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     

Nitrous oxide emission from wetland rice soil as affected by the application of controlled-availability fertilizers and mid-season aeration

 N₂O emission from a wetland rice soil as affected by the application of three controlled-availability fertilizers (CAFs) and urea was investigated through a pot experiment. N₂O fluxes from the N fertilized paddy soil averaged 44.8–69.3 μg N m^–2 h^–1 during the rice growing season, accounting for 0.28–0.51% of the applied N. The emission primarily occurred during the mid-season aeration (MSA) and the subsequent re-flooding period. Fluxes were highly correlated with the NO₃ ^– and N₂O concentrations in the soil water. As there were relatively large amounts of NH₄ ⁺-N present in the soil of the CAF treatments at the beginning of MSA, leading to large amounts of NO₃ ^–-N during the MSA and the subsequent re-flooding period, the tested CAFs were not effective in reducing N₂O emission from this paddy soil. The potential of applied CAFs to reduce N₂O emissions from paddy soil is discussed. Received: 25 May 1999     

Evaluation of 3,4-dimethylpyrazole phosphate as a nitrification inhibitor in a <Emphasis Type="Italic">Citrus</Emphasis>-cultivated soil

 The objectives of this work were to evaluate the inhibitory action on nitrification of 3,4-dimethylpyrazole phosphate (DMPP) added to ammonium sulphate nitrate [(NH₄)₂SO₄ plus NH₄NO₃; ASN] in a Citrus-cultivated soil, and to study its effect on N uptake. In a greenhouse experiment, 2 g N as ASN either with or without 0.015 g DMPP (1% DMPP relative to NH₄ ⁺-N) was applied 6 times at 20-day intervals to plants grown in 14-l pots filled with soil. Addition of DMPP to ASN resulted in higher levels of NH₄ ⁺-N and lower levels of NO₃ ^–-N in the soil during the whole experimental period. The NO₃ ^–-N concentration in drainage water was lower in the ASN plus DMPP (ASN+DMPP)-treated pots. Also, DMPP supplementation resulted in greater uptake of the fertilizer-N by citrus plants. In another experiment, 100 g N as ASN, either with or without 0.75 g DMPP (1% DMPP relative to NH₄ ⁺-N) was applied to 6-year-old citrus plants grown individually outdoors in containers. Concentrations of NH₄ ⁺-N and NO₃ ^–-N at different soil depths and N distribution in the soil profile after consecutive flood irrigations were monitored. In the ASN-amended soil, nitrification was faster, whereas the addition of the inhibitor led to the maintenance of relatively high levels of NH₄ ⁺-N and NO₃ ^–-N in soil for longer than when ASN was added alone. At the end of the experiment (120 days) 68.5% and 53.1% of the applied N was leached below 0.60 m in the ASN and ASN+DMPP treatments, respectively. Also, leaf N levels were higher in plants fertilized with ASN+DMPP. Collectively, these results indicate that the DMPP nitrification inhibitor improved N fertilizer efficiency and reduced NO₃ ^– leaching losses by retaining the applied N in the ammoniacal form. Received: 31 May 1999     

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     

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     

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     

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     

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     

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     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

Spatial variability of N2O emissions and soil parameters of an arable silt loam – a field study

 The spatial in situ variability of soil N₂O emissions (measured by micro-chambers, radius 0.033 m), N₂O content, water content, NO₃ ^–, NH₄ ⁺, inorganic carbon and organic carbon concentrations was investigated on a silt loam by means of geostatistical methods and nonparametric statistics. The sampling grid consisted of different spacings between sampling points which ranged from 0.1 m to 50 m. There were no significant correlations between N₂O emissions and soil parameters (P>0.1) when all the sampling points were considered. In the centre of the grid a "hot area" was localized with significantly higher N₂O emissions, and NO₃ ^– and NH₄ ⁺ concentrations (P≤0.05). Within this hot area the N₂O soil content significantly correlated with N₂O emissions (P≤0.05). When semiovariograms were computed without data of the hot area samples, N₂O emissions showed a weak spatial correlation (range: 4.3 m). The calculations including all data led to pure nugget effects for all parameters except for soil water content (range >40 m) and N₂O soil content (range 16.4 m). Received: 19 December 1997     

Nitrous oxide emissions from a fallow and wheat field as affected by increased soil temperatures

 In order to determine the effects of increased soil temperature resulting from global warming on microbiological reactions, a 21-month field experiment was carried out in the Bavarian tertiary hills. The major objective was to focus on N₂O releases as either a positive or negative feedback in response to global warming. The soils of a fallow field and a wheat field were heated 3  °C above ambient temperature and N₂O fluxes were measured weekly from June 1994 to March 1996. During the experimental period, measured temperature differences between the control plots and the heated plots were 2.9±0.3  °C at a depth of 0.01 m and 1.0–1.8  °C at a depth of 1 m. Soil moisture decreased with the elevated soil temperatures of the heated plots. The mean differences in soil moisture between the treatments were 6.4% (fallow field) and 5.2%_DW (wheat field dry weight, DW), respectively. Overall N₂O releases during the experimental period from the fallow field were 4.8 kg N₂O–N ha^–1 in the control plot against 5.0 kg N₂O–N ha^–1 in the heated plot, and releases from the wheat field were 8.0 N₂O–N ha^–1 in the control plot and 7.6 N₂O–N kg ha^–1 in the heated plot. However, on a seasonal basis, cumulated N₂O emissions differed between the plots. During the summer months (May–October), releases from the heated fallow plot were 3 times the rates from the control plot. In the winter months, N₂O releases increased in both the fallow and wheat fields and were related to the number of freezing and thawing cycles. 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.     

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     

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     

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     

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     

Recovery of nitrification and production of NO and N2O after exposure of soil to acetylene

Incubation of soil under low partial pressures of acetylene (10 Pa) is a widely used method to specifically inhibit nitrification due to the suicide inhibition of ammonium monooxygenase (AMO), the first enzyme in NH₄ ⁺ oxidation by nitrifying bacteria. Although the inhibition of AMO is irreversible, recovery of activity is possible if new enzyme is synthesized. In experiments with three different soils, NH₄ ⁺ concentrations decreased and NO₃ ^– concentrations increased soon after acetylene was removed from the atmosphere. Recovery of NO production started immediately after the removal of acetylene. The release rates of NO and N₂O were higher in soil samples which were only preincubated with 10 Pa acetylene than in those which were kept in the presence of 10 Pa acetylene. In the permanent presence of 10 Pa acetylene, NH₄ ⁺ and NO₃ ^– concentrations stayed constant, and the release rates of NO and N₂O were low. These low release rates were apparently due to processes other than nitrification. Our experiments showed that the blockage of nitrification by low (10 Pa) acetylene partial pressures is only reliable when the soil is kept in permanent contact with acetylene. Received: 17 July 1996     

Exchangeable ammonium and nitrate from different nitrogen fertilizer preparations in polyacrylamide-treated and untreated agricultural soils

 High molecular weight, anionic polyacrylamide (PAM) is currently being used as an irrigation water additive to significantly reduce soil erosion associated with furrow irrigation. PAM contains amide-N, and PAM application to soils has been correlated with increased activity of soil enzymes, such as urease and amidase, involved in N cycling. Therefore we investigated potential impacts of PAM treatment on the rate at which fertilizer N is transformed into NH₄ ⁺ and NO₃ ^– in soil. PAM-treated and untreated soil microcosms were amended with a variety of fertilizers, ranging from common rapid-release forms, such as ammonium sulfate [(NH₄)₂SO₄] and urea, to a variety of slow-release formulations, including polymerized urea and polymer-encapsulated urea. Ammonium sulfate was also tested together with the nitrification inhibitor dicyandiamide (DCD). The fertilizers were applied at a concentration of 1.0 mg g^–1, which is comparable to 100 lb acre^–l, or 112 kg ha^–1. Potassium chloride-extractable NH₄ ⁺-N and NO₃ ^–-N were quantified periodically during 2–4 week incubations. PAM treatment had no significant effect on NH₄ ⁺ release rates for any of the fertilizers tested and did not alter the efficacy of DCD as a nitrification inhibitor. However, the nitrification rate of urea and encapsulated urea-derived NH₄ ⁺-N was slightly accelerated in the PAM-treated soil. Received: 16 January 1998     

Spatial variability and biophysicochemical controls on N<Subscript>2</Subscript>O emissions from differently tilled arable soils

Nitrous oxide (N₂O) emissions, soil microbial community structure, bulk density, total pore volume, total C and N, aggregate mean weight diameter and stability index were determined in arable soils under three different types of tillage: reduced tillage (RT), no tillage (NT) and conventional tillage (CT). Thirty intact soil cores, each in a 25 × 25-m² grid, were collected to a depth of 10 cm at the seedling stage of winter wheat in February 2008 from Maulde (50°3′ N, 3°43′ W), Belgium. Two additional soil samples adjacent to each soil core were taken to measure the spatial variance in biotic and physicochemical conditions. The microbial community structure was evaluated by means of phospholipid fatty acids analysis. Soil cores were amended with 15 kg NO₃⁻-N ha⁻¹, 15 kg NH₄⁺-N ha⁻¹ and 30 kg ha⁻¹ urea-N ha⁻¹ and then brought to 65% water-filled pore space and incubated for 21 days at 15°C, with regular monitoring of N₂O emissions. The N₂O fluxes showed a log-normal distribution with mean coefficients of variance (CV) of 122%, 78% and 90% in RT, NT and CT, respectively, indicating a high spatial variation. However, this variability of N₂O emissions did not show plot scale spatial dependence. The N₂O emissions from RT were higher (p < 0.01) than from CT and NT. Multivariate analysis of soil properties showed that PC1 of principal component analysis had highest loadings for aggregate mean weight diameter, total C and fungi/bacteria ratio. Stepwise multiple regression based on soil properties explained 72% (p < 0.01) of the variance of N₂O emissions. Spatial distributions of soil properties controlling N₂O emissions were different in three different tillages with CV ranked as RT > CT > NT.