Nitrous oxide and nitric oxide fluxes from cornfield,grassland, pasture and forest in a watershed in Southern Hokkaido,Japan

Abstract

To develop an advanced method for estimating nitrous oxide (N₂O) emission from an agricultural watershed, we used a closed-chamber technique to measure seasonal N₂O and nitric oxide (NO) fluxes in cornfields, grassland, pastures and forests at the Shizunai Experimental Livestock Farm (467 ha) in southern Hokkaido, Japan. From 2000 to 2004, N₂O and NO fluxes ranged from –137 to 8,920 µg N m^?2 h^?1 and from –12.1 to 185 µg N m^?2 h^?1, respectively. Most N₂O/NO ratios calculated on the basis of these N₂O and NO fluxes ranged between 1 and 100, and the log-normal N₂O/NO ratio was positively correlated with the log-normal N₂O fluxes (r ² = 0.346, P < 0.01). These high N₂O fluxes, therefore, resulted from increased denitrification activity. Annual N₂O emission rates ranged from –1.0 to 81 kg N ha^?1 year^?1 (average = 6.6 kg N ha^?1). As these emission values varied greatly and included extremely high values, we divided them into two groups: normal values (i.e. values lower than the overall average) and high values (i.e. values higher than average). The normal data were significantly positively correlated with N input (r ² = 0.61, P < 0.01) and the “higher” data from ungrazed fields were significantly positively correlated with N surplus (r ² = 0.96, P < 0.05). The calculated probability that a high N₂O flux would occur was weakly and positively correlated with precipitation from May to August. This probability can be used to represent annual variation in N₂O emission rates and to reduce the uncertainty in N₂O estimation.     

Influence of thiosulfate on nitrification,denitrification, and production of nitric oxide and nitrous oxide in soil

Thiosulfate and CS₂ inhibit nitrification. The effect of the addition of thiosulfate on the turnover of inorganic N compounds was tested in an Egyptian and a German arable soil under nitrifying and denitrifying conditions. For nitrification, the soils were amended with NH _inf4 ^sup+ and incubated under aerobic conditions. For denitrification, the soils were amended with NO _inf3 ^sup- and incubated under anaerobic conditions. In both cases, the thiosulfate decreased with time while tetrathionate accumulated to an intermediate extent. Both compounds disappeared completely after <25 days. Production of CS₂ was not observed. Carbonyl sulfide was produced only in the Egyptian soil, but production decreased with increasing amounts of added thiosulfate. Under nitrifying conditions, the addition of increasing amounts of thiosulfate (25, 50, and 100 g S g^-1 dry weight) resulted in decreasing rates of NH _inf4 ^sup+ oxidation to NO _inf3 ^sup- ; it also resulted in an increasing intermediate accumulation of NO _inf2 ^sup- and NO, and in an increasing production of N₂O. Under denitrifying conditions, the addition of increasing amounts of thiosulfate did not significantly affect the rate of NO _inf3 ^sup- reduction, and resulted in an increasing intermediate accumulation of NO _inf2 ^sup- and of NO only in the German soil in which the production of N₂O was slightly inhibited by thiosulfate. These results demonstrate that the nitrification of NH _inf4 ^sup+ and NO _inf2 ^sup- was inhibited by increasing concentrations of thiosulfate and/or tetrathionate without involving the formation of volatile S compounds as potential nitrification inhibitors. Denitrification was not affected by the addition of thiosulfate.     

Laboratory investigations into the effects of the pesticides mancozeb, chlorothalonil, and prosulfuron on nitrous oxide and nitric oxide production in fertilized soil

Independent soil microcosm experiments were used to investigate the effects of the fungicides mancozeb and chlorothalonil, and the herbicide prosulfuron, on N₂O and NO production by nitrifying and denitrifying bacteria in fertilized soil. Soil cores were amended with NH₄NO₃ or NH₄NO₃ and pesticide, and the N₂O and NO concentrations were monitored periodically for approximately 48 h following amendment. Nitrification is the major source of N₂O and NO in these soils at soil moistures relevant to those observed at the field site where the cores were collected. At pesticide concentrations from 0.02 to 10 times that of a standard single application on a corn crop, N₂O and NO production was inhibited by all three pesticides. Generally N₂O production was inhibited by the pesticides from 10 to 62% and 20 to 98% at the lowest and highest dosages, respectively. Nitric oxide production was generally inhibited from about 5 to 47% and by 20 to 97% at the lowest and highest dosages, respectively. Nitrous oxide and nitric oxide production by nitrification was more susceptible to inhibition by these pesticides than denitrification. Production of both N₂O and NO by nitrification was inhibited by as much as 99%, at the highest concentration of pesticide applied. The net production of N₂O increased as soil moisture increased. The rate of NO production was greatest at the intermediate moistures investigated, between 14 and 19% gravimetric soil moisture, suggestive that nitrification is the dominant source of NO.     

Soil and residue management effects on cropping conditions and nitrous oxide fluxes under controlled traffic in Scotland2. Nitrous oxide, soil N status and weather

Nitrogen from fertilisers and crop residues can be lost as nitrous oxide (N₂O), a greenhouse gas that causes an increase in global warming and also depletes stratospheric ozone. Nitrous oxide emissions, soil chemical status, temperature and N₂O concentration in the soil atmosphere were measured in a field experiment on soil compaction in loam and sandy loam (cambisols) soils in south-east Scotland. The overall objective was to discover how the intensity and distribution of soil compaction by tractor wheels or by roller just before sowing influenced crop performance, soil conditions and production and emissions of N₂O under controlled traffic conditions. Compaction treatments were zero, light compaction by roller (up to 1 Mg per metre of length) and heavy compaction by loaded tractor (up to 4.2 Mg). In this paper we report the effects on production and emissions of N₂O and relate them to soil and crop conditions. Nitrous oxide fluxes were substantial only when the soil water content was high (>27 g per 100 g). Fertiliser application stimulated emissions in the spring whereas crop residues stimulated emissions in autumn and winter. Heavy compaction increased N₂O emissions after fertiliser application or residue incorporation more than light or zero compaction. The bulk densities of the heavily and lightly compacted soils were up to 89% and 82% of the theoretical (Proctor) maxima. Higher soil cone resistances, temperatures and nitrogen availability and lower gas diffusivities and air-filled porosities combined to make the heavily compacted soil more anaerobic and likely to denitrify than the zero or lightly compacted soil. Compaction sufficient to increase N₂O emissions significantly corresponded with adverse soil conditions for winter barley (Hordeum vulgare L.) growth. Soil tillage, which ensures that soil compaction is no greater than in our light treatment and is confined to near the soil surface, may help to mitigate both surface fluxes of N₂O and losses to the subsoil.     

Nitrous oxide emissions and nitrogen cycling in managed grassland in Southern Hokkaido,Japan

Abstract

Nitrous oxide (N₂O) emissions were measured and nitrogen (N) budgets were estimated for 2?years in the fertilizer, manure, control and bare plots established in a reed canary grass (Phalaris arundinacea L.) grassland in Southern Hokkaido, Japan. In the manure plot, beef cattle manure with bark was applied at a rate of 43–44?Mg fresh matter (236–310?kg?N)?ha^?1?year^?1, and a supplement of chemical fertilizer was also added to equalize the application rate of mineral N to that in the fertilizer plots (164–184?kg?N?ha^?1?year^?1). Grass was harvested twice per year. The total mineral N supply was estimated as the sum of the N deposition, chemical fertilizer application and gross mineralization of manure (GMm), soil (GMs), and root-litter (GMl). GMm, GMs and GMl were estimated by dividing the carbon dioxide production derived from the decomposition of soil organic matter, root-litter and manure by each C?:?N ratio (11.1 for soil, 15.5 for root-litter and 23.5 for manure). The N uptake in aboveground biomass for each growing season was equivalent to or greater than the external mineral N supply, which is composed of N deposition, chemical fertilizer application and GMm. However, there was a positive correlation between the N uptake in aboveground biomass and the total mineral N supply. It was assumed that 58% of the total mineral N supply was taken up by the grass. The N supply rates from soil and root-litter were estimated to be 331–384?kg?N?ha^?1?year^?1 and 94–165?kg?N?ha^?1?year^?1, respectively. These results indicated that the GMs and GMl also were significant inputs in the grassland N budget. The cumulative N₂O flux for each season showed a significant positive correlation with mineral N surplus, which was calculated as the difference between the total mineral N supply and N uptake in aboveground biomass. The emission factor of N₂O to mineral N surplus was estimated to be 1.2%. Furthermore, multiple regression analysis suggested that the N₂O emission factor increased with an increase in precipitation. Consequently, soil and root-litter as well as chemical fertilizer and manure were found to be major sources of mineral N supply in the grassland, and an optimum balance between mineral N supply and N uptake is required for reducing N₂O emission.     

Spatial variations in nitrous oxide and nitric oxide emission potential on a slope of Japanese cedar (Cryptomeria japonica) forest

To quantify the spatial variation and spatial structure of nitrous oxide (N₂O) and nitric oxide (NO) emission from forest soils, we measured N₂O and NO emission rates from surface soil cores taken at 1 m intervals on a cross-line transect (65 m × 20 m) on a slope of Japanese cedar ( Cryptomeria japonica ) forest in a temperate region of central Japan and analyzed the spatial dependency of N oxide gas emissions using geostatistics. We divided N₂O emission into N₂O from denitrification and N₂O from nitrification using the acetylene inhibition method. According to the geostatistical analysis, N₂O emission rates on the slope had large spatial variation and weak spatial dependency. This weak spatial dependency was caused by the inordinately high N₂O emissions on the slope, which were derived mainly from denitrification. In contrast, NO emission rate on the slope had large spatial variation, but strong spatial dependency and a distinct spatial distribution related to slope position, that is, high in the middle of the slope and low in the shoulder and the foot of the slope. The CN ratio and water-filled pore space were the dominant factors controlling NO emission rate on a slope. Our results suggest that spatial information about topographic factors helps to improve the estimation of both N₂O emission and NO emission from forest soils.     

Influence of oxygen on production and consumption of nitric oxide in soil

Summary NO and N₂O release rates were measured in an acidic forest soil (pH 4.0) and a slightly alkaline agricultural soil (pH 7.8), which were incubated at different O₂ concentrations (<0.01 – 20% O₂) and at different NO concentrations (40 – 1000 ppbv NO). The system allowed the determination of simultaneously operating NO production rates and NO uptake rate constants, and the calculation of a NO compensation concentration. Both NO production and NO consumption decreased with increasing O₂. NO consumption decreased to a smaller extent than NO production, so that the NO compensation concentrations also decreased. However, the NO compensation concentrations were not low enough for the soils to become a net sink for atmospheric NO. The release of N₂O increased relative to NO release when the gases were allowed to accumulate instead of being flushed out. The forest soil contained only denitrifying, but not nitrifying bacteria, whereas the agricultural soil contained both. Nevertheless, NO release rates were less sensitive to O₂ in the forest soil compared to the agricultural soil.     

Fluxes of nitrous oxide and nitric oxide from experimental excreta patches in boreal agricultural soil

Nitric oxide (NO) and nitrous oxide (N₂O) emissions were measured from experimental dung and urine patches placed on boreal pasture soil during two growing seasons and one autumn period until soil freezing. N₂O emissions in situ were studied by a static chamber method. NO was measured with a dynamic chamber method using a NO analyser in situ. Mean emissions from the control plots were 47.6±4.5 μg N₂ON m⁻² h⁻¹ and 12.6±1.6 μg NON m⁻² h⁻¹. N₂O and NO emissions from urine plots (132±21.2 μg N₂ON m⁻² h⁻¹ and 51.9±7.6 μg NON m⁻² h⁻¹) were higher than those from dung plots (110.0±20.1 μg N₂ON m⁻² h⁻¹ and 14.7±2.1 μg NON m⁻² h⁻¹). There was a large temporal variation in N₂O and NO emissions. Maximum N₂O emissions were measured a few weeks after dung or urine application, whereas the maximum NO emissions were detected the following year. NO was responsible on average 14% (autumn) and 34% (summer) of total (NO+N₂O)N emissions from the pasture soil. NO emissions increased with increasing soil temperature and with decreasing soil moisture. N₂O emissions increased with increasing soil moisture, but did not correlate with soil temperature. Therefore we propose that N₂O and NO were produced mainly during different microbial processes, i.e., nitrification and denitrification, respectively. The results show that the overall conditions and mechanism especially for emissions of NO are still poorly understood but that there are differences in the mechanisms regulating N₂O and NO production.     

Nitrate removal, denitrification and nitrous oxide production in the riparian zone of an ephemeral stream

Riparian zones are important features of the landscape that can buffer waterways from non-point sources of nitrogen pollution. Studies of perennial streams have identified denitrification as one of the dominant mechanisms by which this can occur. This study aimed to assess nitrate removal within the riparian zone of an ephemeral stream and characterise the processes responsible, particularly denitrification, using both in-situ and laboratory techniques. To quantify rates of groundwater nitrate removal and denitrification in-situ, nitrate was added to two separate injection-capture well networks in a perched riparian aquifer of a low order ephemeral stream in South East Queensland, Australia. Both networks also received bromide as a conservative tracer and one received acetylene to inhibit the last step of denitrification. An average of 77 ± 2% and 98 ± 1% of the added nitrate was removed within a distance of 40 cm from the injection wells (networks with acetylene and without, respectively). Based on rates of N₂O production in the network with added acetylene, denitrification was not a major mechanism of nitrate loss, accounting for only 3% of removal. Reduction of nitrate to ammonium was also not a major pathway in either network, contributing <4%. Relatively high concentrations of oxygen in the aquifer following recent filling by stream water may have reduced the importance of these two anaerobic pathways. Alternatively, denitrification may have been underestimated using the in-situ acetylene block technique. In the laboratory, soils taken from two depths at each well network were incubated with four nitrate-N treatments (ranging from ambient concentration to an addition of 15 mg N l⁻¹), with and without added acetylene. Potential rates of denitrification, N₂O production and N₂O:N₂ ratios increased with nitrate additions, particularly in shallow soils. Potential rates of denitrification observed in the laboratory were equivalent in magnitude to nitrate removal measured in the field (mean 0.26 ± 0.12 mg N kg of dry soil⁻¹ d⁻¹), but were two orders of magnitude greater than denitrification measured in the field with added acetylene. The relative importance of assimilatory vs. dissimilatory processes of nitrate removal depends on environmental conditions in the aquifer, particularly hydrology and its effects on dissolved oxygen concentrations. Depending on seasonal conditions, aquifers of ephemeral streams like the study site are likely to fluctuate between oxic and anoxic conditions; nevertheless they may still function as effective buffers. While denitrification to N₂ is a desirable outcome from a management perspective, assimilation into biomass can provide a rapid sink for nitrate, thus helping to reduce short-term delivery of nitrate downstream. Longer-term studies are needed to determine the overall effectiveness of riparian buffers associated with ephemeral streams in mitigating nitrate loads reaching downstream ecosystems.     

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.     

Soils and nitrous oxide research

Between 1990 and 2008, Soil Use and Management has published around 42 articles which have dealt with nitrous oxide (N₂O) emissions from soils. The importance of this subject to readers of the journal has increased rapidly in recent years. A substantial number of these papers have appeared in two supplements. These were ‘Soils and the Greenhouse Effect’, vol. 13 (4) and ‘Soils as Carbon Sinks’, vol. 20. The number of annual citations of articles on N₂O in the journal has risen from zero in the early 1990s to 160 per year in 2008. In this article, we have highlighted some of the more important papers on N₂O that have been published by Soil Use and Management, and explain how they have helped advance our understanding of the role that soil management plays in influencing N₂O emissions.     

Carbon dioxide, nitrous oxide and methane dynamics in boreal organic agricultural soils with different soil characteristics

The annual carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) dynamics were measured with static chambers on two organic agricultural soils with different soil characteristics. Site 1 had a peat layer of 30 cm, with an organic matter (OM) content of 74% in the top 20 cm. Site 2 had a peat layer of 70 cm but an OM content of only 40% in the top 20 cm. On both sites there were plots under barley and grass and also plots where the vegetation was removed. All soils were net sources of CO₂ and N₂O, but they consumed atmospheric CH₄. Soils under barley had higher net CO₂ emissions (830 g CO₂-C m⁻² yr⁻¹) and N₂O emissions (848 mg N₂O-N m⁻² yr⁻¹) than those under grass (395 g CO₂-C m⁻³ yr⁻¹ and 275 mg N₂O-N m⁻² yr⁻¹). Bare soils had the highest N₂O emissions, mean 2350 mg N₂O-N m⁻² yr⁻¹. The mean CH₄ uptake rate from vegetated soils was 100 mg CH₄-C m⁻³ yr⁻¹ and from bare soils 55 mg CH₄-C m⁻² yr⁻¹. The net CO₂ emissions were higher from Site 2, which had a high peat bulk density and a low OM content derived from the addition of mineral soil to the peat during the cultivation history of that site. Despite the differences in soil characteristics, the mean N₂O emissions were similar from vegetated peat soils from both sites. However, bare soils from Site 2 with mineral soil addition had N₂O emissions of 2-9 times greater than those from Site 1. Site 1 consumed atmospheric CH₄ at a higher rate than Site 2 with additional mineral soil. N₂O emissions during winter were an important component of the N₂O budget even though they varied greatly, ranging from 2 to 99% (mean 26%) of the annual emission.     

Influence of soil properties on the turnover of nitric oxide and nitrous oxide by nitrification and denitrification at constant temperature and moisture

 Soils are a major source of atmospheric NO and N₂O. Since the soil properties that regulate the production and consumption of NO and N₂O are still largely unknown, we studied N trace gas turnover by nitrification and denitrification in 20 soils as a function of various soil variables. Since fertilizer treatment, temperature and moisture are already known to affect N trace gas turnover, we avoided the masking effect of these soil variables by conducting the experiments in non-fertilized soils at constant temperature and moisture. In all soils nitrification was the dominant process of NO production, and in 50% of the soils nitrification was also the dominant process of N₂O production. Factor analysis extracted three factors which together explained 71% of the variance and identified three different soil groups. Group I contained acidic soils, which showed only low rates of microbial respiration and low contents of total and inorganic nitrogen. Group II mainly contained acidic forest soils, which showed relatively high respiration rates and high contents of total N and NH₄ ⁺. Group III mainly contained neutral agricultural soils with high potential rates of nitrification. The soils of group I produced the lowest amounts of NO and N₂O. The results of linear multiple regression conducted separately for each soil group explained between 44–100% of the variance. The soil variables that regulated consumption of NO, total production of NO and N₂O, and production of NO and N₂O by either nitrification or denitrification differed among the different soil groups. The soil pH, the contents of NH₄ ⁺, NO₂ ^– and NO₃ ^–, the texture, and the rates of microbial respiration and nitrification were among the important variables. Received: 28 October 1999     

Nitrous oxide emissions under different soil and land management conditions

Nitrous oxide (N₂O) emissions of three different soils – a rendzina on cryoturbed soil, a hydromorphic leached brown soil and a superficial soil on a calcareous plateau – were measured using the chamber method. Each site included four types of land management: bare soil, seeded unfertilized soil, a suboptimally fertilized rapeseed crop and an overfertilized rapeseed crop. Fluxes varied from –1g to 100g N₂O-nitrogen ha^–1 day^–1. The highest rates of N₂O emissions were measured during spring on the hydromorphic leached brown soil which had been fertilized with nitrogen (N); the total emissions during a 5-month period exceeded 3500gNha^–1. Significant fluxes were also observed during the summer. Very marked effects of soil type and management were observed. Two factors – the soil hydraulic behaviour and the ability of the microbial population to reduce N₂O – appear to be essential in determining emissions of N₂O by soils. In fact, the hydromorphic leached brown soil showed the highest emissions, despite having the lowest denitrification potential because of its water-filled pore space and low N₂O reductase activity. Soil management also appears to affect both soil nitrate content and N₂O emissions. Received: 4 April 1997     

Nitrous oxide emissions from grazed grassland

Abstract. Grazing animals on managed pastures and rangelands have been identified recently as significant contributors to the global N₂O budget. This paper summarizes relevant literature data on N₂O emissions from dung, urine and grazed grassland, and provides an estimate of the contribution of grazing animals to the global N₂O budget.
The effects of grazing animals on N₂O emission are brought about by the concentration of herbage N in urine and dung patches, and by the compaction of the soil due to treading and trampling. The limited amount of experimental data indicates that 0.1 to 0.7% of the N in dung and 0.1 to 3.8% of the N in urine is emitted to the atmosphere as N₂O. There are no pertinent data about the effects of compaction by treading cattle on N₂O emission yet. Integral effects of grazing animals have been obtained by comparing grazed pastures with mown-only grassland. Grazing derived emissions, expressed as per cent of the amount of N excreted by grazing animals in dung and urine, range from 0.2 to 9.9%, with an overall mean of 2%. Using this emission factor and data statistics from FAO for numbers of animals, the global contribution of grazing animals was estimated at 1.55 Tg N₂O-N per year. This is slightly more than 10% of the global budget.     

Nitrous oxide and methane transport through rice plants

The separate closed chamber technique was used to study the potential of rice plants for transporting N₂O and CH₄ produced in soil to the atmosphere. The results indicate that N₂O produced in soil can be conducted to the atmosphere via rice plants similarly to CH₄ transport. More than 80% of both N₂O and CH₄ was emitted through rice plants. The rest was emitted through the soil/water/atmosphere interface by ebullition and diffusion. Nitrate addition increased the total N₂O emission rate substantially but decreased the total CH₄ emission. Nitrate addition did not change the CH₄ emission ratio through rice plants, but lowered the percentage of N₂O emission through rice plants. The results suggest that rice plants serve not only as a conduit for most of the CH₄ leaving the soil but also for the N₂O produced in the soil. Received: 31 January 1996     

一氧化氮改善铁胁迫玉米光合组织结构及其活性

一氧化氮(NO)影响植物生长发育过程及其机制是近年来的研究热点,在植物生长发育的多个层面起着重要的作用。以硝普钠(SNP)为一氧化氮发生剂,液体培养20.d龄的玉米幼苗叶片为实验材料进行研究,结果表明,NO可完全逆转玉米幼苗由缺铁引起的脉间失绿现象,极显著地提高叶片叶绿素含量。电镜观察结果证实,NO促进了玉米叶片叶肉细胞和维管束鞘细胞中叶绿体的发育,叶绿体数量增多且体积增大,基质片层和基粒数量明显增多且结构完好。同时,NO促进了缺铁玉米类囊体膜色素蛋白复合体的装配并显著提高了光合链的电子传递速率,使叶片光合活性得到极显著增加。     

The effect of soil texture and soil drainage on emissions of nitric oxide and nitrous oxide

Abstract. Agricultural soils are important sources of the tropospheric ozone precursor NO and the greenhouse gas N₂O. Emissions are controlled primarily by parameters that vary the soil mineral N supply, temperature and soil aeration. In this field experiment, the importance of soil physical properties on emissions of NO and N₂O are identified. Fluxes were measured from 13 soils which belonged to 11 different soil series, ranging from poorly drained silty clay loams to freely drained sandy loams. All soils were under the same soil management regime and crop type (winter barley) and in the same maritime climate zone. Despite this, emissions of NO and N₂O ranged over two orders of magnitude on all three measurement occasions, in spring before and after fertilizer application, and in autumn after harvest. NO emissions ranged from 0.3 to 215 μg NO-N m^–2 h^–1, with maximum emissions always from the most sandy, freely drained soil. Nitrous oxide emissions ranged from 0 to 193 μg N₂O-N m^–2 h^–1. Seasonal shifts in soil aeration caused maximum N₂O emissions to switch from freely drained sandy soils in spring to imperfectly drained soils with high clay contents in autumn. Although effects of soil type on emissions were not consistent, N₂O emission was best related to a combination of bulk density and clay content and the NO/N₂O ratio decreased logarithmically with increasing water filled pore space.