Role of nitrification and denitrification for NO metabolism in soil

Release and uptake of NO was measured in a slightly alkaline (pH 7.8) and an acidic (pH 4.7) cambisol. In the alkaline soil under aerobic conditions, NO release was stimulated by ammonium and inhibited by nitrapyrin. Nitrate accumulated simultaneously and was also inhibited by nitrapyrin.¹⁵NO was released after fertilization with¹⁵NH₄NO₃ but not with NH₄ ¹⁵NO₃. The results indicate that in aerobic alkaline cambisol NO was mainly produced during nitrification of ammonium. The results were different under anaerobic conditions and also in the acidic cambisol. There, NO release was stimulated by nitrate and not by ammonium, and was inhibited by chlorate and not by nitrapyrin indicating that NO production was exclusively due to reduction of nitrate. The results were confirmed by¹⁵NO being released mainly from NH₄ ¹⁵NO₃ rather than from¹⁵NH₄NO₃. The observed patterns of NO release were explained by the NO production processes being stimulated by either ammonium or nitrate in the two different soils, whereas the NO consumption processes being only stimulated by nitrate. NO release was larger than N₂O release, but both were small compared to changes in concentrations of soil ammonium or nitrate.(*request for offprints)     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Contributions of Autotrophic and Heterotrophic Nitrifiers to Soil NO and N2O Emissions

Soil emission of gaseous N oxides during nitrification of ammonium represents loss of an available plant nutrient and has an important impact on the chemistry of the atmosphere. We used selective inhibitors and a glucose amendment in a factorial design to determine the relative contributions of autotrophic ammonium oxidizers, autotrophic nitrite oxidizers, and heterotrophic nitrifiers to nitric oxide (NO) and nitrous oxide (N₂O) emissions from aerobically incubated soil following the addition of 160 mg of N as ammonium sulfate kg⁻¹. Without added C, peak NO emissions of 4 μg of N kg⁻¹ h⁻¹ were increased to 15 μg of N kg⁻¹ h⁻¹ by the addition of sodium chlorate, a nitrite oxidation inhibitor, but were reduced to 0.01 μg of N kg⁻¹ h⁻¹ in the presence of nitrapyrin [2-chloro-6-(trichloromethyl)-pyridine], an inhibitor of autotrophic ammonium oxidation. Carbon-amended soils had somewhat higher NO emission rates from these three treatments (6, 18, and 0.1 μg of N kg⁻¹ h⁻¹ after treatment with glucose, sodium chlorate, or nitrapyrin, respectively) until the glucose was exhausted but lower rates during the remainder of the incubation. Nitrous oxide emission levels exhibited trends similar to those observed for NO but were about 20 times lower. Periodic soil chemical analyses showed no increase in the nitrate concentration of soil treated with sodium chlorate until after the period of peak NO and N₂O emissions; the nitrate concentration of soil treated with nitrapyrin remained unchanged throughout the incubation. These results suggest that chemoautotrophic ammonium-oxidizing bacteria are the predominant source of NO and N₂O produced during nitrification in soil.     

Stimulation of NO and N2O emissions from soils by SO2 deposition

Sulfur dioxide (SO₂) in the atmosphere has been demonstrated to have many adverse impacts on the environment and human health. In this study, deposition of SO₂ ranging from 9.0 to 127.8 mg kg^?1 with an average of 35.7 mg S kg^?1 was found to substantially stimulate NO and N₂O emissions from soils in the humid subtropical areas of Hainan, Fujian, Jiangxi, and Yunnan provinces of China under field conditions. Laboratory tests indicated that the stimulations were mediated biologically as the effects were not observed in sterilized soils. Acidification of soil resulting from SO₂ deposition was not responsible for the stimulated NO and N₂O emissions alone as the stimulation did not occur by acidifying soil with HNO₃ treatment. By using the ¹⁵N tracing method, we found that the N₂O emissions stimulated by SO₂ deposition were from either denitrification, heterotrophic nitrification or both, but not from autotrophic nitrification. Therefore, atmospheric SO₂ deposition would most likely stimulate NO and N₂O emissions in acidic soils in which heterotrophic nitrification dominates NO and N₂O production and waterlogged soils in which denitrification dominates NO and N₂O production.     

Influence of O2 availability on NO and N2O release by nitrification and denitrification in soils

The availability of O₂ is believed to be one of the main factors regulating nitrification and denitrification and the release of NO and N₂O. The availability of O₂ in soil is controlled by the O₂ partial pressure in the gas phase and by the moisture content in the soil. Therefore, we investigated the influence of O₂ partial pressures and soil moisture contents on the NO and N₂O release in a sandy and a loamy silt and differentiated between nitrification and denitrification by selective inhibition of nitrification with 10 Pa acetylene. At 60% whc (maximum water holding capacity) NO and N₂O release by denitrification increased with decreasing O₂ partial pressure and reached a maximum under anoxic conditions. Under anoxic conditions NO and N₂O were only released by denitrification. NO and N₂O release by nitrification also increased with decreasing O₂ partial pressure, but reached a maximum at 0.1–0.5% O₂ and then decreased again. Nitrification was the main source of NO and N₂O at O₂ partial pressures higher than 0.1–0.5% O₂. At lower O₂ partial pressures denitrification was the main source of NO and N₂O. With decreasing O₂ partial pressure N₂O release increased more than NO release, indicating that the N₂O release was more sensitive against O₂ than the NO release. At ambient O₂ partial pressure (20.5% O₂) NO and N₂O release by denitrification increased with increasing soil moisture content. The maximum NO and N₂O release was observed at soil moisture contents of 65–80% whc and 100% whc, respectively. NO and N₂O release by nitrification also increased with increasing soil moisture content with a maximum at 45–55% whc and 90% whc, respectively. Nitrification was the main source of NO and N₂O at soil moisture contents lower than 90% whc and 80% whc, respectively. Higher soil moisture contents favoured NO and N₂O release by denitrification. Soil texture had also an effect on the release of NO and N₂O. The coarse-textured sandy silt released more NO than N₂O compared with the fine-textured loamy silt. At high soil moisture contents (80–100% whc) the fine-textured soil showed a higher N₂O release by denitrification than the coarse-textured soil. We assume that the fine-textured soil became anoxic at a lower soil moisture content than the coarse-textured soil. In conclusion, the effects of O₂ partial pressure, soil moisture and soil texture were consistent with the theory that denitrification increasingly contributes to the release of NO and in particular N₂O when conditions for soil microorganisms become increasingly anoxic.     

Fungal control of nitrous oxide production in semiarid grassland

Fungi are capable of both nitrification and denitrification and dominate the microbial biomass in many soils. Recent work suggests that fungal rather than bacterial pathways dominate N transformation in desert soils. We evaluated this hypothesis by comparing the contributions of bacteria and fungi to N₂O production at control and N fertilized sites within a semiarid grassland in central New Mexico (USA). Soil samples were taken from the rhizosphere of blue grama (B. gracilus) and the microbiotic crusts that grow in open areas between the bunch grasses. Soils incubated at 30% or 70% water holding capacity, were exposed to one of three biocide treatments (control, cycloheximide or streptomycin). After 48 h, N₂O and CO₂ production were quantified along with the activities of several extracellular enzymes. N₂O production from N fertilized soils was higher than that of control soils (165 vs. 41 pmol h⁻¹ g⁻¹), was higher for crust soil than for rhizosphere soil (108 vs. 97 pmol h⁻¹ g⁻¹), and increased with soil water content (146 vs. 60 pmol h⁻¹ g⁻¹). On average, fungicide (cycloheximide) addition reduced N₂O production by 85% while increasing CO₂ production by 69%; bactericide (streptomycin) reduced N₂O by 53% with mixed effects on CO₂ production. N₂O production was significantly correlated with C and N mineralization potential as measured by assays for glycosidic and proteolytic enzymes, and with extractable nitrate and ammonium. Our data indicate that fungal nitrifier denitrification and bacterial autotrophic nitrification dominate N transformation in this ecosystem and that N₂O production is highly sensitive to soil cover, N deposition and moisture.     

Production of nitric oxide in Nitrosomonas europaea by reduction of nitrite

Nitrosomonas europaea and Nitrosovibrio sp. produced NO and N₂O during nitrification of ammonium. Less then 15% of the produced NO was due to chemical decomposition of nitrite. Production of NO and especially of N₂O increased when the bacteria were incubated under anaerobic conditions at decreasing flow rates of air, or at increasing cell densities. Low concentrations of chlorite (10 M) inhibited the production of NO and N₂, but not of nitrite indicating that NO and N₂O were not produced during the oxidative conversion of ammonium to nitrite. NO and N₂O were produced during reduction of nitrite with hydrazine as electron donor in almost stoichiometric quantities indicating that reduction of nitrite was the main source of NO and N₂O.     

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N₂O) and nitric oxide (NO) are not well understood. We examined emissions of N₂O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N₂O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N₂O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha⁻¹· y⁻¹). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha⁻¹· y⁻¹), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha⁻¹· y⁻¹). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.     

Factors promoting emissions of nitrous oxide and nitric oxide from denitrifying sequencing batch reactors operated with methanol and ethanol as electron donors

The emissions of nitrous oxide (N₂O) and nitric oxide (NO) from biological nitrogen removal (BNR) operations via nitrification and denitrification is gaining increased prominence. While many factors relevant to the operation of denitrifying reactors can influence N₂O and NO emissions from them, the role of different organic carbon sources on these emissions has not been systematically addressed or interpreted. The overall goal of this study was to evaluate the impact of three factors, organic carbon limitation, nitrite concentrations, and dissolved oxygen concentrations on gaseous N₂O and NO emissions from two sequencing batch reactors (SBRs), operated, respectively, with methanol and ethanol as electron donors. During undisturbed ultimate‐state operation, emissions of both N₂O and NO from either reactor were minimal and in the range of <0.2% of influent nitrate‐N load. Subsequently, the two reactors were challenged with transient organic carbon limitation and nitrite pulses, both of which had little impact on N₂O or NO emissions for either electron donor. In contrast, transient exposure to oxygen led to increased production of N₂O (up to 7.1% of influent nitrate‐N load) from ethanol grown cultures, owing to their higher kinetics and potentially lower susceptibility to oxygen inhibition. A similar increase in N₂O production was not observed from methanol grown cultures. These results suggest that for dissolved oxygen, but not for carbon limitation or nitrite exposure, N₂O emission from heterotrophic denitrification reactors can vary as a function of the electron donor used. Biotechnol. Bioeng. 2010; 106: 390–398. © 2010 Wiley Periodicals, Inc.     

Nitrous oxide production, its source and distribution in urine patches on grassland on peat soil

Urine patches are considered to be important sites for nitrous oxide (N₂O) production through nitrification and denitrification due to their high concentration of nitrogen (N). The aim of the present study was to determine the microbial source and size of production of N₂O in different zones of a urine patch on grassland on peat soil. Artificial urine was applied in elongated patches of 4.5 m. Four lateral zones were distinguished and sampled for four weeks using an intact soil core incubation method. Incubation of soil cores took place without any additions to the headspace to determine total N₂O production, with acetylene addition to determine total denitrification (N₂O+N₂), and with methyl fluoride to determine the N₂O produced through denitrification.Nitrous oxide production was largest in the centre and decreased towards the edge of the patch. Maximum N₂O production was about 50 mg N m^–2 d^–1 and maximum denitrification activity was 70 mg N m^–2 d^–1. Nitrification was the main N₂O producing process. Nitrous oxide production through denitrification was only of significance when denitrification activity was high. Total N loss through nitrification and denitrification over 31 days was 4.1 g N per patch which was 2.2% of the total applied urine-N.     

Relative Rates of Nitric Oxide and Nitrous Oxide Production by Nitrifiers, Denitrifiers, and Nitrate Respirers

Biogenic emissions of nitric and nitrous oxides have important impacts on the photochemistry and chemistry of the atmosphere. Although biogenic production appears to be the overwhelming source of N₂O, the magnitude of the biogenic emission of NO is very uncertain. In soils, possible sources of NO and N₂O include nitrification by autotrophic and heterotrophic nitrifiers, denitrification by nitrifiers and denitrifiers, nitrate respiration by fermenters, and chemodenitrification. The availability of oxygen determines to a large extent the relative activities of these various groups of organisms. To better understand this influence, we investigated the effect of the partial pressure of oxygen (pO₂) on the production of NO and N₂O by a wide variety of common soil nitrifying, denitrifying, and nitrate-respiring bacteria under laboratory conditions. The production of NO per cell was highest by autotrophic nitrifiers and was independent of pO₂ in the range tested (0.5 to 10%), whereas N₂O production was inversely proportional to pO₂. Nitrous oxide production was highest in the denitrifier Pseudomonas fluorescens, but only under anaerobic conditions. The molar ratio of NO/N₂O produced was usually greater than unity for nitrifiers and much less than unity for denitrifiers. Chemodenitrification was the major source of both the NO and N₂O produced by the nitrate respirer Serratia marcescens. Chemodenitrification was also a possible source of NO and N₂O in nitrifier cultures but only when high concentrations of nitrite had accumulated or were added to the medium. Although most of the denitrifiers produced NO and N₂O only under anaerobic conditions, chemostat cultures of Alcaligenes faecalis continued to emit these gases even when the cultures were sparged with air. Based upon these results, we predict that aerobic soils are primary sources of NO and that N₂O is produced only when there is sufficient soil moisture to provide the anaerobic microsites necessary for denitrification by either denitrifiers or nitrifiers.     

Nitrogen loss from grassland on peat soils through nitrous oxide production

Nitrous oxide (N₂O) in soils is produced through nitrification and denitrification. The N₂O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N₂O or N₂. Aim of the study was to quantify N₂O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N₂O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N₂O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH₃F).Nitrous oxide production over the 2 year period was on average 34 kg N ha^-1 yr^-1 for mown treatments that received no N fertiliser and 44 kg N ha^-1 yr^-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N₂O production to reach 81 kg N ha^-1 yr^-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N₂O production in the 0–40 cm layer. The N₂O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N₂O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N₂O. In conclusion, N₂O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N₂O producing process.     

Quantitative traits associated with adaptation of three barley (Hordeum vulgare L) cultivars to suboptimal iron supply

A laboratory method was developed that allows determination of in situ net nitrification with high sensitivity and at high temporal resolution. Nitrate in soils is quantitatively converted into nitrous oxide under strictly anaerobic conditions in the presence of 10 kPa acetylene by the soil endogenous denitrifier population, with the N₂O detected by a gas chromatograph equipped with a ⁶³Ni electron capture detector. Thus, even low net nitrification rates, i.e. small net increases in soil nitrate concentrations can easily be detected. Comparison of results using this method with results obtained using the classical in situ incubation method (buried bag soil incubation) revealed excellent agreement. Application of the new method allowed both determination of the seasonal pattern of net nitrification as well as correlation analysis between in situ NO and N₂O flux rates and in situ net nitrification rates of the forest soils studied. Regardless of the forest site studied (spruce, spruce limed, beech), and during each year of a 3 years period (1995–1997), net nitrification varied strongly with season and was least during winter and greatest during summer. The long-term annual, mean rate of net nitrification for the untreated spruce site, the limed spruce site and the beech site were 1.54 ± 0.27 mg N kg^–1 sdw d^–1, 1.92 ± 0.23 mg N kg^–1 sdw d^–1 and 1.31 ± 0.23 mg N kg^–1 sdw d^–1, respectively. In situ rates of nitrification and NO and N₂O emission were strongly correlated for all sites suggesting that nitrification was the dominate source of NO as well as N₂O.     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.     

Mechanisms and kinetics of nitric and nitrous oxide production during nitrification in agricultural soil

Laboratory experiments were conducted with three California agricultural soils to examine substrate and process controls over temporal variability of NO and N₂O production during nitrification, and to quantify the kinetics of HNO₂‐mediated chemical reactions. Gross NO production rates were highly correlated (r² = 0.93–0.97) with calculated concentrations of HNO₂, which were shown to originate from autotrophic microbial oxidation of NH₄ ⁺ to NO₂ ^? Production of NO was not correlated with NH₄ ⁺ or NO₃^–, or with the overall nitrification rate. Distinct periods of high NO₂^– accumulation occurred below critical pH values in each soil, apparently due to inhibition of microbial NO₂^– oxidation. Data suggest that even during periods of relatively low NO₂^– accumulation and rapid overall nitrification, HNO₂‐mediated reactions may have been the primary source of NO. Rate coefficients (k_PNO) relating NO production to HNO₂ concentrations were determined for sterile (λ‐irradiated) soils, and were similar to k_PNO values in 2 of 3 nonsterile soils undergoing nitrification. Production of N₂O was correlated with HNO₂ (r² = 0.88–0.99) in sterile soils, and with NO₂^– and NO₃^– (R² = 0.72–0.91) in nonsterile soils. Experiments using ¹⁵N confirmed that dissimilatory NO₃^– reduction contributed to N₂O production even under primarily aerobic conditions. Sterile kPNO and kPN2O values were correlated (r² = 0.90 and 0.82) with soil organic matter content. Overall, the results demonstrate that both steps of the nitrification sequence, together with abiotic reactions involving NO₂^–/HNO₂ need to be considered in developing improved models of NO and N₂O emissions from soils.     

Nitrogen oxide gas emissions from temperate forest soils receiving long-term nitrogen inputs

From spring 2000 through fall 2001, we measured nitric oxide (NO) and nitrous oxide (N₂O) fluxes in two temperate forest sites in Massachusetts, USA that have been treated since 1988 with different levels of nitrogen (N) to simulate elevated rates of atmospheric N deposition. Plots within a pine stand that were treated with either 50 or 150 kg N ha^?1 yr^?1 above background displayed consistently elevated NO fluxes (100–200 µg N m^?2 h^?1) compared to control plots, while only the higher N treatment plot within a mixed hardwood stand displayed similarly elevated NO fluxes. Annual NO emissions estimated from monthly sampling accounted for 3.0–3.7% of N inputs to the high‐N plots and 8.3% of inputs to the Pine low‐N plot. Nitrous oxide fluxes in the N‐treated plots were generally < 10% of NO fluxes. Net nitrification rates (NRs) and NO production rates measured in the laboratory displayed patterns that were consistent with field NO fluxes. Total N oxide gas flux was positively correlated with contemporaneous measurements of NR and concentration. Acetylene inhibited both nitrification and NO production, indicating that autotrophic nitrification was responsible for the elevated NO production. Soil pH was negatively correlated with N deposition rate. Low levels (3–11 µg N kg^?1) of nitrite () were detected in mineral soils from both sites. Kinetic models describing NO production as a function of the protonated form of (nitrous acid [HNO₂]) adequately described the mineral soil data. The results indicate that atmospheric deposition may generate losses of gaseous NO from forest soils by promoting nitrification, and that the response may vary significantly between forest types under similar climatic regimes. The lowering of pH resulting from nitrification and/or directly from deposition may also play a role by promoting reactions involving HNO₂.     

Production of nitrous oxide in artificially flooded and drained soils

Production of nitrous oxide (N₂O) was studied in one peaty and one sandy soil undergoing wetting and drying cycles. The background concentration of N₂O in the soil was compared with the N₂O produced during 4 hours of incubation with and without addition of acetylene. The concentration of N₂O in the soil under flooded conditions was relatively stable, and net consumption of N₂O was observed as often as net production. The reference area and drained soils showed somewhat different patterns compared to the flooded soils, which was probably an effect of intermediate soil water conditions. During flooding, the nitrous oxide made up less than 1% of total denitrification on 50% and 54% of the sampling occasions for the peaty and the sandy soil, respectively, and N₂O/(N₂O+N₂)-ratios exceeded 0.2 on only 6% and 3% of the sampling occasions. Under drained conditions and in the reference areas, the ratios showed a more even frequency distribution. Grouping the nitrous oxide production data for different seasons and field conditions, we found few seasonal trends. At the sandy site, mean production of N₂O was larger during the winter months. There were weak correlations between N₂O production and floodwater nitrate concentration, and between N₂O production and soil temperature. N₂O production in the reference area varied between consumption and 4.6 kg N ha^–1 month^–1 and in flooded and drained soil between consumption and 2.6 kg N ha^–1 month^–1.     

Denitrification rate determined by nitrate disappearance is higher than determined by nitrous oxide production with acetylene blockage

A mixed beech and spruce forest soil was incubated under potential denitrification assay (PDA) condition with 10% acetylene (C₂H₂) in the headspace of soil slurry bottles. Nitrous oxide (N₂O) concentration in the headspace, as well as nitrate, nitrite and ammonium concentrations in the soil slurries were monitored during the incubation. Results show that nitrate disappearance rate was higher than N₂O production rate with C₂H₂ blockage during the incubation. Sum of nitrate, nitrite, and N₂O with C₂H₂ blockage could not recover the original soil nitrate content, showing an N imbalance in such a closed incubation system. Changes in nitrite and ammonium concentration during the incubation could not account for the observed faster nitrate disappearance rate and the N imbalance. Non-determined nitric oxide (NO) and N₂ production could be the major cause, and the associated mechanisms could vary for different treatments. Commonly applied PDA measurement likely underestimates the nitrate removal capacity of a system. Incubation time and organic matter/nitrate ratio are the most critical factors to consider using C₂H₂ inhibition technique to quantify denitrification. By comparing the treatments with and without an antibiotic, the results suggest that microbial N uptake probably played a minor role in N balance, and other denitrifying enzymes but nitrate reductase could be substantially synthesized during the incubation.     

Excessive use of nitrogen in Chinese agriculture results in high N2O/(N2O+N2) product ratio of denitrification,primarily due to acidification of the soils

China is the world's largest producer and consumer of fertilizer N, and decades of overuse has caused nitrate leaching and possibly soil acidification. We hypothesized that this would enhance the soils' propensity to emit N₂O from denitrification by reducing the expression of the enzyme N₂O reductase. We investigated this by standardized oxic/anoxic incubations of soils from five long‐term fertilization experiments in different regions of China. After adjusting the nitrate concentration to 2 mM, we measured oxic respiration (R), potential denitrification (D), substrate‐induced denitrification, and the denitrification product stoichiometry (NO, N₂O, N₂). Soils with a history of high fertilizer N levels had high N₂O/(N₂O+N₂) ratios, but only in those field experiments where soil pH had been lowered by N fertilization. By comparing all soils, we found a strong negative correlation between pH and the N₂O/(N₂O+N₂) product ratio (r² = 0.759, P < 0.001). In contrast, the potential denitrification (D) was found to be a linear function of oxic respiration (R), and the ratio D/R was largely unaffected by soil pH. The immediate effect of liming acidified soils was lowered N₂O/(N₂O+N₂) ratios. The results provide evidence that soil pH has a marginal direct effect on potential denitrification, but that it is the master variable controlling the percentage of denitrified N emitted as N₂O. It has been known for long that low pH may result in high N₂O/(N₂O+N₂) product ratios of denitrification, but our documentation of a pervasive pH‐control of this ratio across soil types and management practices is new. The results are in good agreement with new understanding of how pH may interfere with the expression of N₂O reductase. We argue that the management of soil pH should be high on the agenda for mitigating N₂O emissions in the future, particularly for countries where ongoing intensification of plant production is likely to acidify the soils.     

Variations in soil N cycling and trace gas emissions in wet tropical forests

We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai’i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year⁻¹ MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N₂O and NO, soil inorganic nitrogen pools (NH₄⁺ and NO₃⁻), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed ¹⁵N-labeling experiments to partition sources of N₂O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N₂O emissions were highest at the mesic end of the gradient (8.7±4.6 and 1.1±0.3 ng N cm⁻² h⁻¹, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (≤0.2 ng N cm⁻² h⁻¹). Isotopic labeling showed that with increasing MAP, the source of N₂O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH₄⁺ and decline in NO₃⁻, while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.