Contribution of nitrification and denitrification to N2O emissions from urine patches

Urine deposition by grazing livestock causes an immediate increase in nitrous oxide (N₂O) emissions, but the responsible mechanisms are not well understood. A nitrogen-15 (¹⁵N) labelling study was conducted in an organic grass-clover sward to examine the initial effect of urine on the rates and N₂O loss ratio of nitrification (i.e. moles of N₂O-N produced per moles of nitrate produced) and denitrification (i.e. moles of N₂O produced per moles of N₂O+N₂ produced). The effect of artificial urine (52.9 g N m⁻²) and ammonium solution (52.9 g N m⁻²) was examined in separate experiments at 45% and 35% water-filled pore space (WFPS), respectively, and in each experiment a water control was included. The N₂O loss derived from nitrification or denitrification was determined in the field immediately after application of ¹⁵N-labelled solutions. During the next 24 h, gross nitrification rates were measured in the field, whereas the denitrification rates were measured in soil cores in the laboratory. Compared with the water control, urine application increased the N₂O emission from 3.9 to 42.3 μg N₂O-N m⁻² h⁻¹, whereas application of ammonium increased the emission from 0.9 to 6.1 μg N₂O-N m⁻² h⁻¹. In the urine-affected soil, nitrification and denitrification contributed equally to the N₂O emission, and the increased N₂O loss resulted from a combination of higher rates and higher N₂O loss ratios of the processes. In the present study, an enhanced nitrification rate seemed to be the most important factor explaining the high initial N₂O emission from urine patches deposited on well-aerated soils.     

Nitrogen oxides emission from soils bearing a potato crop as influenced by fertilization with treated pig slurries and composts

Nitrous oxide, nitric oxide and denitrification losses from an irrigated soil amended with organic fertilizers with different soluble organic carbon fractions and ammonium contents were studied in a field study covering the growing season of potato (Solanum tuberosum). Untreated pig slurry (IPS) with and without the nitrification inhibitor dicyandiamide (DCD), digested thin fraction of pig slurry (DTP), composted solid fraction of pig slurry (CP) and composted municipal solid waste (MSW) mixed with urea were applied at a rate of 175 kg available N ha⁻¹, and emissions were compared with those from urea (U) and a control treatment without any added N fertilizer (Control). The cumulative denitrification losses correlated significantly with the soluble carbohydrates, dissolved N and total C added. Added dissolved organic C (DOC) and dissolved N affected the N₂O/N₂ ratio, and a lower ratio was observed for organic fertilizers than from urea or unfertilized controls. The proportion of N₂O produced from nitrification was higher from urea than from organic fertilizers. Accumulated N₂O losses during the crop season ranged from 3.69 to 7.31 kg N₂O-N ha⁻¹ for control and urea, respectively, whereas NO losses ranged from 0.005 to 0.24 kg NO-N ha⁻¹, respectively. Digested thin fraction of pig slurry compared to IPS mitigated the total N₂O emission by 48% and the denitrification rate by 33%, but did not influence NO emissions. Composted pig slurry compared to untreated pig slurry increased the N₂O emission by 40% and NO emission by 55%, but reduced the denitrification losses (34%). DCD partially inhibited nitrification rates and reduced N₂O and NO emissions from pig slurry by at least 83% and 77%, respectively. MSW+U, with a C:N ratio higher than that of the composted pig slurry, produced the largest denitrification losses (33.3 kg N ha⁻¹), although N₂O and NO emissions were lower than for the U and CP treatments.This work has shown that for an irrigated clay loam soil additions of treated organic fertilizers can mitigate the emissions of the atmospheric pollutants NO and N₂O in comparison with urea.     

Evaluation of nitrate and ammonium as sources of NO and N2O emissions from black earth soils (Haplic Chernozem) based on 15N field experiments

Nitrous oxide (N₂O) and nitric oxide (NO) released from soil is a concern since it can act as a potential atmospheric pollutant and it represents a loss of N from the soil. These gases are present in the atmosphere in trace amounts and are important to atmospheric chemistry and earth's radiative balance. Nitric oxide (NO) does not directly contribute to the greenhouse effect, but it contributes to climate forcing through its role in photochemistry of hydroxyl radicals and ozone and plays a key role in air quality issues. Nitrification and denitrification have been identified as major controlling microbial processes in soils responsible for the formation of NO and N₂O. To elucidate the contribution of both processes to the release of NO and N₂O from loess-black earth soils under field conditions—i.e. to evaluate nitrate and ammonium as sources of NO and N₂O emission—two field experiments with either [¹⁵N] nitrate (NO₃^?) or [¹⁵N] ammonium (NH₄⁺) labelling have been conducted at two sites differing in soil organic matter content (high and normal SOM). [¹⁵N] nitrate treatments revealed that denitrification of NO₃^? represents the main pathway of soil N₂O release. On average 76% and 54% of N₂O was emitted during denitrification from soils with high and normal SOM content, respectively. Contrarily, denitrification contributed on average only 17% and 12% of released NO from soil with high and normal SOM content, respectively. The [¹⁵N]ammonium treatments revealed that nitrification of NH₄⁺ is the major process responsible for soil NO emission. SOM content of the loess-black earth soil significantly influenced NO and N₂O emission. The soil with the higher SOM content showed lower NO emission but drastically increased N₂O emission after nitrate fertilisation. In particular the soil with high SOM content exhibited a high sorption capacity for ammonium ions which led to unexpected results after fertilisation with [¹⁵N]ammonium. To explain this results a revised concept containing three different interacting soil ammonium pools have been hypothesised.     

N2O emissions and product ratios of nitrification and denitrification as affected by freezing and thawing

Agricultural soils contribute significantly to atmospheric nitrous oxide (N₂O). A considerable part of the annual N₂O emission may occur during the cold season, possibly supported by high product ratios in denitrification (N₂O/(N₂+N₂O)) and nitrification (N₂O-N/(NO₃⁻-N+NO₂⁻-N)) at low temperatures and/or in response to freeze-thaw perturbation. Water-soluble organic materials released from frost-sensitive catch crops and green manure may further increase winter emissions. We conducted short-term laboratory incubations under standardized moisture and oxygen (O₂) conditions, using nitrogen (N) tracers (¹⁵N) to determine process rates and sources of emitted N₂O after freeze-thaw treatment of soil or after addition of freeze-thaw extract from clover. Soil respiration and N₂O production was stimulated by freeze-thaw or addition of plant extract. The N₂O emission response was inversely related to O₂ concentration, indicating denitrification as the quantitatively prevailing process. Denitrification product ratios in the two studied soils (pH 4.5 and 7.0) remained largely unaltered by freeze-thaw or freeze-thaw-released plant material, refuting the hypothesis that high winter emissions are due to frost damage of N₂O reductase activity. Nitrification rates estimated by nitrate (NO₃⁻) pool enrichment were 1.5-1.8 μg NO₃-N g⁻¹ dw soil d⁻¹ in freeze-thaw-treated soil when incubated at O₂ concentrations above 2.3 vol% and one order of magnitude lower at 0.8 vol% O₂. Thus, the experiments captured a situation with severely O₂-limited nitrification. As expected, the O₂ stress at 0.8 vol% resulted in a high nitrification product ratio (0.3 g g⁻¹). Despite this high product ratio, only 4.4% of the measured N₂O accumulation originated from nitrification, reaffirming that denitrification was the main N₂O source at the various tested O₂ concentrations in freeze-thaw-affected soil. N₂O emission response to both freeze-thaw and plant extract addition appeared strongly linked to stimulation of carbon (C) respiration, suggesting that freeze-thaw-induced release of decomposable organic C was the major driving force for N₂O emissions in our soils, both by fuelling denitrifiers and by depleting O₂. The soluble C (applied as plant extract) necessary to induce a CO₂ and N₂O production rate comparable with that of freeze-thaw was 20-30 μg C g⁻¹ soil dw. This is in the range of estimates for over-winter soluble C loss from catch crops and green manure plots reported in the literature. Thus, freeze-thaw-released organic C from plants may play a significant role in freeze-thaw-related N₂O emissions.     

Nitrous oxide production by nitrification and denitrification in soil aggregates as affected by O2 concentration

Nitrous oxide emitted by soils can be produced either by denitrification in anoxic conditions or by nitrification in presence of O₂. The relative importance of the two processes, particularly under varied partial pressures of O₂, is not always known. This paper focuses on the influence of O₂ concentration on N₂O production by nitrification and denitrification in an arable Orthic Luvisol. Soil aggregates (2-3 mm size), water unsaturated, received 116 mg N kg⁻¹ as ammonium sulphate labelled with ¹⁵N and were incubated during 14 days at different O₂ partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A ¹⁵N tracing technique was used to quantify nitrification and denitrification rates. ¹⁵N₂O and ¹⁵N₂ were measured. Oxygen pressure appeared to strongly influence both nitrification and denitrification rates and also N₂O emissions. Nitrification rates were reduced by a factor of 6-9 when O₂ decreased from 20.4 to 0.35 kPa. They were highly correlated with O₂ consumption rates. Denitrification mainly occurred in complete anoxic conditions. The proportion of N₂O emitted by denitrification was estimated by two independent methods: one based on ¹⁵N tracing using isotope composition of NH₄, NO₃ and N₂O, the other based on the measurement of the ¹⁵N₂O:¹⁵N₂ ratio. The two methods gave close results. The highest N₂O emissions were obtained under complete anoxic conditions and were due to denitrification. However, N₂O emissions almost as important were obtained at day 14 with 1.5 kPa O₂ pressure, and they were due to nitrification. Nitrification was the main source of N₂O at O₂ concentrations greater than 0.35 kPa. The amounts of N₂O-N emitted by nitrification were linearly related to the amounts of N nitrified, but the slope of the regression was highly dependent on O₂ concentration: it varied from 0.16 to 1.48% when O₂ concentration was reduced from 20.4 to 0.76 kPa. Emissions of N₂O by nitrification may then be quite significant if nitrification occurs at a reduced O₂ concentration.     

Gaseous nitrogen losses from fertilized soil during nitrification and denitrification using the acetylene blockage technique

Summary A sandy soil amended with different forms and amounts of fertilizer nitrogen (urea, ammonium sulphate and potassium nitrate) was investigated in model experiments for N₂O emission, which may be evolved during both oxidation of ammonia to nitrate and anaerobic respiration of nitrate. Since C₂H₂ inhibits both nitrification and the reduction of N₂O to N₂ during denitrification, the amount of N₂O evolved in the presence and absence of C₂H₂ represents the nitrogen released through nitrification and denitrification.Results show that amounts of N₂O-N lost from soils incubated anaerobically with 0.1% C₂H₂ and treated with potassium nitrate (23.1 µg N-NO ₃ ^– /g dry soil) exceeded those from soils incubated in the presence of 20% oxygen and treated with even larger amounts of nitrogen as urea and ammonium sulphate. This indicates that nitrogen losses by denitrification may potentially be higher than those occurring through nitrification.     

Rapid shift from denitrification to nitrification in soil after biogas residue application as indicated by nitrous oxide isotopomers

Nitrous oxide (N₂O) is one of the major greenhouse gases emitted from soils, where it is mainly produced by nitrification and denitrification. It is well known that rates of N₂O release from soils are mainly determined by the availability of substrates and oxygen, but N₂O source apportioning, highly needed to advance N₂O mitigation strategies, still remains challenging. In this study, using an automated soil incubation system, the N₂O site preference, i.e. the intramolecular ¹⁵N distribution, was analyzed to evaluate the progression in N₂O source processes following organic soil amendment. Biogas fermentation residue (BGR; originating from food waste fermentation) was applied to repacked grassland soil cores and compared to ammonium sulfate (AS) application, both at rates equivalent to 160 kg NH₄⁺-N ha⁻¹, and to unamended soil (control). The soil cores were incubated in a helium-oxygen atmosphere with 20 kPa O₂ for 43 days at 80% water-filled pore space. 43-day cumulative N₂O emissions were highest with BGR treated soil accounting for about 1.68 kg N₂O-N ha⁻¹ while application of AS caused much lower fluxes of c. 0.23 kg N₂O-N ha⁻¹. Also, after BGR application, carbon dioxide (CO₂) fluxes showed a pronounced initial peak with steep decline until day 21 whereas with ammonium addition they remained at the background level. N₂O dual isotope and isotopomer analysis of gas samples collected from BGR treated soil indicated bacterial denitrification to be the main N₂O generating process during the first three weeks when high CO₂ fluxes signified high carbon availability. In contrast, in the second half after all added labile carbon substrates had been consumed, nitrification, i.e. the generation of N₂O via oxidation of hydroxylamine, gained in importance reaching roughly the same N₂O production rate compared to bacterial denitrification as indicated by N₂O SP. Overall in this study, bacterial denitrification seemed to be the main N₂O forming process after application of biogas residues and fluxes were mainly driven by available organic carbon.     

Production of NO and N2O in the presence and absence of C2H2 by soil slurries and batch cultures of denitrifying bacteria

We evaluated the potential of the C₂H₂-catalyzed NO oxidation reaction to influence N₂O production during denitrification. We measured the total amount of free NO and N₂O produced by slurries of sandy loam soil and by batch cultures of denitrifying bacteria under both anaerobic and low O₂ conditions. The maximum amount of free NO released by anaerobic Nos⁺ (able to reduce N₂O to N₂) and Nos⁻ (unable to reduce N₂O to N₂) batch cultures of Cytophaga johnsonae strains catalyzing denitrification of nitrate was 17-79 nmol NO per bottle. In all cases the maximum headspace concentrations of NO-N measured in anaerobic cultures in the absence of C₂H₂ was less than 0.21% of those of N₂O-N measured in the presence of 10 kPa C₂H₂. Peak NO production was delayed when between 2.0 and 4.5% O₂ was present. Less NO accumulated in cultures in the presence of both O₂ and C₂H₂, and the maximum amount of NO-N measured in the absence of C₂H₂ was less than 0.13% of the total amount of N₂O-N measured in the presence of C₂H₂. For an agricultural sandy loam soil, the maximum concentrations of free NO released from slurries were 598-897 ng NO-N g⁻¹ of dry soil in the absence of C₂H₂ and 118-260 ng NO-N g⁻¹ of dry soil in the presence of 10 kPa C₂H₂. The maximum concentration of NO-N released in the absence of C₂H₂ was between 0.32 and 8.1% of the maximum concentration of N₂O-N accumulated in the presence of 10 kPa C₂H₂. We conclude that scavenging of NO by the C₂H₂-catalyzed NO oxidation reaction in the presence of trace amounts of O₂ does not cause a serious underestimate of long-term measurements of active denitrification in anaerobic soils containing adequate carbon and nitrate sources.     

Effect of cellulose and straw incorporation in soil on total denitrification and nitrogen immobilization at initially aerobic and permanent anaerobic conditions

Summary Laboratory experiments were used to examine the influence of cellulose and straw on denitrification and N immobilization in a sandy loam soil. The soil was mixed with 300 g nitrate-N/g and incubated in a special vessel under conditions that changed from aerobic to anaerobic or in the permanent absence of O₂. Gases (O₂, CO₂, N₂, N₂O, NO and CH₄) were analysed by gas chromatography at regular intervals and the soil was examined for nitrate, nitrite, ammonium and cellulose. Compared with controls, the application of straw and cellulose (0.5% and 1.0%, respectively) enhanced nitrate immobilization and decreased denitrification, under both anaerobic and originally aerobic (PO₂ = 20 vol%) conditions. However, a comparison of results from the aerobic and the anaerobic incubations shows that an increase in denitrification and N immobilization was apparent at an original O₂ concentration of 20 vol%. N₂ was the major product of denitrification in all experiments. Free methane was apparent as soon as nitrate was respired. The stimulating effect of O₂ on total denitrification in the presence of relatively high amounts of easily decomposable cellulose is ascribed to a higher turnover and an intensified mineralization rate (CO₂ production), which increased the total demand for electron acceptors.     

The N2O product ratio of nitrification and its dependence on long-term changes in soil pH

The contribution of nitrification to the emission of nitrous oxide (N₂O) from soils may be large, but its regulation is not well understood. The soil pH appears to play a central role for controlling N₂O emissions from soil, partly by affecting the N₂O product ratios of both denitrification (N₂O/(N₂+N₂O)) and nitrification (N₂O/(NO₂⁻+NO₃⁻). Mechanisms responsible for apparently high N₂O product ratios of nitrification in acid soils are uncertain. We have investigated the pH regulation of the N₂O product ratio of nitrification in a series of experiments with slurries of soils from long-term liming experiments, spanning a pH range from 4.1 to 7.8. ¹⁵N labelled nitrate (NO₃⁻) was added to assess nitrification rates by pool dilution and to distinguish between N₂O from NO₃⁻ reduction and NH₃ oxidation. Sterilized soil slurries were used to determine the rates of chemodenitrification (i.e. the production of nitric oxide (NO) and N₂O from the chemical decomposition of nitrite (NO₂⁻)) as a function of NO₂⁻ concentrations. Additions of NO₂⁻ to aerobic soil slurries (with ¹⁵N labelled NO₃⁻ added) were used to assess its potential for inducing denitrification at aerobic conditions. For soils with pH?5, we found that the N₂O product ratios for nitrification were low (0.2-0.9‰) and comparable to values found in pure cultures of ammonia-oxidizing bacteria. In mineral soils we found only a minor increase in the N₂O product ratio with increasing soil pH, but the effect was so weak that it justifies a constant N₂O product ratio of nitrification for N₂O emission models. For the soils with pH 4.1 and 4.2, the apparent N₂O product ratio of nitrification was 2 orders of magnitude higher than above pH 5 (76‰ and 14‰). This could partly be accounted for by the rates of chemodenitrification of NO₂⁻. We further found convincing evidence for NO₂⁻-induction of aerobic denitrification in acid soils. The study underlines the role of NO₂⁻, both for regulating denitrification and for the apparent nitrifier-derived N₂O emission.     

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

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

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     

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

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

Isotopomer signatures of soil-emitted N2O under different moisture conditions—A microcosm study with arable loess soil

Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N₂O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to evaluate isotopomer analysis of N₂O (intramolecular distribution of ¹⁵N) as well as conventional nitrogen and oxygen isotope ratios (i) as a tool to identify N₂O production processes in soils and (ii) to constrain the isotopic fingerprint of soil-derived N₂O. We conducted a microcosm study with arable loess soil fertilized with 20 mg N kg⁻¹ of ¹⁵NO₃⁻-labeled or non-labeled ammonium nitrate. Soils were incubated for 16 d at varying moisture (55%, 75% and 85% water-filled pore space (WFPS)) in order to establish different levels of nitrification and denitrification. Dual isotope and isotopomer ratios of emitted N₂O were determined by mass spectrometric analysis of δ¹⁸O, average δ¹⁵N (δ¹⁵N^bulk) and ¹⁵N site preference (SP=difference in δ¹⁵N between the central and peripheral N-positions of the asymmetric N₂O molecule). Total rates and N₂O emission of denitrification and nitrification were determined by ¹⁵N analysis of headspace gases and soil extracts of the ¹⁵NO₃⁻ treatment. N₂O emission and denitrification increased with moisture whereas gross nitrification was almost constant. In the 55% WFPS treatment, more than half of the N₂O flux was derived from nitrification, whereas denitrification was the dominant N₂O source in the 75% WFPS and 85% WFPS treatments. Moisture conditions were reflected by the isotopic signatures since highly significant differences were observed for average δ¹⁵N^bulk, SP and δ¹⁸O. Experiment means of the 75% WFPS and 85% WFPS treatments gave negative δ¹⁵N^bulk (−18.0‰ and −34.8‰, respectively) and positive SP (8.6‰ and 15.3‰, respectively), which we explained by the fractionation during N₂O production and partial reduction to N₂. In the 55% WFPS treatment, mean SP was relatively low (1.9‰), which suggests that nitrification produced N₂O with low or negative SP. The observed influence of process condition on isotopomer signatures suggests that the isotopomer approach might be suitable for identifying N₂O source processes. However, more research is needed to determine the impact from process rates and microbial community structure. Isotopomer signatures were within the range reported from previous soil studies which supports the assumption that SP of soil-derived N₂O is lower than SP of tropospheric N₂O.     

Organism specific denitrification in samples of an Udifluvent with different nitrate concentrations

Model experiments were carried out to elucidate the interrelations between the ratio of N₂O to N₂ gases released by the denitrification of different amounts of nitrate on the one hand and of the activity of bacterial populations on the other hand. Soil samples amended with 2 different amounts of nitrate were sterilized, inoculated with either their natural bacterial population or by strains of denitrifying soil bacteria and incubated anaerobically in gas-tight flasks. Results of these experiments are:

• 1 The composition of denitrification gases (N₂O, NO, N₂) was exclusively caused by the species of the active microorganism in the soil: the nitrate concentration only influenced the denitrification rate, but not the N₂O/N₂ ratio of each organism.
• 2 The amounts of nitrite formed transitorily were also specific for the type of organism. These amounts of nitrite influenced neither the emission of N₂O (and NO) nor the ratio of N₂O to N₂ or of N₂O to consumption of nitrate.
• 3 The N₂O formation depended primarily on the microbial soil population and secondly on the nitrate concentration in correlation with the type of the denitrifying population.

     

N2O and NO production in various Chinese agricultural soils by nitrification

Eleven types of agricultural soils were collected from Chinese uplands and paddy fields to compare their N₂O and NO production by nitrification under identical laboratory conditions. Before starting the assays, all air-dried soils were preincubated for 4 weeks at 25 °C and 40% WFPS (water-filled pore space). The nitrification activities of soils were determined by adding (NH₄)₂SO₄ (200 mg N kg⁻¹ soil) and incubating for 3 weeks at 25 °C and 60% WFPS. The net nitrification rates obtained fitted one of two types of models, depending on the soil pH: a zero-order reaction model for acidic soils and one neutral soil (Group 0); or a first-order reaction model for one neutral soil and alkaline soils (Group 1). The results suggest that pH is the most important factor in determining the kinetics of soil nitrification from ammonium. In the Group 1 soils, initial emissions (i.e. during the first week) of N₂O and NO were 82.6 and 83.6%, respectively, of the total emissions during 3 weeks of incubation; in the Group 0 soils, initial emissions of N₂O and NO were 54.7 and 59.9%, respectively, of the total emissions. The net nitrification rate in the first week and second-third weeks were highly correlated with the initial and subsequent emissions (i.e. during the second and third weeks), respectively, of N₂O and NO. The average percentages of emitted (N₂O+NO)-N relative to net nitrification N in initial and subsequent periods were 2.76 and 0.59 for Group 0, and 1.47 and 0.44 for the Group 1, respectively. The initial and subsequent emission ratios of NO/N₂O from Group 0 (acidic) soils were 3.77 and 2.52 times, respectively, higher than those from Group 1 soils (P<0.05).     

N2O and NO emissions from an Andisol field as influenced by pelleted poultry manure

Animal manures from intensive livestock operations can be pelleted to improve handlings and recyclings of embodied nutrients. The aim of this study was to evaluate the influence of pelleted poultry manure on N₂O and NO fluxes from an Andisol field. In autumn 2006 and summer 2007, poultry manure (PM), pelleted poultry manure (PP), and chemical fertilizer (CF) were applied at a rate of 120 kg N ha⁻¹ in each cultivation period to Komatsuna (Brassica rapa var. peruviridis). Nitrous oxide and NO fluxes were measured using an automated monitoring system. A soil incubation experiment was also conducted to determine the influence of intact and ground pelleted manure on N₂O, NO, and CO₂ production with a water-filled pore space (WFPS) of 30 or 50%. In the field measurements, N₂O emission rates from the organic fertilizer treatments were larger than that from the CF treatment, possibly because organic C stimulated denitrification. The highest N₂O flux was observed from the PP treatment after a rainfall following fertilization, and the cumulative emission rate (2.72 ± 0.22 kg N ha⁻¹ y⁻¹) was 3.9 and 7.1 times that from the PM and CF treatments, respectively. In contrast, NO emission rates were highest from the CF treatment. The NO/N₂O flux ratio indicated that nitrification was the dominant process for NO and N₂O production from the CF treatment. Cumulative N₂O emission rates from all treatments were generally higher during the wetter cultivation period (autumn 2006) than during the drier cultivation period (summer 2007). In contrast, NO emission rates were higher in the drier than in the wetter cultivation period. The incubation experiment results showed a synergistic effect of soil moisture and the pelleted manure form on N₂O emission rates. The intact pelleted manure with the 50% WFPS treatment produced the highest N₂O and CO₂ fluxes and resulted in the lowest soil NO₃⁻ content after the incubation. These results indicate that anaerobic conditions inside the pellets, caused by rainfall and heterotrophic microbial activities, led to denitrification, resulting in high N₂O fluxes. Controlling the timing of N application by avoiding wet conditions might be one mitigation option to reduce N₂O emission rates from the PP treatment in this study field.     

Land use effects on gross nitrogen mineralization, nitrification, and N2O emissions in ephemeral wetlands

Stable ¹⁵N isotope dilution and tracer techniques were used in cultivated (C) and uncultivated (U) ephemeral wetlands in central Saskatchewan, Canada to: (1) quantify gross mineralization and nitrification rates and (2) estimate the relative proportion of N₂O emissions from these wetlands that could be attributed to denitrification versus nitrification-related processes. In-field incubation experiments were repeated in early May, mid-June and late July. Mean gross mineralization and nitrification rates (10.3 and 3.1 mg kg⁻¹ d⁻¹, respectively) did not differ between C and U wetlands on any given date. Despite these similarities, the mean NH₄⁺ pool size in the U wetlands (17.2 mg kg⁻¹) was two to three times that of the C wetlands (6.7 mg kg⁻¹) whereas the mean NO₃⁻ pool size in U wetlands (2.2 mg kg⁻¹) was less than half that of C wetlands (5.8 mg kg⁻¹). Mean N₂O emissions from the C wetlands decreased from 112.8 to 17.0 ng N₂O m² s⁻¹ from May to July, whereas mean U-wetland N₂O emissions ranged only from 31.8 to 51.1 ng N₂O m² s⁻¹ over the same period. This trend is correlated to water-filled pore space in C wetlands, demonstrating a soil moisture influence on emissions. Denitrification is generally considered the dominant emitter of N₂O under anaerobic conditions, but in the C wetlands, only 49% of the May emissions could be directly attributed to denitrification, decreasing to 29% in July. In contrast, more than 75% of the N₂O emissions from the U wetlands arose from denitrification of the soil NO₃⁻ pool throughout the season. These land use differences in emission sources and rates should be taken into consideration when planning management strategies for greenhouse gas mitigation.     

Nitrous oxide production in grassland soils: assessing the contribution of nitrifier denitrification

Nitrifier denitrification is the reduction of NO₂⁻ to N₂ by nitrifiers. It leads to the production of the greenhouse gas nitrous oxide (N₂O) as an intermediate and possible end product. It is not known how important nitrifier denitrification is for the production of N₂O in soils. We explored N₂O production by nitrifier denitrification in relation to other N₂O producing processes such as nitrification and denitrification under different soil conditions. The influence of aeration of the soil, different N sources, and pH were tested in four experiments. To differentiate between sources of N₂O, an incubation method with inhibitors was used [Biol. Fertil. Soils 22 (1996) 331]. Sets of four incubations included controls without addition of inhibitors, incubations with addition of small concentrations of C₂H₂ (0.01-0.1 kPa), large concentrations of O₂ (100 kPa), or a combination of C₂H₂ and O₂. The results indicate that the availability of NO₂⁻ stimulated the apparent N₂O production by nitrifier denitrification. A decreasing O₂ content increased the total N₂O production, but decreased N₂O production by nitrifier denitrification. No significant effect of pH could be found. The study revealed problems concerning the use of the inhibitors C₂H₂ and O₂. Almost one-third of all incubations with inhibitors produced more N₂O than the controls. Possible reasons for the problems are discussed. The inhibitors C₂H₂ and O₂ need to be tested thoroughly for their effects on different N₂O producing processes before further application.     

Nitrogen isotope discrimination in denitrification of nitrate in soils

Nitrogen isotope discrimination during denitrification in soils of nitrate containing natural concentrations of ¹⁴N and ¹⁵N was studied by determining the amount and the ¹⁵N content of nitrate-N and (nitrate + nitrite)-N in nitrate-treated soils incubated under anaerobic conditions (He atmosphere) for various times after treatment with glucose to promote denitrification. Analyses performed showed that the nitrate-N lost on incubation of these soils could largely be accounted for as products of denitrification (nitrite, NO. N₂O and N₂).The studies reported show that marked discrimination between ¹⁴N and ¹⁵N occurs during denitrification of nitrate in soils and that significant N isotope effects occur both in reduction of nitrate to nitrite and in reduction of nitrite to gaseous forms of N. They also indicate that the overall N isotope effect during denitrification of nitrate in soil will depend upon the tendency of the soil to accumulate nitrite under conditions that induce denitrification.It is concluded that discrimination between ¹⁴N and ¹⁵N during denitrification in soils of nitrate containing natural concentrations of these isotopes is of sufficient magnitude to invalidate the use of N isotope-ratio analyses for assessment of the contributions of soil and fertilizer N to nitrate in surface or ground waters or to nitrous oxide in the atmosphere.