Evaluation of different nitrous oxide production models with four continuous long-term wastewater treatment process data series

Five activated sludge models describing N₂O production by ammonium oxidising bacteria (AOB) were compared to four different long-term process data sets. Each model considers one of the two known N₂O production pathways by AOB, namely the AOB denitrification pathway and the hydroxylamine oxidation pathway, with specific kinetic expressions. Satisfactory calibration could be obtained in most cases, but none of the models was able to describe all the N₂O data obtained in the different systems with a similar parameter set. Variability of the parameters can be related to difficulties related to undescribed local concentration heterogeneities, physiological adaptation of micro-organisms, a microbial population switch, or regulation between multiple AOB pathways. This variability could be due to a dependence of the N₂O production pathways on the nitrite (or free nitrous acid—FNA) concentrations and other operational conditions in different systems. This work gives an overview of the potentialities and limits of single AOB pathway models. Indicating in which condition each single pathway model is likely to explain the experimental observations, this work will also facilitate future work on models in which the two main N₂O pathways active in AOB are represented together.     

Dominance of bacterial ammonium oxidizers and fungal denitrifiers in the complex nitrogen cycle pathways related to nitrous oxide emission

Organic compounds and mineral nitrogen (N) usually increase nitrous oxide (N₂O) emissions. Vinasse, a by‐product of bio‐ethanol production that is rich in carbon, nitrogen, and potassium, is recycled in sugarcane fields as a bio‐fertilizer. Vinasse can contribute significantly to N₂O emissions when applied with N in sugarcane plantations, a common practice. However, the biological processes involved in N₂O emissions under this management practice are unknown. This study investigated the roles of nitrification and denitrification in N₂O emissions from straw‐covered soils amended with different vinasses (CV: concentrated and V: nonconcentrated) before or at the same time as mineral fertilizers at different time points of the sugarcane cycle in two seasons. N₂O emissions were evaluated for 90 days, the period that occurs most of the N₂O emission from fertilizers; the microbial genes encoding enzymes involved in N₂O production (archaeal and bacterial amoA, fungal and bacterial nirK, and bacterial nirS and nosZ), total bacteria, and total fungi were quantified by real‐time PCR. The application of CV and V in conjunction with mineral N resulted in higher N₂O emissions than the application of N fertilizer alone. The strategy of vinasse application 30 days before mineral N reduced N₂O emissions by 65% for CV, but not for V. Independent of rainy or dry season, the microbial processes were nitrification by ammonia‐oxidizing bacteria (AOB) and archaea and denitrification by bacteria and fungi. The contributions of each process differed and depended on soil moisture, soil pH, and N sources. We concluded that amoA‐AOB was the most important gene related to N₂O emissions, which indicates that nitrification by AOB is the main microbial‐driven process linked to N₂O emissions in tropical soil. Interestingly, fungal nirK was also significantly correlated with N₂O emissions, suggesting that denitrification by fungi contributes to N₂O emission in soils receiving straw and vinasse application.     

Mechanism leading to N<Subscript>2</Subscript>O production in wastewater treating biofilm systems

Wastewater treatment plants are known to be important point sources for nitrous oxide (N₂O) in the anthropogenic N cycle. Biofilm based treatment systems have gained increasing popularity in the treatment of wastewater, but the mechanisms and controls of N₂O formation are not fully understood. Here, we review functional groups of microorganism involved in nitrogen (N) transformations during wastewater treatment, with emphasis on potential mechanism of N₂O production in biofilms. Biofilms used in wastewater treatment typically harbour aerobic and anaerobic zones, mediating close interactions between different groups of N transforming organisms. Current models of mass transfer and biomass interactions in biofilms are discussed to illustrate the complex regulation of N₂O production. Ammonia oxidizing bacteria (AOB) are the prime source for N₂O in aerobic zones, while heterotrophic denitrifiers dominate N₂O production in anoxic zones. Nitrosative stress ensuing from accumulation of NO₂ ^? during partial nitrification or denitrification seems to be one of the most critical factors for enhanced N₂O formation. In AOB, N₂O production is coupled to nitrifier denitrification triggered by nitrosative stress, low O₂ tension or low pH. Chemical N₂O production from AOB intermediates (NH₂OH, HNO, NO) released during high NH₃ turnover seems to be limited to surface-near AOB clusters, since diffusive mass transport resistance for O₂ slows down NH₃ oxidation rates in deeper biofilm layers. The proportion of N₂O among gaseous intermediates (NO, N₂O, N₂) in heterotrophic denitrification increases when NO or nitrous acid (HNO₂) accumulates because of increasing NO₂ ^?, or when transient oxygen intrusion impairs complete denitrification. Limited electron donor availability due to mass transport limitation of organic substrates into anoxic biofilm zones is another important factor supporting high N₂O/N₂ ratios in heterotrophic denitrifiers. Biofilms accommodating Anammox bacteria release less N₂O, because Anammox bacteria have no known N₂O producing metabolism and reduce NO₂ ^? to N₂, thereby lowering nitrosative stress to AOB and heterotrophs.     

Who contributes more to N2O emission during sludge bio-drying with two different aeration strategies,nitrifiers or denitrifiers?

Global warming effects have drawn more and more attention to studying all sources and sinks of nitrous oxide (N₂O). Sludge bio-drying, as an effective sludge treatment technology, is being adopted worldwide. In this study, two aeration strategies (piles I and II) were compared to investigate the primary contributors to N₂O emission during sludge bio-drying through studying the evolution of functional genes involved in nitrification (amoA, hao, and nxrA) and denitrification (narG, nirS, nirK, norB, and nosZ) by quantitative PCR (qPCR). Results showed that the profile of N₂O emission can be divided into three stages, traditional denitrification contributed largely to N₂O emission at stage I (days 1–5), but N₂O emission mainly happened at stage II (days 5–14) due to nitrifier denitrification and NH₂OH accumulation by ammonia-oxidizing bacteria (AOB), accounting for 51.4% and 58.2% of total N₂O emission for piles I and II, respectively. At stage III (days 14–21), nitrifier denitrification was inhibited because sludge bio-drying proceeded mainly by the physical aeration, thus N₂O emission decreased and changed little. The improved aeration strategy availed pile I to reduce N₂O emission much especially at stages II and III, respectively. These results indicated that nitrifier denitrification by AOB and biological NH₂OH oxidation due to AOB made more contribution to N₂O emission, and aeration strategy was crucial to mitigate N₂O emission during sludge bio-drying.

     

Modeling nitrogen removal for a denitrification biofilter

Nitrous oxide (N₂O) is a major greenhouse gas, heavily contributing to global warming. N₂O is emitted from various sources such as wastewater treatment plants, during the nitrification and denitrification steps. ASM models, which are commonly used in wastewater treatment, usually consider denitrification as a one-step process (NO₃ ⁻ directly reduced to N₂) and are as such unable to provide values for intermediate products of the reaction like N₂O. In this study, a slightly modified ASM1 model was implemented in the GPS-X™ software to simulate the concentration of such intermediate products (NO₂ ⁻, NO and N₂O) and to estimate the amounts of gaseous N₂O emitted by the denitrification stage (12 biofilters) of the Seine-Centre WWTP (SIAAP, Paris). Simulations running on a 1-year period have shown good agreements with measured effluent data for nitrate and nitrite. The calculated mean value for emitted N₂O is 4.95 kgN–N₂O/day, which stands in the typical range of estimated experimental values of 4–31 kgN–N₂O/day. Nitrous oxide emissions are usually not measured on WWTPs and so, as obtained results show, there is a certain potential for using models that quantify those emissions using traditionally measured influent data.     

Modelling nitrous oxide emission from water-logged soils of a spruce forest ecosystem using the biogeochemical model Wetland-DNDC

During the last decades, decision makers and policy have increasingly demanded for regional and national inventories of greenhouse gas emission, such as nitrous oxide (N₂O), to develop appropriate strategies and mitigation options. A potential way to derive large-scale estimates of N₂O emission is the use of process-based models, such as PnET-N-DNDC or Wetland-DNDC. While PnET-N-DNDC has been effectively applied for various upland forest ecosystems, the Wetland-DNDC model has not yet been validated with regard to N₂O emission. We calibrated and validated the Wetland-DNDC model on the basis of a 4-year field data set of two water-logged soils (Humic Gleysol and Histic Gleysol) of a spruce forest ecosystem. Model calibration by means of the Levenberg–Marquardt algorithm considerably improved the model performance for the period of calibration (2001–2002). The error variance was reduced by up to a factor of two and the modelling efficiency was increased from −1.24 to −0.15 (Humic Gleysol) and from −0.42 to 0.1 (Histic Gleysol). However, the model performance for the period of validation (2003–2004) and particularly for the extreme dry period in summer 2003 was not fully satisfying, notably with regard to the temporal pattern of the N₂O emission.     

Identifying sources of nitrous oxide emission in anoxic/aerobic sequencing batch reactors (A/O SBRs) acclimated in different aeration rates

Both long term and batch experiments were carried out to identify the sources of the N₂O emission in anoxic/aerobic sequencing batch reactors (A/O SBRs) under different aeration rates. The obtained results showed that aeration rate has an important effect on the N₂O emission of A/O SBR and most of the N₂O was emitted during the aerobic phase. During the anoxic phase, nitrate ammonification was the major source of N₂O emission while denitrification performed as a sink of N₂O, in all three bioreactors. The N₂O emission mechanisms during the aerobic phase differed with the aeration rate. At low and high aeration rates (Run 1 and Run 3), both coupled-denitrification and nitrifier denitrification were ascribed to be the source of N₂O emission. At mild aeration rate (Run 2), nitrifier denitrification by Nitrosomonas-like ammonia oxidizing-bacterial (AOB) was responsible for N₂O emission while coupled-denitrification turned out to be a sink of N₂O because of the presence of inner anaerobic region in sludge flocs.     

Nitrous oxide production in autotrophic nitrogen removal granular sludge: A modeling study

The sustainability of autotrophic granular system performing partial nitritation and anaerobic ammonium oxidation (anammox) for complete nitrogen removal is impaired by the production of nitrous oxide (N₂O). A systematic analysis of the pathways and affecting parameters is, therefore, required for developing N ₂O mitigation strategies. To this end, a mathematical model capable of describing different N ₂O production pathways was defined in this study by synthesizing relevant mechanisms of ammonium-oxidizing bacteria (AOB), nitrite-oxidizing bacteria, heterotrophic bacteria (HB), and anammox bacteria. With the model validity reliably tested and verified using two independent sets of experimental data from two different autotrophic nitrogen removal biofilm/granular systems, the defined model was applied to reveal the underlying mechanisms of N ₂O production in the granular structure as well as the impacts of operating conditions on N ₂O production. The results show that: (a) in the aerobic zone close to the granule surface where AOB contribute to N ₂O production through both the AOB denitrification pathway and the NH ₂OH pathway, the co-occurring HB consume N ₂O produced by AOB but indirectly enhance the N ₂O production by providing NO from NO ₂ ⁻ reduction for the NH ₂OH pathway, (b) the inner anoxic zone of granules with the dominance of anammox bacteria acts as a sink for NO ₂ ⁻ diffusing from the outer aerobic zone and, therefore, reduces N ₂O production from the AOB denitrification pathway, (c) operating parameters including bulk DO, influent NH ₄ ⁺, and granule size affect the N ₂O production in the granules mainly by regulating the NH ₂OH pathway of AOB, accounting for 34–58% of N ₂O turnover, and (d) the competition between the NH ₂OH pathway and heterotrophic denitrification for nitric oxide leads to the positive role of HB in reducing N ₂O production in the autotrophic nitrogen removal granules, which could be further enhanced in the presence of a proper level of influent organics.     

Modeling indirect N2O emissions along the N cascade from cropland soils to rivers

The frequently observed discrepancy between estimations of N₂O emissions at regional or global scale based either on field data or inventories (bottom-up) or on direct atmospheric observations (top-down) suggests that riparian areas and river surfaces play a significant role as hot spots of emission. We developed a modeling procedure to assess N₂O emissions occurring during the transfer of water masses from the subroot water pool of the watershed to the outlet of the river drainage network, including their passage through riparian wetlands. The model was applied to three river basins of increasing size located in the sedimentary geological area of the Paris basin (France) and validated by its capability to predict river N₂O concentrations and fluxes across the river–atmosphere interface. At the scale of the Seine watershed, indirect emissions, i.e. emissions linked to agricultural practices but occurring elsewhere than directly at the field plot, are estimated to represent approximately 20% of the direct emissions from the watershed soils, in good agreement with previous estimates based on empirical accounting approaches. Denitrification in riparian zones is responsible for the largest share of these indirect emissions. The model results are very sensitive to the value of the ratio of N₂O versus (N₂ + N₂O), in the final products of denitrification in rivers and wetlands. By calibration on river N₂O concentrations, a value of 0.015 ± 0.05 is proposed for this ratio, in agreement with recent studies. This represents the main uncertainty factor of the model. In basins with conditions prone to increasing the value of this ratio, higher proportions of indirect N₂O emissions might possibly be observed.

     

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.     

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.     

Denitrification and N<Subscript>2</Subscript>O emission from forested and cultivated alluvial clay soil

Restored forested wetlands reduce N loads in surface discharge through plant uptake and denitrification. While removal of reactive N reduces impact on receiving waters, it is unclear whether enhanced denitrification also enhances emissions of the greenhouse gas N₂O, thus compromising the water-quality benefits of restoration. This study compares denitrification rates and N₂O:N₂ emission ratios from Sharkey clay soil in a mature bottomland forest to those from an adjacent cultivated site in the Lower Mississippi Alluvial Valley. Potential denitrification of forested soil was 2.4 times of cultivated soil. Using intact soil cores, denitrification rates of forested soil were 5.2, 6.6 and 2.0 times those of cultivated soil at 70, 85 and 100% water-filled pore space (WFPS), respectively. When NO₃ was added, N₂O emissions from forested soil were 2.2 times those of cultivated soil at 70% WFPS. At 85 and 100% WFPS, N₂O emissions were not significantly different despite much greater denitrification rates in the forested soil because N₂O:N₂ emission ratios declined more rapidly in forested soil as WFPS increased. These findings suggest that restoration of forested wetlands to reduce NO₃ in surface discharge will not contribute significantly to the atmospheric burden of N₂O.     

Simulation of Effects of Soils,Climate and Management on N<Subscript>2</Subscript>O Emission from Grasslands

Nitrous oxide (N₂O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate, crops and management practices. The N₂O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems. An algorithm for N₂O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources for N₂O production, and the N₂O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N₂O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management. The model simulated the general time course of N₂O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N fertilisation on N₂O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha⁻¹. The simulated N₂O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated emissions increased with increasing soil clay contents. N₂O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing N₂O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils.     

Differences in field‐scale N2O flux linked to crop residue removal under two tillage systems in cold climates

Residue removal for biofuel production may have unintended consequences for N₂O emissions from soils, and it is not clear how N₂O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field‐scale N₂O flux over 5 years (2005–2007, 2010–2011) from an annual crop rotation to evaluate how N₂O emissions are influenced by no‐till (NT) compared to conventional tillage (CV), and how crop residue removal (R?) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all 5 years indicated no differences in N₂O flux between tillage practices at the onset of the growing season, but CT had 1.4–6.3 times higher N₂O flux than NT overwinter. Nitrous oxide emissions were higher due to R? compared to R+, but the effect was more marked under CT than NT and overwinter than during spring. Our results thus challenge the assumption based on IPCC methodology that crop residue removal will result in reduced N₂O emissions. The potential for higher N₂O emission with residue removal implies that the benefit of utilizing biomass as biofuels to mitigate greenhouse gas emission may be overestimated. Interestingly, prior to an overwinter thaw event, dissolved organic C (DOC) was negatively correlated to peak N₂O flux (r = ?0.93). This suggests that lower N₂O emissions with R+ vs. R? may reflect more complete stepwise denitrification to N₂ during winter and possibly relate to the heterotrophic microbial capacity for processing crop residue into more soluble C compounds and a shift in the preferential C source utilized by the microbial community overwinter.     

Soil fauna diversity increases CO2 but suppresses N2O emissions from soil

Soil faunal activity can be a major control of greenhouse gas (GHG) emissions from soil. Effects of single faunal species, genera or families have been investigated, but it is unknown how soil fauna diversity may influence emissions of both carbon dioxide (CO₂, end product of decomposition of organic matter) and nitrous oxide (N₂O, an intermediate product of N transformation processes, in particular denitrification). Here, we studied how CO₂ and N₂O emissions are affected by species and species mixtures of up to eight species of detritivorous/fungivorous soil fauna from four different taxonomic groups (earthworms, potworms, mites, springtails) using a microcosm set‐up. We found that higher species richness and increased functional dissimilarity of species mixtures led to increased faunal‐induced CO₂ emission (up to 10%), but decreased N₂O emission (up to 62%). Large ecosystem engineers such as earthworms were key drivers of both CO₂ and N₂O emissions. Interestingly, increased biodiversity of other soil fauna in the presence of earthworms decreased faunal‐induced N₂O emission despite enhanced C cycling. We conclude that higher soil fauna functional diversity enhanced the intensity of belowground processes, leading to more complete litter decomposition and increased CO₂ emission, but concurrently also resulting in more complete denitrification and reduced N₂O emission. Our results suggest that increased soil fauna species diversity has the potential to mitigate emissions of N₂O from soil ecosystems. Given the loss of soil biodiversity in managed soils, our findings call for adoption of management practices that enhance soil biodiversity and stimulate a functionally diverse faunal community to reduce N₂O emissions from managed soils.     

Characterization of N<Subscript>2</Subscript>O emissions and associated microbial communities from the ant mounds in soils of a humid tropical rainforest

Tropical rainforest soils harbor a considerable diversity of soil fauna that contributes to emissions of N₂O. Despite their ecological dominance, there is limited information available about the contribution of epigeal ant mounds to N₂O emissions in these tropical soils. This study aimed to determine whether ant mounds contribute to local soil N emissions in the tropical humid rainforest. N₂O emission was determined in vitro from individual live ants, ant-processed mound soils, and surrounding reference soils for two trophically distinct and abundant ant species: the leaf-cutting Atta mexicana and omnivorous Solenopsis geminata. The abundance of total bacteria, nitrifiers (AOA and AOB), and denitrifiers (nirK, nirS, and nosZ) was estimated in these soils using quantitative PCR, and their respective mineral N contents determined. There was negligible N₂O emission detected from live ant individuals. However, the mound soils of both species emitted significantly greater (3-fold) amount of N₂O than their respective surrounding reference soils. This emission increased significantly up to 6-fold in the presence of acetylene, indicating that, in addition to N₂O, dinitrogen (N₂) is also produced from these mound soils at an equivalent rate (N₂O/N₂?=?0.57). Functional gene abundance (nitrifiers and denitrifiers) and mineral N pools (ammonium and nitrate) were significantly greater in mound soils than in their respective reference soils. Furthermore, in the light of the measured parameters and their correlation trends, nitrification and denitrification appeared to represent the major N₂O-producing microbial processes in ant mound soils. The ant mounds were estimated to contribute from 0.1 to 3.7% of the total N₂O emissions of tropical rainforest soils.     

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.     

Simulating soil N2O emissions and heterotrophic CO2 respiration in arable systems using FASSET and MoBiLE-DNDC

Modelling of soil emissions of nitrous oxide (N₂O) and carbon dioxide (CO₂) is complicated by complex interactions between processes and factors influencing their production, consumption and transport. In this study N₂O emissions and heterotrophic CO₂ respiration were simulated from soils under winter wheat grown in three different organic and one inorganic fertilizer-based cropping system using two different models, i.e., MoBiLE-DNDC and FASSET. The two models were generally capable of simulating most seasonal trends of measured soil heterotrophic CO₂ respiration and N₂O emissions. Annual soil heterotrophic CO₂ respiration was underestimated by both models in all systems (about 10?C30% by FASSET and 10?C40% by MoBiLE-DNDC). Both models overestimated annual N₂O emissions in all systems (about 10?C580% by FASSET and 20?C50% by MoBiLE-DNDC). In addition, both models had some problems in simulating soil mineral nitrogen, which seemed to originate from deficiencies in simulating degradation of soil organic matter, incorporated residues of catch crops and organic fertilizers. To improve the performance of the models, organic matter decomposition parameters need to be revised.     

The contribution of nitrogen transformation processes to total N2O emissions from soils used for intensive vegetable cultivation

The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N₂O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N₂O production from the intensive vegetable fields and to identify the affecting factors, a ¹⁵N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with ¹⁵NH₄NO₃ or NH ₄ ¹⁵ NO₃. The average N₂O emission rate was 24.2 ng N?kg^?1 h^?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg^?1 h^?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N₂O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N₂O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N₂O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N₂O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N₂O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO ₃ ^- or NH ₄ ⁺ ) could also significantly affect the sources and rates of N₂O emission.     

A state-of-the-art review on nitrous oxide control from waste treatment and industrial sources

This review aims at holistically analyzing the environmental problems associated with nitrous oxide (N₂O) emissions by evaluating the most important sources of N₂O and its environmental impacts. Emissions from wastewater treatment processes and the industrial production of nitric and adipic acid represent nowadays the most important anthropogenic point sources of N₂O. Therefore, state-of-the-art strategies to mitigate the generation and release to the atmosphere of this greenhouse and O₃-depleting gas in the waste treatment and industrial sectors are also reviewed. An updated review of the end-of-the-pipe technologies for N₂O abatement, both in the waste treatment and industrial sectors, is herein presented and critically discussed for the first time. Despite the consistent efforts recently conducted in the development of cost-efficient and eco-friendly N₂O abatement technologies, physical/chemical technologies still constitute the most popular treatments for the control of industrial N₂O emissions at commercial scale. The recent advances achieved on biological N₂O abatement based on heterotrophic denitrification have opened new opportunities for the development of eco-friendly alternatives for the treatment of N₂O emissions. Finally, the main limitations and challenges faced by these novel N₂O abatement biotechnologies are identified in order to pave the way for market implementation.