N2O Production Rates Research Articles

Quantifying Nitrous Oxide Cycling Regimes in the Eastern Tropical North Pacific Ocean With Isotopomer Analysis

AbstractNitrous oxide (N2O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N2O cycling in an ODZ, we analyzed N2O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N2O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 ± 20.5 nM at the coast. Above the σθ = 25.0 kg/m3 isopycnal, Keeling plot analysis revealed two sources to the near‐surface N2O maximum, with different δ15N2Oα and δ15N2Oβ values, but each with a site preference (SP) of 6‰–8‰. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%–26% nitrification (bacterial or archeal), and 74%–83% denitrification (or nitrifier‐denitrification). Below the σθ = 25.0 kg/m3 isopycnal, box model analysis indicated that the observed 46‰–50‰ SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N2O production by denitrification, and may indicate instead a transient net consumption of N2O. Furthermore, time‐dependent model results indicated that while δ15N2Oα and δ18O‐N2O reflect both N2O production and consumption in the anoxic core of the ODZ, δ15N2Oβ predominantly reflects N2O sources. Finally, we infer that the high (N2O) observed at some stations derive from a set of conditions supporting high rates of N2O production that have not been previously encountered in this region.

Assessment of a cylindrical porous radiant burner with internal combustion regime for sustainable energy: Numerical analysis of the radiant efficiency and NO production

A numerical assessment is conducted on a premixed axisymmetric two-dimensional porous radiant burner. Equations of continuity, momentum, energy, turbulence, radiation, progress variable, and mixture fraction are solved using a structured grid with a high degree of accuracy. A good agreement between the results and the available data are observed. The flame cannot pass through the thin porous medium and increases the solid phase temperature, consequently, leads to an increase in radiant efficiency. Temperature contours represent very low temperature for burner exhaust gases, leading to decrease in the energy losses. The effects of three parameters of input power, equivalence ratio, and porosity on radiant efficiency and NO production rate are investigated. Due to the number of parameters studied, the validated numerical solutions can prevent spending extra time and money to produce porous mediums with different properties, and therefore, the operating conditions can be analyzed from environmental and energy saving points of view. A radiant efficiency of above 50% is achieved. In addition, it is concluded that there is an optimum equivalence ratio lower than unity, about 0.83 at P,εp=4.938kW,0.65, which maximizes the radiant efficiency. Furthermore, the results indicate that the NO production rate increases by increasing the input power/equivalence ratio.

Effects of hydraulic retention time on nitrous oxide production rates during nitrification in a laboratory-scale biological aerated filter reactor

This study investigated the effects of hydraulic retention time (HRT) on nitrous oxide (N2O) production during nitrification in a laboratory-scale biological aerated filter (BAF) reactor. Furthermore, its underlying mechanisms were explored by microelectrode technology and PCR-denaturing gradient gel electrophoresis (PCR-DGGE). The results showed that the N2O production rate was at its maximum at HRT of 4 h and at its minimum at HRT of 6 h. And there was only a little increase in N2O production rate at HRT of 8 h compared to HRT of 6 h. The average N2O production rates were 0.2258, 0.0872, and 0.1015 mg/h for HRT of 4, 6, and 8 h, respectively. Results of analysis of the internal biofilm microenvironment were consistent with the results of the N2O production rate. Production of N2O decreased sharply when HRT was increased from 4 to 6 h, whereas a slight increase in N2O production was observed when HRT was further increased from 6 to 8 h. Determination of DO, ORP, and nitrogen by microelectrode also demonstrated that the internal microenvironment of the biofilm was responsible for N2O generation. DNA analysis showed that Proteobacteria and Bacteroidetes might be responsible for N2O emission in the biofilm. These results indicate that HRT significantly affects the microbial community structures between the seed sludge and BAF biofilm, thereby affecting N2O production ability and rates, and hence, should be taken into consideration during BAF application.

Influence of rye cover cropping on denitrification potential and year-round field N2O emissions

Cover cropping is beneficial for reducing soil erosion and nutrient losses, but there are conflicting reports on how cover cropping affects emissions of nitrous oxide (N2O), a potent greenhouse gas. In this study, we measured N2O fluxes over a full year in Illinois corn plots with and without rye cover crop. We compared these year-round measurements to N2O emissions predicted by the Intergovernmental Panel on Climate Change (IPCC) Tier 1 equation and the Denitrification-Decomposition (DNDC) model. In addition, we measured potential denitrification and N2O production rates. The field measurements showed typical N2O peaks shortly after fertilizer application, as well as a significant late-winter peak. Cover cropping significantly reduced all peak N2O fluxes, with decreases ranging from 39 to 95%. Neither model was able to accurately predict annual N2O fluxes or the decrease in N2O emissions from cover-cropped fields. In contrast to field measurements, lab assays found that cover cropping significantly increased potential denitrification by 90–127% and potential N2O production by 54–106%. The rye cover-cropped plots had lower soil nitrate and higher soil carbon. When limiting nitrate and excess carbon were provided in lab assays, the proportion of N2O resulting from denitrification decreased. These results suggest that the discrepancy between the observed decrease in field N2O emissions and the increase in denitrification potential may be due to the difference in available nutrients between the field and laboratory measurements. Overall, these results suggest the importance of late-winter peaks in N2O emissions and the potential of rye cover cropping to reduce N2O emissions from agricultural fields.

Microbial N2O consumption in and above marine N2O production hotspots

The ocean is a net source of N2O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N2O via microbial N2O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N2O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O2 tolerance, and community composition of N2O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N2O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N2O cycling. Microbes from the oxic layer consume N2O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N2O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O2 and N2O gradients right above the ODZ is a previously ignored potential gatekeeper of N2O and should be accounted for in the marine N2O budget.

Discovery and quantification of anaerobic nitrogen metabolisms among oxygenated tropical Cuban stony corals

Coral reef health depends on an intricate relationship among the coral animal, photosynthetic algae, and a complex microbial community. The holobiont can impact the nutrient balance of their hosts amid an otherwise oligotrophic environment, including by cycling physiologically important nitrogen compounds. Here we use 15N-tracer experiments to produce the first simultaneous measurements of ammonium oxidation, nitrate reduction, and nitrous oxide (N2O) production among five iconic species of reef-building corals (Acropora palmata, Diploria labyrinthiformis, Orbicella faveolata, Porites astreoides, and Porites porites) in the highly protected Jardines de la Reina reefs of Cuba. Nitrate reduction is present in most species, but ammonium oxidation is low potentially due to photoinhibition and assimilatory competition. Coral-associated rates of N2O production indicate a widespread potential for denitrification, especially among D. labyrinthiformis, at rates of ~1 nmol cm−2 d−1. In contrast, A. palmata displays minimal active nitrogen metabolism. Enhanced rates of nitrate reduction and N2O production are observed coincident with dark net respiration periods. Genomes of bacterial cultures isolated from multiple coral species confirm that microorganisms with the ability to respire nitrate anaerobically to either dinitrogen gas or ammonium exist within the holobiont. This confirmation of anaerobic nitrogen metabolisms by coral-associated microorganisms sheds new light on coral and reef productivity.

Temporal evolution of cardiac mitochondrial dysfunction in a type 1 diabetes model. Mitochondrial complex I impairment, and H2O2 and NO productions as early subcellular events

The aim of this work was to study the early events that occur in heart mitochondria and to analyse the temporal evolution of cardiac mitochondrial dysfunction in a type 1 diabetes model. Male Wistar rats were injected with Streptozotocin (STZ, single dose, 60 mg × kg−1, i.p.) and hyperglycemic state was confirmed 72 h later. The animals were sacrificed 10 or 14 days after STZ-injection. Heart mitochondrial state 3 O2 consumption sustained by malate-glutamate (21%) or by succinate (16%), and complexes I-III (27%), II-III (24%) and IV (22%) activities were lower in STZ group, when animals were sacrificed at day 14, i.e. ~11 days of hyperglycemia. In contrast, after 10 days of STZ-injection (~7 days of hyperglycemia), only the state 3 O2 consumption sustained by malate-glutamate (23%) and its corresponding respiratory control (30%) were lower in diabetic rats, in accordance with complex I-III activity reduction (17%). Therefore, this time (~7 days of hyperglycemia) has been considered as an “early stage” of cardiac mitochondrial dysfunction. At this point, mitochondrial production rates of H2O2 (117%), NO (30%) and ONOO− (~225%), and mtNOS expression (29%) were higher; and mitochondrial SOD activity (15%) and [GSH + GSSG] (28%) were lower in diabetic rats. Linear correlations between the modified mitochondrial parameters and glycemias were observed. PGC-1α expression was similar between groups, suggesting that mitochondrial biogenesis was not triggered in this initial phase of mitochondrial dysfunction. Consequently, complex I, H2O2 and NO could be considered early subcellular signals of cardiac mitochondrial dysfunction, with NO and H2O2 being located upstream de novo synthesis of mitochondria.

Coupling effects of hydrogen transport and reactivity on NO production in laminar nonpremixed methane/air flames

A computational simulation of counterflow nonpremixed flame was performed to study the effects of hydrogen enrichment on NO chemistry on the basis of the methodology of fabricating hydrogen enrichment. Normal hydrogen and the fictitious hydrogen enrichment with weakened transport properties were both introduced into flame modeling and the comparative analysis was implemented to explore the coupling effects of the physical transport and reactivity of hydrogen enrichment on NO production. The study focused on the variations in NO peak concentration and the total production rate of NO versus hydrogen enrichment rate. The contributions and sensitivities of key reactions to NO production were estimated to determine the influencing routes of hydrogen enrichment on NO chemistry. The flame structures were compared in terms of the convection, conduction, diffusion and reaction in flame energy and hydrogen species conservations. The coupling interaction between hydrogen diffusion and the reaction promotion caused by hydrogen diffusion is the dominant kinetic incentive of hydrogen enrichment on NO production in nonpremixed methane/air flames.

Continental margin sediments underlying the NE Pacific oxygen minimum zone are a source of nitrous oxide to the water column

AbstractContinental margin sediments are important sites of marine nitrogen cycling and potential contributors to atmospheric N2O emissions. We employed trace‐level N2O microsensors to measure vertical N2O profiles at submillimeter resolutions in intact cores from outer continental margin sediments underlying the NE Pacific oxygen minimum zone. We used mathematical modeling to estimate depth‐dependent rates of N2O production and fluxes to the overlying water along a transect of diminishing bottom water oxygen concentrations. Net sediment efflux was observed at all sites on the outer continental margin, with a mean value of 524 nmol m−2 d−1. N2O efflux increased with decreased oxygen penetration depth in sediments. Enhanced N2O production and efflux were obtained when outer continental shelf sediments were experimentally exposed to lower bottom‐water O2 concentrations, to simulate upwelling conditions. Our results underline the need for further investigation of the drivers of N2O production in continental margin sediments, and the relative importance of these environments to the global N2O budget.

Effects of Temperature and Substrate Concentration on N2O Release of ANAMMOX Process

The greenhouse gas N2O is released during the biological nitrogen removal process. ANAMMOX (anaerobic ammonium oxidation) is regarded as a promising nitrogen removal process for treating municipal wastewater, and the N2O emission patterns and mechanisms need further investigation. In this study, batch tests were performed to study the release of N2O at different temperatures and substrate concentrations, and the microbial mechanisms of N2O emission were discussed. The results showed that the increase of the influent substrate concentration of the ANAMMOX process promoted the release of N2O. At 35℃, when the influent nitrite increased from 40 mg·L-1 to 60 mg·L-1, 120 mg·L-1, the maximum accumulated concentration of N2O increased from 0.5 mg·L-1 to 1.5 mg·L-1 and 2.4 mg·L-1, accounting for 0.85%, 1.43%, and 1.11% of the total nitrogen removal, respectively. Lowering temperature had an obvious inhibitory effect on ANAMMOX activity. The specific ANAMMOX activity at 15℃ was only 6% of that at 30℃. Furthermore, the intracellular ATP concentration was reduced. At 15℃, the intracellular ATP concentration was 4% of that at 30℃. The decrease in temperature led to a decrease in the release of N2O in the ANAMMOX process. When the temperature decreased, the denitrification rate would decrease, leading to a lower N2O production rate and lower N2O accumulation. 16S rRNA amplicon sequencing showed that ANAMMOX bacteria Candidatus Brocadia and Ca. Jettenia were enriched, accounting for 6.9%-13.8% and 1.4%-2.6% of microbial community, respectively. Abundant heterotrophic bacteria were also found in the microbial community. The accumulation of N2O in the ANAMMOX process was mainly attributed to denitrifying bacteria producing and consuming N2O. This study provides support for controlling N2O emission during the ANAMMOX process for treating municipal wastewater.

Role of N2O and equivalence ratio on NOx formation of methane/nitrous oxide premixed flames

Nitrous oxide (N2O) is often used as an oxidizer in rocket technology with plenty of potential advantages. However, the N2O/hydrocarbon combustion results in the staggering increase of NOx emission. To scrutinize the role of N2O and equivalence ratio on NO formation characteristics of N2O/CH4 premixed flame, mixtures of N2O/CH4 and N2/O2/CH4 were investigated by employing one-dimensional opposed-jet model with detailed reaction mechanism. The flame structures, NO reaction pathways, and NO formation routes were discussed. In the heat release rate analysis, the heat release is dominated by N2O route, resulting in high flame temperature in N2O/CH4 premixed flame. Regarding the mechanism of NO formation, NO is mainly produced from N2 through the thermal routes and from HNO through the HNO-intermediate routes for the N2/O2/CH4 flame but mainly comes from N2O through the N2O and HNO-intermediate routes in the N2O/CH4 flame. The thermal routes alter to the primary chemical mechanism of NO consumption. The NO production in fuel-lean N2O/CH4 flames is enhanced through the N2O route due to the increase of the mole fraction of O atom. The thermal route becomes forward reactions to produce NO for the extreme fuel-rich N2O/CH4 flame. Regarding the reaction pathways of the prompt route, it can be approximately divided into three categories: (1) N2 → CN → NCN, NO (in terminal) → N2; (2) CN → NCN, NO (in terminal) → NO, CN; (3) N2 → CN → NCN, NO (in terminal) → N → N2. In NO production rate analysis, the main NO formation comes from the N2O and HNO-intermediate routes, while NO consumption associates with the thermal and NNH-intermediate routes. In addition, the prompt-NO formation is dominated by species of N2O instead of CH radical in the extreme fuel-rich N2O/CH4 flames.

Nitrite induced transcription of p450nor during denitrification by Fusarium oxysporum correlates with the production of N2O with a high 15N site preference

The greenhouse gas nitrous oxide (N2O) is produced in soil as a consequence of complex co-occurring processes conducted by diverse microbial species, including fungi. The fungal p450nor gene encodes a nitric oxide reductase associated with fungal denitrification. We thus hypothesized that p450nor gene expression is a marker for ongoing fungal denitrification. Specific PCR primers and quantitative PCR (qPCR) assays were developed targeting p450nor genes and transcripts. The novel PCR primers successfully amplified p450nor from pure cultures, and were used in an mRNA targeted qPCR to quantify p450nor gene transcription (i.e., gene expression) during denitrification activity in cultures of the fungal model denitrifier Fusarium oxysporum. Gene expression was induced by high (5 mM) and low (0.25 mM) nitrite concentrations. Nitrite stimulated N2O production rates by F. oxysporum, which correlated well with an up to 70-fold increase in p450nor gene expression during the first 12–24 h of anoxic incubation. The relative p450nor gene peak expression and peak N2O production rates declined 20- and 2-fold on average, respectively, towards the later phase of incubation (48–120 h). The 15N site preference of N2O (SP(N2O)) was high for F. oxysporum and independent of reaction progress, confirming the fungal origin of N2O produced. In conclusion, the developed fungal p450nor gene expression assay together with the analysis of SP(N2O) values provide a basis to improve current tools for the identification of fungal denitrification and/or N2O production in natural systems like soils.

Nitrous oxide dynamics during denitrification along a hydrological gradient of subtropical grasslands

AbstractNitrous oxide (N2O) dynamics during denitrification, including N2O production and reduction, particularly as related to soil depth, are poorly understood. The objective of this study was to investigate the rates of N2O production and reduction processes at various soil depths along a hydrological gradient in grazed subtropical grasslands. A batch incubation study was conducted on soils collected along a hydrological gradient representing isolated wetland (Center), transient edge (Edge) and pasture upland (Upland) in south‐central Florida. Significantly different N2O production and reduction rates between hydrological zones were observed for surface soils (0–10 cm) under ambient conditions, with average N2O production rates of 0.368, 0.178 and 0.003 N2O‐N kg−1 dry soil h−1 for Center, Edge and Upland, respectively, and average N2O reduction rates of 0.063, 0.132 and 0.002 N2O‐N kg−1 dry soil h−1. Nitrous oxide production and reduction in subsurface soils maintained low rates and showed small variations between depths and hydrological zones. Our results suggest that N2O dynamics were affected by depth, mainly through labile organic carbon (C) and microbial biomass C, being influenced by hydrological zone primarily through soil NO3‐ content. The spatial distribution of N2O fluxes from denitrification along the hydrological gradient is likely attributed to the differences in N2O production and reduction in surface soils.

Dissolved organic carbon enhances both soil N2O production and uptake

Nitrous oxide (N2O) is a potent greenhouse gas with high global warming potential. Soil acts as sources and sinks of N2O. The mechanism of soil N2O uptake is important for both the net emission of N2O and the N2O concentration in the atmosphere. The use of organic matter as an amendment is an effective management practice for soil fertility worldwide, whereas, its effects on soil N2O uptake remain unclear. To understand the responses of N2O uptake with organic carbon incorporation. A laboratory incubation using 15N2O dilution tracing technology with and without acetylene (C2H2) was conducted. The result showed that gross rate of soil N2O production in Glucose treatment (3.33 g kg−1 glucose incorporated with soil) with and without C2H2 were 497.9 and 639.2 μg N kg−1 soil h−1, which were significantly higher than that in CK (no carbon incorporation) and Straw (3.33 g kg−1 straw incorporated with soil) treatments (25.3 and 29.8 μg N kg−1 soil h−1 and 42.0 and 52.5 μg N kg−1 soil h−1, respectively). The gross rates of N2O uptake was significantly increased by Glucose with 221.2 μg N kg−1 soil h−1, whereas, in CK and Straw treatments, they were 21.6 and 28.3 μg N kg−1 soil h−1. However, the gross rates of N2O uptake were decreased to 2.2, 3.2 and 6.2 μg N kg−1 soil h−1 by C2H2 for CK, Straw and Glucose treatments, which indicated that denitrification could be a key pathway of soil N2O uptake. We also found that soil dissolved organic carbon (DOC) content was an important factor that positively correlated with the gross rates of N2O production and uptake. These results indicate that soil DOC enhances both soil N2O production and uptake, and denitrification is an important pathway of soil N2O uptake.

Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia

AbstractNitrous oxide (N2O) is a strong greenhouse gas and an ozone depleting agent. In marine environments, N2O is produced biologically via ammonium oxidation, nitrite, and nitrate reduction. The relative importance of these principle production pathways is strongly influenced by oxygen availability. We conducted 15N tracer experiments of N2O production in parallel with measurements of N2O concentration and natural abundance isotopes/isotopomers in Saanich Inlet, a seasonally anoxic fjord, to investigate how temporal and vertical oxygen gradients regulate N2O production pathways and rates. In April, June, and August 2018, the depth of the oxic‐anoxic interface (dissolved oxygen = 2.5 μmol L−1 isoline) progressively deepened from 110 to 160 m. Within the oxygenated and suboxic water column, N2O supersaturation coincided with peak ammonium oxidation activity. Conditions in the anoxic deep water were potentially favorable to N2O production from nitrate and nitrite reduction, but N2O undersaturation was observed indicating that N2O consumption exceeded rates of production. In October, tidal mixing introduced oxygenated water from outside the inlet, displacing the suboxic and anoxic deep water. This oxygenation event stimulated N2O production from ammonium oxidation and increased water column N2O supersaturation while inhibiting nitrate and nitrite reduction to N2O. Results from 15N tracer incubation experiments and natural abundance isotopomer measurements both implicated ammonium oxidation as the dominant N2O production pathway in Saanich Inlet, fueled by high ammonium fluxes (0.6–3.5 nmol m−2 s−1) from the anoxic depths. Partial denitrification contributed little to water column N2O production because of low availability of nitrate and nitrite.

Experimental shifts of hydrologic residence time in a sandy urban stream sediment–water interface alter nitrate removal and nitrous oxide fluxes

The sediment–water interfaces (SWI) of streams serve as important biogeochemical hotspots in watersheds and contribute to whole-catchment reactive nitrogen budgets and water-quality conditions. Recently, the SWI has been identified as an important source of nitrous oxide (N2O) produced in streams, with SWI residence time among the principal controls on its production. Here, we conducted a series of controlled manipulations of SWI exchange in an urban stream that has high dissolved N2O concentrations and where we concurrently evaluated less-mobile porosity dynamics. Our experiments took place within isolated portions of two sediment types: a coarse sandy stream bed resulting from excess road-sand application in the watershed, and a coarse sand mixed with clay and organic particles. In these manipulation experiments we systematically varied SWI vertical-flux rates and residence times to evaluate their effect on the fate of nitrate and production rates of N2O. Our experiments demonstrate that the fate and transport of nitrate and N2O production are influenced by hydrologic flux rates through SWI sediments and associated residence times. Specifically, we show that manipulations of hydrologic flux systematically shifted the depth of the bulk oxic–anoxic interface in the sediments, and that nitrate removal increased with residence time. Our results also support the emerging hypothesis of a ‘Goldilocks’ timescale for the production of nitrous oxide, when transport and reaction timescales favor incomplete denitrification. Areal N2O production rates were up to threefold higher during an intermediate residence-time experiment, compared to shorter or longer residence times. In our companion study we documented that the studied sediments were dominated by a long-residence-time less-mobile porosity domain, which could explain why we observed N2O production even in bulk-oxic sediments. Overall, we have experimentally demonstrated that changes to SWI hydrologic residence times and SWI substrate associated with urbanization can change the biogeochemical function of the river corridor.

Evaluating the roles of coexistence of sludge flocs on nitrogen removal and nitrous oxide production in a granule-based autotrophic nitrogen removal system

Certain levels of sludge flocs would always coexist in granule-based reactors due to the biomass detachment from granules. Such inevitable coexistence could affect both total nitrogen (TN) removal and nitrous oxide (N2O) production in autotrophic nitrogen removal systems. This work utilized a mathematical approach to systematically study the influence of the coexisting sludge flocs on TN removal and N2O production in a granular nitritation-anaerobic ammonium oxidation (Anammox) process for the first time, based on a 2-pathway N2O production model concept. The modelling results reveal that the highest TN removal efficiency decreases from ca. 87–88% to ca. 41–49% as the fraction of sludge flocs in the system increases from 10% to 40%, while the N2O production rate gradually increases with such increase. Meanwhile, both bulk dissolved oxygen (DO, 0.05–0.3 mg/L) and the size of granule (200–400 μm) could also influence the TN removal efficiency and N2O production. As the fraction of sludge flocs increases from 10% to 40%, the contribution of granular biomass to total N2O production is reduced due to increase of N2O-producing ammonia-oxidizing bacteria (AOB) in the sludge flocs, and the increase of granule size could intensify such decrease. In addition, the hydroxylamine oxidation pathway dominates the nitrifier denitrification pathway in both granules and sludge flocs under various testing conditions, whereas the increasing contribution of the latter would occur at a certain DO range, higher fraction of sludge flocs and smaller granule size. These results disclose an important influence of the coexisting sludge flocs on the performance of granular nitritation-Anammox systems.

Gross N2O Production Process, Not Consumption, Determines the Temperature Sensitivity of Net N2O Emission in Arable Soil Subject to Different Long-Term Fertilization Practices.

Chronic amendment of agricultural soil with synthetic nitrogen fertilization and/or livestock manure has been demonstrated to enhance the feedback intensity of net N2O emission to temperature variation (i.e., temperature sensitivity, TS). Yet few studies have explored the relevance of changes in underlying gross N2O production and consumption processes toward explaining this phenomenon, in particular for the latter. Furthermore, the microbe-based mechanisms associated with the variation of N2O consumption process remain largely unexplored. To address this knowledge gap, a temperature- (15, 25, and 35°C) and moisture-controlled (50% water holding capacity) microcosm incubation experiment was established using an arable soil subject to long-term addition of synthetic fertilizer (NPK), a mixture of synthetic fertilizer with livestock manure (MNPK), or with no fertilizer treatment (CT). Over the incubation time period, the C2H2 inhibition method was adopted to monitor reaction rates of gross N2O production and consumption; the population sizes and community structures of nosZI- and nosZII-N2O reducers were analyzed using quantitative PCR (Q-PCR) and terminal restriction fragment length polymorphism (T-RFLP). The results indicated that only NPK significantly increased the TS of net N2O emission, and gross N2O consumption process consistently occurred under all treatment combinations (temperature and fertilization) at each sampling time point. The responses of gross N2O production and consumption processes to temperature elevation exhibited fertilization- and sampling time-dependent pattern, and the higher net N2O production TS in the NPK treatment was underlain by its higher TS of gross production process and insensitivity of gross consumption process to temperature. The size and structure of nosZII-N2O reducers, as well as the community structure of nosZI-N2O reducers, were positively correlated with variation of gross N2O production and consumption rates across all fertilization regimes. NosZII-N2O reducer abundance was less responsive to temperature change, and its community structure less susceptible to fertilization, as compared with nosZI-N2O reducers. Overall, our results demonstrate that the TS of the gross N2O production process, not gross consumption, is the key step regulating the TS of net N2O production, and both nosZI- and nosZII-N2O clades are likely active N2O reducers in the tested soil.

Duration and frequency of drainage and flooding events interactively affect soil biogeochemistry and N flux in subtropical peat soils

With the demand for restoration and future prediction of climate change effects, subtropical peatlands are expected to be subjected to hydrologic regimes with variable duration and frequency of drained and flooded conditions, but knowledge of their interactive effects on soil biogeochemistry and emission of greenhouse gases including nitrous oxide (N2O) is largely limited. The objective of this study was to investigate how the duration and frequency of drainage and flooding events interactively influence soil biogeochemical properties and denitrification and related net N2O production rates following rewetting. Surface soils are susceptible to different hydrologic regimes. Significantly higher pH, extractable organic carbon (ext. OC), ammonium (NH4+-N), denitrification enzyme activity (DEA), but lower nitrate (NO3−-N), microbial biomass C and N were observed when the peat soils were under flooded conditions compared to drained conditions. Two-week and four-week drainage or flooding duration did not result in statistically significant differences in soil biogeochemical properties. A 24-week prolonged drainage led to an accumulation of NO3−-N and a significantly lower pH. Soil microbial biomass and fungal:bacterial abundance likely increased with the frequency of drainage-flooding cycles. Significant differences in denitrification and net N2O production rates following reflooding were mainly found in the surface soils. Structural equation modeling indicated that hydroperiod and water-filled pore space (WFPS) prior to reflooding is likely to control denitrification and net N2O production through its regulation of NO3−-N and activity of microorganisms involved in denitrification while higher drainage-flooding frequency decreases the availability of organic C and NO3−-N for denitrification. Our results also suggest high NO3−-N and low pH within peat soils caused by prolonged drainage likely leads to a significant N2O emission pulse following reflooding. For peat soils subjected to frequent drainage-flooding cycles, N2O emission pulses following reflooding would decrease with time, attributing to the loss of substrates for denitrification.

C:N ratio is not a reliable predictor of N2O production in acidic soils after a 30-day artificial manipulation

To test the effect of C:N ratio on soil N2O production, N2O production rates and pathways associated with nitrification (AOA-amoA, AOB-amoA, fungal ITS rDNA, bacterial 16S rRNA), and denitrification-related (nirK, nirS, nosZ) genes were investigated in subtropical forest (SF) and cropland (SC) soil in China in a 30-day C:N ratio manipulation. In addition, 24-hour C:N ratio manipulation, including the addition of acetic acid, were conducted to verify the results observed in the 30-day experiment. After 30 days of manipulation, the N2O production rates (N2Ot) increased from 2.46 in CN23 treatment to 4.71 μg N kg−1 day−1 in CN 10 treatment in SF, while it decreased from 4.17 in CN23 treatment to 3.83 μg N kg−1 day−1 in CN10 treatment in SC. The results in 24-hour experiment were consistent with those in 30-day experiment, and the addition of acetic acid increased N2Ot in SC, but not in SF. Soil C:N ratios and inorganic N (NH4+ + NO3−) concentrations influenced the contribution of denitrification to N2O production and the N2O production rate via denitrification. Soil AOA played a dominant role in autotrophic nitrification-derived N2O production, resulting in a high contribution of autotrophic nitrification under low pH. Therefore, pH instead of C:N ratio, is a key parameter for evaluating autotrophic nitrification-derived N2O via AOA and AOB. Soil C:N ratio was significantly and positively correlated with the contribution of heterotrophic nitrification to N2O production, while there was no significant correlation with the N2O production rate via heterotrophic nitrification. This is mainly because the responsible heterotrophs (i.e., fungi and bacteria) were negatively and positively correlated with C:N ratio in SF and SC, respectively. Therefore, C:N ratio is not a strong predictor of soil N2O production, the initial C or N content and composition of functional genes could provide key information in acidic soils after a 30-day artificial C:N ratio manipulation.