Estimates of N2O and CH4 fluxes from agricultural lands in various regions in Europe

According to the revised 1996 IPCC guidelines, several emission factors are needed to calculate national inventories of N₂O emissions from agriculture. To estimate the direct N₂O emissions from mineral soils, an emission factor of 0.0125 kg N₂O-N per kg N applied is currently being used. From recent literature data it was clearly shown that real N₂O emissions could differ substantially from this value. Based on the IPCC methodology an inventory of N₂O emission from agriculture in Europe (EU-15) has been made. In 1996, the N₂O emission was estimated at 672 Gg N₂O-N. The N₂O emission per country varied between 10 and 177 Gg N₂O-N. The N₂O emission per ha agricultural land in the various countries varied between 1.7 and 14.2 kg N₂O-N ha⁻¹. Highest N₂O emissions per ha were found in countries with a high agricultural intensity, such as the Netherlands, Belgium-Luxembourg, Denmark and Germany. Agricultural soils are a sink for atmospheric methane. An oxidation capacity of 2.5 and 1.5 kg CH₄ ha⁻¹ yr⁻¹ was put forward for grasslands and arable land, respectively. Based on land use data of 1993, the CH₄ sink of agricultural lands in EU-15 was estimated at 303.5 Gg CH₄. In general, it could be concluded that N₂O emissions from soils (327 Tg CO₂ equivalents) are far more important than its sink function for CH₄ (6.3 Tg CO₂ equivalents). This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of agriculture on methane oxidation in soil

Aerobic soils are an important sink for methane (CH₄) contributing up to 15% of annual global CH₄ destruction. However, the sink strength is significantly affected by land management, nitrogen (N) fertilizers and acidity. We tested these effects on samples taken from the Broadbalk Continuous Wheat, Park Grass permanent grassland and Broadbalk and Geescroft Wilderness experiments at Rothamsted. The rates of uptake from the atmosphere of both enhanced (10 ppmv) and ambient (2 ppmv) concentrations of CH₄ were measured in laboratory incubations of soil cores under controlled conditions. The most rapid rates of uptake were measured in soil from deciduous woodland at pH 7 (measured in water); acidic (pH 4) woodland soil showed no net CH₄ oxidation. While disturbance of the cores used in the experiments did not affect the rate of CH₄ uptake, extended (150 years) cultivation of land for arable crops reduced uptake rate by 85% compared to that in the soil under calcareous woodland. The long-term application of ammonium- (NH₄) based fertilizer, but not nitrate- (NO₃) based fertilizer, completely inhibited CH₄ uptake, but the application for the same period of farmyard manure that contained more N than the fertilizer had no inhibitory effect. Although the effects of agricultural practice on the oxidation of CH₄ in soil are significant, the differences in oxidation rates between land use types are even greater. The likely effects of forest clearance, agricultural intensification and anthropogenic emissions of CH₄ over the last 2500 years have been estimated for the United Kingdom. The calculations indicate that 54% of the current CH₄ uptake by UK soils is the result of increased CH₄ mixing ratio. They also indicate that land use change has decreased the potential sink strength by 62% or 37 kt CH₄ g^-1. In countries with much larger land areas than the UK, such as China, aerobic soil is likely to be a more significant factor in calculating net fluxes of CH₄. It is important that the impacts of different agricultural managements and land use systems are understood and quantified so that the best possible estimate of CH₄ sinks is calculated for comparison with sources. This revised version was published online in August 2006 with corrections to the Cover Date.     

Assessment of CH4 and N2O fluxes in a Danish beech (Fagus sylvatica) forest and an adjacent N-fertilised barley (Hordeum vulgare) field: effects of sewage sludge amendments

Fluxes of CH₄ and N₂O were measured regularly in an agricultural field treated with 280 g m⁻² of sewage sludge. In a nearby beech forest N₂O and CH₄ fluxes were measured in a well-drained (dry) area and in a wet area adjacent to a drainage canal. We observed brief increases of both CH₄ and N₂O emissions immediately following soil applications of digested sewage sludge. Cumulated values for CH₄ emissions over the course of 328 days after sludge applications indicated a small net source in sludge treated plots (7.6 mg C m⁻²) whereas sludge-free soil constituted a small sink (-0.9 mg C m⁻²). The CH₄ emission amounted 0.01% of the sludge-C. Extrapolated to current rates of sludge applications in Danish agriculture this amounts to 0.1% of the total agricultural derived CH₄. Sludge applications did not affect cumulated fluxes of N₂O showing 312 mg N₂O–N m⁻² and 304 mg N m⁻² with and without sludge, respectively. Four months after the sludge applications a significant effect on CO₂ and NO emissions was still obvious in the field, the latter perhaps due to elevated nitrification. Nitrous oxide emission in the beech forest was about six times smaller (45 mg N m⁻²) than in the field and independent of drainage status. Methane oxidation was observed all-year round in the forest cumulating to -225 mg C m⁻² and -84 mg C m⁻² in dry and wet areas. In a model experiment with incubated soil cores, nitrogen amendment (NH₄Cl) and perturbation significantly reduced CH₄ oxidation in the forest soil, presumably as a result of increased nitrification activity. Sludge also induced net CH₄ production in the otherwise strong CH₄ oxidising forest soil. This emphasises the potential for CH₄ emissions from sewage sludge applications onto land. The study shows, however, that emissions of N₂O and CH₄ induced by sewage sludge in the field is of minor importance and that factors such as land use (agriculture versus forest) is a much stronger controller on the source/sink strengths of CH₄ and N₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Greenhouse gas emissions from tropical peatlands of Kalimantan,Indonesia

Greenhouse gas emissions were measured from tropical peatlands of Kalimantan, Indonesia. The effect of hydrological zone and land-use on the emission of N₂O, CH₄ and CO₂ were examined. Temporal and annual N₂O, CH₄ and CO₂ were then measured. The results showed that the emissions of these gases were strongly affected by land-use and hydrological zone. The emissions exhibited seasonal changes. Annual emission of N₂O was the highest (nearly 1.4 g N m^–2y^–1) from site A-1 (secondary forest), while there was no signi.cant difference in annual N₂O emission from site A-2 (paddy field) and site A-3 (rice-soybean rotation field). Multiplying the areas of forest and non-forest in Kalimantan with the emission of N₂O from corresponding land-uses, the annual N₂O emissions from peat forest and peat non-forest of Kalimantan were estimated as 0.046 and 0.004 Tg N y^–1, respectively. The emissions of CH₄ from paddy field and non-paddy field were estimated similarly as 0.14 and 0.21 Tg C y^–1, respectively. Total annual CO₂ emission was estimated to be 182 Tg C y^–1. Peatlands of Kalimantan, Indonesia, contributed less than 0.3 of the total global N₂O, CO₂ or CH₄ emission, indicating that the gaseous losses of soil N and C from the study area to the atmosphere were small.     

Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics

Losses of carbon (C) stocks in terrestrial ecosystems and increasing concentrations of greenhouse gases in the atmosphere are challenges that scientists and policy makers have been facing in the recent past. Intensified agricultural practices lead to a reduction in ecosystem carbon stocks, mainly due to removal of aboveground biomass as harvest and loss of carbon as CO₂ through burning and/or decomposition. Evidence is emerging that agroforestry systems are promising management practices to increase aboveground and soil C stocks and reduce soil degradation, as well as to mitigate greenhouse gas emissions. In the humid tropics, the potential of agroforestry (tree-based) systems to sequester C in vegetation can be over 70 Mg C ha^–1, and up to 25 Mg ha^–1 in the top 20 cm of soil. In degraded soils of the sub-humid tropics, improved fallow agroforestry practices have been found to increase top soil C stocks up to 1.6 Mg C ha^–1 y^–1 above continuous maize cropping. Soil C accretion is linked to the structural development of the soil, in particular to increasing C in water stable aggregates (WSA). A review of agroforestry practices in the humid tropics showed that these systems were able to mitigate N₂O and CO₂ emissions from soils and increase the CH₄ sink strength compared to cropping systems. The increase in N₂O and CO₂ emissions after addition of legume residues in improved fallow systems in the sub-humid tropics indicates the importance of using lower quality organic inputs and increasing nutrient use efficiency to derive more direct and indirect benefits from the system. In summary, these examples provide evidence of several pathways by which agroforestry systems can increase C sequestration and reduce greenhouse gas emissions.     

CH4 emission and oxidation in Chinese rice paddies

In the paper, the characteristics of CH₄ emission from the rice paddies, its temporary and spatial variations as well as factors regulating CH₄ emission and oxidation are reviewed with an emphasis on CH₄ emission from rice paddies in China. The observed four types of diel variation and two type of seasonal variation can be explained by the variations of methane production in the soil and the transport efficiencies of the three transport routs. The inter-annual variation of CH₄ emission from rice fields is significant, but the process causing this change is very complicated and unclear based on the available data at present. The large special variation, more than 10 times difference, of the total season methane emissions observed in various rice fields in China, is largely attributed to soil type difference although both soil physics and chemistry are important. Rice growing activities regulate the diel and seasonal variation patterns of the methane emissions. Drainage of flooded water may significantly reduce the emission. Organic fertilizer may enhance the emission, while some of the chemical fertilizers may reduce the emission. Local climate conditions, average temperature and annual rainfall, may be responsible for part of the observed year to year differences of the total season emission. Estimates of total emissions of CH₄ from Chinese rice fields, based on field measurement and model calculation, are 9.7–12.7 Tg/year and 8.17–10.52 Tg/year respectively, for the year of 1994. Oxidation of CH₄ reduces the emission of CH₄ produced in the soil of rice field to the atmosphere. The most likely sites for CH₄ oxidation in rice fields are the water–soil interface and the rhizosphere. When the flood water dries up in irrigated fields, the oxidation of CH₄ in the soil is more important and can partially explain the lower emission rates during the last period before harvest in most experiments. The magnitude of oxidation in the rhizosphere is not well known. Good correlation between methane reduction and O₂ mixing ratio in the soil has been found in most soil types. Methane oxidation rate is mainly controlled by the gas transport resistance in the soil. The oxidation rate increases with the increase of temperature in the temperature range of 5–36 °C.     

Nitric oxide emissions from agricultural soils in temperate and tropical climates: sources, controls and mitigation options

Global annual NO emissions from soil are of the order of 10 Tg NO-N. This is about half the amount fossil fuel combustion processes contribute to the annual global NO_x budget. Reducing the emissions of soil derived NO_x requires an understanding of the source of the flux and the processes that determine its magnitude. A thorough investigation of possible mitigation strategies and the consequences of their implementation is also necessary. The ratio of NO and N₂O emissions from soils can be used as an indicator of the dominant NO production pathway operating. Fertilizer application (rate, type and time of application), soil temperature, soil water content and soil management practices all affect the emission rate and are reviewed. Mitigation options include reduction in N fertilizer use through an increase in fertilizer use efficiency, preferential use of NH⁴NO₃ instead of urea, improved timing of fertilizer application, the use of nitrification and urease inhibitors, improving the fertilizer uptake efficiency of crops in tropical agriculture and changes in land management. Several of the viable mitigation strategies, mainly those increasing fertilizer use efficiency, have the capacity to reduce global annual NO emissions by 4% (0.4 Tg NO-N y^-1). For other strategies including use of inhibitors, changing cultivation or land use, the possible reductions are too uncertain to justify quantification on the basis of present knowledge.     

Enhancing the carbon sink in European agricultural soils: including trace gas fluxes in estimates of carbon mitigation potential

The possibility that the carbon sink in agricultural soils can be enhanced has taken on great political significance since the Kyoto Protocol was finalised in December 1997. The Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere. Thus, forestry activities (Article 3.3) and changes in the use of agricultural soils (Article 3.4) that are shown to reduce atmospheric CO₂levels may be included in the Kyoto emission reduction targets. The European Union is committed to a reduction in CO₂ emissions to 92% of baseline (1990) levels during the first commitment period (2008–2012). We have shown recently that there are a number of agricultural land-management changes that show some potential to increase the carbon sink in agricultural soils and others that allow alternative forms of carbon mitigation (i.e. through fossil fuel substitution), but the options differ greatly in their potential for carbon mitigation. The changes examined were, (a) switching all animal manure use to arable land, (b) applying all sewage sludge to arable land, (c) incorporating all surplus cereal straw, (d) conversion to no-till agriculture, (e) use of surplus arable land to de-intensify 1/3 of current intensive crop production (through use of 1/3 grass/arable rotations), (f) use of surplus arable land to allow natural woodland regeneration, and (g) use of surplus arable land for bioenergy crop production. In this paper, we attempt for the first time to assess other (non-CO₂) effects of these land-management changes on (a) the emission of the other important agricultural greenhouse gases, methane and nitrous oxide, and (b) other aspects of the ecology of the agroecosystems. We find that the relative importance of trace gas fluxes varies enormously among the scenarios. In some such as the sewage sludge, woodland regeneration and bioenergy production scenarios, the inclusion of trace gases makes only a small (<10%) difference to the CO₂-C mitigation potential. In other cases, for example the no-till, animal manure and agricultural de-intensification scenarios, trace gases have a large impact, sometimes halving or more than doubling the CO₂-C mitigation potential. The scenarios showing the greatest increase when including trace gases are those in which manure management changes significantly. In the one scenario (no-till) where the carbon mitigation potential was reduced greatly, a small increase in methane oxidation was outweighed by a sharp increase in N₂O emissions. When these land-management options are combined to examine the whole agricultural land area of Europe, most of the changes in mitigation potential are small, but depending upon assumptions for the animal manure scenario, the total mitigation potential either increases by about 20% or decreases by about 10%, shifting the mitigation potential of the scenario from just above the EU's 8% Kyoto emission reduction target (98.9 Tg C y⁻¹) to just below it. Our results suggest that (a) trace gas fluxes may change the mitigation potential of a land management option significantly and should always be considered alongside CO₂-C mitigation potentials and (b) agricultural management options show considerable potential for carbon mitigation even after accounting for trace gas fluxes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Canadian greenhouse gas mitigation options in agriculture

In 1991, on farm management practices contributed 57.6 Tg CO₂ equivalent in greenhouse gas emissions, that is, about 10% of the anthropogenic GHG emissions in Canada. Approximately 11% of these emissions were in the form of CO₂, 36% in the form of CH₄ and 53% in the form of N₂O. The CO₂ emissions were from soils; CH₄ emissions were from enteric fermentation and manure, and N₂O emissions were primarily a function of cropping practices and manure management. With the emissions from all other agricultural practices included, such as the emissions from fossil fuels used for transportation, manufacturing, food processing etc., the agricultural sector's contributions were about 15% of Canada's emissions. In this publication, several options are examined as to their potential for reducing greenhouse gas emissions. These involve soil and crop management, soil nutrient management, improved feeding strategies, and carbon storage in industrial by-products. The Canadian Economic Emissions Model for Agriculture (CEEMA) was used to predict the greenhouse gas emissions for the year 2010, as well as the impact of mitigation options on greenhouse gas emissions from the agricultural sector. This model incorporates the Canadian Regional Agricultural sub-Model (CRAM), which predicts the activities related to agriculture in Canada up to 2010, as well as a Greenhouse Gas Emissions sub-Model (GGEM), which estimates the greenhouse gas emissions associated with the various agricultural activities. The greenhouse gas emissions from all agricultural sources were 90.5 Tg CO₂ equivalent in 1991. Estimates based on CEEMA for the year 2010 indicate emissions are expected to be 98.0 Tg CO₂ equivalent under a business as usual scenario, which assumes that the present trends in management practices will continue. The agricultural sector will then need to reduce its emissions by about 12.9 Tg CO₂ equivalent below 2010 forecasted emissions, if it is to attain its part of the Canadian government commitment made in Kyoto. Technologies focusing on increasing the soil carbon sink, reducing greenhouse gas emissions and improving the overall farming efficiency, need to be refined and developed as best management practices. The soils carbon sink can be increased through reduced tillage, reduced summer fallowing, increased use of grasslands and forage crops, etc. Key areas for the possible reduction of greenhouse gas emissions are improved soil nutrient management, improved manure storage and handling, better livestock grazing and feeding strategies, etc. The overall impact of these options is dependent on the adoption rate. Agriculture's greenhouse gas reduction commitment could probably be met if soils are recognized as a carbon sink under the Kyoto Accord and if a wide range of management practices are adopted on a large scale. None of these options can currently be recommended as measures because their socio-economic aspects have not been fully evaluated and there are still too many uncertainties in the emission estimates. This revised version was published online in August 2006 with corrections to the Cover Date.     

Laboratory study of methane oxidation in paddy soils

Methane oxidation in paddy soils was investigated under laboratory conditions. Paddy soils collected before early rice transplanting could not oxidize atmospheric CH₄ but could oxidize CH₄ when the concentration exceeded 10 μl l^-1. Initial CH₄ oxidation rate increased with the increase of initial CH₄ concentration. Soil with the maximum potential to produce CH₄, also had the maximum CH₄ oxidation activity and the maximum emission flux from paddy soil. High CH₄ concentration stimulated the oxidation of CH₄. After 10 days' incubation under atmosphere containing 1000 μl^-1 or 10⁴ μl l^-1 CH₄, the soil which could not oxidize atmospheric CH₄ was able to oxidize it. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fluxes of CO2 and CH4 in soil profiles of a mountainous watershed of Nepal as influenced by land use, temperature, moisture and substrate addition

Effects of land use, moisture, temperature and substrate on production of CO₂ and consumption of CH₄ were measured in a series of laboratory incubation experiments on bulk soil samples from 0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths under four dominant land uses [forest, grazing land, irrigated rice on level terraces (Khet), and upland maize–millet on sloping terraces (Bari)] of Mardi watershed (area=144 km²), Nepal. In addition, baseline physical and chemical properties of these soils were measured. The production of CO₂-C day^–1 per unit soil organic carbon (SOC) content in topsoil was lowest in grazing land, indicating a possibility of higher C sequestration with this land use than with other land uses. There was a decreasing trend of CO₂ emission with soil depth in all land uses, as was also the case with the SOC content. The CO₂ emission was increased by 90, 58, 27 and 23% for Bari, Khet, grazing and forest soil, respectively, with increase in moisture level from 40 to 60% (w/w). The CO₂ release from forest soil went up from 15 to 50 g CO₂ g^–1 dry soil with increase in temperature from 5 to 15 °C and it further increased to 67 g CO₂ g^–1 at 20 °C with estimated Q ₁₀ values of 3.4 and 1.8, respectively. Significantly higher amounts of CO₂ were emitted from all the land use types upon addition of glucose to the soil, indicating high potential of microbial activity. Consumption of CH₄ was more rapid in the soil from 10 to 20 cm depth for all the land use types. There was a 89% increase in CH₄ consumption in forest soil with increase in moisture level from 40 to 60%, while it was decreased by 38, 73, and 40% for Khet, Bari and grazing soil, respectively. Addition of (NH₄)₂SO₄ inhibited CH₄ oxidation in soils of all land uses, indicating a negative effect of N fertiliser input on CH₄ uptake in soil.     

Land use change and soil organic carbon dynamics

Historically, soils have lost 40–90 Pg carbon (C) globally through cultivation and disturbance with current rates of C loss due to land use change of about 1.6 ± 0.8 Pg C y⁻¹, mainly in the tropics. Since soils contain more than twice the C found in the atmosphere, loss of C from soils can have a significant effect of atmospheric CO₂ concentration, and thereby on climate. Halting land-use conversion would be an effective mechanism to reduce soil C losses, but with a growing population and changing dietary preferences in the developing world, more land is likely to be required for agriculture. Maximizing the productivity of existing agricultural land and applying best management practices to that land would slow the loss of, or is some cases restore, soil C. There are, however, many barriers to implementing best management practices, the most significant of which in developing countries are driven by poverty. Management practices that also improve food security and profitability are most likely to be adopted. Soil C management needs to considered within a broader framework of sustainable development. Policies to encourage fair trade, reduced subsidies for agriculture in developed countries and less onerous interest on loans and foreign debt would encourage sustainable development, which in turn would encourage the adoption of successful soil C management in developing countries. If soil management is to be used to help address the problem of global warming, priority needs to be given to implementing such policies.     

Effects of N management on N2O and CH4 fluxes and15N

Forage production in irrigated mountain meadows plays a vital role in the livestock industry in Colorado and Wyoming. Mountain meadows are areas of intensive fertilization and irrigation which may impact regional CH₄ and N₂O fluxes. Nitrogen fertilization typically increases yields, but N-use efficiency is generally low. Neither the amount of fertilizer-N recovered by the forage nor the effect on N₂O and CH₄ emissions were known. These trace gases are long-lived in the atmosphere and contribute to global warming potential and stratospheric ozone depletion. From 1991 through 1993 studies were conducted to determine the effect of N source, and timing of N-fertilization on forage yield, N-uptake, and trace gas fluxes at the CSU Beef Improvement Center near Saratoga, Wyoming. Plots were fertilized with 168 kg N ha^-1. Microplots labeled with¹⁵N-fertilizer were established to trace the fate of the added N. Weekly fluxes of N₂O and CH₄ were measured during the snow-free periods of the year. Although CH₄ was consumed when soils were drying, flood irrigation converted the meadow into a net source of CH₄. Nitrogen fertilization did not affect CH₄ flux but increased N₂O emissions. About 5% of the applied N was lost as N₂O from spring applied NH₄NO₃, far greater than the amount lost as N₂O from urea or fall applied NH₄NO₃. Fertilizer N additions increased forage biomass to a maximum of 14.6 Mg ha^-1 with spring applied NH₄NO₃. Plant uptake of N-fertilizer was greater with spring applications (42%), than with fall applications (22%).     

Influence of Azolla on CH4 Emission from Rice Fields

Azolla is an aquatic fern that has been used successfully as a dual crop with wetland rice. Rice fields are a major source of atmospheric CH₄, which is an important greenhouse gas. In this study, field and laboratory experiments showed that growing Azolla as a dual crop could enhance CH₄ emission from rice fields. In pot experimen indications showed that Azolla could mediate CH₄ transport from the floodwater of a rice soil into the atmosphere. It was also found that due to the presence of Azolla, chemical soil properties could be developed, stimulating CH₄ production and decreasing in situ CH₄ removal.     

Combining Upscaling and Downscaling of Methane Emissions from Rice Fields: Methodologies and Preliminary Results

The uncertainty in the methane (CH₄) source strength of rice fields is among the highest of all sources in the global CH₄ budget. Methods to estimate the source strength of rice fields can be divided into two scaling categories: bottom-up (upscaling) and top-down (downscaling). A brief review of upscaling and downscaling methodologies is presented. The combination of upscaling and downscaling methodologies is proposed as a potential method to reduce the uncertainty in the regional CH₄ source strength of rice fields. Some preliminary results based on upscaling and downscaling are presented and the limitations of the approaches are discussed. The first case study focuses on upscaling by using a field-scale model in combination with spatial databases to calculate CH₄ emissions for the island of Java. The reliability of upscaling results is limited by the uncertainty in model input parameters such as soil properties and organic carbon management. Because controlling variables such as harvested rice area may change on relatively short time scales, a land use change model (CLUE) was used to quantify the potential land use changes on Java in the period 1994–2010. The predicted changes were evaluated using the CH₄ emission model. Temporal scaling by coupling land use change models and emission models is necessary to answer policy-related questions on future greenhouse gas emissions. In a downscaling case study, we investigate if inverse modeling can constrain the emissions from rice fields by testing a standard CH₄ from rice scenario and a low CH₄ from rice scenario (80 and 30 Tg CH₄ yr^–1, respectively). The results of this study are not yet conclusive; to obtain fine-resolution CH₄ emission estimates over the Southeast Asian continent, the monitoring network atmospheric mixturing ratios need to be extended and located closer to the continental sources.     

Methane oxidation in soils with different textures and land use

Intact core samples from soils with different textures and land use were tested for their capacity to oxidise methane. The soil cores were taken from arable land, grassland and forest. It was found that coarse textured soils (6.74–16.38 μg CH₄ m^-2 h^-1) showed a higher methane uptake rate than fine textured soils (4.66–5.34 μg CH₄ m^-2 h^-1). Increasing soil tortuosity was thought to reduce the methane oxidation rate in fine textured soils. The oxidation rate of forest soils (16.32–16.38 μg CH₄ m^-2 h^-1), even with a pH below 4.5, was very pronounced and higher than arable land (11.40–14.47 μg CH₄ m^-2 h^-1) and grassland (6.74–9.30 μg CH₄ m^-2 h^-1). Within the same textural class arable land showed a faster methane uptake rate than grassland. In grassland with a fine texture, even methane production was observed. Nitrogen availability and turnover in these land use systems were thought to cause the different oxidation rates. Decreasing the moisture content slowed down the oxidation rate in all soils. This could be caused by an increased N turnover and a starvation of the methanotrophic bacteria. This revised version was published online in August 2006 with corrections to the Cover Date.     

Two-year field study on the emission of N2O from coarse and middle-textured Belgian soils with different land use

In the following study N₂O emissions from 3 different grasslands and from 3 different arable lands, representing major agriculture areas with different soil textures and normal agricultural practices in Belgium, have been monitored for 1 to 2 years. One undisturbed soil under deciduous forest was also included in the study. Nitrous oxide emission was measured directly in the field from vented closed chambers through photo-acoustic infrared detection. Annual N₂O emissions from the arable lands ranged from 0.3 to 1.5 kg N ha⁻¹ y⁻¹ and represent 0.3 to 1.0% of the fertilizer N applied. Annual N₂O emissions from the intensively managed grasslands and an arable land sown with grass were significantly larger than those from the cropped arable lands. Emissions ranged from 14 to 32 kg N ha⁻¹ y⁻¹, representing fertilizer N losses between 3 and 11%. At the forest soil a net N₂O uptake of 1.3 kg N₂O-N ha⁻¹ was recorded over a 2-year period. It seems that the N₂O-N loss per unit of fertilizer N applied is larger for intensively managed and heavily fertilized (up to 500 kg N ha⁻¹) grasslands than for arable lands and is substantially larger than the 1.25% figure used for the global emission inventory. Comparison of the annual emission fluxes from the different soils also indicated that land use rather than soil properties influenced the N₂O emission. Our results also show once again the importance of year-round measurements for a correct estimate of N₂O losses from agricultural soils: 7 to 76% of the total annual N₂O was emitted during the winter period (October–February). Disregarding the emission during the off-season period can lead to serious underestimation of the actual annual N₂O flux. This revised version was published online in August 2006 with corrections to the Cover Date.     

Carbon sequestration in soils of cool temperate regions (introductory and editorial)

The cool temperate climate, dominance of perennial land use, and relatively large proportion of peat and organically rich soils, make the northern European regions to have a large potential of soil organic carbon (SOC) sequestration. However, with predicted global warming soils in these areas may become sources of atmospheric CO₂. Quantitative and reliable assessment methods and understanding of the spatial variability of SOC pools are required to make accurate mean estimate of available C and integrate variability into predictive models of SOC reserves and sequestration potential. Advanced analytical methods such as near-infrared spectroscopy and carbon isotope techniques can be used to estimate retention time and C turnover rates in soils. The rehabilitation of peat lands has shown a potential for SOC sequestration ranging from 25 to 45 gCm⁻² yr⁻¹ in Scandinavian countries. The potential of SOC sequestration in agricultural and forestry ecosystems depends on the land use and management practice adopted. Furthermore, the proven land management practices (e.g. conservation tillage, reduced soil erosion, restoring wetlands and degraded lands) coupled with improved cultivation practices (e.g. judicious fertilizer use, crop rotations and cover crops) can make the soil of this region as C sink.     

Effect of Land Management in Winter Crop Season on CH4 Emission During the Following Flooded and Rice-Growing Period

A greenhouse pot experiment was carried out to study the effect of land management during the winter crop season on methane (CH₄) emissions during the following flooded and rice-growing period. Three land management patterns, including water management, cropping system, and rice straw application time were evaluated. Land management in the winter crop season significantly influenced CH₄ fluxes during the following flooded and rice-growing period. Methane flux from plots planted to alfalfa (ALE) in the winter crop season was significantly higher than those obtained with treatments involving winter wheat (WWE) or dry fallow (DFE). Mean CH₄ fluxes of treatments ALE, WWE, and DFE were 28.6, 4.7, and 4.1 mg CH₄ m^–2 h^–1 in 1996 and 38.2, 5.6, and 3.2 mg CH₄ m^–2 h^–1 in 1997, respectively. The corresponding values noted with continuously flooded fallow (FFE) treatment were 6.1 and 5.2 times higher than that of the dry fallow treatment in 1996 and 1997, respectively. Applying rice straw just before flooding the soil (DFL) significantly enhanced CH₄ flux by 386% in 1996 and by 1,017% in 1997 compared with rice straw application before alfalfa seed sowing (DFE). Land management in the winter crop season also affected temporal variation patterns of CH₄ fluxes and soil Eh after flooding. A great deal of CH₄ was emitted to the atmosphere during the period from flooding to the early stage of the rice-growing season; and CH₄ fluxes were still relatively high in the middle and late stages of the rice-growing period for treatments ALE, DFL, and FFE. However, for treatments DFE and WWE, almost no CH₄ emission was observed until the middle stage, and CH₄ fluxes in the middle and late stages of the rice-growing period were also very small. Soil Eh of treatments ALE and DFL decreased quickly to a low value suitable for CH₄ production. Once Eh below –150 mV was established, the small changes in Eh did not correlate to changes in CH₄ emissions. The soil Eh of treatments DFE and WWE did not decrease to a negative value until the middle stage of the rice-growing period, and it correlated significantly with the simultaneously measured CH₄ fluxes during the flooded and rice-growing period.     

Methane emission from rice paddy fields in all of Japanese prefecture

Field measurements of CH₄ emission from rice paddy field during cultivation periods were performed at all of 47 Japanese prefectures under the project of ‘Research for evaluation of CH₄ and N₂O emissions from agricultural land, and improvement methods of soil, water and fertilizer management’ conducted by Agricultural Production Bureau, the Ministry of Agriculture, Forestry and Fisheries. Although this project was carried out at 159 fields, the data of 132 fields were used for this report because other 27 fields had not enough data to be suitable for the statistics analyses. The measurements at rice paddy fields in various locations in Japan showed that there were large temporal variations of CH₄ flux and that the fluxes differed markedly with climate, characteristics of soil and paddy, application of organic matter and mineral fertilizer, and agricultural management practices. These data mainly indicated that CH₄ emission from Gley soils was greater than those from other soil types such as Andosols, Upland soils, fine-textured Lowland soils, medium and coarse-textured Lowlands soils and gravelly Lowland soils, and that water and organic matter managements influenced CH₄ emission. It is suggested that midsummer drainage treatment suppressed while the application of fresh organic matter such as rice straw and wheat straw enhanced CH₄ emission, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.