Effect of oxic and anoxic conditions on nitrous oxide emissions from nitrification and denitrification processes

A lab-scale sequencing batch reactor fed with real municipal wastewater was used to study nitrous oxide (N(2)O) emissions from simulated wastewater treatment processes. The experiments were performed under four different controlled conditions as follows: (1) fully aerobic, (2) anoxic-aerobic with high dissolved oxygen (DO) concentration, (3) anoxic-aerobic with low DO concentration, and 4) intermittent aeration. The results indicated that N(2)O production can occur from both incomplete nitrification and incomplete denitrification. N(2)O production from denitrification was observed in both aerobic and anoxic phases. However, N(2)O production from aerobic conditions occurred only when both low DO concentrations and high nitrite concentration existed simultaneously. The magnitude of N(2) O produced via anoxic denitrification was lower than via oxic denitrification and required the presence of nitrite. Changes in DO, ammonium, and nitrite concentrations influenced the magnitude of N(2)O production through denitrification. The results also suggested that N(2)O can be produced from incomplete denitrification and then released to the atmosphere during aeration phase due to air stripping. Therefore, biological nitrogen removal systems should be optimized to promote complete nitrification and denitrification to minimize N(2)O emissions.     

Evaluating four mathematical models for nitrous oxide production by autotrophic ammonia‐oxidizing bacteria

There is increasing evidence showing that ammonia‐oxidizing bacteria (AOB) are major contributors to N₂O emissions from wastewater treatment plants (WWTPs). Although the fundamental metabolic pathways for N₂O production by AOB are now coming to light, the mechanisms responsible for N₂O production by AOB in WWTP are not fully understood. Mathematical modeling provides a means for testing hypotheses related to mechanisms and triggers for N₂O emissions in WWTP, and can then also become a tool to support the development of mitigation strategies. This study examined the ability of four mathematical model structures to describe two distinct mechanisms of N₂O production by AOB. The production mechanisms evaluated are (1) N₂O as the final product of nitrifier denitrification with NO as the terminal electron acceptor and (2) N₂O as a byproduct of incomplete oxidation of hydroxylamine (NH₂OH) to NO. The four models were compared based on their ability to predict N₂O dynamics observed in three mixed culture studies. Short‐term batch experimental data were employed to examine model assumptions related to the effects of (1) NH concentration variations, (2) dissolved oxygen (DO) variations, (3) NO accumulations and (4) NH₂OH as an externally provided substrate. The modeling results demonstrate that all these models can generally describe the NH, NO, and NO data. However, none of these models were able to reproduce all measured N₂O data. The results suggest that both the denitrification and NH₂OH pathways may be involved in N₂O production and could be kinetically linked by a competition for intracellular reducing equivalents. A unified model capturing both mechanisms and their potential interactions needs to be developed with consideration of physiological complexity. Biotechnol. Bioeng. 2013; 110: 153–163. © 2012 Wiley Periodicals, Inc.     

Modeling nitrous oxide emission from rivers: a global assessment

Estimates of global riverine nitrous oxide (N₂O) emissions contain great uncertainty. We conducted a meta‐analysis incorporating 169 observations from published literature to estimate global riverine N₂O emission rates and emission factors. Riverine N₂O flux was significantly correlated with NH₄, NO₃ and DIN (NH₄ + NO₃) concentrations, loads and yields. The emission factors EF(a) (i.e., the ratio of N₂O emission rate and DIN load) and EF(b) (i.e., the ratio of N₂O and DIN concentrations) values were comparable and showed negative correlations with nitrogen concentration, load and yield and water discharge, but positive correlations with the dissolved organic carbon : DIN ratio. After individually evaluating 82 potential regression models based on EF(a) or EF(b) for global, temperate zone and subtropical zone datasets, a power function of DIN yield multiplied by watershed area was determined to provide the best fit between modeled and observed riverine N₂O emission rates (EF(a): R² = 0.92 for both global and climatic zone models, n = 70; EF(b): R² = 0.91 for global model and R² = 0.90 for climatic zone models, n = 70). Using recent estimates of DIN loads for 6400 rivers, models estimated global riverine N₂O emission rates of 29.6–35.3 (mean = 32.2) Gg N₂O–N yr⁻¹ and emission factors of 0.16–0.19% (mean = 0.17%). Global riverine N₂O emission rates are forecasted to increase by 35%, 25%, 18% and 3% in 2050 compared to the 2000s under the Millennium Ecosystem Assessment's Global Orchestration, Order from Strength, Technogarden, and Adapting Mosaic scenarios, respectively. Previous studies may overestimate global riverine N₂O emission rates (300–2100 Gg N₂O–N yr⁻¹) because they ignore declining emission factor values with increasing nitrogen levels and channel size, as well as neglect differences in emission factors corresponding to different nitrogen forms. Riverine N₂O emission estimates will be further enhanced through refining emission factor estimates, extending measurements longitudinally along entire river networks and improving estimates of global riverine nitrogen loads.     

Nitrous oxide and dinitrogen emissions from urine-affected soil under controlled conditions

A ¹⁵N labelling technique was used to measure N₂O and N₂ emissions from an undisturbed grassland soil treated with cow urine and held at 30 cm water tension and 20°C in a laboratory. Large emissions of dinitrogen were detected immediately following urine application to pasture. These coincided with a rapid and large increase in soil water-soluble carbon levels, some of this increase being attributed to solubilization of soil organic matter by high pH and ammonia concentrations. Emissions of nitrous oxide generally increased with time in contrast to dinitrogen fluxes which decreased as time progressed. Estimated losses of N₂O and N₂ over a 30 day period were between 1 to 5% and 30 to 65% of the urine N applied plus N mineralized from soil organic matter, respectively. Most of the N₂ and N₂O originated from denitrification with nitrification-denitrification being of minor significance as a source of N₂O. Comparisons of the ¹⁵N enrichments in the soil mineral N pools and the evolved N₂O suggested that much of the N₂O was produced in the 5–8 cm zone of the soil. It is concluded that established grassland soils contain large amounts of readily-oxidizable organic carbon which may be used by soil denitrifying organisms when nitrate is non-limiting and soil redox potential is lowered due to high rates of biological activity and high soil moisture contents. ei]{gnR}{fnMerckx}     

Estimation of nitrous oxide, nitric oxide and ammonia emissions from croplands in East, Southeast and South Asia

Agricultural activities have greatly altered the global nitrogen (N) cycle and produced nitrogenous gases of environmental significance. More than half of all chemical N fertilizer produced globally is used in crop production in East, Southeast and South Asia, where rice is central to nutrition. Emissions of nitrous oxide (N₂O), nitric oxide (NO) and ammonia (NH₃) from croplands in this region were estimated by considering background emission and emissions resulting from N added to croplands, including chemical N, animal manure, biologically fixed N and N in crop residues returned to fields. Background emission fluxes of N₂O and NO from croplands were estimated to be 1.22 and 0.57 kg N ha^?1 yr^?1, respectively. Separate fertilizer‐induced emission factors were estimated for upland fields and rice fields. Total N₂O emission from croplands in the study region was estimated to be 1.19 Tg N yr^?1, with 43% contributed by background emissions. The average fertilizer‐induced N₂O emission, however, accounts for only 0.93% of the applied N, which is less than the default IPCC value of 1.25%, because of the low emission factor from paddy fields. Total NO emission was 591 Gg N yr^?1 in the study region, with 40% from background emissions. The average fertilizer‐induced NO emission factor was 0.48%. Total NH₃ emission was estimated to be 11.8 Tg N yr^?1. The use of urea and ammonium bicarbonate and the cultivation of rice led to a high average NH₃ loss rate from chemical N fertilizer in the study region. Emissions were displayed at a 0.5° × 0.5° resolution with the use of a global landuse database.     

Effect of ammonium and nitrate application on the NO and N2O emission out of different soils

The effect of nitrate and ammonium application (0, 50, 100 and 150 mg N kg^-1 soil) was studied in an incubation experiment. Four Belgian soils, selected for different soil characteristics, were used. The application of both nitrate and ammonium caused an increase of the NO and N₂O emission. The NO production from nitrate and ammonium was found to be of the same order of magnitude. At low pH the NO production was found to be highest from nitrate, at higher pH values the production was found to be higher from ammonium. This seems to be the result of the negative effect of low pH on nitrification.The ANOVA analysis was carried out to separate the effect of the form of nitrogen, quantily of N applied and soil characteristics. The total production of NO was found to depend for 97% on the soil characteristics and for 3% on the quantity of N added. The total N₂O production depended for 100% on the soil characteristics.Stepwise regression analysis showed that the total NO production was best predicted by a combination of the factors CaCO₃ content and NH₄ ⁺ concentration in the soil. Total N₂O production was best described by a combination of CaCO₃, water soluble carbon (WSC) and sand-content.The N₂O/NO ratio was found to be highly variable, indicating that their productions react differently to changes in conditions, or are partly independent.It may be concluded that to NO and N₂O from soils both nitrification and denitrification may be equally important, their relative importance depending on local conditions such as substrate availability, water content of the soil etc. However, the NO production seems to be more nitrification dependent than the N₂O production. ei]{gnE}{fnMerckx}{edSection editor}     

Nitrification and nitrous oxide production potentials in aerobic soil samples from the soil profile of a Finnish coniferous site receiving high ammonium deposition

Abstract Using aerobic soil slurry technique nitrification and nitrous oxide production were studied in samples from a pine site in Western Finland. The site received atmospheric ammonium deposition of 7–33 kg N ha⁻¹ a⁻¹ from a mink farm. The experiments with soil slurries showed that the nitrification potential in the litter layer was higher at pH 6 than at pH 4. However, the nitrification potentials in the samples from the organic and mineral horizons at pH 6 and 4 were almost equal. Also N₂O was produced at a higher rate at pH 6 than at pH 4 in slurries of the litter layer samples. The reverse was true for samples from the organic and mineral horizons. The highest N₂O production and nitrification rates were measured in the suspensions of litter layer samples. Nitrification activity in field-moist soil samples was lower than the activity in the slurries indicating that the availability of ammonium limited nitrification in these soils. Acetylene (2.5 kPa) retarded nitrification activity (70-–100%) and N₂O production (40 – 90%) in soil slurries. Acetylene inhibited the N₂O production by 40–60% during the first 3 days after its addition to field-moist samples incubated in aerobic atmosphere. After 3 days the inhibition became much lower (4–5%). The results indicate that, in soil profiles of boreal coniferous forests receiving ammonium deposition, chemolithotrophic nitrification may have importance in the N₂O production, and that changes in soil pH affect differently nitrification as well as N₂O production in litter and deeper soil layers.     

Challenges for simultaneous nitrification, denitrification, and phosphorus removal in microbial aggregates: mass transfer limitation and nitrous oxide production

The microbial community composition and activity was investigated in aggregates from a lab-scale bioreactor, in which nitrification, denitrification and phosphorus removal occurred simultaneously. The biomass was highly enriched for polyphosphate accumulating organisms facilitating complete removal of phosphorus from the bulk liquid; however, some inorganic nitrogen still remained at the end of the reactor cycle. This was ascribed to incomplete coupling of nitrification and denitrification causing NO(3)(-) accumulation. After 2 h of aeration, denitrification was dependent on the activity of nitrifying bacteria facilitating the formation of anoxic zones in the aggregates; hence, denitrification could not occur without simultaneous nitrification towards the end of the reactor cycle. Nitrous oxide was identified as a product of denitrification, when based on stored PHA as carbon source. This observation is of critical importance to the outlook of applying PHA-driven denitrification in activated sludge processes.     

Nitrate leaching and soil nitrous oxide emissions diminish with time in a hybrid poplar short‐rotation coppice in southern Germany

Hybrid poplar short‐rotation coppices (SRC) provide feedstocks for bioenergy production and can be established on lands that are suboptimal for food production. The environmental consequences of deploying this production system on marginal agricultural land need to be evaluated, including the investigation of common management practices i.e., fertilization and irrigation. In this work, we evaluated (1) the soil‐atmosphere exchange of carbon dioxide, methane, and nitrous oxide (N₂O); (2) the changes in soil organic carbon (SOC) stocks; (3) the gross ammonification and nitrification rates; and (4) the nitrate leaching as affected by the establishment of a hybrid poplar SRC on a marginal agricultural land in southern Germany. Our study covered one 3‐year rotation period and 2 years after the first coppicing. We combined field and laboratory experiments with modeling. The soil N₂O emissions decreased from 2.2 kg N₂O‐N ha^?1 a^?1 in the year of SRC establishment to 1.1–1.4 kg N₂O‐N ha^?1 a^?1 after 4 years. Likewise, nitrate leaching reduced from 13 to 1.5–8 kg N ha^?1 a^?1. Tree coppicing induced a brief pulse of soil N₂O flux and marginal effects on gross N turnover rates. Overall, the N losses diminished within 4 years by 80% without fertilization (irrespective of irrigation) and by 40% when 40–50 kg N ha^?1 a^?1 were applied. Enhanced N losses due to fertilization and the minor effect of fertilization and irrigation on tree growth discourage its use during the first rotation period after SRC establishment. A SOC accrual rate of 0.4 Mg C ha^?1 a^?1 (uppermost 25 cm, P = 0.2) was observed 5 years after the SRC establishment. Overall, our data suggest that SRC cultivation on marginal agricultural land in the region is a promising option for increasing the share of renewable energy sources due to its net positive environmental effects.     

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.     

The production and emission of nitrous oxide from headwater streams in the Midwestern United States

The emission of nitrous oxide (N₂O) from streams draining agricultural landscapes is estimated by the Intergovernmental Panel on Climate Change (IPCC) to constitute a globally significant source of this gas to the atmosphere, although there is considerable uncertainty in the magnitude of this source. We measured N₂O emission rates and potential controlling variables in 12 headwater streams draining a predominantly agricultural basin on glacial terrain in southwestern Michigan. The study sites were nearly always supersaturated with N₂O and emission rates ranged from ?8.9 to 266.8 μg N₂O‐N m^?2 h^?1 with an overall mean of 35.2 μg N₂O‐N m^?2 h^?1. Stream water NO₃^? concentrations best‐predicted N₂O emission rates. Although streams and agricultural soils in the basin had similar areal emission rates, emissions from streams were equivalent to 6% of the anthropogenic emissions from soils because of the vastly greater surface area of soils. We found that the default value of the N₂O emission factor for streams and groundwater as defined by the IPCC (EF5‐g) was similar to the value observed in this study lending support to the recent downward revision to EF5‐g. However, the EF5‐g spanned four orders of magnitude across our study sites suggesting that the IPCC's methodology of applying one emission factor to all streams may be inappropriate.     

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.     

Metabolism of nitric oxide and nitrous oxide during nitrification and denitrification in soil at different incubation conditions

Abstract NO production and consumption rates as well as N₂O accumulation rates were measured in a loamy cambisol which was incubated under different conditions (i.e. soil moisture content, addition of nitrogen fertilizer and/or glucose, aerobic or anaerobic gas phase). Inhibition of nitrification with acetylene allowed us to distinguish between nitrification and denitrification as sources of NO and N₂O. Under aerobic conditions untreated soil showed very low release of NO and N₂O but high consumption of NO. Fertilization with NH₄⁺ or urea stimulated both NO and N₂O production by nitrification. Addition of glucose at high soil moisture contents led to increased N₂ and N₂O production by denitrification, but not to increased NO production rates. Anaerobic conditions, however, stimulated both NO and N₂O production by denitrification. The production of NO and N₂O was further stimulated at low moisture contents and after addition of glucose or NO₃⁻. Anaerobic consumption of NO by denitrification followed Michaelis-Menten kinetics and was stimulated by addition of glucose and NO₃⁻. Aerobic consumption of NO followed first-order kinetics up to mixing ratios of at least 14 ppmv NO, was inhibited by autoclaving but not by acetylene, and decreased with increasing soil moisture content. The high NO-consumption activity and the effects of soil moisture on the apparent rates of anaerobic and aerobic production and consumption of NO suggest that diffusional constraints have an important influence on the release of NO, and may be a reason for the different behaviour of NO release vs N₂O release.     

Nitrous oxide production by ammonia oxidizers: Physiological diversity,niche differentiation and potential mitigation strategies

Oxidation of ammonia to nitrite by bacteria and archaea is responsible for global emissions of nitrous oxide directly and indirectly through provision of nitrite and, after further oxidation, nitrate to denitrifiers. Their contributions to increasing N₂O emissions are greatest in terrestrial environments, due to the dramatic and continuing increases in use of ammonia‐based fertilizers, which have been driven by requirement for increased food production, but which also provide a source of energy for ammonia oxidizers (AO), leading to an imbalance in the terrestrial nitrogen cycle. Direct N₂O production by AO results from several metabolic processes, sometimes combined with abiotic reactions. Physiological characteristics, including mechanisms for N₂O production, vary within and between ammonia‐oxidizing archaea (AOA) and bacteria (AOB) and comammox bacteria and N₂O yield of AOB is higher than in the other two groups. There is also strong evidence for niche differentiation between AOA and AOB with respect to environmental conditions in natural and engineered environments. In particular, AOA are favored by low soil pH and AOA and AOB are, respectively, favored by low rates of ammonium supply, equivalent to application of slow‐release fertilizer, or high rates of supply, equivalent to addition of high concentrations of inorganic ammonium or urea. These differences between AOA and AOB provide the potential for better fertilization strategies that could both increase fertilizer use efficiency and reduce N₂O emissions from agricultural soils. This article reviews research on the biochemistry, physiology and ecology of AO and discusses the consequences for AO communities subjected to different agricultural practices and the ways in which this knowledge, coupled with improved methods for characterizing communities, might lead to improved fertilizer use efficiency and mitigation of N₂O emissions.     

土壤水分对土壤产生气态氮的厌氧微生物过程的影响

气态氮[一氧化氮(NO)、氧化亚氮(N2O)和氮气(N2)]的释放是土壤氮损失的一种重要途径.硝化和反硝化作用是土壤气态氮损失的主要微生物过程,但是异养硝化作用、共反硝化作用和厌氧氨氧化过程对土壤气态氮损失的贡献尚不清楚.本研究利用15N标记和配对法,结合硝化抑制剂双氰胺(DCD),通过土壤培养试验来量化厌氧条件下各种...     

Nitric oxide reduction in BioDeNOx reactors: kinetics and mechanism

Biological reduction of nitric oxide (NO) to di-nitrogen (N(2)) gas in aqueous Fe(II)EDTA(2-) solutions is a key reaction in BioDeNOx, a novel process for NOx removal from flue gases. The mechanism and kinetics of the first step of NO reduction, that is, the conversion of NO to N(2)O, was determined in batch experiments using various types of inocula. Experiments were performed in Fe(II)EDTA(2-) medium (5-25 mM) under BioDeNOx reactor conditions (55 degrees C, pH 7.2 +/- 0.2) with ethanol as external electron donor. BioDeNOx reactor mixed liquor gave the highest NO reduction rates (+/-0.34 nmol s(-1) mg(prot)(-1)) with an estimated K(m) value for NO lower than 10 nM. The specific NO (to N(2)O) reduction rate depended on the NO (aq) and Fe(II)EDTA(2-) concentration as well as the temperature. The experimental results, complemented with kinetic and thermodynamic considerations, show that Fe(II)EDTA(2-), and not ethanol, is the primary electron donor for NO reduction, that is, the BioDeNOx reactor medium (the redox system Fe(II)EDTA(2-)/Fe(III)EDTA(-)) interferes with the NO reduction electron transfer chain and thus enhances the NO denitrification rate.     

Microbial oxidation of methane,ammonium and carbon monoxide,and turnover of nitrous oxide and nitric oxide in soils

The effect of soil microbial processes on production and/or consumption of atmospheric trace gases was studied in four different soils which were preincubated in the presence of elevated concentrations of CH₄, NH ₄ ⁺ or CO, to simulate the growth of the resident populations of methanotrophic, nitrifying, or carboxydotrophic bacteria, respectively. Oxidation of CH₄, both at atmospheric (1.8 ppmv) and at elevated (3500 ppmv) CH₄ mixing ratios, was stimulated after preincubation with CH₄, but not with NH ₄ ⁺ or CO, indicating that CH₄ was oxidized by methanotrophic, but not by nitrifying or carboxydotrophic bacteria. However, the oxidation of CH₄ was partially inhibited by addition of NH ₄ ⁺ and CO. Analogously, oxidation of NH ₄ ⁺ was partially inhibited by addition of CH₄. Oxidation of CO at elevated mixing ratios (2300 ppmv) was stimulated after preincubation with CO, indicating oxidation by carboxydotrophs, but was also stimulated at a small extent after preincubation with CH₄, suggesting the involvement of methanotrophs. At atmospheric CO mixing ratios (0.13 ppmv), on the other hand, oxidation of CO was stimulated after preincubation with NH ₄ ⁺ , indicating that the activity was due to nitrifiers. NO uptake was stimulated in soils preincubated with CH₄, indicating the involvement of methanotrophs. However, production of N₂O was only stimulated, if CH₄ was added as a substrate. The results indicate that especially the methanotrophic and nitrifying populations in soil not only oxidize their specific substrates, but are also involved in the metabolism of other compounds.     

Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation

Nitrous oxide (N₂O) is a greenhouse gas that also plays the primary role in stratospheric ozone depletion. The use of nitrogen fertilizers is known as the major reason for atmospheric N₂O increase. Empirical bottom‐up models therefore estimate agricultural N₂O inventories using N loading as the sole predictor, disregarding the regional heterogeneities in soil inherent response to external N loading. Several environmental factors have been found to influence the response in soil N₂O emission to N fertilization, but their interdependence and relative importance have not been addressed properly. Here, we show that soil pH is the chief factor explaining regional disparities in N₂O emission, using a global meta‐analysis of 1,104 field measurements. The emission factor (EF) of N₂O increases significantly (p < .001) with soil pH decrease. The default EF value of 1.0%, according to IPCC (Intergovernmental Panel on Climate Change) for agricultural soils, occurs at soil pH 6.76. Moreover, changes in EF with N fertilization (i.e. ΔEF) is also negatively correlated (p < .001) with soil pH. This indicates that N₂O emission in acidic soils is more sensitive to changing N fertilization than that in alkaline soils. Incorporating our findings into bottom‐up models has significant consequences for regional and global N₂O emission inventories and reconciling them with those from top‐down models. Moreover, our results allow region‐specific development of tailor‐made N₂O mitigation measures in agriculture.     

Diurnal changes in nitric oxide emissions from conventional tillage and pasture sites in the Amazon Basin: influence of soil temperature

We have investigated a subset of restoration practices applied to a degraded pasture at Fazenda Nova Vida, a 22000 ha cattle ranch in Rond^onia, Brazil. Nitric oxide (NO) and carbon dioxide (CO₂) emissions from soils were measured in conventional tillage and current pasture sites to assess N and C losses. Mean daily NO emissions from tilled plots were at least twice those from the pasture. Nitric oxide emissions from the tilled sites showed a strong diurnal pattern, while those from the pasture sites did not. Mean daytime NO emissions from the tilled sites were 9.7 g NO-N m^–2 h^–1, while mean nighttime emissions were 29.7 g NO-N m^–2 h^–1. In the pasture sites, NO emissions were 7.6 g NO-N m^–2 h^–1 during the day, and 7.7 g NO-N m^–2 h^–1 at night. Surface soil temperature was a good inverse predictor (r ²=0.75) of NO emissions from the tilled sites. Carbon dioxide emissions from the tilled sites were generally larger than CO₂ emissions from the pasture sites. The mean CO₂ emission rate from the tilled sites was 179 mg C m^–2 h^–1, while it was 123 mg C m^–2 h^–1 from the pasture sites. There was no distinct diurnal pattern for CO₂ emissions. We found that the very high temperatures measured at the soil surface in the tillage plots, in the range of 40–45°C, reduced the rate of NO emission. The reduction in NO emissions may be because of the sensitivity of autotrophic nitrifiers to high temperatures. This study provides insights on how land-use change may alter regional NO fluxes by exposing certain microbial communities to extreme environmental conditions. Future studies of NO emissions in tropical agricultural systems where soils are bare for extend periods need to make diurnal measurements or the daily fluxes will be substantially underestimated.