Preliminary studies on N2O emission fluxes from upland soils and paddy soils in China

In this paper, we presented the preliminary results of N₂O fluxes from Chinese upland and rice paddy fields. The mean N₂O flux from upland fields of North China is 30.6 μg N₂O-N m^-2 h^-1; the average N₂O flux from Chinese rice paddy field is 39.5 μg N₂O-N m^-2 h^-1. The effects of cropping system, water management and application of N fertilizer and organic manure on N₂O emission from rice paddy field have also been presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fluxes of methane and nitrous oxide in water-saving rice production in north China

Lowland rice production is currently facing serious water shortages in numerous Asian countries. In the North China Plain water limitations are severe. Water-saving rice production techniques are therefore increasingly searched for. Here we present the first study of trace gas emissions from a water-saving rice production system where soils are mulched and are kept close to field capacity in order to compare their contribution to global warming with traditional paddy rice. In a two-year field experiment close to Beijing, CH₄ and N₂O fluxes were monitored in two forms of the Ground Cover Rice Production System (GCRPS) and in traditional paddy fields using closed chambers. With paddy rice the observed CH₄ emissions were very low, about 0.3 g CH₄ m⁻² a⁻¹ in 2001 and about 1 g CH₄ m⁻² a⁻¹ in 2002. In GCRPS, the CH₄ emissions were negligible. N₂O fluxes in GCRPS were similar, 0.5 to 0.6 g N₂O m⁻² a⁻¹ in 2001 and 2002, and emission peaks mainly followed fertilizer applications. In paddy rice, N₂O fluxes were unexpectedly low throughout the year 2001 (0.03 g N₂O m⁻² a⁻¹), and in 2002 larger emissions occurred during the drainage period. So with 0.4 g N₂O m⁻² a⁻¹ the cumulative flux was similar to emissions in GCRPS. Total CO₂ equivalent fluxes calculated according to IPCC methodology were tenfold higher in GCRPS compared to paddy in 2001. In 2002, fluxes from both systems were similar with 175 and 141 g CO₂ equivalents m⁻² a⁻¹ from GCRPS and paddy. Burkhard Sattelmacher deceased.     

Impact of different forms of N fertilizer on N2O emissions from intensive grassland

Nitrous oxide (N₂O) emissions were measured over two years from an intensively managed grassland site in the UK. Emissions from ammonium nitrate (AN) and urea (UR) were compared to those from urea modified by various inhibitors (a nitrification inhibitor, UR(N), a urease inhibitor, UR(U), and both inhibitors together, SU), as well as a controlled release urea (CR). N₂O fluxes varied through time and between treatments. The differences between the treatments were not consistent throughout the year. After the spring and early summer fertilizer applications, fluxes from AN plots were greater than fluxes from UR plots, e.g. the cumulative fluxes for one month after N application in June 1999 were 5.2 ± 1.1 kg N₂O-N ha^–1 from the AN plots, compared to 1.4 ± 1.0 kg N₂O-N ha^–1 from the UR plots. However, after the late summer application, there was no difference between the two treatments, e.g. cumulative fluxes for the month following N application in August 2000 were 3.3 ± 0.7 kg N₂O-N ha^–1 from the AN plots and 2.9 ± 1.1 kg N₂O-N ha^–1 from the UR plots. After all N applications, fluxes from the UR(N) plots were much less than those from either the AN or the UR plots, e.g. 0.2 ± 0.1 kg N₂O-N ha^–1 in June 1999 and 1.1 ± 0.3 kg N₂O-N ha^–1 in August 2000. Combining the results of this experiment with earlier work showed that there was a greater N₂O emission response to rainfall around the time of fertilizer application in the AN plots than in the UR plots. It was concluded that there is scope for reducing N₂O emissions from N-fertilized grassland by applying UR instead of AN to wet soils in cool conditions, e.g. when grass growth begins in spring. Applying UR with a nitrification inhibitor could cut emissions further.     

Atmospheric methane and nitrous oxide: sources, sinks and strategies for reducing agricultural emissions

I discuss production, emission and oxidation of CH₄ in rice paddy fields and N₂O in fertilized soils. The quantity of CH₄ emitted from rice paddy fields depends upon several important factors including soil factors, nutrient management, water regimes, cultivation practices and others. Important factors for N₂O emitted from fertilized soils are soil water content, temperature, nitrate or ammonium concentration, available organic carbon for denitrification and pH. I provide an estimate of mitigation potential in agricultural systems based on this estimate and the management technology. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous oxide and nitric oxide fluxes from an upland field in Japan: effect of urea type, placement, and crop residues

Fertilizer type and application mode may influence nitrous oxide(N₂O) and nitric oxide (NO) emissions as well as crop yield. Using astatic chamber method, fluxes of both gases from a Chinese cabbage field inJapan were measured in situ following the application of easily decomposableurea by broadcasting (U-BC) and banding (U-B) and coated urea by banding(CU-B),respectively, at an application rate of 250 kg Nha^–1. The measurements were made throughout the growingseason and continued 3 more months after harvest to determine the effect ofcropresidues on the emissions. Large N₂O fluxes from U-BC occurredwithinabout 2 weeks after the application of the N fertilizer, while that from bothU-B and CU-B was prolonged by about 2 weeks, and significant emissions lasted alonger time but with a smaller emission size. Substantial N₂O fluxesderived from crop residues were observed in the late growing season (especiallyfollowing rainfall) as well as after harvest, at all treatments including thecontrol plots (CK). Large NO fluxes occurred only at U-BC within the first 2weeks through the measurements. Total emissions were estimated to be 38.1,78.3,77.8, and 100.4 mg N₂O-N m^–2 and 0.7,194.9, 8.5, and 11.4 mg NO-N m^–2 at CK, U-BC,U-B,and CU-B, respectively. Statistical analyses indicate that neither the bandmodenor the coated urea was able to significantly reduce the total N₂Oemission through the season, but the band mode substantially reduced the NOemission. However, the application of urea by the band mode presented a 22.8%increase in crop yield as compared with urea applied by broadcasting.Therefore,by improving fertilizer use efficiency to decrease the amount of N needed tobetter meet the crop growing demand, the band mode may be a good agriculturalpractice to also reduce N₂O emission. In addition, the experimentdemonstrated that crop residue is a large source of N₂O emission.     

N2O emission from cropland in China

Based on the regionalization of uplands and paddy fields in China, the crop intensity in each region and the available field measurements, N₂O emission from cropland in China in 1995 was estimated to be 398 Gg N, in which, 310 Gg N was from uplands, accounting for 78% of the total. 88 Gg N–N₂O was emitted from paddy fields with 35 Gg N emitted during the rice growing season and 53 Gg N emitted during upland crop season. N₂O emission from upland and from paddy field during upland land crop season accounted for 91% of the total emission.     

Effect of changing groundwater levels caused by land-use changes on greenhouse gas fluxes from tropical peat lands

Monthly measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes in peat soils were carried out and compared with groundwater level over a year at four sites (drained forest, upland cassava,upland and lowland paddy fields) located in Jambi province, Indonesia. Fluxes from swamp forest soils were also measured once per year as the native state of this investigated area. Land-use change from drained forest to lowland paddy field significantly decreased the CO₂ (from 266 to 30 mg C m^–2 h^–1) and N₂O fluxes (from 25.4 to 3.8 g N m^–2 h^–1), but increased the CH₄ flux (from 0.1 to 4.2 mg C m^–2 h^–1) in the soils. Change from drained forest to cassava field significantly increased N₂O flux (from 25.4 to 62.2 g N m^–2 h^–1), but had no significant influence on CO₂ (from 266 to 200 mg C m^–2 h^–1) and CH₄ fluxes (from 0.1 to 0.3 mg C m^–2 h^–1) in the soils. Averaged CO₂ fluxes in the swamp forests (94 mg C m^–2 h^–1) were estimated to be one-third of that in the drained forest. Groundwater levels of drained forest and upland crop fields had been lowered by drainage ditches while swamp forest and lowland paddy field were flooded, although groundwater levels were also affected by precipitation. Groundwater levels were negatively related to CO₂ flux but positively related to CH₄ flux at all investigation sites. The peak of the N₂O flux was observed at –20 cm of groundwater level. Lowering the groundwater level by 10 cm from the soil surface resulted in a 50 increase in CO₂ emission (from 109.1 to 162.4 mg C m^–2 h^–1) and a 25% decrease in CH₄ emission (from 0.440 to 0.325 mg C m^–2 h^–1) in this study. These results suggest that lowering of groundwater level by the drainage ditches in the peat lands contributes to global warming and devastation of fields. Swamp forest was probably the best land-use management in peat lands to suppress the carbon loss and greenhouse gas emission. Lowland paddy field was a better agricultural system in the peat lands in terms of C sequestration and greenhouse gas emission. Carbon loss from lowland paddy field was one-eighth of that of the other upland crop systems, although the Global Warming Potential was almost the same level as that of the other upland crop systems because of CH₄ emission through rice plants.     

Effects of fertiliser type and the presence or absence of plants on nitrous oxide emissions from irrigated soils

Nitrous oxide (N₂O) emissions and denitrification losses from an irrigated sandy loam soil amended with composted municipal solid waste (MSW), sheep manure (SM), surface applied pig slurry (SPS), incorporated pig slurry (IPS) or urea (U) were studied under Mediterranean conditions. We quantified emissions, in both the presence and absence of maize and N₂O production, via denitrification and nitrification pathways using varying concentrations of acetylene. Discounting the N₂O lost in the Control, the percentages of N₂O lost in relation to the total N applied were greater for urea (1.80%) than for MSW (0.50%), SM (0.46%), SPS (1.02%) or IPS (1.27%). In general, plots treated with organic fertilisers emitted higher amounts of N₂O when under maize than bare soil plots. On the other hand, greater denitrification losses were also recorded for plots in the absence of plants (between 9.7 and 29.3 kg N₂O-N ha⁻¹) than for areas with plants (between 7.1 and 24.1 kg N₂O-N ha⁻¹). The proportion of N₂O produced via denitrification was greater from fertiliser treatments than for the controls and also greater without plants (between 66 and 91 % of the N₂O emitted) than with plants (between 48 and 81%).     

Effect of chemical fertilizer form on N2O, NO and NO2 fluxes from Andisol field

N₂O, NO and NO₂ fluxes from an Andosol soil in Japan after fertilization were measured 6 times per day for 10 months from June 1997 to April 1998 with a fully automated flux monitoring system in lysimeters. Three nitrogen chemical fertilizers were applied to the soil–calcium nitrate (NI), controlled-release urea (CU), and controlled-release calcium nitrate (CN), and also no nitrogen fertilizer (NN). The total amount of nitrogen applied was 15 g N m^–2 in the first and the second cultivation period of Chinese vegetable. In the first measuremnt period of 89 days, the total N₂O emissions from NI, CN, CU, and NN were 18.4, 16.3, 48.7, and 9.60 mgN m^–2, respectively. The total NO emissions from NI, CN, CU, and NN were 48.4, 33.7, 149, and 13.7 mgN m^–2, respectively. In the second measurement period of 53 days, the total N₂O emissions from NI, CN, and CU were 9.66, 7.23, and 20.6 mgN m^–2, respectively. The total NO emissions from NI, CN, and CU were 24.7, 2.60 and 34.2 mgN m^–2, respectively. The total N₂O emission from CU was significantly higher than CN. In the third cultivation period, all plots were applied with 10 g N m^–2 of ammonium phosphate (AP) and winter barley was cultivated. In the third measurement period of 155 days, the total N₂O and NO emissions were 9.02 mgN m^–2 and 10.2 mgN m^–2, respectively. N₂O and NO peaks were observed just after the fertilization for 30 days and 15 days, respectively. N₂O, NO and NO₂ fluxes for the year were estimated to be 38.6 81.5, 48.2 181, and –24.8 to –39.3 mgN m^–2, respectively. NO₂ was absorbed in all the plots, and a negative correlation was found between NO₂ flux and the NO₂ concentration just after the chamber closed. NO was absorbed in the winter period, and a negative correlation was found between NO flux and the NO concentration just after the chamber closed. A diurnal pattern was observed in N₂O and NO fluxes in the summer, similar to air and soil temperature. We could find a negative relationship between flux ratio of NO-N to N₂O-N and water-filled pore space (WFPS), and a positive relationship between NO-N and N₂O-N fluxes and temperature. Q₁₀ values were 3.1 for N₂O and 8.7 for NO between 530 °C.     

Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions

Annual cycles of NO, NO₂ and N₂O emission rates from soil were determined with high temporal resolution at a spruce (control and limed plot) and beech forest site (Höglwald) in Southern Germany (Bavaria) by use of fully automated measuring systems. The fully automated measuring system used for the determination of NO and NO₂ flux rates is described in detail. In addition, NO, NO₂ and N₂O emission rates from soils of different pine forest ecosystems of Northeastern Germany (Brandenburg) were determined during 2 measuring campaigns in 1995. Mean monthly NO and N₂O emission rates (July 1994–June 1995) of the untreated spruce plot at the Höglwald site were in the range of 20–130 µg NO-N m^-2 h^-1 and 3.5–16.4 µg N₂O-N m^-2 h^-1, respectively. Generally, NO emission exceeded N₂O emission. Liming of a spruce plot resulted in a reduction of NO emission rates (monthly means: 15–140 µg NO-N m^-2 h^-1) by 25-30% as compared to the control spruce plot. On the other hand, liming of a spruce plot significantly enhanced over the entire observation period N₂O emission rates (monthly means: 6.2–22.1 µg N₂O-N m^-2 h^-1). Contrary to the spruce stand, mean monthly N₂O emission rates from soil of the beech plot (range: 7.9–102 µg N₂O-N m^-2 h^-1) were generally significantly higher than NO emission rates (range: 6.1–47.0 µg NO-N m^-2 h^-1). Results obtained from measuring campaigns in three different pine forest ecosystems revealed mean N₂O emission rates between 6.0 and 53.0 µg N₂O-N m^-2 h^-1 and mean NO emission rates between 2.6 and 31.1 µg NO-N m^-2 h^-1. The NO and N₂O flux rates reported here for the different measuring sites are high compared to other reported fluxes from temperate forests. Ratios of NO/N₂O emission rates were >> 1 for the spruce control and limed plot of the Höglwald site and << 1 for the beech plot. The pine forest ecosystems showed ratios of NO/N₂O emission rates of 0.9 ± 0.4. These results indicate a strong differentiating impact of tree species on the ratio of NO to N₂O emitted from soil.     

Simultaneous Records of Methane and Nitrous Oxide Emissions in Rice-Based Cropping Systems under Rainfed Conditions

Rainfed rice (Oryza sativa L.)-based cropping systems are characterized by alternate wetting and drying cycles as monsoonal rains come and go. The potential for accumulation and denitrification of NO₃ ^– is high in these systems as is the production and emission of CH₄ during the monsoon rice season. Simultaneous measurements of CH₄ and N₂O emissions using automated closed chamber methods have been reported in irrigated rice fields but not in rainfed rice systems. In this field study at the International Rice Research Institute, Philippines, simultaneous and continuous measurements of CH₄ and N₂O were made from the 1994 wet season to the 1996 dry season. During the rice-growing seasons, CH₄ fluxes were observed, with the highest emissions being in organic residue-amended plots. Nitrous oxide fluxes, on the other hand, were generally nonexistent, except after fertilization events where low N₂O fluxes were observed. Slow-release N fertilizer further reduced the already low N₂O emissions compared with prilled urea in the first rice season. During the dry seasons, when the field was planted to the upland crops cowpea [Vigna unguiculata (L.) Walp] and wheat (Triticum aestivum L.), positive CH₄ fluxes were low and insignificant except after the imposition of a permanent flood where high CH₄ fluxes appeared. Evidences of CH₄ uptake were apparent in the first dry season, especially in cowpea plots, indicating that rainfed lowland rice soils can act as sink for CH₄ during the upland crop cycle. Large N₂O fluxes were observed shortly after rainfall events due to denitrification of accumulated NO₃ ^–. Cumulative CH₄ and N₂O fluxes observed during this study in rainfed conditions were lower compared with previous studies on irrigated rice fields.     

Nitrous oxide and methane emissions from soil–plant systems

The closed chamber method was used to measure the N₂O and CH₄ emissions from rice, maize, soybean and spring wheat fields in Northeast China. Rice field almost did not emit or deposit N₂O in total during flooding period, whereas N₂O was substantially emitted during non-flooding period. The annual emission amount of N₂O was 1.70 kg N₂O ha^-1, but that in flooding period was only 0.04 kg N₂O ha^-1. Daily average and seasonal total CH₄ emission in rice field were 0.07 and 7.40 g CH₄m^-2, respectively. A trade-off between N₂O and CH₄ emissions from rice field was found. The growth of Azolla in rice field greatly stimulated both N₂O and CH₄ emissions. Total N₂O emissions (270 days) from maize and soybean fields were 7.10 and 3.12 kg N₂O ha^-1, respectively. The sink function of the uplands monitored as the atmospheric CH₄ was not significant. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous oxide emission from a rice paddy field in Japan

The flux of nitrous oxide (N₂O) from a rice paddy field to the atmosphere was measured at Ryuhgasaki experiment station in Ibaraki Prefecture of Japan by closed chamber method, from the summer of 1992 to the summer of 1993. During the rice-cultivated and flooding periods when methane (CH₄) was emitted, no emission or uptake of N₂O was measured because the flux values were below the detection limits. After the final water drainage for harvest in August or September, N₂O began to emit from the soil surface while the emission of CH₄ was stopped, and N₂O was emitted continually until the re-flooding day in the following spring. In the first few months after the final water drainage, the N₂O flux was in the range of 10–20 μgN/m²/hour, then in the latter several months during the cold season, the N₂O flux was less than 10 μgN/m²/hour. The vertical profiles of N₂O, CO₂ and CH₄ concentrations in the plowed layer of the soil down to a depth of 20 cm, were also measured six times in the fallow season. The maximum concentrations of N₂O and CO₂ were found in the plowed layer in the early period, and which demonstrates that most of the N₂O was produced in the plowed layer through nitrification, due to the decomposition of organic matter accumulated in the plowed layer during the rice-growing and water-flooding period. On the contrary, the vertical profiles in the cold season showed a gradual increase in the concentrations of N₂O and CO₂ in the plowed layer. It clearly indicates that a small amount of N₂O was emitted to the atmosphere by diffusion through the plowed layer from the sub-soil layer where a large source of N₂O was expected to exist. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of timing and duration of midseason aeration on CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions from irrigated lowland rice paddies in China

Midseason aeration (MSA) of rice paddy fields functions to mitigate CH₄ emission by a large margin, while simultaneously promoting N₂O emission. Alternation of timing and duration of MSA would affect CH₄ and N₂O emissions from intermittently irrigated rice paddies. A pot trial and a field experiment were conducted to study the effect of timing and duration of MSA on CH₄ and N₂O emissions from irrigated lowland rice paddy soils in China. Four different water regimes, i.e., early aeration, normal aeration (the same as the local practice in timing and duration of aeration), delayed aeration, and prolonged aeration, were adopted separately and compared with respect to global warming potential (GWP) of CH₄ and N₂O emissions and rice yields as well. Total emission of CH₄ from the rice fields ranged from 28.6 to 64.1 kg CH₄ ha⁻¹, while that of N₂O did from 1.71 to 6.30 kg N₂O–N ha⁻¹ during the study periods. Compared with the local practice, early aeration reduced CH₄ emission by 13.3–16.2% and increased N₂O emission by 19.1–68.8%, while delayed aeration reduced N₂O emission by 6.8–26.0% and increased CH₄ emission by 22.1–47.3%. The lowest GWP of CH₄ and N₂O emissions occurred in prolonged aeration treatment, however, rice grain yield was reduced by 15.3% in this condition when compared with normal practice. It was found in the experiments that midseason aeration starting around D 30 after rice transplanting, just like the local practice, would optimize rice yields while simultaneously limiting GWPs of CH₄ and N₂O emissions from irrigated lowland rice fields in China.     

Nitrous oxide emission from temperate meadow grassland and emission estimation for temperate grassland of China

Nitrous oxide emission from temperate meadow grassland was measured using a closed chamber method at two experimental sites in China. In the four-month measurement period, the N₂O fluxes in mown meadow grasslands of the Songnen Plain and of the Kerqin Steppe were on average 41.1 and 7.9 g N₂O-N m^–2 h^–1, respectively. Considering the influence of grassland type and degradation extent, an empirical formula was constructed, with which the annual N₂O emission from temperate grassland of China was estimated as 40.4 Gg N. Meadow grassland, accounting for 14.0% of the total grassland area, contributed 28.4% of the total N₂O emission.     

Field Validation of DNDC Model for Methane and Nitrous Oxide Emissions from Rice-based Production Systems of India

The DNDC (DeNitrification and DeComposition) model was tested against experimental data on CH₄ and N₂O emissions from rice fields at different geographical locations in India. There was a good agreement between the simulated and observed values of CH₄ and N₂O emissions. The difference between observed and simulated CH₄ emissions in all sites ranged from −11.6 to 62.5 kg C ha⁻¹ season⁻¹. Most discrepancies between simulated and observed seasonal fluxes were less than 20% of the field estimate of the seasonal flux. The relative deviation between observed and simulated cumulative N₂O emissions ranged from −237.8 to 28.6%. However, some discrepancies existed between observed and simulated seasonal patterns of CH₄ and N₂O emissions. The model simulated zero N₂O emissions from continuously flooded rice fields and poorly simulated CH₄ emissions from Allahabad site. For all other simulated cases, the model satisfactorily simulated the seasonal variations in greenhouse gas emission from paddy fields with different land management. The model also simulated the C and N balances in all the sites, including other gas fluxes, viz. CO₂, NO, NO₂, N₂ and NH₃ emissions. Sensitivity tests for CH₄ indicate that soil texture and pH significantly influenced the CH₄ emission. Changes in organic C content had a moderate influence on CH₄ emission on these sites. Introducing the mid-season drainage reduced CH₄ emissions significantly. Process-based biogeochemical modeling, as with DNDC, can help in identifying strategies for optimizing resource use, increasing productivity, closing yield gaps and reducing adverse environmental impacts.     

Nitrous oxide flux from a solid dairy manure pile measured using a micrometeorological mass balance method

A micrometeorological mass balance technique was used to quantify the N₂O flux from a solid dairy manure pile under field conditions. Flux was determined using time-averaged measurements of wind speed, and nitrous oxide concentration using a tunable diode laser trace gas analyzer. A total of 66 hourly flux averages were collected and values were never lower than 200 ng N₂O-N m^–2 s^–1. The mean hourly N₂O flux was 4865 ng N₂O-N m^–2 s^–1 (0.42 g N m^–2 day^–1), which is of the same order of magnitude, albeit higher, as previously observed for a similar solid pig manure storage.     

Methane and Nitrous Oxide Emissions from Rice Field and Related Microorganism in Black Soil, Northeastern China

Methane (CH₄) and nitrous oxide (N₂O) emissions from rice field in black soil were measured in situ by using static chamber techniques during crop growth season in 2001. The experiment fields were divided into three plots for three different treatments, one with continuous flooded and applying urea (CU), one with continuous flooded and applying slow-releasing urea (CS), and one with intermittent irrigation and applying urea (IU). Under the same fertilization application, compared with continuous flooded, intermittent irrigation can significantly reduce CH₄ emission and increase N₂O emission. But, integrated global warming potentials (GWP_S) of CH₄ and N₂O emission were reduced greatly, while rice yield was not affected. So, the intermittent irrigation is an effective measure to reduce greenhouse gas emissions from paddy fields. The amount of CH₄ emission during rice-growing season for the three treatments was all much lower than that from any other region in China. There was a trade-off relationship between CH₄ and N₂O emissions. We also measured the numbers of methanogens, methanotrophs, nitrifiers and denitrifers from rice field at various growth stages in 2001. Bacteria populations were estimated by the most probable number (MPN) method. Regression analyses show CH₄ emissions were closely related to methanogens population for all the three treatments. There was a positive correlation between denitrifiers population level and N₂O emission in the treatment of IU.     

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.     

Subsoil <Superscript>15</Superscript>N-N<Subscript>2</Subscript>O Concentrations in a Sandy Soil Profile After Application of <Superscript>15</Superscript>N-fertilizer

Studies on emissions of nitrous oxide (N₂O) from agricultural soils mostly focus on fluxes between the soil and the atmosphere or are limited to the atmosphere in the topsoil. However, in soils with shallow water tables, significant N₂O formation may occur closer to the groundwater. The aims of this study were (i) to determine the importance of subsoil N₂O formation in a sandy soil; and (ii) to obtain a quantitative insight in the contribution of subsoil N₂O to the overall losses of N₂O to the environment. We applied ¹⁵N labeled fertilizer at a rate of 5.22 kg ¹⁵N ha⁻¹; 50% as Ca(NO₃)₂ and 50% as NH₄Cl, on a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.), and traced soil N₂O concentrations and fluxes over a one-year period. Throughout the year, total N₂O and the amount of ¹⁵N recovered in soil N₂O were highest in the subsoil, with a maximum concentration at 48 cm depth in mid-February of 19900 μl m⁻³ and 24 μg ¹⁵N m⁻³, respectively. The maximum concentration coincided with the highest water-filled pore space of 71%. The cumulative flux of N₂O was 446 g N₂O-N ha⁻¹, the recovery of ¹⁵N in this flux was 0.06%. During the summer, maximum fluxes followed high soil N₂O concentrations. During winter, no such relation was found. We concluded that the formation of N₂O was the highest in the subsoil, largely controlled by water-filled pore space rather than NO₃⁻ concentration or temperature. Although high subsoil N₂O concentrations did not lead to high surface fluxes of N₂O in the winter, artificial draining may lead to high indirect N₂O emissions through supersaturated drainage water.