Direct field measurement of N2 and N2O evolution from soil

A gas lysimeter has been designed and used to measure directly the evolution of N₂and N₂O in a soil profile under field conditions. Concentrations of N₂ in the soil atmosphere within the lysimeter as low as 2000–5000 p.p.m. have been achieved by flushing with N-free gas. A flow of gas into the base of the lysimeter forms a barrier against diffusion of soil air into the lysimeter during measurements. After reducing the N₂ concentration in the soil core, a low concentration of N₂enriched in N-15 is introduced. By monitoring changes in the 8 N value using a high-precision isotope mass spectrometer, rates of N₂ evolution down to 6 kg N₂-N ha^?1 a^?1 can be detected. N₂O evolution was determined at the same time using the mass spectrometer in the single-beam mode.     

Strong pH influence on N2O and CH4 fluxes from forested organic soils

Greenhouse gas (GHG) emissions from farmed organic soils can have a major impact on national emission budgets. This investigation was conducted to evaluate whether afforestation of such soils could mitigate this problem. Over the period 1994–1997, emissions of methane (CH₄) and nitrous oxide (N₂O) were recorded from an organic soil site in Sweden, forested with silver birch (Betula pendula Roth), using static field chambers. The site was used for grazing prior to forestation. Soil pH and soil carbon content varied greatly across the site. The soil pH ranged from 3.6 to 5.9 and soil carbon from 34 to 42%. The mean annual N₂O emission was 19.4 (± 6.7) kg N₂O‐N ha^?1 and was strongly correlated with soil pH (r = ?0.93, P < 0.01) and soil carbon content (r = 0.97, P < 0.001). The N₂O emissions showed large spatial and temporal variability with greatest emissions during the summer periods. The site was a sink for CH₄ (i.e. ?0.8 (± 0.5) kg CH₄ ha^?1 year^?1) and the flux correlated well with the C/N ratio (r = 0.93, P < 0.01), N₂O emission (r = 0.92, P < 0.01), soil pH (r = ?0.95, P < 0.01) and soil carbon (r = 0.97, P < 0.001). CH₄ flux followed a seasonal pattern, with uptake dominating during the summer, and emission during winter. This study indicates that, because of the large N₂O emissions, afforestation may not mitigate the GHG emissions from fertile peat soils with acidic pH, although it can reduce the net GHG because of greater CO₂ assimilation by the trees compared with agricultural crops.     

Fluxes of N2O from farmed peat soils in Finland

Agricultural peat soils are important sources of nitrous oxide (N₂O). Emissions of N₂O were measured from field plots of grass, barley, potatoes and fallow on a peat field in northern Finland during 2000–2002 and in southern Finland in 1999–2002. In the north the mean annual fluxes of N₂O (with their standard errors) during 2 years were 4.0 (±1.2), 13 (±3.0) and 4.4 (±0.8) kg N ha^?1 from the plots of grass, barley and fallow, respectively. In the north there were no significant thaw periods in the middle of winter. As a result, the thawing in the spring did not induce especially large N₂O emissions. Emissions of N₂O were larger in the south than in the north. In the southern peat field the mean annual fluxes during 3 years were 7.3 (±1.2), 15 (±2.6), 10 (±1.9) and 25 (±6.9) kg N₂O‐N ha^?1 for grass, barley, potato and fallow plots, respectively. Here, the largest single episodes of emission occurred during the spring thaw each year, following winter thaw events. An emission factor of 10.4 kg N₂O‐N ha^?1 year^?1 for the N₂O emission from the decomposition of the peat results from these data if the effect of fertilization according to the IPCC default emission factor is omitted. The direct effect of adding N as fertilizer on N₂O emissions was of minor importance. On average, 52% of the annual N₂O flux entered the atmosphere outside the cropping season (October–April) in the north and 55% in the south. The larger N₂O fluxes from the peat soil in the south might be due to the more humified status of the peat, more rapid mineralization and weather with more cycles of freezing and thawing in the winter.     

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.     

Brief and vigorous N2O production by soil at spring thaw

In an acid sandy loam soil (pH 3.8), field production of N²O was two orders of magnitude higher at thaw in the spring than at any time during the rest of the year. Soil thaw in midwinter did not result in any increase in N²O flux. Soil water content remained at, or above field capacity during measurements; nitrate was added in excess. This effect could be reproduced in the laboratory: thawing soil cores at controlled temperature, nitrate and moisture yielded a large flush of N²O compared to an unfrozen control. The results indicate the importance of microbial N²O production during thaw for total annual N²O-emission.     

Relationship between N2O and NO emission potentials and soil properties in Japanese forest soils

To determine the relationship between nitrous oxide (N₂O) and nitric oxide (NO) emission rates and soil properties in forest soils, N₂O and NO emission rates in soils were measured in incubation experiments under standardized temperature and water conditions (water content at a water-holding capacity of 60%) using soils packed into a cylindrical core, and variations in the soil properties were also determined. The N₂O emission rates from nitrification and from denitrification were determined separately using a nitrification inhibitor (10 Pa acetylene). Soil samples were taken from 25 forest stands in a central temperate area of Japan. The N₂O and NO emission rates were highly variable, even under the standardized temperature and water-holding capacity (60%) conditions. According to a partial least squared regression model analysis, the C:N ratio and pH strongly affected the N₂O emission rate, whereas     , water-soluble Al and the C:N ratio strongly affected the NO emission rate. The C:N ratio negatively affected the emission rate of both N oxide gases, suggesting that N mineralization is an important factor in the rates of N oxide gas emission. The acetylene inhibition experiment showed that N₂O emission from denitrification was positively affected by pH, water-filled pore space and filling density, and negatively affected by the C:N ratio, total carbon and total nitrogen.     

Mitigation of N2O emissions from grassland by nitrification inhibitor and Actilith F2 applied with fertilizer and cattle slurry

Abstract. Nitrous oxide (N₂O) is involved in both ozone destruction and global warming. In agricultural soils it is produced by nitrification and denitrification mainly after fertilization. Nitrification inhibitors have been proposed as one of the management tools for the reduction of the potential hazards of fertilizer-derived N₂O. Addition of nitrification inhibitors to fertilizers maintains soil N in ammonium form, thereby gaseous N losses by nitrification and denitrification are less likely to occur and there is increased N utilization by the sward. We present a study aimed to evaluate the effectiveness of the nitrification inhibitor dicyandiamide (DCD) and of the slurry additive Actilith F2 on N₂O emissions following application of calcium ammonium nitrate or cattle slurry to a mixed clover/ryegrass sward in the Basque Country. The results indicate that large differences in N₂O emission occur depending on fertilizer type and the presence or absence of a nitrification inhibitor. There is considerable scope for immediate reduction of emissions by applying DCD with calcium ammonium nitrate or cattle slurry. DCD, applied at 25 kg ha^–1, reduced the amount of N lost as N₂O by 60% and 42% when applied with cattle slurry and calcium ammonium nitrate, respectively. Actilith F2 did not reduce N₂O emissions and it produced a long lasting mineralization of previously immobilized added N.     

Assessment of N2O, NOx and NH3 emissions from a typical rural catchment in Eastern China

To evaluate the atmospheric load of reactive gaseous nitrogen in the fast-developing Eastern China region, we compiled inventories of nitrous oxide (N₂O), nitrogen oxide (NO_x) and ammonia (NH₃) emissions from a typical rural catchment in Jiangsu province, China, situated at the lower reach of the Yangtze River. We considered emissions from synthetic N fertilizer, human and livestock excreta, decomposition of crop residue returned to cropland and residue burning, soil background and household energy consumption. The results showed that, for the 45.5 km² catchment, the annual reactive gaseous emission was 279 ton N, of which 7% was N₂O, 16% was NO_x and 77% was NH₃. Synthetic N fertilizer application was the dominant source of N₂O and NH₃ emissions and crop residue burning was the dominant source of NO_x emission. Sixty-seven percent of the total reactive gaseous N was emitted from croplands, but on a per unit area basis, NO_x and NH₃ emissions in residential areas were higher than in croplands, probably as a result of household crop residue burning and extensive human and livestock excreta management systems. Emission per capita was estimated to be 18.2 kg N year⁻¹ in the rural catchment, and emission per unit area was 56.9 kg N ha⁻¹year⁻¹ for NH₃ + NO_x, which supports the observed high atmospheric N deposition in the catchment. Apparently, efficient use of N fertilizer and biological utilization of crop straw are important measures to reduce reactive gases emissions in this rural catchment.     

Temperature responses of NO and N2O emissions from boreal organic soil

Both NO and N₂O are produced in soil microbial processes and have importance in atmospheric physics and chemistry. In recent years several studies have shown that N₂O emissions from organic soils can be high at low temperatures. However, the effects of low temperature on NO emissions from soil are unknown. We studied in laboratory conditions, using undisturbed soil cores, the emissions of NO and N₂O from organic soils at various temperatures, with an emphasis on processes and emissions during soil freezing and thawing periods. We found no soil freezing- or thawing-related emission maxima for NO, while the N₂O emissions were higher both during soil freezing and thawing periods. The results suggest that different factors are involved in the regulation of NO and N₂O emissions at low temperatures.     

Emission of NH3 and N2O after spreading of pig slurry by broadcasting or band spreading

Abstract. The recommended method of reducing the emission of NH₃ while spreading manure is to plough or harrow the manure into the soil. This in turn increases the possibility of N₂O emission. At two sites in southern Sweden emissions of NH₃ and N₂O were measured after spreading pig slurry by broadcasting and band spreading. The band spreading technique can be used in growing crops i.e. when nitrogen is most needed, and it is thought that the NH₃ emission is smaller with this technique compared to broadcasting. The average NH₃ loss was 50% of applied NH₄⁺ during warm/dry conditions and 10% during cold/wet conditions. The N₂O emission was always less than 1% of applied NH₄⁺. When the NH₃ emission decreased, the direct N₂O emission increased. However, when taking into account the indirect N₂O emission due to deposition of NH₃ outside the field, the spreading techniques all produced similar total N₂O emissions. The ammonia emission was not much lower for the band spreading technique compared to broadcasting, when compared on seven occasions.     

Emissions of N2O from soils during cycles of freezing and thawing and the effects of soil water, texture and duration of freezing

Freezing and thawing influence many physical, chemical and biological processes in soils, including the production of trace gases. We studied the effects of freezing and thawing on three soils, one sandy, one silty and one loamy, on the emissions of N₂O and CO₂. We also studied the effect of varying the water content, expressed as the percentage of the water‐filled pore space (WFPS). Emissions of N₂O during thawing decreased in the order 64% > 55% > 42% WFPS, which suggests that the retardation of the denitrification was more pronounced than the acceleration of the nitrification with increasing oxygen concentration in the soil. However, emissions of N₂O at 76% WFPS were less than at 55% WFPS, which might be caused by an increased ratio of N₂/N₂O in the very moist conditions. The emission of CO₂ was related to the soil water, with the smallest emissions at 76% WFPS and largest at 42% WFPS. The emissions of CO₂ during thawing exceeded the initial CO₂ emissions before the soils were frozen, which suggests that the supply of nutrients was increased by freezing. Differences in soil texture had no marked effect on the N₂O emissions during thawing. The duration of freezing, however, did affect the emissions from all three soils. Freezing the soil for less than 1 day had negligible effects, but freezing for longer caused concomitant increases in emissions. Evidently the duration of freezing and soil water content have important effects on the emission of N₂O, whereas the effects of texture in the range we studied were small.     

The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol

To investigate the effect of soil physical conditions and land use on emissions of nitrous oxide (N₂O) to the atmosphere, soil cores of an imperfectly drained gleysol were taken from adjacent fields under perennial ryegrass and winter wheat. The cores were fertilized with ammonium nitrate and incubated at three different temperatures and water‐filled pore space (WFPS) values, and N₂O emissions were measured by gas chromatography. Emissions showed a very large response to temperature. Apparent values of Q₁₀ (emission rate at (T + 10)°C/emission rate at T°C) for the arable soil were about 50 for the 5–12°C interval and 8.9 for 12–18°C; the corresponding Q₁₀s for the grassland soil were 3.7 and 2.3. Emissions from the grassland soil were always greater than those from the arable soil, although the ratio narrowed with increasing temperature. Changes in soil WFPS also had a profound effect on emissions. Those from the arable soil increased about 30‐fold as the WFPS increased from 60 to 80%, while that from the grassland soil increased 12‐fold. This latter response was similar to earlier field measurements. The N₂O emissions were considered to be produced primarily by denitrification. We concluded that the impacts of temperature and WFPS on emissions could both be explained on the basis of existing models relating increasing respiration or decreased oxygen diffusivity, or both, to the development of anaerobic zones within the soil.     

Mitigation options for N2O emission from a corn field in Kalimantan, Indonesia

Field experiments were designed to quantify N₂O emissions from corn fields after the application of different types of nitrogen fertilizers. Plots were established in South Kalimantan, Indonesia, and given either urea (200 kg ha⁻¹), urea (170 kg ha⁻¹) + dicyandiamide ([DCD] 20 kg ha⁻¹) or controlled-release fertilizer LP-30 (214 kg ha⁻¹) prior to the plantation of corn seeds (variety BISI 2). Each fertilizer treatment was equivalent to 90 kg N ha⁻¹. Plots without chemical N fertilizer were also prepared as a control. The field was designed to have three replicates for each treatment with a randomized block design. Nitrous oxide fluxes were measured at 4, 8, 12, 21, 31, 41, 51, 72 and 92 days after fertilizer application (DAFA). Total N₂O emission was the highest from the urea plots, followed by the LP-30 plots. The emissions from the urea + DCD plots did not differ from those from the control plots. The N₂O emission from the urea + DCD plots was approximately one thirtieth of that from the urea treatment. However, fertilizer type had no effect on grain yield. Thus, the use of urea + DCD is considered to be the best mitigation option among the tested fertilizer applications for N₂O emission from corn fields in Kalimantan, Indonesia.