N2 Fluxes Research Articles

Nitrification and denitrification in estuarine sediments with tube-dwelling benthic animals

Burrowing benthic animals belonging to the same functional group may produce species-specific effects on microbially mediated nitrogen (N) processes depending upon different ecological traits. We investigated the effects of two tube-dwelling organisms, amphipods (Corophium insidiosum) and chironomid larvae (Chironomus plumosus), on benthic N cycling in bioturbated estuarine sediments. Aims of this work were to analyze the interactions among burrowers and N-related microbial processes in two distinct sedimentary environments colonized by benthic animals with different ecological traits. We hypothesized higher rates of nitrification and higher coupled nitrification–denitrification in sediments with C. insidiosum due to continuous ventilation rates. We expected higher denitrification of water column nitrate in sediments with C. plumosus due to lower and intermittent ventilation activity and lower oxygen levels in burrows. To this purpose, we combined process–specific (nitrification and denitrification) with net N flux measurements in intact and reconstructed sediments. Sediments with C. insidiosum had higher rates of oxygen demand and of potential nitrification and higher concentration of pore water NH4+ as compared to sediments with C. plumosus. Sediments with both species displayed comparable net N2 fluxes, mostly sustained by respiration of water column NO3− in sediments with chironomid larvae and by NO3− produced within sediments in sediments with corophiid amphipods. Corophium insidiosum stimulated nitrification nearly 15-fold more as compared to C. plumosus. Overall, our results demonstrate that sediments with burrowing fauna may display similar rates of denitrification, but underlying mechanisms may deeply vary and be species-specific.

Plasma emission spectroscopy and its relation to the refractive index of silicon nitride thin films deposited by reactive magnetron sputtering

In this work, we present a thorough study on the relation between the plasma emission and the change of the silicon nitride thin films refractive index. Thin films were grown by reactive magnetron direct current sputtering technique and deposited onto silicon wafers at different fluxes of Ar and N2 and at different working pressures. This procedure, at certain deposition parameters, produced poor quality films, i.e. films with refractive index other than pure Si3N4 films. The emission of the plasma was interrogated in real time by means of optical emission spectroscopy (OES) observing at the vicinity of the trget location. In addition, optical properties of the films were measured by in situ ellipsometric-spectroscopy and then correlated with OES observations. Changes in the film refractive index could be deduced from changes in plasma emission applying a principal component analysis.

Methodology for soil respirometric assays: Step by step and guidelines to measure fluxes of trace gases using microcosms

This methodology is proposed to measure the fluxes of trace gases among microcosms and the atmosphere. As microcosm respiration we include both aerobic and anaerobic respiration, which may results in CO2, CH4, NO, N2O, N2, H2S and H2 fluxes. Its applicability includes the assessment of products biodegradability and toxicity, the effect of treatments and products on greenhouse gases fluxes, and the mineralization of organic fertilizers. A step by step procedure; the complementary parameters and good practices that might be taken into account to perform a microcosm experiment; and the tools nowadays available that could be useful in this respirometric methodology are presented. We included a spreadsheet with calculus examples. Samples were taken at 1; 30; 60 and 90?min after closing the microcosms to determine the gases fluxes. The dilution effect was negligible, as we present. Besides CO2, we have successfully quantified the fluxes of CH4 and N2O from the microcosms in a broad range of concentrations. This method is useful in technical and scientific studies, for instances to test new products and improve the understanding of microbial processes, respectively.•Simple materials are required to set up the microcosm.•Examples of (pre) treatments are given regarding water availability, fertilizer doses, pH adjustment and nutrients amendments.•The method was suitable to directly measure multiple trace gases fluxes, either produced or consumed during microcosm respiration.

Anomalous molecular infiltration in graphene laminates.

Graphene laminated (GL) coatings formed by stacked few layer graphene (FLG) nanocrystals were deposited on low-density polyethylene (PE) films by the mechanical rubbing technique. Molecular transport through the bilayer membrane was studied by the gas phase permeation technique by monitoring the CO2, N2 and 2H2 transport fluxes in transient conditions. The results evidenced that the transport exhibited anomalous character. The experimental data could be reproduced assuming that the penetrant concentration in the GL coating, cint(t), reached a saturation value cs following compressed exponential kinetics cint(t) = cs[1 - e-(λrelt)β]. The relaxation time τrel = 1/λrel showed thermally activated behavior, and its value increased with the kinetic diameter of the penetrant molecules. The critical exponent β = 1.5 ± 0.1 for CO2 and N2 and β = 2.0 ± 0.1 for 2H2 did not change with temperature. Positron annihilation lifetime spectroscopy (PALS) analysis indicated that the average cross-section (hg) of the cavities in the GL coating exhibited comparable size to the kinetic diameter (σk) of the penetrant molecules. The results could be explained by assuming that the molecular infiltration in the GL structure occurred in nano-channels having distributed path lengths where the penetrant transport obeyed a configurational diffusion mechanism.

Fungal and bacterial contributions to codenitrification emissions of N2O and N2 following urea deposition to soil

Grazed pastures contribute significantly to anthropogenic emissions of N2O but the respective contributions of archaea, bacteria and fungi to codenitrification in such systems is unresolved. This study examined the relative contributions of bacteria and fungi to rates of denitrification and codenitrification under a simulated ruminant urine event. It was hypothesised that fungi would be primarily responsible for both codenitrification and total N2O and N2 emissions. The effects of bacterial (streptomycin), fungal (cycloheximide), and combined inhibitor treatments were measured in a laboratory mesocosm experiment, on soil that had received 15N labelled urea. Soil inorganic-N concentrations, N2O and N2 gas fluxes were measured over 51 days. On Days 42 and 51, when nitrification was actively proceeding in the positive control, the inhibitor treatments inhibited nitrification as evidenced by increased soil NH 4 + -N concentrations and decreased soil NO 2 − -N and NO 3 − -N concentrations. Codenitrification was observed to contribute to total fluxes of both N2O (≥ 33%) and N2 (≥ 3%) in urine-amended grassland soils. Cycloheximide inhibition decreased NH 4 + –15N enrichment and reduced N2O fluxes while reducing the contribution of codenitrification to total N2O fluxes by ≥ 66 and ≥ 42%, respectively. Thus, given archaea do not respond to significant urea deposition, it is proposed that fungi, not bacteria, dominated total N2O fluxes, and the codenitrification N2O fluxes, from a simulated urine amended pasture soil.

Cleaning level of the target before deposition by reactive direct current magnetron sputtering

We present a thorough study of the target-cleaning phase to estimate the healthiness of the target in a direct current (DC) magnetron sputtering deposition. The study is based on real-time plasma monitoring by means of optical emission spectroscopy during a traditional cleaning phase in an Ar atmosphere. In this work we demonstrate that intensities of Ar emission lines are sufficient indicators of the target cleanliness degree. To derive these results SiOxNy thin films were grown by reactive DC magnetron sputtering on silicon wafers for different deposition configurations of Ar, O2 and N2 fluxes. Refractive index of the resulting films is measured by in-situ spectroscopic-ellipsometry. A simple but robust estimator is used to determine the time when the target is ready to start deposition. Hence, this approach can be suited for an industrial environment since the time invested in the cleaning phase can be minimized avoiding the waste of material and time.

Benthic N pathways in illuminated and bioturbated sediments studied with network analysis

AbstractThe regulation of benthic nitrogen (N) cycling by multiple interactions among bacteria, macrofauna, and primary producers is poorly understood. We hypothesized that a biodiverse benthic system should better exploit the benthic N‐availability and retain N than a simpler one. Retention occurs by avoiding losses both to the water column via increased recycling and to the atmosphere via decreased N2 fluxes and by limiting energy‐costly processes as N‐fixation. We also hypothesized that primary producer‐bacterial competition is reduced in the presence of macrofauna due to mobilization of refractory N pools. To this purpose, the effects of two bioturbators (the detritivorous Sparganophilus tamesis and the filter‐feeding Corbicula spp.) and two primary producer growth forms (the rooted macrophyte Vallisneria spiralis and microphytobenthos) on benthic N cycling were studied. An array of N‐processes were measured along a complexity gradient (from bare sediments to all combinations of the above mentioned organisms), and experimental outcomes were analyzed via ecological network analysis (ENA). This suite of algorithms, applied to the microscale, revealed differential partitioning of N fluxes among bare sediments (highest denitrification rates), sediments with macrofauna (highest recycling), and sediments with rooted plants (highest N‐fixation). N2 losses and inputs were significantly reduced when all components were represented, and N requirements by primary producers were to a large extent supported by the activity of macrofauna. Ecological interactions in biodiverse benthic systems promoted an efficient exploitation of sedimentary N pools, increased the coupling between recycling and uptake, and maximized N use efficiency at the expenses of losses and imports.

Denitrification potential of the eastern oyster microbiome using a 16S rRNA gene based metabolic inference approach.

The eastern oyster (Crassostrea virginica) is a foundation species providing significant ecosystem services. However, the roles of oyster microbiomes have not been integrated into any of the services, particularly nitrogen removal through denitrification. We investigated the composition and denitrification potential of oyster microbiomes with an approach that combined 16S rRNA gene analysis, metabolic inference, qPCR of the nitrous oxide reductase gene (nosZ), and N2 flux measurements. Microbiomes of the oyster digestive gland, the oyster shell, and sediments adjacent to the oyster reef were examined based on next generation sequencing (NGS) of 16S rRNA gene amplicons. Denitrification potentials of the microbiomes were determined by metabolic inferences using a customized denitrification gene and genome database with the paprica (PAthway PRediction by phylogenetIC plAcement) bioinformatics pipeline. Denitrification genes examined included nitrite reductase (nirS and nirK) and nitrous oxide reductase (nosZ), which was further subdivided by genotype into clade I (nosZI) or clade II (nosZII). Continuous flow through experiments measuring N2 fluxes were conducted with the oysters, shells, and sediments to compare denitrification activities. Paprica properly classified the composition of microbiomes, showing similar classification results from Silva, Greengenes and RDP databases. Microbiomes of the oyster digestive glands and shells were quite different from each other and from the sediments. The relative abundance of denitrifying bacteria inferred by paprica was higher in oysters and shells than in sediments suggesting that oysters act as hotspots for denitrification in the marine environment. Similarly, the inferred nosZI gene abundances were also higher in the oyster and shell microbiomes than in the sediment microbiome. Gene abundances for nosZI were verified with qPCR of nosZI genes, which showed a significant positive correlation (F1,7 = 14.7, p = 6.0x10-3, R2 = 0.68). N2 flux rates were significantly higher in the oyster (364.4 ± 23.5 μmol N-N2 m-2 h-1) and oyster shell (355.3 ± 6.4 μmol N-N2 m-2 h-1) compared to the sediment (270.5 ± 20.1 μmol N-N2 m-2 h-1). Thus, bacteria carrying nosZI genes were found to be an important denitrifier, facilitating nitrogen removal in oyster reefs. In addition, this is the first study to validate the use of 16S gene based metabolic inference as a method for determining microbiome function, such as denitrification, by comparing inference results with qPCR gene quantification and rate measurements.

High-temperature stability of Pd alloy membranes containing Cu and Au

High-temperature stability of Pd-based membranes benefits their application in steam reformers and sulfur-contaminated H2 streams because both membrane reforming efficiency and sulfur tolerance of Pd alloys increase much with temperature. Hence, we investigated PdCu, PdAu, and PdCuAu membranes supported on porous ceramic tubes between 500°C and 650°C. Remarkably, PdCu membranes were much more stable than Au-containing ones. The H2 permeation rates of some PdAu and PdCuAu membranes declined at 550°C with substantially increasing N2 fluxes. This was triggered by severe morphological deformation of the Au alloy films into stoichiometrically inhomogeneous, cavernous structures. The H2 fluxes of the PdCu membranes started to decline at 650°C with leak flows increasing slightly. Moreover, the PdCu layer morphology remained dense and compositionally homogeneous even after testing for up to 4800h between 500 and 650°C. The strikingly different high-temperature stability can be understood by considering the divergent surface segregation tendencies of Cu and Au and their differing impact on hydrogen solubility in Pd alloys. As a result, Au may desorb much more easily from membranes than Cu leading to structural instability above 500°C during operation in H2. The instability of PdAu membranes at high temperatures may be mitigated by addition of sufficient Cu to obtain ternary membranes with good H2 permeability and better thermal stability.

Arctic shelves as platforms for biogeochemical activity: Nitrogen and carbon transformations in the Chukchi Sea, Alaska

Continental shelves comprise <5% of global ocean area but may account for a disproportionate 30% of primary production, 80% of organic matter burial, and >50% of marine denitrification. The Hanna Shoal region, part of the continental shelf system in the northeast Chukchi Sea, Alaska, is recognized for its high biodiversity and productivity. We investigated the role of sediments in organic matter decomposition and nutrient cycling at five stations on the shallow Hanna Shoal. In particular, we asked (1) how much sediment organic matter is remineralized in the Chukchi Sea, and what factors drive this degradation, (2) do sediments function as a net source for fixed nitrogen (thus fueling primary production in the overlying water), or as a net sink for fixed nitrogen (thereby removing it from the system), and (3) what is the balance between sediment NH4+ uptake and regeneration, and what factors drive NH4+ cycling? We conducted dark sediment core incubations to measure sediment O2 consumption, net N2 and nutrient (NH4+, NO3-, NO2-, PO43-) fluxes, and rates of sediment NH4+ cycling, including uptake and regeneration. Rates of sediment O2 consumption and NH4+ and PO43- efflux suggest that high organic matter remineralization rates occurred in these cold (−2°C) sediments. We estimated that total organic carbon remineralization accounted for 20–57% of summer export production measured on the Chukchi Shelf. Net N2 release was the dominant nitrogen flux, indicating that sediments acted as a net sink for bioavailable nitrogen via denitrification. Organic carbon remineralization via denitrification accounted for 6–12% of summer export production, which made up ~25% of the total organic carbon oxidized in Hanna Shoal sediments. These shallow, productive Arctic shelves are “hotspots” for organic matter remineralization.

Eelgrass meadows, Zostera marina (L.), facilitate the ecosystem service of nitrogen removal during simulated nutrient pulses in Shinnecock Bay, New York, USA

Seagrass meadows are important sites of nitrogen (N) transformations in estuaries, however, the role of N loading in driving relative rates of N fixation and denitrification in seagrass habitats is unclear. The current study quantified N fluxes in eelgrass meadows (Zostera marina (L.)) and nearby unvegetated sand in trials representing in situ and N enriched conditions. Net N2 fluxes were low or negative under in situ conditions in both eelgrass and sand. Under N enriched conditions, denitrification was higher than N-fixation, and denitrification in eelgrass was significantly higher than sand. Denitrification of water column NO3− was more significant than coupled nitrification-denitrification in the eelgrass. Denitrification was likely supported by greater organic carbon and N within the eelgrass sediment compared to sand. Eelgrass meadows in Shinnecock Bay may facilitate the ecosystem service of N removal and retention during short-term nutrient pulses that can originate from groundwater discharge and stormwater runoff.

Coastal stormwater wet pond sediment nitrogen dynamics

Wet ponds are a common type of stormwater control measure (SCM) in coastal areas of the southeastern US, but their internal nitrogen dynamics have not been extensively studied. Using flow-through intact sediment core incubations, net sediment N2 fluxes before and after a nitrate addition from five wet ponds spanning a range of ages (3.25–10years old) were quantified through membrane inlet mass spectrometry during early summer. Multiple locations within a single wet pond (6.16years old) were also sampled during ambient conditions in late summer to determine the combined effects of depth, vegetation, and flow path position on net N2 fluxes at the sediment-water interface. All pond sediments had considerable rates of net nitrogen fixation during ambient conditions, and net N2 fluxes during nitrate-enriched conditions were significantly correlated with pond age. Following a nitrate addition to simulate storm conditions, younger pond sediments shifted towards net denitrification, but older ponds exhibited even higher rates of net nitrogen fixation. The pond forebay had significantly higher rates of net nitrogen fixation compared to the main basin, and rates throughout the pond were an order of magnitude higher than the early summer experiment. These results identify less than optimal nitrogen processing in this common SCM, however, data presented here suggest that water column mixing and pond sediment excavation could improve the capacity of wet ponds to enhance water quality by permanently removing nitrogen.

Water diversion projects negatively impact lake metabolism: A case study in Lake Dazong, China

Water diversion projects are one of the major tools for counteracting water resource shortage in China. Many diversion projects aimed at diverting water from the Yangtze River or other rivers. This study, using one of the lakes receiving Yangtze River water as a case study, illustrated how nutrient, O2, and N2 fluxes across the sediment-water interface (SWI) changed with increasing nitrate loads during in situ benthic chamber incubation. Increasing nitrate loads in the overlying water caused complex changes in the SWI fluxes. Furthermore, responses of the SWI fluxes to increasing nitrate loads were largely driven by season. Dissolved inorganic carbon (DIC), ammonium (NH4+), CO2, and N2 fluxes increased with enhanced nitrate loads, likely induced by stimulated anaerobic mineralization. DIC had a stronger response to nitrate additions in late summer versus in winter. Nitrate influxes were enhanced by increasing nitrate loads in winter and showed a similar but stronger response in summer, with a shift from sediment release to influx. Sediment oxygen consumption decreased with increasing nitrate loads in both late summer and, especially, winter. Nitrate addition may have changed the proportions of aerobic and anaerobic respiration, i.e., nitrate competed with O2 as an electron acceptor. With increasing nitrate loads, anaerobic respiration was strengthened in the dark benthic chamber. Water diversion from the heavily polluted Yangtze River to Lake Dazong may be disadvantageous for controlling eutrophication, especially during the summer.

Influence of soil moisture on codenitrification fluxes from a urea-affected pasture soil

Intensively managed agricultural pastures contribute to N2O and N2 fluxes resulting in detrimental environmental outcomes and poor N use efficiency, respectively. Besides nitrification, nitrifier-denitrification and heterotrophic denitrification, alternative pathways such as codenitrification also contribute to emissions under ruminant urine-affected soil. However, information on codenitrification is sparse. The objectives of this experiment were to assess the effects of soil moisture and soil inorganic-N dynamics on the relative contributions of codenitrification and denitrification (heterotrophic denitrification) to the N2O and N2 fluxes under a simulated ruminant urine event. Repacked soil cores were treated with 15N enriched urea and maintained at near saturation (−1 kPa) or field capacity (−10 kPa). Soil inorganic-N, pH, dissolved organic carbon, N2O and N2 fluxes were measured over 63 days. Fluxes of N2, attributable to codenitrification, were at a maximum when soil nitrite (NO2−) concentrations were elevated. Cumulative codenitrification was higher (P = 0.043) at −1 kPa. However, the ratio of codenitrification to denitrification did not differ significantly with soil moisture, 25.5 ± 15.8 and 12.9 ± 4.8% (stdev) at −1 and −10 kPa, respectively. Elevated soil NO2− concentrations are shown to contribute to codenitrification, particularly at −1 kPa.

The nitrification inhibitor DMPP (3,4-dimethylpyrazole phosphate) reduces N2 emissions from intensively managed pastures in subtropical Australia

Intensively managed pastures receive high inputs ofnitrogen (N) fertilizer, rendering them prone to high N losses via denitrification following intensive rainfall events. Intense rainfall events, which are predicted to increase in frequency with changes in global climate, increase the risk of denitrification losses from agro-ecosystems. Nitrification inhibitors (NI) have been promoted to mitigate N losses, however their effect on total denitrification (N2 and N2O) and therefore their agronomic viability remains largely unknown. This study investigated the efficacy of the NI 3,4-dimethylpyrazole phosphate (DMPP) to reduce N2 and N2O emissions after heavy rainfall from three pastures with different soil textures in subtropical Australia. Emissions of N2 and N2O were measured over three weeks following the application of 15N urea (36.8 kg N ha−1) with and without DMPP. An intense rainfall event was simulated 10 days after fertilization to create saturated conditions in the topsoil. Emissions of N2O decreased after day 1, reflecting a rapid shift towards complete denitrification. Unlike N2O, N2 fluxes responded to the rainfall event with peak fluxes up to 3.8 kg N2-N ha−1 day−1. The main product of denitrification was N2, which accounted for more than 95% of cumulative denitrification losses across sites. Cumulative N2 losses over 21 days remained below 4 kg N ha−1 for the well-drained sandy loam, while up to 28 kg N2-N ha−1 were emitted from the clay and loam soils, demonstrating N2 emissions as a major pathway of N loss from intensively managed pastures. The magnitude of N2 losses across pasture sites reflects the combined effect of reduced soil gas diffusivity and microbial oxygen consumption on denitrification. DMPP reduced these N2 losses by more than 70%, but had no effect on N2O emissions, providing the first field based evidence that DMPP can substantially reduce N2 emissions. The reduction of agronomically significant N2 losses highlights the potential of DMPP to mitigate the impact of increased rainfall intensity on denitrification losses thereby improving N use efficiency for intensively managed pastures.

Effects of seasonality, transport pathway, and spatial structure on greenhouse gas fluxes in a restored wetland.

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open-water diffusion and ebullition fluxes of CO2 , CH4 , and N2 O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy-covariance measurements of whole-ecosystem CO2 and CH4 exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open-water and vegetated cover types. Annual open-water CO2 , CH4 , and N2 O emissions were 915±95g C-CO2 m-2 yr-1 , 2.9±0.5g C-CH4 m-2 yr-1 , and 62±17mgN-N2 Om-2 yr-1 , respectively. Diffusion dominated open-water GHG transport, accounting for >99% of CO2 and N2 O emissions, and ~71% of CH4 emissions. Seasonality was minor for CO2 emissions, whereas CH4 and N2 O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open-water fluxes (3.5±0.3kgCO2 -eqm-2 yr-1 ) exceeded that of vegetated zones (1.4±0.4kgCO2 -eqm-2 yr-1 ) due to high ecosystem respiration. After scaling results to the entire wetland using object-based cover classification of remote sensing imagery, net uptake of CO2 (-1.4±0.6ktCO2 -eqyr-1 ) did not offset CH4 emission (3.7±0.03ktCO2 -eqyr-1 ), producing an overall positive radiative forcing effect of 2.4±0.3ktCO2 -eqyr-1 . These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.

Seasonal and short-term variations in atmospheric potential oxygen at Ny-Ålesund, Svalbard

Oxygen in the atmosphere undergoes variations and changes in response to biospheric activities, ocean–atmosphere exchange and fossil fuel combustion. Continuous in situ measurements of atmospheric δ(O2/N2) and CO2 mole fraction were started at Ny-Ålesund, Svalbard (78.93°N, 11.83°E, 40 m a.s.l.) in November 2012. Atmospheric potential oxygen (APO) calculated from the measured O2 and CO2 values during November 2012–January 2015 show a clear seasonal cycle with a peak-to-peak amplitude of approximately 50 per meg. The seasonal cycle of APO simulated using an atmospheric transport model, with prescribed oceanic O2, N2 and CO2 fluxes at monthly time intervals, is in excellent agreement with the observed APO. However, in spring and early summer, high values of APO are observed irregularly on a timescale of hours to days. By comparing backward trajectories of air parcels released from the site with distributions of marine net primary production (NPP), and tagged tracer experiments made using the atmospheric transport model for APO, it is found that these high APO fluctuations are primarily attributable to O2 emissions from the Greenland Sea, the Norwegian Sea and the Barents Sea, due to marine biological productivity. Marine net community production, estimated based on the sea-to-air O2 flux derived from observed APO fluctuations, agrees with NPP obtained from satellite observations within an order of magnitude. The results obtained in this study have still some uncertainties, but our continuous observations of atmospheric δ(O2/N2) and CO2 mole fraction at Ny-Ålesund can play an important role in detecting possible changes in the carbon cycle in the near future.

Direct Raman Spectroscopic Measurements of Biological Nitrogen Fixation under Natural Conditions: An Analytical Approach for Studying Nitrogenase Activity.

Biological N2 fixation is a major input of bioavailable nitrogen, which represents the most frequent factor limiting the agricultural production throughout the world. Especially, the symbiotic association between legumes and Rhizobium bacteria can provide substantial amounts of nitrogen (N) and reduce the need for industrial fertilizers. Despite its importance in the global N cycle, rates of biological nitrogen fixation have proven difficult to quantify. In this work, we propose and demonstrate a simple analytical approach to measure biological N2 fixation rates directly without a proxy or isotopic labeling. We determined a mean N2 fixation rate of 78 ± 5 μmol N2 (g dry weight nodule)-1 h-1 of a Medicago sativa-Rhizobium consortium by continuously analyzing the amount of atmospheric N2 in static environmental chambers with Raman gas spectroscopy. By simultaneously analyzing the CO2 uptake and photosynthetic plant activity, we think that a minimum CO2 mixing ratio might be needed for natural N2 fixation and only used the time interval above this minimum CO2 mixing ratio for N2 fixation rate calculations. The proposed approach relies only on noninvasive measurements of the gas phase and, given its simplicity, indicates the potential to estimate biological nitrogen fixation of legume symbioses not only in laboratory experiments. The same methods can presumably also be used to detect N2 fluxes by denitrification from ecosystems to the atmosphere.

Perturbation-free measurement of in situ di-nitrogen emissions from denitrification in nitrate-rich aquatic ecosystems

Increased production of reactive nitrogen (Nr) from atmospheric di-nitrogen (N2) has greatly contributed to increased food production. However, enriching the biosphere with Nr has also caused a series of negative effects on global ecosystems, especially aquatic ecosystems. The main pathway converting Nr back into the atmospheric N2 pool is the last step in the denitrification process. Despite several attempts, there is still a need for perturbation-free methods for measuring in situ N2 fluxes from denitrification in aquatic ecosystems at the field scale. Such a method is needed to comprehensively quantify the N2 fluxes from aquatic ecosystems. Here we observed linear relationships between the δ15N-N2O signatures and the logarithmically transformed N2O/(N2+N2O) emission ratios. Through independent measurements, we verified that the perturbation-free N2 flux from denitrification in nitrate-rich aquatic ecosystems can be inferred from these linear relationships. Our method allowed the determination of field-scale in situ N2 fluxes from nitrate-rich aquatic ecosystems both with and without overlaying water. The perturbation-free in situ N2 fluxes observed by the new method were almost one order of magnitude higher than those by the sediment core method. The ability of aquatic ecosystems to remove Nr may previously have been severely underestimated.

Disentangling gross N2O production and consumption in soil.

The difficulty of measuring gross N2O production and consumption in soil impedes our ability to predict N2O dynamics across the soil-atmosphere interface. Our study aimed to disentangle these processes by comparing measurements from gas-flow soil core (GFSC) and 15N2O pool dilution (15N2OPD) methods. GFSC directly measures soil N2O and N2 fluxes, with their sum as the gross N2O production, whereas 15N2OPD involves addition of 15N2O into a chamber headspace and measuring its isotopic dilution over time. Measurements were conducted on intact soil cores from grassland, cropland, beech and pine forests. Across sites, gross N2O production and consumption measured by 15N2OPD were only 10% and 6%, respectively, of those measured by GFSC. However, 15N2OPD remains the only method that can be used under field conditions to measure atmospheric N2O uptake in soil. We propose to use different terminologies for the gross N2O fluxes that these two methods quantified. For 15N2OPD, we suggest using ‘gross N2O emission and uptake’, which encompass gas exchange within the 15N2O-labelled, soil air-filled pores. For GFSC, ‘gross N2O production and consumption’ can be used, which includes both N2O emitted into the soil air-filled pores and N2O directly consumed, forming N2, in soil anaerobic microsites.