N2O Production Pathways Research Articles

Ammonia-oxidizing bacteria-driven autotrophic nitrification dominated nitrous oxide production in calcareous soil under long term plastic film mulching

Fertilized cropland soils are a major source of anthropogenic N2O emissions in terrestrial ecosystems. However, the main N2O production pathways and microbes underlying N2O production in calcareous cropland soil under long-term plastic film mulching and nitrogen (N) management have not yet been clarified. Here, we used 15N tracing combined with quantitative polymerase chain reaction to evaluate the relative contributions of different N2O production pathways (i.e., autotrophic nitrification, heterotrophic nitrification, and denitrification) to N2O production in a calcareous maize field soil; we also used specific inhibitors (acetylene and 1-octyne) to determine the functional importance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N2O production. Autotrophic nitrification accounted for 47–63% and 76–100% of total N2O emissions in unfertilized (F0) and N-fertilized soils, respectively. AOB accounted for 62% and 91–98% of total N2O production in F0 and N-fertilized soils, respectively. N2O production was significantly lower in soils following long-term film mulching (F225) compared with the bare land soil (N225), and this was due to the reduced abundance and activity of AOB and N2O derived from autotrophic nitrification. Autotrophic nitrification-derived N2O emissions were 2.2-fold, 3.6-fold, and 6.3-fold higher in the F225 treatment (appropriate N level), the treatment in which the level of N was the same as that applied by local farmers (F380), and the manure addition treatment (F225+M) compared with F0, respectively, indicating that autotrophic nitrification was the main driver of increases in N2O emissions. These findings provide the first direct evidence showing that AOB-driven autotrophic nitrification is the dominant N2O production pathway in the plastic film mulched calcareous soils under different agricultural management practices. Our results also show that autotrophic nitrification is depressed by long-term mulching and promoted by N fertilization. These findings have implications for the development of targeted N2O mitigation strategies.

How does the phytoplankton–light feedback affect the marine N2O inventory?

Abstract. The phytoplankton–light feedback (PLF) describes the interaction between phytoplankton biomass and the downwelling shortwave radiation entering the ocean. The PLF allows the simulation of differential heating across the ocean water column as a function of phytoplankton concentration. Only one third of the Earth system models contributing to the 6th phase of the Coupled Model Intercomparison Project (CMIP6) include a complete representation of the PLF. In other models, the PLF is either approximated by a prescribed climatology of chlorophyll or not represented at all. Consequences of an incomplete representation of the PLF on the modelled biogeochemical state have not yet been fully assessed and remain a source of multi-model uncertainty in future projection. Here, we evaluate within a coherent modelling framework how representations of the PLF of varying complexity impact ocean physics and ultimately marine production of nitrous oxide (N2O), a major greenhouse gas. We exploit global sensitivity simulations at 1∘ horizontal resolution over the last 2 decades (1999–2018), coupling ocean, sea ice and marine biogeochemistry. The representation of the PLF impacts ocean heat uptake and temperature of the first 300 m of the tropical ocean. Temperature anomalies due to an incomplete PLF representation drive perturbations of ocean stratification, dynamics and oxygen concentration. These perturbations translate into different projection pathways for N2O production depending on the choice of the PLF representation. The oxygen concentration in the North Pacific oxygen-minimum zone is overestimated in model runs with an incomplete representation of the PLF, which results in an underestimation of local N2O production. This leads to important regional differences of sea-to-air N2O fluxes: fluxes are enhanced by up to 24 % in the South Pacific and South Atlantic subtropical gyres but reduced by up to 12 % in oxygen-minimum zones of the Northern Hemisphere. Our results, based on a global ocean–biogeochemical model at CMIP6 state-of-the-art level, shed light on current uncertainties in modelled marine nitrous oxide budgets in climate models.

Microbial and isotopomer analysis of N2O production pathways in a calcareous film-mulched farmland

Microbial and isotopomer analysis of N2O production pathways in a calcareous film-mulched farmland

Quantitative discrimination of algae multi-impacts on N2O emissions in eutrophic lakes: Implications for N2O budgets and mitigation

It is generally accepted that eutrophic lakes significantly contribute to nitrous oxide (N2O) emissions. However, how these emissions are affected by the formation, disappearance, and mechanisms of algal blooms in these lakes has not been systematically investigated. This study examined and determined the relative contribution of spatiotemporal N2O production pathways in hypereutrophic Lake Taihu. Synchronously, the multi-impacts of algae on N2O production and release potential were measured in the field and in microcosms using isotope ratios of oxygen (δ18O) and bulk nitrogen (δ15N) to N2O and to intramolecular 15N site preference (SP). Results showed that N2O production in Lake Taihu was derived from microbial effects (nitrification and incomplete denitrification) and water air exchanges. N2O production was also affected by the N2O reduction process. The mean dissolved N2O concentrations in the water column during the pre-outbreak, outbreak, and decay stages of algae accumulation were almost the same (0.05 μmol·L–1), which was 2–10 times higher than in lake areas algae was not accumulating. However, except for the central lake area, all surveyed areas (with and without accumulated algae) displayed strong release potential and acted as the emission source because of dissolved N2O supersaturation in the water column. The mean N2O release fluxes during the pre-outbreak, outbreak, and decay stages of algae accumulation areas were 17.95, 26.36, and 79.32 μmol·m–2·d–1, respectively, which were 2.0–7.5 times higher than the values in the non-algae accumulation areas. In addition, the decay and decomposition of algae released large amounts of nutrients and changed the physiochemical properties of the water column. Additionally, the increased algae biomass promoted N2O release and improved the proportion of N2O produced via denitrification process to being 9.8–20.4% microbial-derived N2O. This proportion became higher when the N2O consumption during denitrification was considered as evidenced by isotopic data. However, when the algae biomass was excessive in hypereutrophic state, the algae decomposition also consumed a large amount of oxygen, thus limiting the N2O production due to complete denitrification as well as due to the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Further, the excessive algae accumulation on the water surface reduced N2O release fluxes via hindering the migration of the dissolved N2O into the atmosphere. These findings provide a new perspective and understanding for accurately evaluating N2O release fluxes driven by algae processes in eutrophic lakes.

Pathways of N2O production by marine ammonia-oxidizing archaea determined from dual-isotope labeling

The ocean is a net source of the greenhouse gas and ozone-depleting substance, nitrous oxide (N2O), to the atmosphere. Most of that N2O is produced as a trace side product during ammonia oxidation, primarily by ammonia-oxidizing archaea (AOA), which numerically dominate the ammonia-oxidizing community in most marine environments. The pathways to N2O production and their kinetics, however, are not completely understood. Here, we use 15N and 18O isotopes to determine the kinetics of N2O production and trace the source of nitrogen (N) and oxygen (O) atoms in N2O produced by a model marine AOA species, Nitrosopumilus maritimus. We find that during ammonia oxidation, the apparent half saturation constants of nitrite and N2O production are comparable, suggesting that both processes are enzymatically controlled and tightly coupled at low ammonia concentrations. The constituent atoms in N2O are derived from ammonia, nitrite, O2, and H2O via multiple pathways. Ammonia is the primary source of N atoms in N2O, but its contribution varies with ammonia to nitrite ratio. The ratio of 45N2O to 46N2O (i.e., single or double labeled N) varies with substrate ratio, leading to widely varying isotopic signatures in the N2O pool. O2 is the primary source for O atoms. In addition to the previously demonstrated hybrid formation pathway, we found a substantial contribution by hydroxylamine oxidation, while nitrite reduction is an insignificant source of N2O. Our study highlights the power of dual 15N-18O isotope labeling to disentangle N2O production pathways in microbes, with implications for interpretation of pathways and regulation of marine N2O sources.

Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments.

Estuarine and coastal environments are assumed to contribute to nitrous oxide (N2O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N2O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N2O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N2O molecule increased due to increasing nitrate concentration, suggesting the changes in N2O production pathways. Enhanced N2O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N2O production by 11-25%, whereas it decreased bacterial N2O production by 16-33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13-28% of N2O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N2O production. Collectively, these results suggest that increasing nitrate concentration can shift N2O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N2O production and have widespread implications for N2O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.

Fertilizer N triggers native soil N-derived N2O emissions by priming gross N mineralization

Recent 15nitrogen (N) tracing studies have shown that unlabeled N2O emissions (UNEs) following 15N fertilizer application tend to be higher than those from zero-N controls, which indicates a potential risk for elevated N2O emissions from native soil. However, researchers do not clearly understand whether these increased UNEs are derived from a priming effect or how interactions between N in fertilizer and native soil affect various sources of N2O emissions. Here, we combined 15N tracing and 15N pool dilution in an incubation study to track gross N transformations, specific sources of N2O emissions, and N2O-production pathways following N fertilization. Four treatments, including two fertilizer N levels (0 and 120 mg N kg−1) in conjunction with two native soil N levels (0.56 and 1.25 g kg−1), were established in the study. Soil gross N mineralization (GM) increased by 33.0%–98.5% (4.59–14.97 mg N kg−1) after N fertilizer addition, which indicated a positive priming effect on soil N turnover and resulted in additional native soil N-derived N2O emissions. In addition to priming GM, the pool substitution effect after 15N addition, which was not triggered by stimulated soil N turnover, also generated substrates for the UNEs. Therefore, overlooking the pool substitution effect might overestimate native soil N-derived emissions by 104%–180%. Differentiation of the specific N2O emission sources showed that both the fertilizer N- and primed native soil N-derived N2O emissions were dominated by nitrifier-mediated processes, which were governed by ammonia-oxidizing bacteria. At a specified N fertilizer addition rate, soils with higher native N levels had greater levels of fertilizer N- and native soil N-derived N2O emissions due to the higher GM and gross nitrification rates. The study reveals that fertilizer N application triggers potential N2O emissions from native soil N by stimulating the GM and nitrifier-mediated N2O-production processes.

Different responses of nitrous oxide emissions to liming and manure amendment of an acidic ultisol are controlled by autotrophic and heterotrophic nitrification

Long-term lime and manure amendment can alleviate soil acidification, but their impacts on nitrous oxide (N2O) emissions and their production pathways remain largely unclear. Here, a 15N tracing study was conducted to explore the effect of lime and manure amendment on N2O production pathways and gross nitrogen (N) transformation rates of an acidic Ultisol. Soils were sampled from a 33-year field fertilization experiment, including three treatments of mineral NPK fertilizer (NPK), NPK plus lime (NPKL), and NPK plus manure (NPKM). Compared to NPK, N2O emissions increased by 25.2% for NPKL but decreased by 22.1% for NPKM, indicating that different pH-raising substances may have different effects on N2O emissions. The 15N tracing analysis indicated that heterotrophic nitrification-derived N2O played a dominant role in N2O emissions while NPKL significantly increased autotrophic nitrification-derived N2O production. The increased relative contribution of autotrophic nitrification to N2O emissions by NPKL resulted in an increase of N2O emissions in the acidic Ultisol. In contrast, NPKM decreased heterotrophic nitrification and autotrophic nitrification-derived N2O production, leading to a decrease of N2O emissions. Our results show that lime and manure amendment can have different impacts on N2O emissions due to their differential effects on autotrophic and heterotrophic nitrification, providing important insights for the sustainable management of acidic arable soils.

Quantifying nitrous oxide production rates from nitrification and denitrification under various moisture conditions in agricultural soils: Laboratory study and literature synthesis.

Biogenic nitrous oxide (N2O) from nitrification and denitrification in agricultural soils is a major source of N2O in the atmosphere, and its flux changes significantly with soil moisture condition. However, the quantitative relationship between N2O production from different pathways (i.e., nitrification vs. denitrification) and soil moisture content remains elusive, limiting our ability of predicting future agricultural N2O emissions under changing environment. This study quantified N2O production rates from nitrification and denitrification under various soil moisture conditions using laboratory incubation combined with literature synthesis. 15N labeling approach was used to differentiate the N2O production from nitrification and denitrification under eight different soil moisture contents ranging from 40 to 120% water-filled pore space (WFPS) in the laboratory study, while 80 groups of data from 17 studies across global agricultural soils were collected in the literature synthesis. Results showed that as soil moisture increased, N2O production rates of nitrification and denitrification first increased and then decreased, with the peak rates occurring between 80 and 95% WFPS. By contrast, the dominant N2O production pathway switched from nitrification to denitrification between 60 and 70% WFPS. Furthermore, the synthetic data elucidated that moisture content was the major driver controlling the relative contributions of nitrification and denitrification to N2O production, while NH4 + and NO3 - concentrations mainly determined the N2O production rates from each pathway. The moisture treatments with broad contents and narrow gradient were required to capture the comprehensive response of soil N2O production rate to moisture change, and the response is essential for accurately predicting N2O emission from agricultural soils under climate change scenarios.

Soil pH and long-term fertilization affect gross N transformation and N2O production pathways in Chinese and UK croplands

Soil pH and long-term fertilization affect gross N transformation and N2O production pathways in Chinese and UK croplands

Mathematical modeling of the dynamic effect of denitrifying glycogen-accumulating organisms on nitrous oxide production during denitrifying phosphorus removal

The large amount of intermediate nitrous oxide (N2O) production from denitrifying phosphorus removal (DPR) increases the carbon footprint of wastewater treatment. However, there is a lack of detailed understanding of the interrelationships between denitrifying polyphosphate-accumulating organisms (DPAOs) and denitrifying glycogen-accumulating organisms (DGAOs) on N2O production during DPR. In this work, a mathematical model was developed for the first time to describe dynamic N2O production in the DPR system coexisting DPAOs and DGAOs. The model took into account a four-step subsequent denitrification of nitrate, nitrite, nitric oxide, and N2O. The validity of model was fully tested by comparing simulation studies with experimental data from three independent reports on DPR, which satisfactorily described the dynamics of N2O production, nitrogen oxide reduction, phosphate release and uptake, and intracellular polymers turnover. The validated model could clarify the source and pathways of N2O production. Subsequently, the combined effects of key operational conditions on the overall N2O production, DPAOs and DGAOs competition, and nutrients removal efficiency were investigated by model simulation. Simulation results showed higher polyhydroxyalkanoate storage rate of DGAOs than that of DPAOs during anaerobic stage and the preference of DGAOs for nitrate electron acceptor during anoxic stage. DGAOs dominated the growth competition over DPAOs at low COD (<150 mg/L) and high nitrate (>35 mg/l) conditions, leading to the anoxic storage of glycogen by DGAOs as the main pathway of N2O production. In addition, both N2O production and generation pathways in the system possessed greater variability compared to single DGAOs or DPAOs system, further indicating the necessity of this model.

Impacts of slurry application methods and inhibitors on gaseous emissions and N2O pathways in meadow-cinnamon soil

This study aimed to evaluate the impact of mitigation practices (slurry application methods and inhibitors applications) on gas emissions and identify the soil N2O production pathways in cattle slurry applied soil using isotopocule mapping approach. First, we compared the NH3 and N2O emissions of cattle slurry applied soil in a summer maize field experiment in north China plain (NCP) with four treatments: control (CK, no fertilization), slurry application using surface (SA-S), slurry application using band application (BA-S), and chemical fertilizer application using band application (BA-C). Then, an incubation experiment was conducted to investigate the mitigation effect of nitrification inhibitors (dicyandiamide, DCD) and denitrification inhibitors (procyanidins, PC) and their combination (DCD+PC) on gaseous N emissions with slurry applied using incorporation (IA) or surface application (SA) methods. The results showed that the total gaseous N emissions (N2O-N and NH3-N) in field were in the order of SA-S (1534mgm-2)>BA-S (338mgm-2)>BA-C (128mgm-2)>CK (55mgm-2), and the dominant N loss contributor varied from NH3 in SA-S (∼89%) to N2O in BA-S (∼94%) and BA-C (∼88%). Moreover, the isotopocule mapping approach indicated that emitted N2O of the slurry applied soil in field appeared to have lower rN2O values and led to more N2O+N2 emissions at the initial fertilization period. The incubation experiment indicated that the N2O emissions of slurry-applied soil were significantly reduced by DCD (∼45%) and DCD+PC (∼67%) application in comparison with CK (p<0.05), and the stronger contributions of bacterial denitrification/nitrifier denitrification to N2O production were revealed by the lower δ15NSP in N2O using the isotopocule mapping approach. In conclusion, in NCP the gaseous losses of the slurry applied field can be largely reduced by using incorporation method, and greater reduction could be achieved given the simultaneous application of nitrification/denitrification inhibitors.

Insights into N2O turnovers under polyethylene terephthalate microplastics stress in mainstream biological nitrogen removal process

The ubiquitous microplastics in wastewater have raised growing concerns due to their unintended effects on microbial activities. However, whether and how microplastics affect nitrous oxide (N2O) (a potent greenhouse gas) turnovers in mainstream biological nitrogen removal (BNR) process remain unclear. This work therefore aimed to fill such knowledge gap by conducting both long-term and batch tests. After over 100 days of feeding with wastewater containing polyethylene terephthalate (PET) microplastics (0–500 μg/L), the long-term results showed that both production and reduction of N2O during denitrification were reduced, as well as the N2O production during nitrification. Accordingly, 60% reduction in N2O accumulation and 70% reduction in N2O production were observed in the denitrification and nitrification batch tests, respectively. Nevertheless, the long-term N2O emission factors under PET microplastics stress were comparable to that in the control reactor, mainly because PET microplastics led to more nitrite accumulation in anoxic period. With the aid of online N2O sensors and site-preference analysis, it was demonstrated that the heterotrophic bacteria pathway and ammonia oxidizing bacteria denitrification pathway for N2O production were negatively affected by PET microplastics, whereas a clear increase in the contribution of hydroxylamine pathway (+ 22.9%) was observed. Further investigation revealed that PET microplastics even at environmental level (i.e. 10 μg/L) significantly reshaped the BNR sludge characteristics (e.g. much larger particle size) and microbial communities (e.g. Thauera, Rhodobacte and Nitrospira) as well as the nitrogen metabolism pathways, which were chiefly responsible for the changes of N2O turnovers and N2O production pathways under the PET microplastics stress.

N2O production and emission pathways in anammox biofilter for treating wastewater with low nitrogen concentrations

Although anaerobic ammonia oxidation (anammox) is a cost-effective nitrogen removal process, nitrous oxide (N2O) production will greatly reduce the advantages of this process. It is important to identify the N2O emission pathways and then reduce the N2O production in anammox system. To date, very limited research has been done to investigate the N2O production and N2O emission pathways in anammox biofilter. In this study, N2O production were investigated under different filtration rates in anammox biofilter for treating wastewater with low nitrogen concentrations, and N2O emission pathways were analyzed with batch tests using N2O microsensor and stable isotope mass spectrometry. The results showed N2O production increased with the increase of filtration rates in anammox biofilter, where the N2O emission factor increased from 0.012 % at 1.0 m/h to 0.496 % at 3.0 m/h. And the optimal operation condition was at filtration rate of 1.5 m/h, where NH4+-N and NO2−-N removal efficiencies reached 99 % and N2O concentration was the lowest. qPCR showed that anammox bacteria, nitrifying bacteria and denitrifying bacteria were all present in anammox biofilter, with anammox bacteria in the highest abundance. And nitrifying bacteria and denitrifying bacteria provided the possibility of N2O production. The batch tests and stable isotope mass spectrometry analysis indicated that nitrifier denitrification, hydroxylamine oxidation and endogenous heterotrophic denitrification were N2O production pathways in aerobic zone and anoxic zone of anammox biofilter, respectively. In addition, batch tests under different conditions showed no oxygen environment could reduce N2O production. Therefore, the production of N2O in anammox system is a problem that cannot be ignored and should be paid more attention to.

Combined effects of biochar and biogas slurry on soil nitrogen transformation rates and N2O emission in a subtropical poplar plantation

It has been widely accepted that biochar has a great potential of mitigating soil nitrous oxide (N2O) emission. However, the underlying mechanism about how biochar affects nitrogen transformation and the pathways of soil N2O production is under discussion. A 15N-tracer incubation experiment was conducted to investigate the short-term effects of biochar on soil N transformation rates and source partitioning of N2O emissions in soils from a poplar plantation system. A two-factor experimental design was adopted using biogas digestate slurry and biochar as soil amendments. In total, there were 12 treatments, including three rates of biochar: B0 (control), B2 (80 t ha−1), and B3 (120 t ha−1), and four rates of biogas digestate slurry: C (0 m3 ha−1), L (125 m3 ha−1), M (250 m3 ha−1), and H (375 m3 ha−1). We observed significantly lower rates of net nitrification (Nn) and mineralization (Mn) in biochar-treated soils. The 15N tracer analysis revealed a significant decrease in gross autotrophic (ONH4), heterotrophic nitrification (ONrec), and mineralization (MNorg) rates while an increase in gross immobilization (INH4 and INO3) rates in biochar amended soils. When biogas slurry was applied, biochar only significantly reduced ONH4 except in the moderate slurry treatment. Regardless of the slurry application, biochar consistently suppressed N2O emission by 58–89 %, and nitrification was the dominant pathway accounting contributing >90 % to cumulative N2O emissions. Moreover, soil cumulative N2O emissions significantly negatively correlated with soil ammonium contents and positively with MNorg, Mn, and Nn, showing that biochar decreased N2O emission via a reducing effect on nitrification rates and associated N2O emissions. Our results also highlight that application of N fertilizer greatly influence the biochar's impacts on soil N transformation rates and N2O emission, calling for further studies on their interactions to develop mitigate options and to improve N use efficiency.

Nitrous oxide production in the Chesapeake Bay

AbstractEstuaries at the global scale are significant but highly uncertain sources of atmospheric nitrous oxide (N2O), which is an intense greenhouse gas and ozone depletion agent. As the largest estuary in the United States, the Chesapeake Bay is suggested to be a spatially and temporally variable source and sink of N2O. However, limited observations of N2O cycling preclude us from estimating and predicting its net N2O flux. To improve our mechanistic understanding of the processes that control the N2O flux at the point of production, we applied multiple 15N tracers (, 15N‐urea, and ) to separately track N2O production from nitrification and denitrification under in situ and manipulated O2 concentrations in the Chesapeake Bay. Nitrification was the major N2O production pathway in oxic waters (up to 7.5 nmol N2O L−1 d−1). In contrast, denitrification dominated N2O production from hypoxic/anoxic waters (up to 20 nmol N2O L−1 d−1). N2O production from urea was observed for the first time in estuarine waters. The contribution from urea was small, but interestingly, showed a depth pattern distinct from other N2O precursors. Experimentally lowering the O2 concentration substantially enhanced N2O production. Therefore, the expansion of hypoxic and anoxic zones in the Chesapeake Bay under climate change as suggested by some climate models may favor the production of N2O, potentially providing positive feedback on warming. Overall, our study provides mechanistic constraints on N2O dynamics that could benefit modeling studies to better estimate the N2O flux in the Chesapeake Bay and other coastal environments.

Nitrogen-Induced Changes in Soil Environmental Factors Are More Important Than Nitrification and Denitrification Gene Abundance in Regulating N2O Emissions in Subtropical Forest Soils.

Subtropical regions are currently experiencing a dramatic increase in nitrogen (N) deposition; however, the contributions of nitrification and denitrification processes to soil N2O emissions and the underlying mechanisms under increasing N deposition remain unclear. Therefore, a 15N-tracing laboratory experiment with four N application rates (0, 12.5, 25, and 50 μg 15N g–1 soil) was conducted to investigate the response of nitrification- and denitrification-derived N2O to N additions in an evergreen broad-leaved forest (BF) and a Pinus forest (PF) in the Wuyi Mountains in southeastern China. Moreover, the abundance of functional genes related to nitrification (amoA), denitrification (nirK, nirS, and nosZ), and soil properties were measured to clarify the underlying mechanisms. Results showed that nitrification-derived N2O emissions were generally decreased with increasing N input. However, denitrification-derived N2O emissions were a non-linear response to N additions, with maximum N2O emissions at the middle N application rate. Denitrification-derived N2O was the dominant pathway of N2O production, accounting for 64 to 100% of the total N2O fluxes. Soil NH4+-N content and pH were the predominant factors in regulating nitrification-derived N2O emissions in BF and PF, respectively. Soil pH and the nirS abundance contributed the most to the variations of denitrification-derived N2O emissions in BF and PF, respectively. Our results suggest that N application has the potential to increase the contribution of denitrification to N2O production in subtropical forest soils. Changes in soil chemical properties induced by N addition are more important than the abundance of nitrification and denitrification functional genes in regulating soil nitrification- and denitrification-derived N2O emissions.

Isotopic signatures of biotic and abiotic N2O production and consumption in the water column of meromictic, ferruginous Lake La Cruz (Spain)

AbstractLakes can be important sources of the potent greenhouse gas nitrous oxide (N2O) to the atmosphere, but to what extent abiotic processes may contribute to lacustrine N2O production remains uncertain. We assessed pathways of N2O production and reduction in the water column of meromictic and iron‐rich Lake La Cruz, Spain, including chemodenitrification‐induced N2O formation via the reaction of reactive nitrogen (N) (e.g., ) with ferrous iron (Fe[II]). In the oxic waters (~8–10 m), N2O concentrations above atmospheric equilibrium were associated with comparatively low δ15N‐N2O, high , and high N2O 15N‐site‐preference (SP) values (up to ~29‰), suggesting N2O production by nitrification. N2O concentrations were highest (23–33 nM) near the depth of oxygen depletion (~11–14.5 m), likely due to production by nitrifier denitrification and/or denitrification, as indicated by decreasing SP values (as low as 12‰). Further below (~14.5–17 m), N2O consumption was indicated by increasing SP values and a δ18O‐vs.‐δ15N relationship (1.8–2.9) typical for stand‐alone N2O reduction. The coupled N‐vs.‐O isotope signatures thus highlight the spatial, redox‐dependent separation of incomplete and complete denitrification. In incubations with sterile‐filtered lake water and 15N‐labeled or unlabeled substrate, was reduced by Fe2+ to N2O, even at low nitrite concentrations (5 μM ). In the water column, the spatial separation of and Fe(II) during our samplings appears to preclude elevated rates of chemodenitrification, but during periods of overlapping and Fe(II) in Lake La Cruz, and potentially in other lakes, its distinct N2O δ18O‐vs.‐δ15N relationship of ~1 : 1, as experimentally determined, could help to detect it.

Antibiotics sulfamethoxazole alter nitrous oxide production and pathways in estuarine sediments: Evidenced by the N15-O18 isotopes tracing

Estuarine antibiotic residues are profoundly impacting microbial nitrogen (N) cycling and associated N2O production, but the response of N2O production pathways to antibiotics remains poorly understood. Here, 15N–18O labeling technique combined with molecular methods were used to investigate the impacts of sulfamethoxazole on the contribution of ammonia oxidation (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production in estuarine sediments. Results showed that environmental concentration of sulfamethoxazole (4 ng/g) promoted the total N2O production by 17.1% through nitrifier denitrification. Environmentally relevant (40–4000 ng/g) and irrelevant (40,000 ng/g) concentration of sulfamethoxazole drove nitrification denitrification to gradually lose the dominant role in total N2O production and ammonia oxidation-derived N2O, replaced by HD and nitrifier nitrification, while total N2O production were inhibited. Furthermore, when HD dominated the total N2O production, the HD-derived N2O increased by 63.6% with sulfamethoxazole concentration reaching 40,000 ng/g. The mechanistic investigation further showed that nitrifying bacteria were more susceptible to sulfamethoxazole than nitrifying archaea and denitrifiers. The increased expression of nirS gene carried by non-dominant denitrifiers improved the ratio of nirS:nosZ and hence increased HD-derived N2O under high sulfamethoxazole stresses. Overall, our results provide a comprehensive view into how antibiotics regulate N2O production and its pathways in estuarine sediments.

N2O and NO production and functional microbes responding to biochar aging process in an intensified vegetable soil

Vegetable soils with high nitrogen input are hotspots of nitrous oxide (N2O) and nitric oxide (NO), and biochar amended to soil has been documented to effectively decrease N2O and NO emissions. However, the aging effects of biochar on soil N2O and NO production and the relevant mechanisms are not thoroughly understood. A15N tracing microcosm study was conducted to clarify the responses of N2O and NO production pathways to the biochar aging process in vegetable soil. The results showed that autotrophic nitrification was the predominant source of N2O production. Biochar aging increased the O-containing functional groups while lowering the aromaticity and pore size. Fresh biochar enhanced the AOB-amoA gene abundance and obviously stimulated N2O production by 15.5% via autotrophic nitrification and denitrification. In contrast, field-aged biochar markedly weakened autotrophic nitrification and denitrification and thus decreased N2O production by 17.0%, as evidenced by the change in AOB-amoA and nosZI gene abundances. However, the amendment with artificially lab-aged biochar had no effect on N2O production. With the extension of aging time, biochar application reduced the soil NO production dominated by nitrification. Changes in the N2O and NO fluxes were closely associated with soil NH4+-N and NO2−-N contents, indicating that autotrophic nitrification played a critical role in NO production. Overall, our study demonstrated that field-aged biochar suppressed N2O production via autotrophic nitrification and denitrification by regulating associated functional genes, but not for lab-aged biochar or fresh biochar. These findings improved our insights regarding the implications of biochar aging on N2O and NO mitigation in vegetable soils.