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     

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     

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     

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     

The effect of reduced tillage on nitrous oxide emissions of silt loam soils

The effect of reduced tillage (RT) on nitrous oxide (N₂O) emissions of soils from fields with root crops under a temperate climate was studied. Three silt loam fields under RT agriculture were compared with their respective conventional tillage (CT) field with comparable crop rotation and manure application. Undisturbed soil samples taken in September 2005 and February 2006 were incubated under laboratory conditions for 10 days. The N₂O emission of soils taken in September 2005 varied from 50 to 1,095 μg N kg⁻¹ dry soil. The N₂O emissions of soils from the RT fields taken in September 2005 were statistically (P < 0.05) higher or comparable than the N₂O emissions from their respective CT soil. The N₂O emission of soils taken in February 2006 varied from 0 to 233 μg N kg⁻¹ dry soil. The N₂O emissions of soils from the RT fields taken in February 2006 tended to be higher than the N₂O emissions from their respective CT soil. A positive and significant Pearson correlation of the N₂O–N emissions with nitrate nitrogen (NO₃ ⁻–N) content in the soil was found (P < 0.01). Leaving the straw on the field, a typical feature of RT, decreased NO₃ ⁻–N content of the soil and reduced N₂O emissions from RT soils.     

Effects of biochar addition on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> emissions from two paddy soils

Impacts of biochar addition on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from paddy soils are not well documented. Here, we have hypothesized that N₂O emissions from paddy soils could be depressed by biochar incorporation during the upland crop season without any effect on CO₂ emissions. Therefore, we have carried out the 60-day aerobic incubation experiment to investigate the influences of rice husk biochar incorporation (50 t ha⁻¹) into two typical paddy soils with or without nitrogen (N) fertilizer on N₂O and CO₂ evolution from soil. Biochar addition significantly decreased N₂O emissions during the 60-day period by 73.1% as an average value while the inhibition ranged from 51.4% to 93.5% (P < 0.05–0.01) in terms of cumulative emissions. Significant interactions were observed between biochar, N fertilizer, and soil type indicating that the effect of biochar addition on N₂O emissions was influenced by soil type. Moreover, biochar addition did not increase CO₂ emissions from both paddy soils (P > 0.05) in terms of cumulative emissions. Therefore, biochar can be added to paddy fields during the upland crop growing season to mitigate N₂O evolution and thus global warming.     

Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions

 N₂O emissions were periodically measured using the static chamber method over a 1-year period in a cultivated field subjected to different agricultural practices including the type of N fertilizer (NH₄NO₃, (NH₄)₂SO₄, CO(NH₂)₂ or KNO₃ and the type of crop (rapeseed and winter wheat). N₂O emissions exhibited the same seasonal pattern whatever the treatment, with emissions between 1.5 and 15 g N ha^–1 day^–1 during the autumn, 16–56 g N ha^–1 day^–1 in winter after a lengthy period of freezing, 0.5–70 g N ha^–1 day^–1 during the spring and lower emissions during the summer. The type of crop had little impact on the level of N₂O emission. These emissions were a little higher under wheat during the autumn in relation to an higher soil NO₃ ^– content, but the level of emissions was similar over a 7-month period (2163 and 2093 g N ha^–1 for rape and wheat, respectively). The form of N fertilizer affected N₂O emissions during the month following fertilizer application, with higher emissions in the case of NH₄NO₃ and (NH₄)₂SO₄, and a different temporal pattern of emissions after CO(NH₂)₂ application. The proportion of applied N lost as N₂O varied from 0.42% to 0.55% with the form of N applied, suggesting that controlling this agricultural factor would not be an efficient way of limiting N₂O emissions under certain climatic and pedological situations. Received: 1 December 1997     

Short-term effects of ammonium nitrogen in upland pasture soil affected or not affected by cattle

The short-term effects of excessive NH₄⁺-N on selected characteristics of soil unaffected (low annual N inputs) and affected (high annual N inputs) by cattle were investigated under laboratory conditions. The major hypothesis tested was that above a theoretical upper limit of NH₄⁺ concentration, an excess of NH₄⁺-N does not further increase NO₃⁻ formation rate in the soil, but only supports accumulation of NO₂⁻-N and gaseous losses of N as N₂O. Soils were amended with 10 to 500 μg NH₄⁺-N g⁻¹ soil. In both soils, addition of NH₄⁺-N increased production of NO₃⁻-N until some limit. This limit was higher in cattle-affected soil than in unaffected soil. Production of N₂O increased in the whole range of amendments in both soils. At the highest level of NH₄⁺-N addition, NO₂⁻-N accumulated in cattle-affected soil while NO₃⁻-N production decreased in cattle-unaffected soil. Despite being statistically significant, observed effects of high NH₄⁺-N addition were relatively weak. Uptake of mineral N, stimulated by glucose amendment, decreased the mineral N content in both soils, but it also greatly increased production of N₂O.     

Within-field variability of mineral nitrogen in grassland

 The within-field variability of soil mineral nitrogen (N_min) in a grazed grassland of 8000 m² was examined. NO₃ ^–-N concentrations were characterized by a high spatial variability. This can be explained by the uneven deposition of animal excreta. All NH₄ ⁺-N as well as NO₃ ^–-N values were lognormally distributed, before and after the grazing season. At the end of the grazing season the largest part of the variability of NO₃ ^–-N was found for NO₃ ^–-N concentrations measured within a distance of a few metres. A high variability for NO₃ ^–-N over very short distances was also indicated by a large nugget variance. During the grazing season, observed mean N_min values increased from 22 to 132 kg N ha^–1. Regions with clearly higher NO₃ ^–-N concentrations could be identified. These zones matched with the drinking place and the entrance of the pasture, places which were more frequently visited than others. High residual N levels in autumn led to relatively high losses of N, mostly by leaching, during the subsequent drainage period. Knowing the variability of N_min, the number of samples needed to estimate the average N_min in a field could be calculated for different probabilities and various degrees of precision. From the spatial distribution of the N_min concentrations and the restrictions imposed by the new European decree, adapted fertilizer strategies can be proposed at least for places where systematically higher N_min concentrations can be expected. Received: 14 December 1999     

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     

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     

Nitrate Transformation and N2O Emission in a Typical Intensively Managed Calcareous Fluvaquent Soil: A 15-Nitrogen Tracer Incubation Study

A 56-day aerobic incubation experiment was performed with 15-nitrogen (N) tracer techniques after application of wheat straw to investigate nitrate-N (NO₃-N) immobilization in a typical intensively managed calcareous Fluvaquent soil. The dynamics of concentration and isotopic abundance of soil N pools and nitrous oxide (N₂O) emission were determined. As the amount of straw increased, the concentration and isotopic abundance of total soil organic N and newly formed labeled particulate organic matter (POM-N) increased while NO₃-N decreased. When ¹⁵NO₃-N was applied combined with a large amount of straw at 5000 mg carbon (C) kg^?1 only 1.1 ± 0.4 mg kg^?1 NO₃-N remained on day 56. The soil microbial biomass N (SMBN) concentration and newly formed labeled SMBN increased significantly (P < 0.05) with increasing amount of straw. Total N₂O-N emissions were at levels of only micrograms kg^?1 soil. The results indicate that application of straw can promote the immobilization of excessive nitrate with little emission of N₂O.     

Long-term large N and immediate small N addition effects on trace gas fluxes in the Colorado shortgrass steppe

 Land use changes in semiarid grasslands have long-lasting effects. Reversion to near-original conditions with respect to plant populations and productivity requires more than 50 years following plowing. The impact of more subtle management changes like small, annual applications of N fertilizer or changing cattle stocking rates, which alters N redistribution caused by grazing and cattle urine deposition, is not known. To investigate the long-term effects of N addition to the Colorado shortgrass steppe we made weekly, year-round measurements of N₂O and CH₄ from the spring of 1990 through June 1996. Fluxes of NO_x (NO plus NO₂) were measured from October 1995 through June 1996. These measurements illustrated that large N applications, either in a single dose (45 g N m^–2), simulating cattle urine deposition, or in small annual applications over a 15-year period (30 g N m^–2) continued to stimulate N₂O emissions from both sandy loam and clay loam soils 6–15 years after N application. In sandy loam soils last fertilized 6 years earlier, average NO_x emissions were 60% greater than those from a comparable, unfertilized site. The long-term impact of these N additions on CH₄ uptake was soil-dependent, with CH₄ uptake decreased by N addition only in the coarser textured soils. The short-term impact of small N additions (0.5–2 g N m^–2) on N₂O, NO_x emissions and CH₄ uptake was observed in field studies made during the summer of 1996. There was little short-term effect of N addition on CH₄ uptake in either sandy loam or clay loam soils. Small N additions did not result in an immediate increase in N₂O emissions from the sandy loam soil, but did significantly increase N₂O flux from the clay loam soil. The reverse soil type, N addition interaction occurred for NO_x emissions where N addition increased NO_x emissions in the coarser textured soil 10–20 times those of N₂O. Received: 31 October 1997     

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     

Nitrous oxide release from arable soil: importance of perennial forage crops

 N₂O emission rates from a sandy loam soil were measured in a field experiment with 2 years of perennial forage crops (ryegrass, ryegrass-red clover, red clover) and 1 year of spring barley cultivation. Spring barley was sown after the incorporation of the forage crop residues. All spring barley plots received 40 kg N ha^–1 N fertiliser. Ryegrass, ryegrass-red clover and red clover plots were fertilised with 350 kg N ha^–1, 175 kg N ha^–1 and 0 kg N ha^–1, respectively. From June 1994 to February 1997, N₂O fluxes were continuously estimated using very large, closed soil cover boxes (5.76 m²). In order to compare the growing crops, the 33 months of investigation were separated into three vegetation periods (March–September) and three winter periods (October–February). All agronomic treatments (fertilisation, harvest and tillage) were carried out during the vegetation period. Large temporal changes were found in the N₂O emission rates. The data were approximately log-normally distributed. Forty-seven percent of the annual N₂O losses were observed to occur during winter, and mainly resulted from N₂O production during daily thawing and freezing cycles. No relationship was found between the N₂O emissions during the winter and the vegetation period. During the vegetation period, N₂O losses and yields were significantly different between the three forage crops. The unfertilised clover plot produced the highest yields and the lowest N₂O losses on this soil compared to the highly fertilised ryegrass plot. Total N₂O losses from soil under spring barley were higher than those from soil under the forage crops; this was mainly a consequence of N₂O emissions after the incorporation of the forage crop residues. Received: 31 October 1997     

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     

Soil mineral nitrogen dynamics under bare fallow and wheat in vertisols of semi-arid Mediterranean Morocco

 The evoluion of NH₄ ⁺-N and NO₃ ^–-N was monitored during three growing seasons, 1992–1993, 1993–1994, 1994–1995 in the soil profile (0–60 or 0–90 cm) under bare fallow and wheat on a vertisol site of the Sais plateau, Morocco. The aim of this study was to relate the soil mineral N dynamics to crop N uptake and soil N transformation processes. The efficacy of the current N fertilisation rate (100 kg N ha^–1) for wheat production in the region was evaluated. The high level of residual mineral N in the soil profile resulted from a low N plant uptake relative to the soil N supply and N fertilisation, and masked the effect of N fertilisation on dry matter accumulation. NH₄ ⁺-N was present in considerable amounts, suggesting a low nitrification rate under the given pedo-climatic conditions. An artefact due to the sampling procedure was encountered shortly after the application of N fertiliser. Losses through leaching and denitrification occurred after heavy rainfall, but were limited. At least part of the exchangeable NH₄ ⁺-N seemed to be barely taken up by the crop. NO₃ ^–-N was therefore considered to be a better indicator of plant-available N than total mineral N for this type of soil. The low N fertiliser use efficiencies demonstrated clearly that the current fertilisation rate (100 kg N ha^–1) for wheat production in this region is unsustainable. The maximum N uptake ranged from 40 kg N ha^–1 to 180 kg N ha^–1. The estimation of the seasonal production potential is considered to be the main prerequisite for the determination of the best rates and timing of N fertiliser application in this region. Received: 9 December 1997     

DENITRIFICATION IN A VERY ACID TROPICAL SOIL

In a very acid upland clay surface soil and with glucose added to give initial C/N weight ratios (added glucose-C: NO₃-N) in the soil of 0, 2 and 5, the rates of evolution of N₂ and N₂O were maximum at C/N = 2 but were significantly less at 0 and 5. The total N₂ and N₂O production was highest at C/N = 0, confirming that increasing amounts of glucose immobilised more nitrate into the biomass. As with added NO^?₃-N, the time lag, preceding a maximum ‘steady state’ rate of N₂0 evolution, increased regularly with increasing glucose. Within this ‘steady state’ period, the gaseous CO₂-C/(N₂+ N₂O)-N weight ratio in the effluent gas are between 1.0 and 1.3, which corresponds well with the stoichiometric ratios of 1.07 and 1.29 for the reduction of NO^?₃ to N₂O and N₂ respectively. Before and after this period, this gaseous C/N ratio was much higher. Denitrification was not observed in subsurface soil even after adding 100 mg kg^?1glucose-C although it contained 4 times as much indigenous nitrate as the surface soil. Inoculating this soil with increasing amounts of the surface soil, up to 15 per cent by weight, induced substantial increases in the rates and amounts of denitrification. The effects of increasing the soil pH. of introducing increasing oxygen concentrations in the influent gas. and the fate of added NH⁺₄-N, are briefly reported here. In these experiments. NO^?₂-N did not accumulate in the incubated soil nor was there any NH₃in the effluent gas. Evolution of N₂ only occurred when N₂O evolution was in its final stages.     

生物炭改性黏土的渗水渗气特性及其函数关系

黏土中施加生物炭可改变土体的孔隙结构。生物炭掺量和干密度均会对土体的渗透系数产生影响,准确确定生物炭-黏土混合土的渗透系数对满足填埋场上覆层的功能需求就显得格外重要。采用自主研发设计的柔性壁水-气联合渗透测试装置,测定不同生物炭掺量和干密度的生物炭-黏土混合土的饱和渗透系数和渗气系数,得到生物炭掺量、干密度与渗气系数和渗水系数间的关系曲线。建立生物炭掺量和干密度双变化条件下的渗气渗水函数,并通过验证组验证该函数的适用性。研究结果表明：在干密度较小时,对比纯黏土的渗水率,添加5%、10%、15%和20%生物炭处理后的土样渗水系数k_w值分别为8.25×10^-17、8.89×10^-17、10.40×10^-17和18.25×10^-17 m²,掺20%生物炭土样的渗透率增加了将近一个数量级。渗气渗水函数基于易测定的渗气率作为自变量,同时又考虑了干密度和生物炭掺量的影响,能快速、准确地确定土样的渗水系数。结合验证组试验得出,利用该函数计算得到的渗水系数和试验实测值吻合程度较好,表明该函数具有一定的适用性。本研究结果可为快速、准确确定渗水系数,定量描述非饱和土孔隙中水气运动之间的相互影响提供理论支撑。