N2O Sink Research Articles

Carbon substrate selects for different lineages of N2O reducing communities in soils under anoxic conditions

Agricultural soils are a main source of nitrous oxide (N2O), a potent greenhouse gas and the dominant ozone-depleting substance emitted to the atmosphere. The only known sink of N2O in soil is the microbial reduction of N2O to N2. Carbon (C) availability is a key factor in determining microbial community composition in soil. However, its role in shaping the structure of N2O reducing communities in soil is unexplored. In this study, a microcosm experiment was set up in which two arable soils with contrasting edaphic properties were incubated anaerobically for 83 days with four different C substrates: glucose, acetate, hydroxyethylcellulose (HEC) and mixture of the three. We show that the effect of C addition on the abundance and diversity of clade I and clade II nosZ genes, encoding different variants of the N2O reductase, varies across the different C substrates differently in contrasting soil types, yet still plays an important role in selecting specific taxa of N2O reducers under denitrifying conditions. We observed an increase of betaproteobacterial clade I and II N2O reducing species with addition of HEC, whereas alphaproteobacterial clade I species and clade II species within other Proteobacteria and Bacteriodetes were associated with glucose and acetate. These results show that specific C-substrates select for certain lineages of nitrous oxide reducers and influence patterns of niche partitioning within clades of N2O reducers, whereas other soil factors drive differences between clade I and II nosZ communities.

Two temperate seagrass meadows are negligible sources of methane and nitrous oxide

AbstractSeagrasses are globally important ecosystems that can help mitigate climate change by sequestering carbon (C). The net impact seagrass meadows have on the climate, however, also depends on methane (CH4) and nitrous oxide (N2O) fluxes. By not accounting for CH4 and N2O fluxes, we may be overestimating or underestimating the true C sequestration capacity of seagrasses. Yet, few observations of seagrass CH4 and N2O fluxes are available. Here, we quantified summer, dark/light CH4 and N2O fluxes across the sediment–water interface from seagrass meadows (Zostera marina) and adjacent nonvegetated sediments in two temperate bays with different environmental characteristics. On two occasions, we also estimated system wide air‐sea CH4 and N2O fluxes. We found the CH4 fluxes across the sediment–water interface varied between the two sites with one site emitting CH4 from vegetated sediments and a net zero flux at the other site. N2O fluxes across the sediment–water interface were not different from zero regardless of seagrass presence, although when we did measure a flux, there was more often a net uptake of N2O. We estimated that both systems were small net sources of CH4 and N2O to the atmosphere; however, the sediments are not likely the source of CH4 and N2O emitted to the atmosphere in the systems. Although the diffusive fluxes measured here are lower than those reported in the literature, they are consistent with our current understanding of seagrass sediments being variable sources of CH4 and potentially a negligible source or sink of N2O.

Nitrous oxide concentration and flux in Min River Basin of southeast China: Effects of land use, stream order and water variables

Estimates of riverine nitrous oxide (N2O) emissions have great uncertainty partly due to the fact that the data is still sparse for stream sizes and land uses. Here we determined water dissolved N2O concentration and water–air interface N2O flux across the land uses and stream orders in Min River Basin of southeast China. N2O concentrations ranged from 11.1 to 27.4 nmol/L, increasing from first- to eighth-order rivers. The first- and second-order streams showed negative N2O flux (−0.33 and −0.05 µg m−2h−1, respectively) compared to third- to eighth-order rivers (from 0.29 to 2.85 µg m−2h−1). Average N2O flux was significantly higher in urban (1.60 µg m−2h−1) and cropland rivers (0.89 µg m−2h−1) than in forest rivers (−0.47 µg m−2h−1), which could be due to the main effect of land use was on the nitrogen load. Indirect N2O emission factor varied between 0.035 and 0.14 % across the rivers, which decreased from the urban to forest rivers and were lower than the IPCC default value (0.26 %). N2O concentration and flux were positively correlated with NO3−, NH4+, depth, velocity and temperature, suggesting the importance of stream surface hydromoprhology to N2O emissions. Land use indirectly affected N2O flux by affecting water NO3−, NH4+, DO, DOC, pH and temperature, which could further explain the land use dependence of N2O emissions on water variables. High DOC:NO3− ratio (>6) was the main factor of N2O sink in the small streams, which may be due to the enhanced N2O consumption and low N2O production. Therefore, riverine N2O emissions are highly variable across stream sizes and land uses, which are not only increased by inorganic nitrogen load and are also dependent on stream hydraulics and morphology.

Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: Implications for eutrophication management of deep-water ecosystems.

Water eutrophication is a serious global issue because of excess external and internal nutrient inputs. Understanding the intensity and contribution of internal nitrogen (N) and phosphorus (P) loading in deep-water ecosystems is of great significance for water body eutrophication management. In this study, we combined intact sediment core incubation, high-resolution peeper (HR-Peeper) sampling, and analysis of N and P forms and other environmental factors in the water column and sediments to evaluate the contributions of internal N and P loading to water eutrophication by N and P fluxes across the sediment-water interface (SWI) of the Panjiakou Reservoir (PJKR), a deep-water ecosystem where eutrophication threatens the security of the local drinking water supply in North China. The results indicated that the PJKR showed obvious thermal and dissolved oxygen (DO) stratification in the warm seasons and full mixing in the cold seasons. The mean DO concentration was 9.9 and 3.55mg/L in the epilimnion and hypolimnion, respectively, in warm seasons and 10.7mg/L in cold seasons. The sediment acted as a source of soluble reactive phosphorus (SRP), NH4+-N, and NO2--N and a sink of NO3--N. The SRP fluxes were 5.28±4.34 and 2.30±1.93mgm-2·d-1 in warm and cold seasons, respectively, and the dissolved inorganic nitrogen (DIN) fluxes were -0.66±47.84 and 44.04±84.05mgm-2·d-1. Seasonal hypoxia accelerated the release of P rather than N from the sediments in warm seasons, which came mainly from Fe-P and Org-P under anoxic conditions. The strong negative NO3--N flux (diffusion from the water column to the sediment) implied an intensive denitrification process at the SWI, which can counteract the release flux of NH4+-N and NO2--N, resulting in the sediment acting as a weak dissolved inorganic nitrogen (DIN) source for the overlying water. We also found that internal N loading accounted for only ∼9% of the total N loading, while internal P loading accounted for 43% of the total P loading of the reservoir. Our results highlight that efforts to manage the internal loading of deep-water ecosystems should focus on P and seasonal hypoxia.

Model-based identification and testing of appropriate strategies to minimize N2O emissions from biofilm deammonification.

Based on a one-year pilot plant operation of a two-step biofilm nitritation-anammox pilot plant, N2O mitigation strategies were identified by applying a newly developed biofilm modeling approach. Due to adapted plant operation, the N2O emission could be diminished by 75% (8.8% → 2.3% of NH4-Noxidized_AOB). The results (measurement and simulation) confirm the huge importance of denitrification as an N2O source or N2O sink, depending on the boundary conditions. A significant reduction of N2O emissions could only be achieved with a one-step deammonification system, which is related to low nitrite and HNO2 concentrations. Increased oxygen concentrations in the bulk phase are not related to decreased emissions. N2O formation by ammonium-oxidizing bacteria (AOB) just shifts deeper into the biofilm; zones with low oxygen concentrations are not avoidable in biofilm systems. Low oxygen concentrations in the bulk phase, however, result in a reduction of the total net N2O formation due to increased activity of heterotrophic bacteria directly at the source of N2O formation (outer biofilm layer). For the model-based identification of mitigation strategies, the standard modeling approaches for biofilms were expanded by including the factor-based N2O formation and emission approach. The new model 'Biofilm/N2OISAH' was successfully validated using data from pilot-scale measurement campaigns. Altogether, the investigation confirms that the employed digital model can strongly support the development of N2O mitigation strategies without the need for specialized measurement inside the biofilm.

Stable nitrogen-cycling capacity in relation to fertilization and intercropping in a sub-boreal grassland

Grasslands are important in sub-boreal climate agricultural systems and are managed with various combinations of N fertilization and plant species. Ammonia-oxidizing and denitrifying microorganisms are key players in determining the fate of nitrogen (N) and thereby also the yield in grassland systems and their impact on gaseous N losses and leaching. We established a three-year field study in southern Finland with fertilizer treatment as a main-plot factor, including organic and synthetic fertilizers and plant species and mixtures thereof as the sub-plot factor. We quantified six genes encoding key N-cycling enzymes by quantitative PCR to determine the abundance of the communities involved in N-transformation processes and also included previously published data on crop yield, soil properties and the overall bacterial community composition. With the exception of ammonia oxidizing bacteria (AOB), which were primarily affected by fertilization, the abundances of all other N-cycling communities changed over time with either an increase or decrease from summer to autumn. Differences in gene abundances between plant species treatments and in fertilizer by plant species interactions were detected mainly in the beginning of the cropping season during the first year. The nirS-type denitrifiers and nosZII nitrous oxide reducers responded more to changes in soil properties than their functional counterpart nirK and nosZI communities. Using structural equation modeling, we show that the overall microbial community composition and diversity played an important role in mediating the management effects on crop yield, genetic potential for N retention and N2O sink capacity. However, a trade-off between the genetic potential for N retention and N2O sink capacity was detected, indicating the challenges in managing grasslands in a sustainable way.

190 Effects of Flooding on Grazed Pastures: Forage Quality, GHG Emissions and C Stocks

Abstract Manure creates hot spots of nutrients concentration, and consequently nutrient losses to air and water. In flood-prone areas these losses represent a major concern for environment. However, there is a gap on the understanding of the effects of recurring, short-term flooding on environmental and forage characteristics. This study evaluated the effects flooding on soil carbon (C) stock; and greenhouse gas (GHG) emissions (methane, CH4; nitrous oxide, N2O; and carbon dioxide, CO2) from soil; and forage dry matter and quality in flooded and non-flooded grazed pastures during four sampling periods (spring, early summer, late summer, and fall) of 2021. Soil samples were collected from 0-100cm, forage samples were cut at stubble height and soil greenhouse gases (GHG) emissions were monitored using the static chamber methodology. Preliminary results show trends of reduced forage digestibility in flooded pastures. No differences were observed on forage dry matter. Overall, there was no differences between CO2 emission from soil in flooded and non-flooded areas. Soil CH4 fluxes were primarily negative, and flooding seems to reduce the sink potential for CH4 from soil, except during early summer. For N2O the effects of season were very pronounced, increased emissions from spring to early summer and throughout fall in both flooded and non-flooded pastures. We did not observe any N2O sinks, and no difference between treatments. Flood-prone pastures had decreased C stock in comparison with non-flood pastures. Preliminary results show that forage production may represent a good option for flooded areas, since no effects were observed on mass and few effects on quality. However, flooding reduced C stock potential and CH4 sinks. A second sampling season will be performed in 2022 to further observe these results and allow for C-equivalent flux calculations. The balance between productivity and environmental impacts of flood-prone pastures will then be assessed.

Stable Isotope Analysis of Greenhouse Gases Requires Analyte Preconcentration

Nitrous oxide (N2O) is an important trace gas contributing to global warming and depletion of ozone in the stratosphere. Its increasing abundance is caused mainly by anthropogenic sources, such as application of fertilizers in agriculture or emissions from industry. To understand the N2O global budget, its sources and sinks need to be well-described and quantified. In this project, a new method for N2O source appointment was developed that can help with this task. The method is based on analysis of the eight most abundant isotopic molecules of N2O, using quantum cascade laser absorption spectroscopy (QCLAS). The applicability of the method towards the N2O biogeochemical cycle was demonstrated on a prominent N2O source (bacterial denitrification) and the most important N2O sink (UV photolysis) on samples prepared in laboratory experiments. An extension of the QCLAS method to natural samples can be achieved by hyphenation with a preconcentration technique that increases concentration of the analyte and standardizes the sample matrix. This article provides an overview of currently applied preconcentration techniques in the field of greenhouse-gas analysis and a description of the preconcentration device TREX that will be employed in future projects with the developed QCLAS method.

Assembly of root-associated N2O-reducing communities of annual crops is governed by selection for nosZ clade I over clade II.

The rhizosphere is a hotspot for denitrification. The nitrous oxide (N2O) reductase among denitrifiers and nondenitrifying N2O reducers is the only known N2O sink in the biosphere. We hypothesized that the composition of root-associated N2O-reducing communities when establishing on annual crops depend on soil type and plant species, but that assembly processes are independent of these factors and differ between nosZ clades I and II. Using a pot experiment with barley and sunflower and two soils, we analyzed the abundance, composition, and diversity of soil and root-associated N2O reducing communities by qPCR and amplicon sequencing of nosZ. Clade I was more abundant on roots compared to soil, while clade II showed the opposite. In barley, this pattern coincided with N2O availability, determined as potential N2O production rates, but for sunflower no N2O production was detected in the root compartment. Root and soil nosZ communities differed in composition and phylogeny-based community analyses indicated that assembly of root-associated N2O reducers was driven by the interaction between plant and soil type, with inferred competition being more influential than habitat selection. Selection between clades I and II in the root/soil interface is suggested, which may have functional consequences since most clade I microorganisms can produce N2O.

Exploring the Functions of Efficient Canonical Denitrifying Bacteria as N2O Sinks: Implications from 15N Tracer and Transcriptome Analyses.

In denitrifying reactors, canonical complete denitrifying bacteria reduce nitrate (NO3-) to nitrogen via N2O. However, they can also produce N2O under certain conditions. We used a 15N tracer method, in which 15N-labeled NO3-/nitrite (NO2-) and nonlabeled N2O were simultaneously supplied with organic electron donors to five canonical complete denitrifying bacteria affiliated with either Clade I or Clade II nosZ. We calculated their NO3-, NO2-, and N2O consumption rates. The Clade II nosZ bacterium Azospira sp. strain I13 had the highest N2O consumption rate (3.47 ± 0.07 fmol/cell/h) and the second lowest NO3- consumption rate (0.20 ± 0.03 fmol/cell/h); hence, it is a N2O sink. A change from peptone- to acetate/citrate-based organic electron donors increased the NO3- consumption rate by 4.8 fold but barely affected the N2O consumption rate. Electron flow was directed to N2O rather than NO3- in Azospira sp. strain I13 and Az. oryzae strain PS only exerting a N2O sink but to NO3- in the Clade I nosZ N2O-reducing bacteria Pseudomonas stutzeri strain JCM 5965 and Alicycliphilus denitrificans strain I51. Transcriptome analyses revealed that the genotype could not fully describe the phenotype. The results show that N2O production and consumption differ among canonical denitrifying bacteria and will be useful for developing N2O mitigation strategies.

Land‐Use Intensity Increases Benthic N2O Emissions Across Three Sub‐Tropical Estuaries

AbstractEstuaries play an important role in regulating nitrous oxide (N2O) fluxes to the atmosphere, but little is known about how catchment land‐use changes influence benthic N2O cycling. We measured seasonal benthic N2O fluxes and constructed N2O budgets in three sub‐tropical estuaries draining catchments with contrasting levels of land‐use intensity. Benthic habitats were a net N2O sink in the minimally impacted Noosa River Estuary (−287 nmol m−2 h−1) and a net source of N2O in the highly impacted Brisbane River Estuary (126 nmol m−2 h−1). Vegetated habitats can act as an important sink of N2O with uptakes of −286 and −35 nmol m−2 h−1 in the Noosa and Maroochy River Estuaries, respectively. Benthic N2O fluxes were significantly correlated with benthic NO3− fluxes, suggesting NO3− availability was an important control on benthic N2O fluxes. Combining benthic flux data with surface water N2O emissions measurements showed that increased benthic N2O fluxes helped drive increasing water–air N2O emissions over the land‐use intensity gradient. Overall, our results show that land‐use driven changes to both the diversity and sediment composition of benthic habitats play an important role in regulating N2O dynamics in estuarine ecosystems. This highlights that both sediment quality and nitrogen loading need to be considered in order to reduce emissions of greenhouse gases in the coastal ecosystems.

Patterns of denitrifier communities assembly and co-occurrence network regulate N2O emissions in soils with long-term contrasting tillage histories

Denitrification is reducing nitrate to nitrous oxide (N2O) and nitrogen (N2) under the action of denitrifying microorganisms, which are the main source of soil N2O emissions. However, the co-occurrence pattern and community assembly process of denitrifier communities have not been studied in agricultural soils with long-term contrasting tillage histories. Based on a continuous (> 11 yr) conservation experiment, we aimed to understand the community assembly process and co-occurrence network of nirK-, nirS-, nosZI- and nosZII-type denitrifier communities under conservation tillage (i.e., zero tillage (ZT) and chisel plough tillage (CPT)) and conventional tillage (i.e., plow tillage (PT)). Continuous in situ measurements showed that long-term conservation tillage significantly decreased the denitrification potential and N2O emission fluxes. CPT and ZT significantly increased the taxonomic and phylogenetic diversity of nirK, nosZI, and nosZII type communities, whereas consistently decreased the diversity of nirS-type communities, compared with PT. Although the assembly of nirK, nirS, nosZI, and nosZII type denitrifier communities were dominated by stochastic processes, the relative importance of stochastic assembly of denitrifying microbial communities was higher in conservation tillage soil. The co-occurrence network analyses further revealed clear tillage-induced ecological functions of N2O emissions, as evidenced by lower key nodes abundance and robustness of nirS and nirK communities network and higher key nodes abundance and robustness of nosZI and nosZII communities in conservation tillage soils. Variation partitioning analysis further indicated that the assembly processes significantly changed soil N2O emission intensity by regulating α-diversity and the key species abundances in the co-occurrence network of denitrifying communities. Overall, conservation tillage practices increased the diversity and the key nodes abundance of nosZI- and nosZII-type communities and decreased the diversity and the key nodes abundance of nirS- and nirK-type communities by regulating the relative importance of the stochastic assembly process of denitrifier communities, which is eventually likely to curb N2O emissions by strengthening the capacity of the N2O sink and weakening the capacity of the N2O source. These findings provide new insights into the formation of dryland microbial communities in agricultural soils with long-term contrasting tillage histories. Especially considering future climate change, this knowledge would prove useful in improving agroecosystem productivity and curbing greenhouse gas emissions.

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.

Contribution from a eutrophic temperate estuary to the landscape flux of nitrous oxide

For mitigation of climate change, all sources and sinks of greenhouse gases from the environment must be quantified and their driving factors identified. Nitrous oxide (N2O) is a strong greenhouse gas, and the contribution of aquatic systems to the global N2O budget remains poorly constrained. In this study, we measured N2O concentrations in a eutrophic coastal system, Roskilde Fjord (Denmark), and combined measurements with statistical modeling to quantify the N2O fluxes and budget in the system over a period of six months. To do so, we collected water at 15 sampling points and measured N2O concentrations along with physico-chemical water quality parameters, e.g. temperature, salinity, dissolved inorganic nitrogen and phosphorus, and silicon. We used mixed-effect regression models to predict N2O concentrations in the water from water quality parameters. We then derived N2O fluxes using well-established equations of N2O solubility and water-atmosphere exchanges. These fluxes were then put in perspective with those measured at the landscape scale by eddy-covariance at a 96 m nearby tall tower, and to those estimated from the agricultural land next to the fjord using Intergovernmental Panel on Climate Change (IPCC) guidelines. N2O concentrations in the Roskilde Fjord ranged between 2.40 and 8.05 nmol l−1. The best fitting model between water parameters and N2O concentrations in water included phosphorus and temperature. We estimated that (i) Roskilde Fjord was a sink of N2O, with a median inward flux of -0.04 nmol m−2 s−1, (ii) while the surrounding median agricultural flux was 0.13-0.18 nmol m−2 s−1, and (iii) the median landscape flux was 0.07 nmol m−2 s−1. All estimates of N2O fluxes were of the same magnitude and consistent with each other. These preliminary results need to be consolidated by further research.

Stronger link of nosZI than nosZII to the higher total N2O consumption in anoxic paddy surface soils

Rice paddy surface soils emit substantial amounts of atmospheric nitrous oxide (N2O). However, the soil can also act as sites for the abiotic and biological consumption of some of the N2O produced in the soil profile, thus, reducing net N2O emissions. Determining the proportion of N2O consumed in the soil, as well as the amount of N2O microbially reduced to nitrogen (N2) is critical for developing N2O mitigating strategies in paddy soils. However, few studies have focused on these processes. Furthermore, the association of microorganisms containing nosZI and nosZII genes with variations in biological N2O consumption potential remains largely unexplored. Here, moisture-controlled (60% gravimetric water content, GWC) and completely anoxic (helium [He] system) microcosm experiments were conducted on paddy surface soils (0–5 cm deep) treated with N2O. During a 96-h incubation period, the N2O cumulative emissions were monitored, the total N2O consumption and N2 production were quantified, and the population sizes of nosZI- and nosZII-containing microorganisms were analyzed. The results showed that 97.87%–99.99% of the N2O that accumulated in the 5 cm soil profile were consumed before emission, and the N2 increase in these soils accounted for 64.50%–82.44% of the total N2O consumption, indicating that the total N2O consumption potential and biological N2O sink capacity of paddy surface soils were considerable under experimental conditions. Furthermore, the production of N2 from N2O seemed to be positively related to soil pH, with less N2 being produced in more acidic soil. In addition, nosZI gene abundance had a strong positive correlation with N2 production, while nosZII had a much weaker correlation. Considering the positive correlation between N2 production and soil dissolved organic carbon (DOC) consumption and available potassium content (AK), it is speculated that complete denitrification by nosZI-N2O reducers dominate the N2O biological sink capacity of the paddy surface soils investigated in this study.

In-depth characterization of denitrifier communities across different soil ecosystems in the tundra

BackgroundIn contrast to earlier assumptions, there is now mounting evidence for the role of tundra soils as important sources of the greenhouse gas nitrous oxide (N2O). However, the microorganisms involved in the cycling of N2O in this system remain largely uncharacterized. Since tundra soils are variable sources and sinks of N2O, we aimed at investigating differences in community structure across different soil ecosystems in the tundra.ResultsWe analysed 1.4 Tb of metagenomic data from soils in northern Finland covering a range of ecosystems from dry upland soils to water-logged fens and obtained 796 manually binned and curated metagenome-assembled genomes (MAGs). We then searched for MAGs harbouring genes involved in denitrification, an important process driving N2O emissions. Communities of potential denitrifiers were dominated by microorganisms with truncated denitrification pathways (i.e., lacking one or more denitrification genes) and differed across soil ecosystems. Upland soils showed a strong N2O sink potential and were dominated by members of the Alphaproteobacteria such as Bradyrhizobium and Reyranella. Fens, which had in general net-zero N2O fluxes, had a high abundance of poorly characterized taxa affiliated with the Chloroflexota lineage Ellin6529 and the Acidobacteriota subdivision Gp23.ConclusionsBy coupling an in-depth characterization of microbial communities with in situ measurements of N2O fluxes, our results suggest that the observed spatial patterns of N2O fluxes in the tundra are related to differences in the composition of denitrifier communities.

Identification of nosZ-expressing microorganisms consuming trace N2O in microaerobic chemostat consortia dominated by an uncultured Burkholderiales

Microorganisms possessing N2O reductases (NosZ) are the only known environmental sink of N2O. While oxygen inhibition of NosZ activity is widely known, environments where N2O reduction occurs are often not devoid of O2. However, little is known regarding N2O reduction in microoxic systems. Here, 1.6-L chemostat cultures inoculated with activated sludge samples were sustained for ca. 100 days with low concentration (<2 ppmv) and feed rate (<1.44 µmoles h−1) of N2O, and the resulting microbial consortia were analyzed via quantitative PCR (qPCR) and metagenomic/metatranscriptomic analyses. Unintended but quantified intrusion of O2 sustained dissolved oxygen concentration above 4 µM; however, complete N2O reduction of influent N2O persisted throughout incubation. Metagenomic investigations indicated that the microbiomes were dominated by an uncultured taxon affiliated to Burkholderiales, and, along with the qPCR results, suggested coexistence of clade I and II N2O reducers. Contrastingly, metatranscriptomic nosZ pools were dominated by the Dechloromonas-like nosZ subclade, suggesting the importance of the microorganisms possessing this nosZ subclade in reduction of trace N2O. Further, co-expression of nosZ and ccoNO/cydAB genes found in the metagenome-assembled genomes representing these putative N2O-reducers implies a survival strategy to maximize utilization of scarcely available electron acceptors in microoxic environmental niches.

A Dual Enrichment Strategy Provides Soil- and Digestate-Competent Nitrous Oxide-Respiring Bacteria for Mitigating Climate Forcing in Agriculture.

ABSTRACTManipulating soil metabolism through heavy inoculation with microbes is feasible if organic wastes can be utilized as the substrate for growth and vector as a fertilizer. This, however, requires organisms active in both digestate and soil (generalists). Here, we present a dual enrichment strategy to enrich and isolate such generalists among N2O-respiring bacteria (NRB) in soil and digestates, to be used as an inoculum for strengthening the N2O-reduction capacity of soils. The enrichment strategy utilizes sequential batch enrichment cultures alternating between sterilized digestate and soil as substrates, with each batch initiated with limited O2 and unlimited N2O. The cultures were monitored for gas kinetics and community composition. As predicted by a Lotka-Volterra competition model, cluster analysis identified generalist operational taxonomic units (OTUs) which became dominant, digestate/soil-specialists which did not, and a majority that were gradually diluted out. We isolated several NRBs circumscribed by generalist OTUs. Their denitrification genes and phenotypes predicted a variable capacity to act as N2O-sinks, while all genomes predicted broad catabolic capacity. The latter contrasts with previous attempts to enrich NRB by anaerobic incubation of unsterilized digestate only, which selected for organisms with a catabolic capacity limited to fermentation products. The two isolates with the most promising characteristics as N2O sinks were a Pseudomonas sp. with a full-fledged denitrification-pathway and a Cloacibacterium sp. carrying only N2O reductase (clade II), and soil experiments confirmed their capacity to reduce N2O-emissions from soil. The successful enrichment of NRB with broad catabolic spectra suggests that the concept of dual enrichment should also be applicable for enrichment of generalists with traits other than N2O reduction.

Reference for different sensitivities of greenhouse gases effluxes to warming climate among types of desert biological soil crust

There is much uncertainty about how climate warming will impact greenhouse gases (GHG) budget in dry environments due to the lack of available data for desert biocrust soil. We implemented a 2.5-year field measurement of CO2, CH4 and N2O effluxes in cyanobacteria-dominated, moss-dominated and mixed (cyanobacteria, moss and lichen) biocrust soils using open-top-chambers to simulate climate warming (1.2 °C on average). Desert biocrust soils generally acted as a weak sink of atmospheric CH4 and N2O. Although warming effects on daily CO2, CH4, and N2O effluxes varied depending on sampling date and biocrust soil, there was no significant difference in daily, monthly and seasonal average CO2, CH4 and N2O effluxes between warming and control in most cases for three biocrust soils. However, warming caused a marginal (p = 0.06) decrease (14.2%) in annual accumulative CO2 efflux in moss-dominated biocrust soil due to the drought effects caused by warming indirectly and OTC sheltering of precipitation directly, while there was no significant difference between warming and control for cyanobacteria-dominated and mixed biocrust soils, implying a neutral response of GHG effluxes to climate warming. These results suggest that the GHG budget in arid desert biocrust soil would not be significantly changed in the warmer future when the direct negative effects of drought on CO2 effluxes were excluded. Therefore, a marginal decrease of accumulative CO2 effluxes in response to warming coupled with drought for moss-dominated biocrust soil might offer a weak negative feedback to warming and drier climate change pattern.

Nitrous oxide in the central Bay of Bengal during the summer monsoon

Although oceans contribute significantly to the global nitrous oxide (N2O) emissions, there are less than desired data related to N2O concentrations and fluxes in the global oceans and, in particular, the Indian Ocean. Within the Indian Ocean, the quantification and understanding of N2O dynamics in the open Bay of Bengal (BOB) is lacking, specifically during the southwest monsoon. The present study focused on quantifying the dissolved N2O concentrations in the water column along with water–atmosphere fluxes of N2O from the central Bay of Bengal during peak monsoon. For this purpose, a study was conducted during July–August 2018, where dissolved N2O concentrations were measured from the surface to 2000 m depth at eight stations. The concentrations of dissolved N2O in the surface waters ranged from 4.93 to 6.33 nM with saturation levels varying from 81 to 108%. Dissolved N2O showed a small variation with depth and appeared to be undersaturated at all depths except at the surface and at the depth of deep chlorophyll maximum. Flux calculations revealed that out of eight, four stations acted as a minor source, whereas three were minor sink for N2O. The low concentrations of surface N2O and small vertical gradients of N2O in the water column suggested either low production of N2O or significant consumption in the water column. However, the lack of comprehensive complementary data limits us to conclude the exact interplay of the mechanism leading to such observation. Comparison of the present data with available studies in the coastal BOB suggested significantly lower concentrations and fluxes in the open BOB. However, the range reported during the present study is closer to the only study carried out in the central BOB around twenty years ago; suggesting that N2O dynamics in the BOB have not changed drastically in recent times.