Emissions of nitrous oxide from a constructed wetland using a groundfilter and macrophytes in waste-water purification of a dairy farm

 In less populated rural areas constructed wetlands with a groundfilter made out of the local soil mixed with peat and planted with common reed (Phragmites australis) are increasingly used to purify waste water. Particularly in the rhizosphere of the reed, nitrification and denitrification processes take place varying locally and temporally, and the question arises to what extent this type of waste-water treatment plant may contribute to the release of N₂O. In situ N₂O measurements were carried out in the two reed beds of the Friedelhausen dairy farm, Hesse, Germany, irrigated with the waste water from a cheese dairy and 70 local inhabitants (12 m³ waste water or 6 kg BOD₅ or 11 kg chemical O₂ demand (COD_Mn) day^–1). During November 1995 to March 1996, the release of N₂O was measured weekly at 1 m distances using eight open chambers and molecular-sieve traps to collect and absorb the emitted N₂O. Simultanously, the N₂O trapped in the soil, the soil temperature, as well as the concentrations of NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, water-soluble C and the pH were determined at depths of 0–20, 20–40 and 40–60 cm. In the waste water from the in- and outflow the concentrations of COD_Mn, BOD₅, NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, as well as the pH, were determined weekly. Highly varying amounts of N₂O were emitted at all measuring dates during the winter. Even at soil temperatures of –1.5  °C in 10 cm depth of soil or 2  °C at a depth of 50 cm, N₂O was released. The highest organic matter and N transformation rates were recorded in the upper 20 cm of soil and in the region closest to the outflow of the constructed wetland. Not until a freezing period of several weeks did the N₂O emissions drop drastically. During the period of decreasing temperatures less NO₃ ^–-N was formed in the soil, but the NH₄ ⁺-N concentrations increased. On average the constructed wetlands of Friedelhausen emitted about 15 mg N₂O-N inhabitant equivalent^–1 day^–1 during the winter period. Nitrification-denitrification processes rather than heterotrophic denitrification are assumed to be responsible for the N₂O production. Received: 28 October 1998     

Estimation of simultaneous nitrification and denitrification in grassland soil associated with urea-N using 15N and nitrification inhibitor

 A low efficiency of use of N fertilisers has been observed in mid-Wales on permanent pasture grazed intensively by cattle. Earlier laboratories studies have suggested that heterogeneity in redox conditions at shallow soil depths may allow nitrification and denitrification to occur concurrently resulting in gaseous losses of N from both NH₄ ⁺ and NO₃ ^–. The objective of the investigation was to test the hypothesis that both nitrification and denitrification can occur simultaneously under simulated field capacity conditions (∼5 kPa matric potential). Intact soil cores were taken from grassland subjected to both grazing and amenity use. The fate of applied NH₄ ⁺ was examined during incubation. ¹⁵N was used as a tracer. Nitrapyrin was used as a nitrification inhibitor and acetylene was used to block N₂O reductase. More than 50% of N applied as NH₄ ⁺ disappeared over a period of 42 days from the soil mineral-N pool. Some of this N was evolved as N₂O. Accumulation of NO₃ ^––N in the surface 0–2.5 cm indicated active nitrification. Addition of nitrapyrin increased N recovery by 26% and inhibited both the accumulation of NO₃–N and emission of N₂O. When intact field cores were incubated after addition of ¹⁵N-urea, all of the N₂O evolved was derived from added urea-N. It was concluded that nitrification and denitrification do occur simultaneously in the top 7.5 cm or so, of the silty clay loam grassland topsoils of mid-Wales at moisture contents typical of field capacity. The quantitative importance of these concurrent processes to N loss from grassland systems has not yet been assessed. Received: 15 December 1998     

Nitrogen fertilizers promote denitrification

A laboratory study was conducted to compare the effects of different N fertilizers on emission of N₂ and N₂O during denitrification of NO₃ ^– in waterlogged soil. Field-moist samples of Drummer silty clay loam soil (fine-silty, mixed, mesic Typic Haplaquoll) were incubated under aerobic conditions for 0, 2, 4, 7, 14, 21, or 42 days with or without addition of unlabelled (NH₄)₂SO₄, urea, NH₄H₂PO₄, (NH₄)₂HPO₄, NH₄NO₃ (200 or 1000 mg N kg^–1 soil), or liquid anhydrous NH₃ (1000 mg N kg^–1 soil). The incubated soil samples were then treated with ¹⁵N-labelled KNO₃ (250 mg N kg^–1 soil, 73.7 atom% ¹⁵N), and incubation was carried out under waterlogged conditions for 5 days, followed by collection of atmospheric samples for ¹⁵N analyses to determine labelled N₂ and N₂O. Compared to samples incubated without addition of unlabelled N, all of the fertilizers promoted denitrification of ¹⁵NO₃ ^–. Emission of labelled N₂ and N₂O decreased in the order: Anhydrous NH₃>urea<$>\gg<$> (NH₄)₂HPO₄>(NH₄)₂SO₄≃NH₄NO₃≃NH₄H₂PO₄. The highest emissions observed with anhydrous NH₃ or urea coincided with the presence of NO₂ ^–, and ¹⁵N analyses indicated that these emissions originated from NO₂ ^– rather than NO₃ ^–. Emissions of labelled N₂ and N₂O were significantly correlated with fertilizer effects on soil pH and water-soluble organic C. Received: 17 January 1996     

Field method to determine N2O emission from nitrification and denitrification

 Nitrous oxide (N₂O) emissions via the nitrification (I _nit) and denitrification (I _den) pathways were successfully measured with in-field incubation of soil cores in preserving jars at 0 Pa and 5–10 Pa acetylene. From the incubations, fractions of nitrification – N₂O over total N₂O (I _nit / I _tot) – and denitrification – N₂O over total N₂O (I _den / I _tot) – were obtained. Actual field emissions of N₂O via nitrification (F _nit) and denitrification (F _den) were calculated by multiplying the fractions from the incubation technique with the daily N₂O emission (F _day) determined with a direct soil cover method. The approach presented here was successful for a whole range of soil moisture conditions in intensive grassland. F _nit and F _den followed the trends of soil ammonium and soil nitrate. Received: 31 October 1997     

Denitrification enzyme activity and potential of subsoils under grazed grasslands assayed by membrane inlet mass spectrometer

The importance of subsoil denitrification on the fate of agriculturally derived nitrate (NO₃) leached to groundwater is crucial for budgeting N in an ecosystem and for identifying areas where the risk of excess NO₃ is reduced. However, the high atmospheric background of di-nitrogen (N₂) causes difficulties in assessing denitrification enzyme activity (DEA) and denitrification potential (DP) in soils directly. Here, we apply Membrane Inlet Mass Spectrometry (MIMS) technique to investigate indirectly DEA and DP in soils by measuring N₂/Ar ratio changes in headspace water over soil. Soils were collected from 0-10, 15-25 and 60-70 cm depths of a grazed ryegrass and grass-clover. The samples were amended with helium-flushed deionized water containing ranges of NO₃ and carbon (glucose-C) and were incubated for six hours in the dark at 21 °C. The peaks for N₂/Ar ratio, declined with increasing soil depth, indicating a reduced substrate requirements to initiate DEA en-masse (15-30 mg NO₃-N alone or with 60-120 mg glucose-C, kg⁻¹ soil). The dissolved N₂O concentrations were very small (0.004-0.269 μg N kg⁻¹ soil) but responded well to the added N and C, showing a reduction in DEA with soil depth. In three separate studies, only subsoils were incubated for 3 days at 12 °C with 20-30 mg NO₃-N ± 40-60 mg glucose-C, kg⁻¹ soil. Denitrification capacity (DC, NO₃ only treatment) was not statistically different to the control (no amendment) within a land use (0.03-0.05 vs. 0.07-0.22 mg N kg⁻¹ soil d⁻¹), the highest being in ryegrass subsoils receiving groundwater. The DP was significantly (P < 0.0001) higher in subsoils under ryegrass than under grass-clover (0.50-0.71 vs. 1.15 mg N kg⁻¹ soil d⁻¹). The rates of DP (NO₃ + glucose-C) increased significantly (P < 0.0001) in unsaturated and saturated subsoils (0.92 and 2.19 mg N kg⁻¹ soil d⁻¹, respectively) of grass-clover, due to the higher reductive state resulting from the 10 day pre-incubation. Available C accelerated denitrification in soils and superseded the temporary elevation in oxidative state due to NO₃ addition. The substrates load differences between the land uses regulated the degree of denitrification rates. Results suggest that both dissolved N₂O measured by gas chromatography and N₂/Ar ratio measured by MIMS to indirectly determine DEA, and the latter to quantify total DC/DP in soils can be used. However, interference of oxygen in the MIMS system should be considered if available C is added or is naturally elevated in soil or groundwater.     

Nitrification and denitrification in the rhizosphere of rice: the detection of processes by a new multi-channel electrode

 N turnover in flooded rice soils is characterized by a tight coupling between nitrification and denitrification. Nitrification is restricted to the millimetre-thin oxic surface layer while denitrification occurs in the adjacent anoxic soil. However, in planted rice soil O₂ released from the rice roots may also support nitrification within the otherwise anoxic bulk soil. To locate root-associated nitrification and denitrification we constructed a new multi-channel microelectrode that measures NH₄ ⁺, NO₂ ^–, and NO₃ ^– at the same point. Unfertilized, unplanted rice microcosms developed an oxic-anoxic interface with nitrification taking place above and denitrification below ca. 1 mm depth. In unfertilized microcosms with rice plants, NH₄ ⁺, NO₂ ^– and NO₃ ^– could not be detected in the rhizosphere. Assimilation by the rice roots reduced the available N to a level where nitrification and denitrification virtually could not occur. However, a few hours after injecting (NH₄)₂HPO₄ or urea, a high nitrification activity could be detected in the surface layer of planted microcosms and in a depth of 20–30 mm in the rooted soil. O₂ concentrations of up to 150 μM were measured at the same depth, indicating O₂ release from the rice roots. Nitrification occurred at a distance of 0–2 mm from the surface around individual roots, and denitrification occurred at a distance of 1.5–5.0 mm. Addition of urea to the floodwater of planted rice microcosms stimulated nitrification. Transpiration of the rice plants caused percolation of water resulting in a mass flow of NH₄ ⁺ towards the roots, thus supporting nitrification. Received: 23 July 1999     

Diversity of denitrifying microflora and ability to reduce N2O in two soils

 The ozone-depleting gas N₂O is an intermediate in denitrification, the biological reduction of NO₃ ^– to the gaseous products N₂O and N₂ gas. The molar ratio of N₂O produced (N₂O/N₂O+N₂) varies temporally and spatially, and in some soils N₂O may be the dominant end product of denitrification. The fraction of NO₃ ^–-N emitted as N₂O may be due at least in part to the abundance and activity of denitrifying bacteria which possess N₂O reductase. In this study, we enumerated NO₃ ^–-reducing and denitrifying bacteria, and compared and contrasted collections of denitrifying bacteria isolated from two agricultural soils, one (Auxonne, soil A) with N₂O as the dominant product of denitrification, the other (Chalons, soil C) with N₂ gas as the dominant product. Isolates were tested for the ability to reduce N₂O, and the presence of the N₂O reductase (nosZ)-like gene was evaluated by polymerase chain reaction (PCR) using specific primers coupled with DNA hybridization using a specific probe. The diversity and phylogenetic relationships of members of the collections were established by PCR/restriction fragment length polymorphism of 16s rDNA. The two soils had similar numbers of bacteria which used NO₃ ^– as a terminal electron acceptor anaerobically. However, the soil A had many more denitrifiers which reduced NO₃ ^– to gaseous products (N₂O or N₂) than did soil C. Collections of 258 and 281 bacteria able to grow anaerobically in the presence of NO₃ ^– were isolated from soil A and soil C, respectively. These two collections contained 66 and 12 denitrifying isolates, respectively, the others reducing NO₃ ^– only as far as NO₂ ^–. The presence of nosZ sequences was generally a poor predictor of N₂O reducing ability: there was agreement between the occurrence of nosZ sequences and the N₂O reducing ability for only 42% of the isolates; 35% of the isolates (found exclusively in soil A) without detectable nosZ sequences reduced N₂O whereas 21% of the isolates carrying nosZ sequences did not reduce this gas under our assay conditions. Twenty-eight different 16S rDNA restriction patterns (using two restriction endonucleases) were distinguished among the 78 denitrifying isolates. Two types of patterns appeared to be common to both soils. Twenty-three and three types of patterns were found exclusively among bacteria isolated from soils A and C, respectively. The specific composition of denitrifying communities appeared to be different between the two soils studied. This may partly explain the differences in the behaviour of the soils concerning N₂O reduction during denitrification. Received: 31 October 1997     

N dynamics and sources of N2O production following pig slurry application to a loamy soil

Carbon (C) and Nitrogen dynamics and sources of nitrous oxide (N₂O) production were investigated in a loamy soil amended with pig slurry. Pig slurry (40000kgha^–1) or distilled H₂O was applied to intact soil cores of the upper 5cm of a loamy soil which were incubated under aerobic conditions for 28 days at 25°C. Treatments were with or without acetylene (C₂H₂), which is assumed to inhibit the reduction of N₂O to dinitrogen (N₂), and with or without dicyandiamide (DCD), which is thought to inhibit nitrification. Volatilization of ammonia (NH₃), pH, carbon dioxide (CO₂) and N₂O production, and ammonium (NH₄ ⁺) and nitrate NO₃ ^–) concentrations were monitored. The pH of the pig slurry amended soil increased from an initial value of 7.1 to pH 8.3 within 3 days; it then decreased slowly but was still at a value of 7.4 after 28 days. Twenty percent of the NH₄ ⁺ applied volatilized within 28 days. Sixty percent of the C applied in the pig slurry evolved as CO₂, if no priming effect was assumed, but only 38% evolved when the soil was amended with DCD. Pig slurry significantly increased denitrification and the ratio between its gaseous products, N₂O and N₂, was 0.21. No significant increases in NO₃ ^– concentration occurred, and N₂O produced through nitrification was 0.07mg N₂O-N kg^–1 day^–1 or 33% of the total N₂O produced. C₂H₂ was used as a C substrate by microorganisms and increased the production of N₂O. Received: 12 May 1997     

Nitrogen dynamics in different types of pasture in the Austrian Alps

 Soil N dynamics were compared in Alpine pastures on two mountains. N-pool sizes and N fluxes were measured relative to N losses via leaching and denitrification in summer. On each mountain, four types of pasture were studied: (1) forest pastures, (2) recently developed pastures formed by forest clearance ("new pastures"), (3) older established pastures, and (4) pastures planted with clover. At both study sites (Scheuchegg and Teufelstein) we obtained similar results. Compared with forest pasture soils, open pasture soils were found to have greater microbial biomass and faster mineralisation potentials, but net field mineralisation rates were slower. In the forest pastures, highest N losses via denitrification were found. Higher potential leaching of NO₃ ^–, estimated by accumulation of NO₃ ^– on ion-exchange resins, in the forest pasture soils suggests lower N uptake by microbes and herbaceous plants compared with open pastures. N₂O-production rates of the forest pasture soils at the Scheuchegg site (11.54 μg N₂O-N m^–2 h^–1) were of similar magnitude to those reported for spruce forests without pastures, but at Teufelstein (53.75 μg N₂O-N m^–2 h^–1) they were higher. However, if forest pastures are not overgrazed, no elevated N loss through N₂O production and leaching of NO₃ ^– is expected. Denitrification rates in the open pastures (0.83–7.50 μg N₂O-N m^–2 h^–1) were low compared with reports on lowland pastures. In soils of the new pastures, rates of microbial N processes were similar to those in the established pastures, indicating a high capacity of soils to restore their internal N cycle after forest clearance. Received: 19 August 1999     

Comparison of field and laboratory measurement of denitrification and N2O production in the saturated zone of hydromorphic soils

We evaluated a new method to measure in situ denitrification under field conditions in a number of water-saturated subsoils that had a broad range of biogeochemical properties. A test solution containing ¹⁵NO₃⁻ and/or C₂H₂ was introduced to the subsoil and the subsequent production of dissolved denitrification products was measured to quantify denitrification activity. The method showed a clear production of denitrification products over time. Results were compared to laboratory-based measurements from the same soil incubated as anaerobic slurries with added ¹⁵NO₃⁻. Rates of denitrification with the in situ and the laboratory methods ranged from 1-2800 and 1-1700 μg N kg⁻¹ d⁻¹, respectively. Generally the methods gave good agreement and we consider both to be valid. However, there were some significant deviations, which we attribute to spatial heterogeneity and laboratory effects. Because the laboratory method is so much easier to perform, we suggest it should be the preferred method for large-scale studies of denitrification from the soil types we investigated. However, the two methods showed poor agreement in determining the proportion of N₂O in the total denitrification output. This was because this proportion is subject to delicate and complex control. We conclude that neither method was suitable for quantifying N₂O emission from the denitrification measurements.     

Nitrous oxide emissions from an irrigated sandy-clay loam cropped to maize and wheat

 Nitrous oxide (N₂O) emissions were measured from an irrigated sandy-clay loam cropped to maize and wheat, each receiving urea at 100 kg N ha^–1. During the maize season (24 August–26 October), N₂O emissions ranged between –0.94 and 1.53 g N ha^–1 h^–1 with peaks during different irrigation cycles (four) ranging between 0.08 and 1.53 g N ha^–1 h^–1. N₂O sink activity during the maize season was recorded on 10 of the 29 sampling occasions and ranged between 0.18 and 0.94 g N ha^–1 h^–1. N₂O emissions during the wheat season (22 November–20 April) varied between –0.85 and 3.27 g N ha^–1 h^–1, whereas peaks during different irrigation cycles (six) were in the range of 0.05–3.27 g N ha^–1 h^–1. N₂O sink activity was recorded on 14 of the 41 samplings during the wheat season and ranged between 0.01 and 0.87 g N ha^–1 h^–1. Total N₂O emissions were 0.16 and 0.49 kg N ha^–1, whereas the total N₂O sink activity was 0.04 and 0.06 kg N ha^–1 during the maize and wheat seasons, respectively. N₂O emissions under maize were significantly correlated with denitrification rate and soil NO₃ ^–-N but not with soil NH₄ ⁺-N or soil temperature. Under wheat, however, N₂O emissions showed a strong correlation with soil NH₄ ⁺-N, soil NO₃ ^–-N and soil temperature but not with the denitrification rate. Under either crop, N₂O emissions did not show a significant relationship with water-filled pore space or soil respiration. Received: 11 June 1997     

Chemical and biological characteristics of alkaline saline soils from the former Lake Texcoco as affected by artificial drainage

 Soils from the former Lake Texcoco are alkaline saline and were artificially drained and irrigated with sewage effluents since the late 1980s. Undrained soil and soil drained for 1, 5 and 8 years were sampled, characterized and incubated aerobically for 90 days at 22±1  °C while production of CO₂, available P and concentrations of NH₄ ⁺, NO₂ ^– and NO₃ ^– were monitored. Artificial drainage decreased pH_H2O, water holding capacity, organic C, total N, and Na⁺, K⁺, Mg²⁺, B, Cl^– and SO₄ ^2– concentrations, increased inorganic C and Ca²⁺ concentrations more than 5-fold while total P was not affected. Microbial biomass C decreased with increased length of drainage but bacteria, actinomycetes, denitrifiers and cellulose-utilizing bacteria tended to show opposite trends. CO₂ production was less in soils drained ≥5 years compared to undrained soil but more than in soils drained for 1 year. Emission of NH₃ was negligible and concentrations of NH₄ ⁺ remained constant over time in each soil. Nitrification, as witnessed by increases in NO₃ ^– concentrations, occurred in soil drained for 8 years. NO₂ ^– concentrations decreased in soils drained ≤1 year in the first 7 days of the incubation and remained constant thereafter. It was found that artificial drainage of soils from the former Lake Texcoco profoundly affected soil characteristics. Decreases in pH and Na⁺, K⁺, Cl^– and SO₄ ^2– concentrations made conditions more favourable for plant growth, although low concentrations of inorganic N and available P might be limiting factors. Received: 1 December 1999     

Responses of nitrous oxide, dinitrogen and carbon dioxide production and oxygen consumption to temperature in forest and agricultural light-textured soils determined by model experiment

 The experiment, carried out on a forest and arable light-textured soil, was designed to study the temperature response of autotrophic and heterotrophic N₂O production and investigate how the N₂O flux relates to soil respiration and O₂ consumption. Although N₂O production seemed to be stimulated by a temperature increase in both soils, the relationship between production rate and temperature was different in the two soils. This seemed to depend on the different contribution of nitrification and denitrification to the overall N₂O flux. In the forest soil, almost all N₂O was derived from nitrification, and its production rate rose linearly from 2  °C to 40  °C. A stronger effect of temperature on N₂O production was observed in the arable soil, apparently as a result of an incremental contribution of denitrification to the overall N₂O flux with rising temperature. The soil respiration rate increased exponentially with temperature and was significantly correlated with N₂O production. O₂ consumption stimulated denitrification in both soils. In the arable soil, N₂O and N₂ production increased exponentially with decreasing O₂ concentration, though N₂O was the main gas produced at any temperature. In the forest soil, only the N₂ flux was related exponentially to O₂ consumption and it outweighed the rate of N₂O production only at >34  °C. Thus, it appears that in the forest soil, where nitrification was the main source of N₂O, temperature affected the N₂O flux less dramatically than in the arable soil, where a temperature increase strongly stimulated N₂O production by enhancing favourable conditions for denitrification. Received: 26 August 1998     

Denitrification and total fertilizer-N losses from an irrigated cotton field

 In a 2-year field study, denitrification loss was measured from an irrigated sandy-clay loam under cotton receiving urea-N at 158–173 kg ha^–1. An acetylene inhibition-soil core method was employed for the direct measurement of denitrification, considering also the N₂O entrapped in the soil. Taking into account the N₂O evolved from soil cores and that entrapped in the soil, a total of 65.7 kg N ha^–1 and 64.4 kg N ha^–1 was lost due to denitrification during the 1995 and 1996 cotton-growing seasons, respectively. Most (>70%) of the denitrification loss occurred during June–August, a period characterized by high soil temperatures and heavy monsoon rains. On average, 35% of the denitrification-N₂O was found entrapped in the soil and the amount of entrapped N₂O was significantly correlated with head space N₂O concentration and with water-filled pore space. ¹⁵N-balance during the 1996 growing season revealed a loss of 71.8 kg N ha^–1. It was concluded that a substantial proportion of the fertilizer-N applied to irrigated cotton is lost under the semiarid subtropical climatic conditions prevailing in the Central Punjab region of Pakistan and that denitrification is the major N loss process under irrigated cotton in this region. Received: 8 March 1999     

Factors influencing nitrous oxide and methane emissions from minerotrophic fens in northeast Germany

 At two field sites representing northeastern German minerotrophic fens (Rhin-Havelluch, a shallow peat site; Gumnitz, a partially drained peat site) the influence of different factors (N fertilization, groundwater table, temperature) on N₂O and CH₄ emissions was investigated. The degraded fens were sources or sinks of the radiatively active trace gases investigated. The gas fluxes measured were much higher than those found in other terrestrical ecosystems such as forests. Lowering the groundwater table increased the release of N₂O and the oxidation of CH₄. High CH₄ emission rates occurred when the groundwater tables and soil temperatures were high (>12  °C). N fertilization stimulated the release of N₂O only when application rates were very high (480 kg N ha^–1). A moderate N supply (60 or 120 kg N ha^–1) hardly increased the release of N₂O in spite of high soluble soil NO₃ ^– contents. Received: 31 October 1997     

Effects of salts and moisture content on N2O emission and nitrogen dynamics in Yellow soil and Andosol in model experiments

 The effects of salt type and its concentration on nitrification, N mineralization and N₂O emission were examined under two levels of moisture content in Yellow soil and Andosol samples as simulated to agriculture under arid/semi-arid conditions and under heavy application of fertilizer in a glass-house, respectively. The salt mixtures were composed of chlorides (NaCl and NH₄Cl) or sulphates [Na₂SO₄ and (NH₄)₂SO₄] and were added at various concentrations (0, 0.1, 0.2, 0.4 and 0.6 M as in the soil solution). These salts were added to non-saline Yellow soil at different moisture contents (45 or 40 and 65% of maximum water-holding capacity; WHC) and their effects on the changes in mineral N (NH₄ ⁺-N and NO₃ ^–-N) concentration as well as N₂O emission were examined periodically during laboratory incubation. We also measured urease activities to know the effect of salts on N mineralization. Furthermore, Ca(NO₃)₂ solution was added at various concentrations (0, 0.1, 0.3, 0.5 and 0.8 M as in the soil solution) to a non-saline Andosol taken from the subsurface layer in a glass-house and incubated at different moisture contents (50% and 70% of WHC) to examine their effects on changes in mineral N. Nitrification was inhibited by high, but remained unaffected by low, salt concentrations. These phenomena were shown in both the model experiments. It was considered that the salinity level for inhibition of nitrification was an electric conductivity (1 : 5) of 1 dS m^–1. This level was independent of the type of salts or soil, and was not affected by soil moisture content. The critical level of salts for urease activities was about 2 dS m^–1. The emission rate of N₂O was maximum at the beginning of the incubation period and stabilized at a low level after an initial peak. There was no significant difference in N₂O emission among the treatments at different salt concentrations, while higher moisture level enhanced N₂O emission remarkably. Received: 29 July 1998     

Nitrogen monoxide production and consumption in an organic soil

 Factors controlling NO production, consumption, and emission rates were examined in an organic soil. Emission rates were measured in the enclosed headspaces of intact soil cores under three fertilisation treatments (unfertilised or 100 kg N ha^–1 as NH₄Cl or as NaNO₃), with and without the nitrification inhibitor C₂H₂ (20–70 μl l^–1). Nitrification was always the main source of NO emitted across the soil surface, even when the soil was nearly saturated. Fertilisation of soil with NH₄Cl increased NO emission both by stimulating NO production from nitrification, and by decreasing the NO consumption rate constant. Addition of NaNO₃ also stimulated the production of NO and N₂O during nitrification in aerobic soil slurry experiments. This effect was eliminated by adding C₂H₂ and was therefore not related to denitrification. In loose soil samples, the increase in NO-N production after NH₄Cl addition represented as much as 26% of the added N. However, in intact cores, 95% of the NO produced through nitrification was oxidised within the soil column rather than emitted to the atmosphere. We concluded that nitrification is the primary NO source from this organic soil, that surface NO emissions are much lower than gross NO production rates, and that gaseous N oxide (NO and N₂O) losses during nitrification can be affected by both soil NH₄ ⁺ and NO₃ ^–. Received: 15 December 1998     

Effect of cycloheximide on N2O and NO3– production in a forest and an agricultural soil

The present work aims at evaluating the effect of cycloheximide at concentrations of between 0.5 and 5mgg^–1 on N₂O and NO₃ ^– production in two slightly alkaline soils, sampled from deciduous woodland and arable cultivation. In the first experiment, peptone was used as the “inducing substrate” for heterotrophic activity, and soil was incubated with cycloheximide (at different concentrations) and/or acetylene (1mll^–1) to block induced eukaryotic protein synthesis and ammonia monooxygenase activity, respectively. Peptone addition stimulated N₂O and NO₃ ^– production significantly in woodland soil, whereas arable soil showed no significant N₂O emissions and low NO₃ ^– production. Low cycloheximide concentrations drastically reduced N₂O emissions in woodland soil, suggesting a potential role of fungi in N₂O emissions. However, acetylene was equally effective in blocking N₂O emissions and part of NO₃ ^– production, so that a possible role of ammonia monooxygenase in an organic-inorganic pathway of N nitrification in fungal metabolism can be hypothesized. A second experiment was carried out on the woodland soil to check if low cycloheximide concentrations had non-target biocidal effects on soil microorganisms. Attention was focused on the range of concentrations which had reduced N₂O emission in the woodland soil. The results suggested that at concentrations of cycloheximide between 0.5 and 2mgg^–1 any biocidal effect on microbial biomass was negligible in the first 48h; therefore only selective inhibition of protein synthesis could be expected. The whole nitrifier population seemed to be particularly sensitive to cycloheximide concentrations higher than 2.5mgg^–1. Received: 4 July 1997     

Nitrous oxide emission from herbicide-treated soybean

 The emission of N₂O from soybean plants treated with the herbicides dichlorophenoxyacetic acid (2,4-D) and bromoxynil was studied. The N₂O flux from 2,4-D- and bromoxynil-treated soybean was 14.1 ng N₂O-N g^–1 fresh weight h^–1 and 19.7 ng N₂O-N g^–1 fresh weight h^–1, respectively, i.e. approximately twice that of the controls. The NO₂ ^–-N concentration in 2,4-D- and in bromoxynil-treated soybean was about 8 μg N g^–1 fresh weight, i.e. fivefold the concentration found in control plants. The NO₃ ^– content in herbicide-treated soybean did not differ significantly from that of the control plants. Consequently, the accumulation of NO₂ ^–-N during the assimilation of NO₃ ^–-N was thought to cause the observed N₂O release. Probably, N₂O is a by-product produced during either the reaction of NO₂ ^–-N with plant metabolites or NO₂ ^–-N decomposition. Final conclusions must await further experiments. Received: 5 November 1999     

Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

 Two versions of the acetylene inhibition (AI)/soil core method were compared for the measurement of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha^–1. With AI/soil core method A, the denitrification rate was measured by analysing the headspace N₂O, followed by estimation of N₂O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N₂O entrapped in the soil was measured in addition to that released from soil cores into the headspace of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N₂O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO₂, the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N₂O was significantly correlated with the water-filled pore space (WFPS) and with the N₂O concentration in the headspace, whereas CO₂ entrapment was dependent on the headspace CO₂ concentration but not on the WFPS. Due to the entrapment of N₂O and CO₂ in soil, the denitrification rate on several (18 of the 41) sampling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha^–1 day^–1) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha^–1 day^–1) was 1.6 times the rate measured with method A. Results of this study suggest that the N₂O and CO₂ entrapped in soil should also be measured to ensure the recovery of the gaseous products of denitrification by the soil core method. Received: 12 May 1998