Nitrifier dominance of Arctic soil nitrous oxide emissions arises due to fungal competition with denitrifiers for nitrate

Arctic soils emit nitrous oxide, which is a potent greenhouse gas and also represents an important loss of nitrogen to oligotrophic Arctic ecosystems. However, little is known about the temperature sensitivity of nitrous oxide release in Arctic soils or the organisms mainly responsible for it. We investigated controls on nitrous oxide emissions in an Arctic soil across a typical temperature range (between 4 and 13 °C) on Truelove Lowland, Devon Island, Canada (75°40′N 84°35′W) at two different moisture contents. When fertilized with ammonia or nitrate, nitrous oxide emissions and temperature dependence of nitrous oxide emissions were insensitive to soil moisture content but linked to nitrification rates. Stable isotope analysis revealed that nitrous oxide was predominantly released by nitrifiers. However, nitrous oxide emissions were not linked to nitrifier prevalence with an insignificant (P < 0.219) increase in amoA genes and a (P < 0.01) decrease in archaeal nitrifiers. In contrast, denitrifier nosZ prevalence was 10,000 times greater than that of nitrifiers and was related to nitrous oxide emission potential when soils were fertilized with nitrate. Manipulating water-filled pore space should have changed the pattern of N₂O emissions. We used selective inhibitors to further explore why denitrification did not occur under field conditions when we manipulated water-filled pore space or when we used ¹⁵N analysis. When fungi were inhibited in the soil, nitrous oxide emissions from denitrifiers increased with no change in nitrous oxide released by nitrifiers. When fungi were active in the soil, there was little available nitrate but when fungi were inhibited, available soil nitrate increased over the incubation period. The dominance of nitrifiers in nitrous oxide emissions from Arctic soils under field conditions is linked to the competition for nitrate between fungi and denitrifiers.     

Time-lagged induction of N2O emission and its trade-off with NO emission from a nitrogen fertilized Andisol

Abstract

To understand the influence of basal application of N fertilizer on nitrification potential and N₂O and NO emissions, four soil samples were collected from an upland Andisol field just before (sample 1) and 4 (sample 2), 36 (sample 3) and 72 (sample 4) days after the basal application of N fertilizer during the Chinese cabbage growing season from 12 September to 30 November 2005. The potentials of N₂O production and nitrification of the soils were determined using a ¹⁵N tracer technique and the soils were incubated for 25 days at 25°C and 60% water-filled pore space (WFPS). The results revealed that as much as 84–97% N₂O and almost all NO were produced by nitrification. The ¹⁵N₂O emission peak occurred approximately 350 h after the beginning of incubation for samples 1 and 2, but just 48 h later in samples 3 and 4. Total ¹⁵N₂O emission during the 25-day incubation of samples 3 and 4 ranged from 190 to 198 µg N kg^?1 soil, which was significantly higher than the 99–108 µg N kg^?1 soil recorded in samples 1 and 2. Basal application of N fertilizer did not immediately increase the nitrification potential and the ratio of N₂O to N added, but did dramatically increase the nitrification potential and the ratio of N₂O to N added as (¹⁵NH₄)₂SO₄ 36–72 days after the basal N fertilizer was added. In contrast, NO emission was negatively correlated with nitrification potential and total N₂O emission. As a result, a trade-off relationship between total NO and N₂O emissions was identified. The results indicated that there was a time-lagged induction of the change of N turnover in the soil, which was possibly caused by slow population growth of the nitrifiers and/or a slow shift in the microbial community in the soil.     

Comparison of the effectiveness of a nitrification inhibitor, dicyandiamide, in reducing nitrous oxide emissions in four different soils under different climatic and management conditions

Nitrous oxide (N₂O) is a potent greenhouse gas and, in New Zealand, about one‐third of the total greenhouse gas emissions from the agricultural sector are of N₂O, mostly derived from animal excreta in grazed pasture soils. The aim of this study was to determine the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in reducing N₂O emissions from animal urine patches in four different soils located in different regions of New Zealand with different soil, climatic and management conditions. The four soils are Templeton fine sandy loam and Lismore stony silt loam in Canterbury in the South Island, Horotiu silt loam in the Waikato region and Taupo pumice sand near Lake Taupo, both in the North Island. Results showed that the application of a fine‐particle suspension nitrification inhibitor, DCD, to grazed pasture soils was very effective in reducing N₂O emissions in all four different soils. Total N₂O emissions (over 69–137 days) from animal urine patches ranged from 1 to 20.9 kg N₂O‐N ha^?1 without DCD. These were reduced to 0.31–5.7 kg N₂O‐N ha^?1 by the use of DCD, representing 61–73% reductions (with an average of 70% reduction). The N₂O‐N emission factor from animal urine N, EF₃, was reduced from an average of 0.9 to 0.3% by the use of DCD. These results demonstrate the potential of using nitrification inhibitors to mitigate N₂O emissions in a wide range of grazed pasture soils under different climatic and management conditions.     

Effect of encapsulated calcium carbide on dinitrogen,nitrous oxide,methane, and carbon dioxide emissions from flooded rice

Summary The efficiency of N use in flooded rice is usually low, chiefly due to gaseous losses. Emission of CH₄, a gas implicated in global warming, can also be substantial in flooded rice. In a greenhouse study, the nitrification inhibitor encapsulated calcium carbide (a slow-release source of acetylene) was added with 75, 150, and 225 mg of 75 atom % ¹⁵N urea-N to flooded pots containing 18-day-old rice (Oryza sativa L.) plants. Urea treatments without calcium carbide were included as controls. After the application of encapsulated calcium carbide, 3.6 g N₂, 12.4 g N₂O-N, and 3.6 mg CH₄ were emitted per pot in 30 days. Without calcium carbide, 3.0 mg N₂, 22.8 g N₂O-N, and 39.0 mg CH₄ per pot were emitted during the same period. The rate of N added had a positive effect on N₂ and N₂O emissions, but the effect on CH₄ emissions varied with time. Carbon dioxide emissions were lower with encapsulated calcium carbide than without. The use of encapsulated calcium carbide appears effective in eliminating N₂ losses, and in minimizing emissions of the greenhouse gases N₂O and CH₄ in flooded rice.     

Evolution of dinitrogen and nitrous oxide from the soil to the atmosphere through rice plants

Summary It is commonly assumed that a large fraction of fertilizer N applied to a rice (Oryza sativa L.) field is lost from the soil-water-plant system as a result of denitrification. Direct evidence to support this view, however, is limited. The few direct field, denitrification gas measurements that have been made indicate less N loss than that determined by ¹⁵N balance after the growing season. One explanation for this discrepancy is that the N₂ produced during denitrification in a flooded soil remains trapped in the soil system and does not evolve to the atmosphere until the soil dries or is otherwise disturbed. It seems likely, however, that N₂ produced in the soil uses the rice plants as a conduit to the atmosphere, as does methane. Methane evolution from a rice field has been demonstrated to occur almost exclusively through the rice plants themselves. A field study in Cuttack, India, and a greenhouse study in Fort Collins, Colorado, were conducted to determine the influence of rice plants on the transport of N₂ and N₂O from the soil to the atmosphere. In these studies, plots were fertilized with 75 or 99 atom % ¹⁵N-urea and ¹⁵N techniques were used to monitor the daily evolution of N₂ and N₂O. At weekly intervals the amount of N₂+N₂O trapped in the flooded soil and the total-N and fertilized-N content of the soil and plants were measured in the greenhouse plots. Direct measurement of N₂+N₂O emission from field and greenhouse plots indicated that the young rice plant facilitates the efflux of N₂ and N₂O from the soil to the atmosphere. Little N gas was trapped in the rice-planted soils while large quantities were trapped in the unplanted soils. N losses due to denitrification accounted for only up to 10% of the loss of added N in planted soils in the field or greenhouse. The major losses of fertilizer N from both the field and greenhouse soils appear to have been the result of NH₃ volatilization.     

Chemical fixation of ammonia to soil organic matter after application of urea

Chemical fixation of NH₃ to soil organic matter was studied in two Swedish soils with different contents of organic matter: a clay soil with 2.3% C and an organic soil with 36.6% C. ¹⁵N‐labelled urea was applied at different rates to both sterilized and non‐sterilized soils. After 10 days, the soils were extracted and washed with K₂SO₄ and determined for total N and atom% ¹⁵N excess. Urea N was recovered as non‐extractable N in sterilized soil corresponding to 9.7% of supplied ^l5N‐labelled urea in the organic soil and 2.2% in the clay soil. Since no biological immobilization is thought to occur in the sterile soil, this non‐extractable N is suggested to be chemically fixed to soil organic matter. Owing to urea hydrolysis in the clay soil, pH increased from 6.3 to 9.3 and in the organic soil from 5.7 to 6.9 and 8.8, respectively, at the low and high urea supply.     

Assessing the potential of ammonia oxidizing bacteria to produce nitrous oxide in soils of a high arctic lowland ecosystem on Devon Island, Canada

The contribution of nitrifiers (ammonia-oxidizing bacteria (AOB)) and denitrifiers to nitrous oxide (N₂O) emission from arctic soils remains inconclusive. Based on preliminary experiments, we hypothesized that AOB are the primary producers of N₂O in a high arctic lowland ecosystem on Devon Island, Nunavut, Canada. In part 1 of the study, flux chambers were installed in a catena to determine in situ fluxes of gases (N₂O and carbon dioxide (CO₂)) from 16 June to 13 July 2004. Although fluxes were low, N₂O production occurred in the wettest area of the landscape when ammonium levels were high. As ammonium, but not nitrate, levels declined in the wet sedge meadow, N₂O emissions correspondingly decreased. In part 2, the contribution of nitrification and denitrification to N₂O production was assessed by Acetylene Inhibition Assay and ¹⁵N isotopically enriched incubations. Ammonium fertilization stimulated N₂O emissions to a greater extent than nitrate, and acetylene had a greater impact on N₂O emissions in ammonium-fertilized soils than in nitrate-amended soils. Stable isotope analysis indicated that at 50-55% water filled pore space, nitrification was the dominant (>80%) N₂O emitting process. In part 3, molecular analyses of the two N₂O producing groups indicated the both nitrifiers and denitrifiers did not differ between landforms. Our results suggest nitrifier denitrification is the dominant process occurring in these arctic soils and that the role of denitrifiers in N₂O release from arctic soils needs to be re-evaluated.     

中国东北苹果园中土壤总硝化作用和氧化亚氮排放状况

A better understanding of nitrogen (N) transformation in agricultural soils is crucial for the development of sustainable and environmental-friendly N fertilizer management and the proposal of effective N₂O mitigation strategies. This study aimed: i) to elucidate the seasonal dynamic of gross nitrification rate and N2O emission, ii) to determine the influence of soil conditions on the gross nitrification, and iii) to confirm the relationship between gross nitrification and N₂O emissions in the soil of an apple orchard in Yantai, Northeast China. The gross nitrification rates and N₂O fluxes were examined from March to October in 2009, 2010, and 2011 using the barometric process separation (BaPS) technique and the static chamber method. During the wet seasons gross nitrification rates were 1.64 times higher than those under dry season conditions. Multiple regression analysis revealed that gross nitrification rates were significantly correlated with soil temperature and soil water-filled pore space (WFPS). The relationship between gross nitrification rates and soil WFPS followed an optimum curve peaking at 60% WFPS. Nitrous oxide fluxes varied widely from March to October and were stimulated by N fertilizer application. Statistically significant positive correlations were found between gross nitrification rates and soil N₂O emissions. Further evaluation indicated that gross nitrification contributed significantly to N₂O formation during the dry season (about 86%) but to a lesser degree during the wet season (about 51%). Therefore, gross nitrification is a key process for the formation of N₂O in soils of apple orchard ecosystems of the geographical region.     

Effects of urease and nitrification inhibitors added to urea on nitrous oxide emissions from a loess soil

Urea fertilizer‐induced N₂O emissions from soils might be reduced by the addition of urease and nitrification inhibitors. Here, we investigated the effect of urea granule (2–3 mm) added with a new urease inhibitor, a nitrification inhibitor, and with a combined urease inhibitor and nitrification inhibitor on N₂O emissions. For comparison, the urea granules supplied with or without inhibitors were also used to prepare corresponding supergranules. The pot experiments without vegetation were conducted with a loess soil at (20 ± 2)°C and 67% water‐filled pore space. Urea was added at a dose of 86 kg N ha^–1 by surface application, by soil mixing of prills (<1 mm) and granules, and by point‐placement of supergranules (10 mm) at 5 cm soil depth. A second experiment was conducted with spring wheat grown for 70 d in a greenhouse. The second experiment included the application of urea prills and granules mixed with soil, the point‐placement of supergranules and the addition of the urease inhibitor, and the combined urease plus nitrification inhibitors at 88 kg N ha^–1. In both experiments, maximum emissions of N₂O appeared within 2 weeks after fertilization. In the pot experiments, N₂O emissions after surface application of urea were less (0.45% to 0.48% of total fertilization) than from the application followed by mixing of the soil (0.54% to 1.14%). The N₂O emissions from the point‐placed‐supergranule treatment amounted to 0.64% of total fertilization. In the pot experiment, the addition of the combined urease plus nitrification inhibitors, nitrification inhibitor, and urease inhibitor reduced N₂O emissions by 79% to 87%, 81% to 83%, and 15% to 46%, respectively, at any size of urea application. Also, the N₂O emissions from the surface application of the urease‐inhibitor treatment exceeded those of the granules mixed with soil and the point‐placed‐supergranule treatments receiving no inhibitors by 32% to 40%. In the wheat growth experiment, the N₂O losses were generally smaller, ranging from 0.16% to 0.27% of the total fertilization, than in the pot experiment, and the application of the urease inhibitor and the combined urease plus nitrification inhibitors decreased N₂O emissions by 23% to 59%. The point‐placed urea supergranule without inhibitors delayed N₂O emissions up to 7 weeks but resulted in slightly higher emissions than application of the urease inhibitor and the urease plus nitrification inhibitors under cropped conditions. Our results imply that the application of urea fertilizer added with the combined urease and nitrification inhibitors can substantially reduce N₂O emissions.     

Nitrous oxide production at different soil moisture contents in an arable soil in China

Abstract

Laboratory incubations were conducted to investigate nitrous oxide (N₂O) production from a subtropical arable soil (Typic Plinthodults) incubated at different soil moisture contents (SMC) and with different nitrogen sources using a 10% (v/v) acetylene (C₂H₂) inhibitory technique at 25°C. The production of N₂O and CO₂ was monitored during the incubations and changes in the contents of KCl-extractable NO^? ₃-N and NH⁺ ₄-N were determined. The production of N₂O increased slightly with an increase in SMC from 40% water-holding capacity (WHC) to 70% WHC, but increased dramatically at 100% WHC. After incubation the NO^? ₃-N content increased even at a SMC of 100% WHC. At a SMC of 100% WHC, the addition of NH⁺ ₄-N promoted the production of N₂O and CO₂, whereas the addition of NO^? ₃-N decreased N₂O production. Compared with the incubation without C₂H₂, the presence of C₂H₂ increased NH⁺ ₄-N content, but decreased NO^? ₃-N content, and there was no significant difference in N₂O production. These results indicate that heterotrophic nitrification contributes to N₂O production in the soil.     

Yield-scaled nitrous oxide emissions from nitrogen-fertilized croplands in China: A meta-analysis of contrasting mitigation scenarios

Nitrogen(N) losses in cropland resulting from the application of synthetic fertilizers decrease crop productivity and exacerbate environmental pollution.Mitigation measures, such as reduction in N fertilizer application rates, can have unintentional adverse effects on crop yield. We conducted a meta-analysis of soil N₂O emissions from agricultural fields across China under contrasting mitigation scenarios as a novel approach to identify the most effective strategy for the mitigation of emissions of N₂O derived from N fertilizer use in China. Current standard agricultural practice was used as a baseline scenario(BS), and 12 potential mitigation scenarios(S1–S12) were derived from the available literature and comprised single and combinations of management scenarios that accounted for crop yield. Mitigation scenarios S6(nitrification inhibitor 3,4-dimethylpyrazole phosphate) and S11(20% reduction in N application rate plus nitrification inhibitor dicyandiamide) in maize, rice, and wheat crops led to an average 56.0% reduction in N₂O emissions at the national level, whereas scenario S4(nitrification inhibitor dicyandiamide) led to yield optimization, with a 14.0% increase for maize and 8.0% increase for rice as compared to the BS. Implementation of these most effective mitigation scenarios(S4, S6, and S11) might help China, as a signatory to the 2015 United Nations Framework Convention on Climate Change(Paris Agreement), to achieve a 30% reduction in N₂O emissions by 2030.     

Sources and rates of nitrous oxide emissions from grazed grassland after application of N-labelled mineral fertilizer and slurry

Nitrogen (N) fertilizer application and grazing are known to induce nitrous oxide (N₂O) emissions from grassland soils. In a field study, general information on rates of N₂O emission, the effect of cattle grazing and the type (mineral fertilizer, cattle slurry) and amount of N supply on the flux of N₂O from a sandy soil were investigated. N₂O emissions from permanent grassland managed as a mixed system (two cuts followed by two grazing cycles) were monitored over 11 months during 2001-2002 in northern Germany using the closed chamber method. The field experiment consisted of four regionally relevant fertilizer combinations, i.e. two mineral N application rates (0 and 100 kg N ha⁻¹ yr⁻¹) and two slurry levels (0 and 74 kg N ha⁻¹ yr⁻¹).Mean cumulative N₂O-N loss was 3.0 kg ha⁻¹ yr⁻¹, and the cumulative ¹⁵N-labelled N₂O emissions varied from 0.03% to 0.19% of the ¹⁵N applied. ¹⁵N labelling indicated that more N₂O was emitted from mineral N than from slurry treated plots, and in all treatments the soil N pool was always clearly the major source of N₂O. Regarding the total cumulative N₂O losses, differences among treatments were not significant, which was caused by: (i) a high variance in emissions during and after cattle grazing due to the random distribution of excrements and by (ii) high N₂ fixation of white clover in the 0 kg N ha⁻¹ treatments, which resulted in similar N status of all treatments. However before grazing started, treatments showed significant differences. After cattle grazing in summer, N₂O emission rates were higher than around the time of spring fertilizer application, or in winter. Grazing resulted in N₂O flux rates up to 489 μg N₂O-N m⁻² h⁻¹ and the grazing period contributed 31-57% to the cumulative N₂O emission. During freeze-thaw cycles in winter (December-February) N₂O emission rates of up to 147 μg N₂O-N m⁻² h⁻¹ were measured, which contributed up to 26% to the annual N₂O flux. The results suggest that N fertilizer application and grazing caused only short-term increases of N₂O flux rates whereas the major share of annual N₂O emission emitted from the soil N pool. The significantly increased N₂O fluxes during freeze-thaw cycles show the importance of emission events in winter which need to be covered by measurements for obtaining reliable estimates of annual N₂O emissions.     

Phytotron studies to compare nitrogen losses from corn-planted soil by the 15-N balance or direct dinitrogen and nitrous oxide measurements

Summary Containers filled with soil mixed with potassium nitrate highly enriched in ¹⁵N were planted with corn (Zea mays L.) and kept in a phytotron under controlled conditions for 79 days. Soil water content was normally maintained at exactly 60% water-holding capacity (–33 kPa), but it was increased several times to 85% (–5 kPa) for short periods to favour denitrification. The soil headspace was sealed from the phytotron atmosphere and aerated by a continuous stream of air. Nitrous oxide emission was measured by estimating the N₂O concentration differences in the air entering and leaving the containers. Emission of N₂ was estimated by mass spectroscopy from changes in the N₂ composition in the temporarily enclosed soil headspace. Both methods were carefully checked for accuracy by different tests. At specific times during the experiment the distribution of ¹⁵N between plants and soil was determined and a ¹⁵N balance established. Emission of N gases peaked at times of increased water content and reached maxima of 149 and 142 g N pot^–1 day^–1 for N₂O and N₂, respectively. While N losses of 5% ± 2% were indicated by the ¹⁵N balance, only 1.1% ± 0.3% loss from 2.7 g applied N was estimated from the N₂O and N₂ measurements after 79 days. Possible reasons for these differences are discussed.     

Temporal changes in distribution and composition of N from labeled fertilizer in soil organic matter fractions

Variations in the amount and composition of immobilized nitrogen (N) in major soil organic matter fractions were investigated in a 730-day soil incubation experiment using ¹⁵N-labeled urea and ¹⁵N nuclear magnetic resonance spectroscopy with the cross polarization/magic angle spinning (¹⁵N CPMAS NMR) method. After 730 days, 24.7% of the applied N was recovered from the soil as organic N. The urea-derived N recovered from humic acids and humin decreased from 11.2 and 33.8% of the applied amount after 14 days to 1.6 and 20.4% after 730 days, respectively. When these values were corrected for the microbial biomass (MB) N, they ranged from 9.0 to 1.2% and 28 to 18%, respectively. The proportion of urea-derived N recovered from fulvic acids was low, ranging between 0.4 and 5.8% (with MB N) or 5.6% (without MB N) of the applied amount, whereas that from water-soluble nonhumic substances (WS-NHS; NHS in the fulvic acid fraction) remained high, 28–33% of the applied amount after correction for the contribution of MB N up to day 365, and decreased to 0.9% thereafter. The ¹⁵N CPMAS NMR spectra of humic acids, fulvic acids, and humin showed the largest signal at −254 to −264 ppm, corresponding to peptide/amide N. The proportions of heterocyclic, peptide/amide, guanidine/aniline, and free amino N in the urea-derived humic acid N were 3–7, 83–90, 5–7, and 2–4%, respectively. More than 80% loss of the urea-derived humic acid N did not markedly alter their composition. No time-dependent variations were also observed for the proportions of respective N functional groups in humin N, which were 3–5, 71–78, 12–17, and 6–10% in the same order as above. These results suggest the greater importance of physical stability than structural variation for the initial accumulation of organic N in soil.     

Heterotrophic and autotrophic nitrification in two acid pasture soils

Laboratory incubation experiments, using ¹⁵N-labeling techniques and simple analytical models, were conducted to measure heterotrophic and autotrophic nitrification rates in two acid soils (pH 4.8-5.3; 1/5 in H₂O) with high organic carbon contents (6.2-6.8% in top 5 cm soil). The soils were from pastures located near Maindample and Ruffy in the Northeast Victoria, Australia. Gross rates of N mineralization, nitrification and immobilization were measured. The gross rates of autotrophic nitrification were 0.157 and 0.119 μg N g⁻¹ h⁻¹ and heterotrophic nitrification rates were 0.036 and 0.009 μg N g⁻¹ h⁻¹ for the Maindample and Ruffy soils, respectively. Heterotrophic nitrification accounted for 19% and 7% of the total nitrification in the Maindample and Ruffy soils, respectively. The heterotrophic nitrifiers used organic N compounds and no as the substrate for nitrification.     

N2O产生法测定土壤无机态氮15N丰度

用化学方法分别将土壤中微量的铵、硝酸盐和亚硝酸盐转化为N2O气体,然后用带自动预浓缩装置的同位素比值质谱仪测定N2O中的15N丰度.N2O中的15N丰度测量值完全符合铵、硝酸盐和亚硝酸盐的15N参考值.方法快速、简单和准确,不受空气氮的污染.特别是方法的检测限很低,每批次样品中只需含5～ 20μg N.它将有助于土壤氮素的矿化作用、硝化作用和反硝化作用的研究.     

Nitrogen and oxygen isotope ratios of nitrous oxide emitted from soil and produced by nitrifying and denitrifying bacteria

The isotopic composition at natural abundance levels of nitrous oxide emitted from a sandy loam, neutral pH soil under a range of soil water contents (matric potentials of-0.1,-1.0 and-5.0 kPa), from soil amended with sodium succinate and sodium ethanoate, and produced by pure cultures of the nitrifying bacteria Nitrosomonas europaea and Nitrosolobus multiformis, and by the denitrifying bacterium Pseudomonas putida, has been determined in laboratory experiments. N₂O from all sources was depleted in the ¹⁵N and ¹⁸O isotopes relative to the conventional references [atmospheric N₂ and standard mean ocean water (SMOW), respectively]. N₂O from soil was depleted in ¹⁵N and ¹⁸O to increasing extents with increasing soil water content. The isotopic composition of N₂O produced by N. europaea and N. multiformis was similar to that emitted from drier soil (matric potential of-1.0 kPa) and the N₂O produced by P. putida was similar to that emitted from wetter soil (matric potential of-0.1 kPa). N₂O emitted from the wetter soil was enriched in ¹⁵N and ¹⁸O compared with that emitted from the drier soil. The differences in isotopic composition between N₂O from the wetter and drier soil were attributed principally to isotopic fractionation during N₂O reduction to N₂ in the terminal step of denitrification. The effect of both sodium succinate and sodium ethanoate amendment was to increase the overall rate of N₂O emission, much of which arose from denitrification, as revealed by incubation in 100 kPa O₂. In addition, in the sodium ethanoate amended soil N₂O reduction to N₂ did not occur, as revealed by incubation in 10 kPa C₂H₂. The N₂O from the sodium ethanoate amended soil was depleted in ¹⁵N to a greater extent than the sodium succinate amended soil, which is consistent with the observation that N₂O reduction to N₂ leaves residual N₂O relatively enriched in ¹⁵N.     

Effect of acetylene provided by wax-coated calcium carbide on transformations of urea nitrogen applied to an irrigated wheat crop

The effect of acetylene (provided by wax-coated calcium carbide, CaC₂) on N transformations in a red-brown earth was measured in a field experiment with irrigated wheat by determining the change in the concentration and ¹⁵N enrichment of the organic N and mineral N pools with time. The study was conducted in the Goulburn-Murray Irrigation region of south-eastern Australia using 0.3 m by 0.3 m microplots fertilized with ¹⁵N-labelled urea (10 g N m^-2; 5 atom% ¹⁵N). Acetylene was effective in slowing the nitrification of both unlabelled and labelled N. Nitrate derived from the added fertilizer reached a maximum 19 days after sowing in the treatment without CaC₂, whereas little nitrate accumulated in the 8 g CaC₂ m^-2 treatment. There was significant immobilization of the urea N by 19 days after sowing in all treatments, but the extent of immobilization was not affected by the acetylene. The addition of acetylene slowed net mineralization of labelled and unlabelled N from the organic N pool, and resulted in increased accumulation of both unlabelled and labelled N in wheat tops.     

Emissions and spatial variability of N2O, N2 and nitrous oxide mole fraction at the field scale, revealed with N isotopic techniques

The accurate measurement of nitrous oxide (N₂O) and dinitrogen (N₂) during the denitrification process in soils is a challenge which will help to estimate the contribution of soil N₂O emissions to global warming. Oxygen concentration, nitrate concentration and carbon availability are generally the main factors that control soil denitrification rate and the amount of N₂O or N₂ emitted. The aim of this paper is to present a database of the N₂O mole fraction measured at the field scale, and to test hypotheses concerning its regulation. A ¹⁵N-nitrate tracer solution was added to 36 undisturbed soil cores on a 20 m×20 m cultivated field plot. Fluxes of CO₂, N₂O and N₂ from the soil surface were monitored for 24 h. Soil moisture, bulk density, carbon, nitrogen and mineral nitrogen concentration were also measured to investigate possible spatial relationships between their variations and those of N₂O, N₂ and nitrous oxide mole fraction. Under high water content, nitrous oxide and N₂ emissions were highly variable with variation coefficients of 70-140%. N₂O emission rates were about twice as high as those of N₂, with a total denitrification rate ranging from 269 to 3843 g N ha⁻¹ d⁻¹. After 24 h of incubation, the values of nitrous oxide mole fraction ranged from 0.15 to 0.94 and no significant decline during incubation time was observed. Spatial variability of N₂O, N₂ and nitrous oxide mole fraction was high and no spatial dependence was observed at the scale of the experimental plot. Only tenuous relationships between gaseous nitrogen emissions and soil properties (mainly nitrate concentration and moisture content) were found. Meanwhile, a positive correlation was observed between N₂ and CO₂ emissions. This result supports the hypothesis that an increase in soil available organic carbon leads to N₂ emissions as the end product of denitrification.