrogate for sampling immediately after closure (T₀). Data showed differences of up to more than 50‐fold between the maximum and minimum N₂O flux from five chambers within one plot on a single sampling occasion, and that reliability of flux measurements increased with greater numbers of chambers. In more than 90% of the 1970 cases where linearity of headspace N₂O accumulation was measured (with four or more sampling points), linear accumulation was observed; however, where non‐linear accumulation was seen this could result in a 26% under‐estimate of the flux. Statistical analysis demonstrated that the use of ambient air as a surrogate for T₀ headspace samples did not result in any consistent bias in calculated fluxes. Spatial variability has the potential to result in erroneous flux estimates if not taken into account, and generally introduces a far larger uncertainty into the calculated flux (commonly orders of magnitude more) than any uncertainties introduced through reduced headspace sampling or assumption of linearity of headspace accumulation. Hence, when deploying finite resources, maximizing chamber numbers should be given priority over maximizing the number of headspace samplings per enclosure period.     

A comparison of linear and exponential regression for estimating diffusive CH4 fluxes by closed-chambers in peatlands

The closed-chamber method is the most common approach to determine CH₄ fluxes in peatlands. The concentration change in the chamber is monitored over time, and the flux is usually calculated by the slope of a linear regression function. Theoretically, the gas exchange cannot be constant over time but has to decrease, when the concentration gradient between chamber headspace and soil air decreases. In this study, we test whether we can detect this non-linearity in the concentration change during the chamber closure with six air samples. We expect generally a low concentration gradient on dry sites (hummocks) and thus the occurrence of exponential concentration changes in the chamber due to a quick equilibrium of gas concentrations between peat and chamber headspace. On wet (flarks) and sedge-covered sites (lawns), we expect a high gradient and near-linear concentration changes in the chamber. To evaluate these model assumptions, we calculate both linear and exponential regressions for a test data set (n = 597) from a Finnish mire. We use the Akaike Information Criterion with small sample second order bias correction to select the best-fitted model. 13.6%, 19.2% and 9.8% of measurements on hummocks, lawns and flarks, respectively, were best fitted with an exponential regression model. A flux estimation derived from the slope of the exponential function at the beginning of the chamber closure can be significantly higher than using the slope of the linear regression function. Non-linear concentration-over-time curves occurred mostly during periods of changing water table. This could be due to either natural processes or chamber artefacts, e.g. initial pressure fluctuations during chamber deployment. To be able to exclude either natural processes or artefacts as cause of non-linearity, further information, e.g. CH₄ concentration profile measurements in the peat, would be needed. If this is not available, the range of uncertainty can be substantial. We suggest to use the range between the slopes of the exponential regression at the beginning and at the end of the closure time as an estimate of the overall uncertainty.     

Vents and seals in non-steady-state chambers used for measuring gas exchange between soil and the atmosphere

Despite decades of research to define optimal chamber design and deployment protocol for measuring gas exchange between the Earth's surface and the atmosphere, controversy still surrounds the procedures for applying this method. Using a numerical simulation model we demonstrated that (i) all non‐steady‐state chambers should include a properly sized and properly located vent tube; (ii) even seemingly trivial leakiness of the seals between elements of a multiple‐component chamber results in significant risk of measurement error; (iii) a leaking seal is a poor substitute for a properly designed vent tube, because the shorter path length through the seal supports much greater diffusive gas loss per unit of conductance to mass flow; (iv) the depth to which chamber walls must be inserted to minimize gas loss by lateral diffusion is smaller than is customary in fine‐textured, wet or compact soil, but much larger than is customary in highly porous soils, and (v) repetitive sampling at the same location is not a major source of error when using non‐steady‐state chambers. Finally, we discuss problems associated with computing the flux of a gas from the non‐linear increase in its concentration in the headspace of a non‐steady‐state chamber.     

Soil structure and greenhouse gas emissions: a synthesis of 20 years of experimentation

Soil structure affects microbial activity and thus influences greenhouse gas production and exchange in soil. Structure is variable and increasingly vulnerable to compaction and erosion damage as agriculture intensifies and climate changes. Few studies have specifically related the impact of structure and its variability to greenhouse gas (GHG) emissions over a wide range of soils and management treatments. The objective of this study was to draw from research in Scotland, Japan and New Zealand, which examined how soil structures affected by wheel compaction, animal trampling, tillage and land‐use change influence GHG emissions in order to help identify key controlling properties. Nitrous oxide (N₂O) is the main focus, though carbon dioxide (CO₂), methane (CH₄) and nitric oxide (NO) are included. Gas emissions were measured by using static chambers in the field or incubated intact cores. Poor structure, measured as small relative gas diffusivities and air permeabilities, restricted aeration, resulting in N₂O emission or consumption dependent on mineral nitrogen contents. Structural damage (identifiable using the Visual Evaluation of Soil Structure) was especially important near the soil surface where microsites of microbial activity were exposed and aeration was impaired. Moist, well‐aerated soils favoured CH₄ oxidation and CO₂ exchange. N₂O emissions were not necessarily increased in anaerobic soils because of possible N₂O consumption and microbial adaptation. Soil matric potential, volumetric water content, relative diffusivity, air permeability and water‐filled pore space are relevant indicators for N₂O and CH₄ flux and aeration status. As pore continuity and size are so relevant, pore‐scale models are likely to have an increasing role in understanding mechanisms of GHG production, transport and release.     

Nitrous oxide emission from the soil surface: Continuous measurement by gas chromatography

Nitrous oxide flux from the soil is determined by drawing atmospheric air through chambers on the soil surface and measuring the N₂O-content, using a gas Chromatograph (GC). The N₂O-concentration can be at least 2 times higher within the box than outside without altering the measured N₂O-flux. The flow through the chamber is not sufficient to produce pressure effects on the N₂O-concentration in the box. The effects of the chamber on soil temperature are negligible.     

Transient N2O accumulation and emission caused by O2 depletion in soil after liquid manure injection

Emission of nitrous oxide (N₂O) from soils is the net result of N₂O‐producing and consuming processes within the soil, and studying the regulation of these processes in the real soil environment is essential to the understanding of the factors governing N₂O emission. In this study, microscale distributions of O₂ and N₂O in the soil were investigated to describe how N₂O production within, and emission from, soils are regulated by anoxic volumes created by injection of liquid manure. An application device simulating field injection methodology was developed and liquid pig manure was injected at a depth of 5 cm into boxes containing soil. Microsensors with <0.12 mm tip diameter were used to measure high‐resolution vertical N₂O and O₂ concentration profiles though the centre of the horizontally positioned soil‐manure core and up to 4 cm laterally away from the centre. Both microsensor measurements and N₂O emission rate determinations, with a closed chamber, were performed daily. Injected manure filled the original air‐filled pore space of a 6‐cm‐wide cylindrical core and created anoxia. Nitrous oxide was detected in the anoxic part of the core, indicating N₂O production by denitrification in the entire anoxic volume. Although anoxia was present in the core during all 3 days of the experiment, a peak rate of net N₂O production was detected after 1 day, with a maximum N₂O accumulation of 500–700 Pa in the core. Comparison of the cumulated N₂O net production and emission revealed a delay of N₂O emission, as N₂O was trapped inside the saturated core.     

In-field management of corn cob and residue mix: Effect on soil greenhouse gas emissions

In-field management practices of corn cob and residue mix (CRM) as a feedstock source for ethanol production can have potential effects on soil greenhouse gas (GHG) emissions. The objective of this study was to investigate the effects of CRM piles, storage in-field, and subsequent removal on soil CO₂ and N₂O emissions. The study was conducted in 2010–2012 at the Iowa State University, Agronomy Research Farm located near Ames, Iowa (42.0°′N; 93.8°′W). The soil type at the site is Canisteo silty clay loam (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquolls). The treatments for CRM consisted of control (no CRM applied and no residue removed after harvest), early spring complete removal (CR) of CRM after application of 7.5 cm depth of CRM in the fall, 2.5 cm, and 7.5 cm depth of CRM over two tillage systems of no-till (NT) and conventional tillage (CT) and three N rates (0, 180, and 270 kg N ha⁻¹) of 32% liquid UAN (NH₄NO₃) in a randomized complete block design with split–split arrangements. The findings of the study suggest that soil CO₂ and N₂O emissions were affected by tillage, CRM treatments, and N rates. Most N₂O and CO₂ emissions peaks occurred as soil moisture or temperature increased with increase precipitation or air temperature. However, soil CO₂ emissions were increased as the CRM amount increased. On the other hand, soil N₂O emissions increased with high level of CRM as N rate increased. Also, it was observed that NT with 7.5 cm CRM produced higher CO₂ emissions in drought condition as compared to CT. Additionally, no differences in N₂O emissions were observed due to tillage system. In general, dry soil conditions caused a reduction in both CO₂ and N₂O emissions across all tillage, CRM treatments, and N rates.     

Effects of biochar amendment on greenhouse gas emissions,net ecosystem carbon budget and properties of an acidic soil under intensive vegetable production

Biochar addition to soils has been frequently proposed as a means to increase soil fertility and carbon (C) sequestration. However, the effect of biochar addition on greenhouse gas emissions from intensively managed soils under vegetable production at the field scale is poorly understood. The effects of wheat straw biochar amendment with mineral fertilizer or an enhanced‐efficiency fertilizer (mixture of urea and nitrapyrin) on N₂O efflux and the net ecosystem C budget were investigated for an acidic soil in southeast China over a 1‐yr period. Biochar addition did not affect the annual N₂O emissions (26–28 kg N/ha), but reduced seasonal N₂O emissions during the cold period. Biochar increased soil organic C and CO₂ efflux on average by 61 and 19%, respectively. Biochar addition greatly increased C gain in the acidic soil (average 11.1 Mg C/ha) compared with treatments without biochar addition (average ?2.2 Mg C/ha). Biochar amendment did not increase yield‐scaled N₂O emissions after application of mineral fertilizer, but it decreased yield‐scaled N₂O by 15% after nitrapyrin addition. Our results suggest that biochar amendment of acidic soil under intensive vegetable cultivation contributes to soil C sequestration, but has only small effects on both plant growth and greenhouse gas emissions.     

Above and Below Ground Measurements of Greenhouse Gases from Swine Effluent Amended Soil

Abstract

As a means of economic disposal and to reduce need for chemical fertilizer, waste generated from swine production is often applied to agricultural land. However, there remain many environmental concerns about this practice. Two such concerns, contribution to the greenhouse effect and stratospheric ozone depletion by gases emitted from waste‐amended soils, have not been thoroughly investigated. An intact core study at Auburn University (32 36′N, 85 36′W) was conducted to determine the source‐sink relationship of three greenhouse gases in three Alabama soils (Black Belt, Coastal Plain, and Appalachian Plateau regions) amended with swine waste effluent. Soil cores were arranged in a completely random design, and treatments used for each soil type consisted of a control, a swine effluent amendment (112 kg N ha^?1), and an ammonium nitrate (NH₄NO₃) fertilizer amendment (112 kg N ha^?1). During a 2‐year period, a closed‐chamber technique was used to determine rates of emission of nitrous oxide (N₂O)–nitrogen (N), carbon dioxide (CO₂)–carbon (C), and methane (CH₄)–C from the soil surface. Gas probes inserted into the soil cores were used to determine concentrations of N₂O‐N and CO₂‐C from depths of 5, 15, and 25 cm. Soil water was collected from each depth using microlysimeters at the time of gas collection to determine soil‐solution N status. Application of swine effluent had an immediate effect on emissions of N₂O‐N, CO₂‐C, and CH₄‐C from all soil textures. However, greatest cumulative emissions and highest peak rates of emission of all three trace gases, directly following effluent applications, were most commonly observed from sandier textured Coastal Plain and Appalachian Plateau soils, as compared to heavier textured Black Belt soil. When considering greenhouse gas emission potential, soil type should be a determining factor for selection of swine effluent waste disposal sites in Alabama.     

A comparison of regression methods for estimating soil-atmosphere diffusion gas fluxes by a closed-chamber technique

Continuous changes in methane (CH₄) and carbon dioxide (CO₂) concentrations inside a closed chamber were measured on the forest floor at three sites: a deciduous forest and a coniferous forest in Hokkaido, Japan, and a birch forest in West Siberia, Russian Federation. Flux estimations by three types of regression methods, exponential, nonlinear, and linear, were examined using field-collected concentration data. The pattern of change with time of the gas concentration in the headspace differed, mainly according to site but also, to a lesser extent, according to the gas. This was a function of both the chamber height and surface soil property relating to soil gas diffusion and the gas concentration profile. Flux estimations did not differ statistically between the exponential and nonlinear methods for either gas at any site, because both of those regression methods were based on diffusion theory. However, the flux values estimated by linear regression were significantly different from those estimated by the other two methods for both CH₄ and CO₂ at the deciduous forest site and for CO₂ at the coniferous forest site. Shortening the chamber deployment period improved the linearity of the curve, but did not completely eliminate the error. Our results suggest that linear regression is not a good model of the change in headspace concentration with time.     

Denitrification and nitrous oxide to nitrous oxide plus dinitrogen ratios in the soil profile under three tillage systems

There is a growing interest in the adoption of conservation tillage systems [no-till (NT) and reduced tillage (RT)] as alternatives to conventional tillage (CT) systems. A 2-year study was conducted to investigate possible environmental consequences of three tillage systems on a 2.4-ha field located at Macdonald Research Farm, McGill University, Montreal. The soil was a sandy loam (0.5 m depth) underlain by a clay layer. Treatments consisted of a factorial combination of CT, RT, and NT with the presence or absence of crop residue. Soil NO₃^--N concentrations tended to be lower in RT than NT and CT tillage treatments. Denitrification and N₂O emissions were similar among tillage systems. Contrary to the popular assumption that denitrification is limited to the uppermost soil layer (0–0.15 m), large rates of N₂O production were measured in the subsurface (0.15–0.45 m) soil, suggesting that a significant portion of produced N₂O may be missed if only soil surface gas flux measurements are made. The N₂O mole fraction (N₂O:N₂O+N₂) was higher in the drier season of 1999 under CT than in 2000, with the ratio occasionally exceeding 1.0 in some soil layers. Dissolved organic C concentrations remained high in all soil depths sampled, but were not affected by tillage system.     

Measuring N2O emissions from organic soils by closed chamber or soil/snow N2O gradient methods

Drained organic soils contribute substantial amounts of nitrous oxide to the global atmosphere, and we should be able to estimate this contribution. We have investigated when the fluxes of N₂O from drained forested or cultivated organic soils could be determined by calculating the fluxes from the concentration gradients of the gas in soil or snow according to Fick's law of diffusion. A static chamber method was applied as a control technique for the gas gradient method. Concentrations of N₂O in soil varied from 296 nl l^?1 to 8534 nl l^?1 during the snow‐free periods and were greatest in the early summer. Our results suggest that the gas gradient method can be used to estimate N₂O emissions from drained organic soils. There was some systematic difference in the N₂O fluxes measured with these two methods, which we attributed to the differences in weather between years 1996 and 1997. In the wet summer of 1996 the chamber method gave greater flux rates than the gas gradient method, and the reverse was true in the dry summer of 1997. In the forest the N₂O fluxes measured with the two methods agreed well. The gas gradient is convenient and fast for measuring N₂O emissions from fairly dry organic unfrozen soil. In winter the diffusion calculation based on the N₂O gradients in snow and the chamber method gave fairly similar flux rates and provided adequate estimates of the fluxes of N₂O in winter.     

Effects of N placement,carbon distribution and temperature on N2O emissions in clay loam and loamy sand soils

Changes in the profile distribution of soil C stocks for conventional versus no‐tillage can affect N₂O losses. Uncertainty remains whether deep N placement into a wetter layer in humid areas would affect N₂O losses. This study evaluated the effects of soil carbon profile distribution (inverted, normal), depth of nitrogen placement (5 cm, 15 cm), temperature (10, 20 and 30 °C) and soil texture (clay loam, loamy sand) on N₂O emissions from soil cores in a 216‐h incubation after simulated rainfall. N₂O losses were larger from the clay loam than from the loamy sand, and cumulative N₂O emissions from the inverted profile, with greater C levels at depth, were more than those from the profile with more C near the upper surface. Cumulative N₂O losses from the inverted clay loam profile with deep N placement (1.16 mg N per kg dry soil; 0.71% of applied N) on average were almost double those in the loamy sand (0.62 mg N per kg dry soil; 0.42%). The smallest N₂O losses were measured from the profiles with more C close to the upper surface with a shallow placement of N for the clay loam (0.19 mg N per kg dry soil; 0.12%) and loamy sand (0.33 mg N per kg dry soil; 0.23%). An exponential relationship between N₂O fluxes and temperature was measured. We conclude that large N₂O losses may occur under the combination of greater soil C content at deeper layers (ploughed soils) and moist profiles after N application (humid regions). Deep N placement appears to aggravate rather than ameliorate these concerns.     

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.