Isotopomer Ratios Of N2O Research Articles

Lower soil nitrogen-oxide emissions associated with enhanced denitrification under replacing mineral fertilizer with manure in orchard soils

Emerging evidence suggests that replacing mineral fertilizers with organic livestock manure can effectively suppress reactive gaseous nitrogen (N) emissions from soils. However, the extent of this mitigation potential and the underlying microbial mechanisms in orchards remain unclear. To address this knowledge gap, we measured nitrous and nitric oxide (N2O and NO) emissions, microbial N cycling gene abundance, and N2O isotopomer ratios in pear and citrus orchards under three different fertilization regimes: no fertilization, mineral fertilizer, and manure plus mineral fertilizer. The results showed that although manure application caused large transient peaks of N2O, it reduced cumulative emissions of N2O and NO by an average of 20 % and 17 %, respectively, compared to the mineral fertilizer treatment. Partial replacement of mineral fertilizers with manure enhanced the contribution of AOA to nitrification and reduced the contribution of AOB, thus reducing N2O emissions from nitrification. Isotope analysis suggested that the pathway for N2O production in the soils of both orchards was dominated by bacterial denitrification and nitrifier denitrification. The manure treatment reduced the ratio of denitrification products. Additionally, the dual isotope mixing model results indicated that partially replacing mineral fertilizers with manure could promote soil denitrification, resulting in more N2O being reduced. N-oxide emissions were on average 67 % higher in the pear orchard than in the citrus orchard, probably due to the differences in soil physicochemical properties and growth habits between the two orchards. These findings underscore the potential of partially replacing mineral fertilizers with organic manure in orchards to reduce gaseous N emissions, contributing to the transition towards environmentally sustainable and climate-smart agricultural practices.

The isotopomer ratios of N2O in the Shaying River, the upper Huai River network, Eastern China: The significances of mechanisms and productions of N2O in the heavy ammonia polluted rivers

In order to figure out the effects of nitrogen pollution and dams on N2O production paths in the river systems, the isotopomer ratios in N2O, NH4+ and NO3−, as well as N2 concentrations and physico-chemical characteristics of water and sediments from the Shaying River system, which is the biggest tributary and major NH4+ contributor of the Huai River, were analyzed in the years 2015 and 2016. The results showed that the net productions of N2O (△N2O) in the river were pretty high, ranging from 12.9 to 440 nmol/L. N2O exhibited a narrow range in δ15Nbulk (−0.04 to 13.51‰), nevertheless a wide range in δ18O (22.54 to 59.90‰). Isotopocule diagram and Pearson correlation analysis indicated that isotopomer ratios of N2O were significantly affected by the mixing of N2O from difference production paths, not by N2O reduction. Relative contributions of nitrification and denitrification to N2O in the Shaying river system were deduced from the two end-members model. The contribution of nitrification to gross N2O was 58.5% on average, almost equal to the contribution of denitrification in summer, although denitrification was the dominant N2O source with average contribution of 75.6% in winter. No significant relationship was found either between △N2O and NH4+ or between △N2O and NO3− in the Shaying River. Heavy NO3− and COD loading reduced nitrification and increased the relative contribution of denitrification to N2O in winter. Heavy ammonia pollution caused pH values to decrease apparently, from 7.5 ± 0.3 in July to 6.3 ± 0.1 in December, resulting in denitrification being the dominant source to N2O in winter. Assimilation enhanced by the construction of dams had weakened the contribution of nitrification to N2O in summer in the Shaying River.

Partitioning of sources of N2O from soil treated with different types of fertilizers by the acetylene inhibition method and stable isotope analysis

Nitrous oxide (N2O) is one of the major human‐induced greenhouse gases. It has a large potential for global warming, a long lifetime and can deplete ozone. In China, nitrogenous fertilizer is an important source of N2O in agricultural soils, especially in vegetable production, where large amounts of nitrogen fertilizers are commonly used. Isotopomer ratios of N2O, oxygen and bulk nitrogen isotope values (δ18O and δ15Nbulk) and intramolecular 15N site preferences (SP) have recently been used to identify the sources of N2O. In this study, vegetable farm soil was incubated with manure (M), compound fertilizers (CF) and no fertilizers (NF). The acetylene inhibition method and a 15N isotopic technique were used to reveal the most likely microbial processes leading to N2O emissions. The results showed that the M treatment produced smaller N2O emissions than the CF treatment of the soil. Nitrifier denitrification or denitrifier denitrification made the greatest contribution to N2O production in the M treatment, accounting for approximately 60–89% of N2O production, whereas it was 25–55% and 37–66% in the CF and NF treatments under non‐acetylene conditions, respectively. Under non‐acetylene conditions, N2O reduction to N2 was largest for treatment M (32–90%), intermediate for NF (42–77%) and smallest for CF (20–51%). The results also showed that nitrifier nitrification and N2O reduction were partly inhibited under acetylene conditions, with a relative contribution of 9–20% and 0–83% for M, 39–48% and 0–46% for CF, and 25–35% and 0–56% for NF, respectively. Our results suggest that analysing isotopomer ratios of N2O with SP would be a practical approach for determining the contributions of microbial processes to N2O emissions from different pools in soil. The results revealed that manure produced smaller N2O emissions and promoted relatively larger rates of denitrification and N2O reduction than compound fertilizers with the same amount of N, which indicated that complementing chemical with manure fertilizers might help to reduce N2O emissions on vegetable‐producing land.Highlights We investigated emissions and sources of N2O with different types of fertilizers. For the same amount of N, manure produced less N2O than compound fertilizer. Stable isotope analysis revealed that denitrification was the dominant process in the manure treatment. The acetylene inhibition method can supplement δ15Nbulk compared with the SP model to assess N2O reduction in soil.

Estimate of bacterial and fungal N2O production processes after crop residue input and fertilizer application to an agricultural field by 15N isotopomer analysis

Nitrogen fertilizer and crop residue are important sources of N2O emission in agricultural ecosystems. This study aims to investigate N2O emission and microbial N2O production process after potato crop residue input and fertilizer application to an Andosol field. The experiment comprised crop residue treatments as follows: with residue (WR) and without residue (NR). We measured N2O emission, N2O isotopomer ratios (bulk N isotope ratios and intramolecular 15N site preference), and environmental parameters to estimate microbial N2O production processes. Comparative analysis of N2O isotopomer ratios and environmental parameters suggested that nitrification and bacterial denitrification were main N2O production processes after N application. Nitrifier denitrification would also contribute to N2O production after N application. In contrast, bacterial denitrification and fungal denitrification were important N2O production processes in the WR treatment after crop residue input, while nitrification also contributed to N2O production. Fungal denitrification was possibly important process during N2O peak after crop residue input. These findings reveal that real-time measurements of N2O isotopomers performed using a quantum cascade laser spectrometry system are useful for estimating microbial N2O production on a field scale.

Source identification of N2O produced during simulated wastewater treatment under different oxygen conditions using stable isotopic analysis

Nitrous oxide (N2O), a potent greenhouse gas which is important in climate change, is predicted to be the most dominant ozone depleting substance. It is mainly produced by oxidation of hydroxylamine (NH2OH) or reduction of nitrite (NO2-) during microbiological processes such as nitrification and denitrification. Wastewater treatment plant (WWTP) is one of the anthropogenic N2O sources because inorganic and organic nitrogen compounds are converted to nitrate (NO3-, in the case of standard system) or N2 (in the case of advanced system) by bacterial nitrification and denitrification in WWTP. We investigated the N2O production mechanisms during batch experiments that simulate wastewater treatment with activated sludge under various dissolved oxygen (DO) concentrations by stable isotope analysis. About 125mL of water was sampled from 30L incubation chamber for several times during the incubation, and concentration and isotopomer ratios of N2O and N-containing species were measured using gas chromatography/isotope ratio mass spectrometry (GC/IRMS). Ammonium (NH4+) consumption was accompanied by increment of nitrite (NO2-), and at the same time dissolved N2O concentration gradually increased to 4850 and 5650 nmol kg-1, respectively, during the four-hour incubation when DO concentrations were 0.2 and 0.5 mg L-1. Observed low SP values (0.2-8.9‰ at DO-0.2 mg L-1, -5.3-6.3‰ at DO-0.5 mg L-1, -1.0-8.3‰ at DO-0.8 mg L-1) in N2O and relationship of nitrogen isotope ratios between N2O and its potential substrates (NH4+, NO3-) suggested that N2O produced under the aerobic condition derived mainly from NO2- reduction by ammonia-oxidizing bacteria (nitrifier–denitrification).DOI: http://doi.dx.org/10.5564/mjc.v15i0.313Mongolian Journal of Chemistry 15 (41), 2014, p4-10

Soil denitrification potential and its influence on N2O reduction and N2O isotopomer ratios

N2O isotopomer ratios may provide a useful tool for studying N2O source processes in soils and may also help estimating N2O reduction to N2. However, remaining uncertainties about different processes and their characteristic isotope effects still hamper its application. We conducted two laboratory incubation experiments (i) to compare the denitrification potential and N2O/(N2O+N2) product ratio of denitrification of various soil types from Northern Germany, and (ii) to investigate the effect of N2O reduction on the intramolecular (15)N distribution of emitted N2O. Three contrasting soils (clay, loamy, and sandy soil) were amended with nitrate solution and incubated under N2 -free He atmosphere in a fully automated incubation system over 9 or 28 days in two experiments. N2O, N2, and CO2 release was quantified by online gas chromatography. In addition, the N2O isotopomer ratios were determined by isotope-ratio mass spectrometry (IRMS) and the net enrichment factors of the (15)N site preference (SP) of the N2O-to-N2 reduction step (η(SP)) were estimated using a Rayleigh model. The total denitrification rate was highest in clay soil and lowest in sandy soil. Surprisingly, the N2O/(N2O+N2) product ratio in clay and loam soil was identical; however, it was significantly lower in sandy soil. The IRMS measurements revealed highest N2O SP values in clay soil and lowest SP values in sandy soil. The η(SP) values of N2O reduction were between -8.2 and -6.1‰, and a significant relationship between δ(18)O and SP values was found. Both experiments showed that the N2O/(N2O+N2) product ratio of denitrification is not solely controlled by the available carbon content of the soil or by the denitrification rate. Differences in N2O SP values could not be explained by variations in N2O reduction between soils, but rather originate from other processes involved in denitrification. The linear δ(18)O vs SP relationship may be indicative for N2O reduction; however, it deviates significantly from the findings of previous studies.

Lime-nitrogen application affects nitrification, denitrification, and N2O emission in an acidic tea soil

Lime-N (calcium cyanamide, CaCN2) acts as both fertilizer and pesticide. Lime-N may reduce nitrous oxide (N2O) emission from soil, although its effectiveness and the relative mechanisms are not well understood. The aim of the study was to quantify the effect of lime-N on N2O emission from the acidic soil of tea fields. The study design consisted of two treatments: conventional fertilizer (CF) (application of conventional organo-chemical fertilizer) and lime-N (LN) (application of approximately 53 % of the applied N as lime-N and the remaining as conventional organo-chemical fertilizer). Both treatments had the same amount of N, P2O5, and K2O applied to soil between plant canopies; fertilizer was incorporated into soil. We measured N2O emissions and environmental and microbial parameters of soil between plant canopies and under the canopy of tea plants, including the concentrations of dicyandiamide and cyanamide derived from lime-N. Nitrous oxide emission from soil between plant canopies was lower in the LN treatment than in the CF treatment, and soil ammonium oxidation activity and soil denitrification rate decreased after lime-N application. We applied the acetylene inhibition technique and analyzed isotopomer ratios of N2O; the results of both techniques suggested that denitrification was the major process of N2O production in the soil between plant canopies, despite relatively low water-filled pore space. Cumulative N2O emission over the 366 days of the experiment was 36.0 % lower in the LN treatment than in the CF treatment (P < 0.05). Our results suggest that lime-N application decreases N2O emission by inhibiting both nitrification and denitrification processes in the acidic soil.

Distribution of nitrous oxide dissolved in water masses in the eastern subtropical North Pacific and its origin inferred from isotopomer analysis

N2O concentration and its isotopomer ratios were measured over a wide area from San Diego to Honolulu in the eastern subtropical North Pacific (ESNP). Waters in the study area had an N2O maximum (38.2–50.5 nmol kg−1) at 600–1000 m depth, which is similar to the profiles obtained previously in other areas in the North Pacific. We separated the seawater into five water masses (two for the surface layer, two for the middle layer, and one for the deep layer) and deduced N2O production–consumption mechanisms in each water body by use of N2O isotopomer ratios. The results showed that the mechanisms differ slightly among water masses. In the “coastal” surface layer, N2O is produced by nitrification (NH2OH oxidation). In the “open ocean” surface layer, it is produced mainly by nitrifier denitrification and to a lesser extent by nitrification under substrate-limited conditions. In both “upwelling” and “open ocean” middle layers it is produced mainly by denitrification and to a lesser extent by nitrifier denitrification. It is also partly reduced. In the deep layer, it is produced predominantly by denitrification with partial reduction. In this way, isotopomers aid elucidation of production–consumption mechanisms of N2O in the sea even though the mechanisms cannot always be ascertained.

Characterization and production and consumption processes of N2O emitted from temperate agricultural soils determined via isotopomer ratio analysis

[1] Isotopomer ratios of N2O (bulk nitrogen and oxygen isotope ratios, δ15Nbulk and δ18O, and intramolecular 15N site preference, SP) are useful parameters that characterize sources of this greenhouse gas and also provide insight into production and consumption mechanisms. We measured isotopomer ratios of N2O emitted from typical Japanese agricultural soils (Fluvisols and Andisols) planted with rice, wheat, soybean, and vegetables, and treated with synthetic (urea or ammonium) and organic (poultry manure) fertilizers. The results were analyzed using a previously reported isotopomeric N2O signature produced by nitrifying/denitrifying bacteria and a characteristic relationship between δ15Nbulk and SP during N2O reduction by denitrifying bacteria. Relative contributions from nitrification (hydroxylamine oxidation) and denitrification (nitrite reduction) to gross N2O production deduced from the analysis depended on soil type and fertilizer. The contribution from nitrification was relatively high (40%–70%) in Andisols amended with synthetic ammonium fertilizer, while denitrification was dominant (50%–90%) in the same soils amended with poultry manure during the period when N2O production occurred in the surface layer. This information on production processes is in accordance with that obtained from flux/concentration analysis of N2O and soil inorganic nitrogen. However, isotopomer analysis further revealed that partial reduction of N2O was pronounced in high-bulk density, alluvial soil (Fluvisol) compared to low-bulk density, volcanic ash soil (Andisol), and that the observed difference in N2O flux between normal and pelleted manure could have resulted from a similar mechanism with different rates of gross production and gross consumption. The isotopomeric analysis is based on data from pure culture bacteria and would be improved by further studies on in situ biological processes in soils including those by fungi. When flux/concentration-weighted average isotopomer ratios of N2O from various fertilized soils were examined, linear correlations were found between δ15Nbulk and δ18O, and between SP and δ15Nbulk. These relationships would be useful to parameterize isotopomer ratios of soil-emitted N2O for the modeling of the global N2O isotopomer budget. The results obtained in this study and those from previous firn/ice core studies confirm that the principal source of anthropogenic N2O is fertilized soils.

Isotopomeric analysis of N2O dissolved in a river in the Tokyo metropolitan area

River water has been suggested as a potential source of nitrous oxide (N2O), which is a greenhouse gas that is accumulating rapidly in the troposphere and which is a precursor to stratospheric NOx that depletes ozone. Previous studies on freshwater N2O sources have specifically examined estuaries where sedimentary N2O production might be important and a few points near anthropogenic nitrogen sources such as agricultural or municipal wastewater areas. Here we present the first observation of a temporal and horizontal distribution of N2O and its isotopomers between the midstream and estuary of an urban river. Surface water was supersaturated (100-6800%) with N2O at all stations during the study period. The average or maximum saturation value was greater than described in most previous reports. High N2O concentrations were observed near sewage plants and the unique signature of isotopomer ratios implied direct N2O addition from the plants. The isotopomer ratios also suggested N2O production/consumption at the sediment-water interface. Fluxes and isotopomer ratios of N2O, from the river to the atmosphere, estimated from our observations, indicated that the urban river is indeed a source of atmospheric N2O and that its production could be distinguished from other natural or anthropogenic sources using isotopomer ratios.

Isotopomeric characterization of N2O produced, consumed, and emitted by automobiles

Fossil fuel combustion is the second largest anthropogenic source of nitrous oxide (N2O) after agriculture. The estimated global N2O flux from combustion sources, as well as from other sources, still has a large uncertainty. Herein, we characterize automobile sources using N2O isotopomer ratios (nitrogen and oxygen isotope ratios and intramolecular site preference of 15N, SP) to assess their contributions to total global sources and to deconvolute complex production/consumption processes during combustion and subsequent catalytic treatments of exhaust. Car exhaust gases were sampled under running and idling state, and N2O isotopomer ratios were measured by mass spectrometry. The N2O directly emitted from an engine of a vehicle running at constant velocity had almost constant isotopomer ratios (delta15Nbulk = -28.7 +/- 1.2 per thousand, delta18O = 28.6 +/- 3.3 per thousand, and SP = 4.2 +/- 0.8 per thousand) irrespective of the velocity. After passing through catalytic converters, the isotopomer ratios showed an increase which varied with the temperature and the aging of the catalysts. The increase suggests that both production and consumption of N2O occur on the catalyst and that their rates can be comparable. It was noticed that in the idling state, the N2O emitted from a brand new car has higher isotopomer ratios than that from used cars, which indicate that technical improvements in catalytic converters can reduce the N2O from mobile combustion sources. On average, the isotopomeric signatures of N2O finally emitted from automobiles are not sensitive to running/idling states or to aging of the catalysts. Characteristic average isotopomer ratios of N2O from automobile sources are estimated at -4.9 +/- 8.2 per thousand, 43.5 +/- 13.9 per thousand, and 12.2 +/- 9.1 per thousand for delta15Nbulk, delta18O, and SP, respectively.

Role of nitrification and denitrification on the nitrous oxide cycle in the eastern tropical North Pacific and Gulf of California

Nitrous oxide (N2O) is an important atmospheric greenhouse gas and is involved in stratospheric ozone depletion. Analysis of the isotopomer ratios of N2O (i.e., the intramolecular distribution of 15N within the linear NNO molecule and the conventional N and O isotope ratios) can elucidate the mechanisms of N2O production and destruction. We analyzed the isotopomer ratios of dissolved N2O at a site in the eastern tropical North Pacific (ETNP) and a site in the Gulf of California (GOC). At these sites, the flux of N2O to the atmosphere is extremely high but denitrification activity in the oxygen minimum zone (OMZ) also reduces N2O to N2. We estimated the isotopomeric enrichment factors for N2O reduction by denitrification. The factor was −11.6 ± 1.0‰ for the bulk (average) N, −19.8 ± 2.3‰ for the center N (α‐site nitrogen), −3.4 ± 0.3‰ for the end N (β‐site nitrogen), and −30.5 ± 3.2‰ for the 18O of N2O. Isotopomer analysis of N2O suggests that nitrifiers should contribute to N2O production more than denitrifiers at the oxycline above the OMZs in the ETNP (50–80 m) and in the GOC (80–300 m). In contrast, denitrifiers should largely contribute to the N2O production and consumption in the OMZs both in the ETNP (120–130 m) and in the GOC (600–800 m). The N2O isotopomer analysis will be a useful tool for resolving the distribution of water masses that carry a signal of N loss by denitrification.

Contributions of denitrification and mixing on the distribution of nitrous oxide in the North Pacific

We analyzed N2O isotopomer ratios (distribution of isotopes within N2O molecules) in the eastern tropical North Pacific. The N2O isotopomer ratios indicate the contribution of denitrification in the oxygen minimum zone (OMZ, ∼600 m in depth) in the western North Pacific, which is not consistent with the widely accepted nitrification hypothesis. Our models indicate that the N2O yield per mole O2 consumed (dN2O/−dO2) is 0.008 (0–0.015) nmol/μmol during remineralization and nitrification in the western North Pacific. Nitrification in aerobic deep waters is a minor source of oceanic N2O, whereas the N2O production in the OMZ is the dominant factor for the oceanic N2O distribution. The denitrification in the OMZ is consistent with the correlation between ΔN2O (level above atmospheric equilibrium) and AOU (apparent oxygen utilization), and the parallel 18O‐enrichment of N2O and O2 in the OMZ, which have been believed to support the nitrification hypothesis.

Dual isotope and isotopomer ratios of N2O emitted from a temperate grassland soil after fertiliser application.

The N2O and N2 fluxes emitted from a temperate UK grassland soil after fertiliser application (equivalent to 25 and 75 kg N ha(-1)) were simultaneously measured, using a new automated soil incubation system, which replaces soil atmosphere (N2 dominated) with a He+O2 mixture. Dual isotope and isotopomer ratios of the emitted N2O were also determined. Total N2O and N2 fluxes were significantly lower (P<0.001) in the control (0 kg N) than in the 25 and 75 kg N treatments. The total N2O flux was significantly higher (P<0.001) in the 75 kg N than in the 25 kg N treatment. The general patterns of N2O and N2 fluxes were similar for both fertiliser treatments. The total gaseous N loss in the control treatment was nearly all N2, whereas in the fertiliser treatment more N2O than N2 was emitted from the soil. The ratio N2O/N2 fluxes as measured during the experiment suggested three phases in N2O production, in phase 1 nitrification>denitrification, in phase 2 denitrification>nitrification, and in phase 3 denitrification (and total denitrification)>>nitrification. Dual delta15N and delta18O isotope and isotopomer (delta15Nalpha and delta15Nbeta) value ratios of emitted N2O also pointed towards an increasing dominance of the production of N2O by denitrification and total denitrification. The site preference value from the soil-emitted N2O was lower than the troposphere value. This confirmed that the enhanced troposphere N2O site preference could result from back injection of N2O from the stratosphere. The measurements of N2O/N2 flux ratio and the isotopic content of emitted N2O pointed, independently, to similar temporal trends in N2O production processes after fertiliser application to grassland soil. This confirmed that both measurements are suitable diagnostic tools to study the N2O production process in soils.