Isotope fractionation factors controlling isotopocule signatures of soil-emitted N₂O produced by denitrification processes of various rates

Microbial nitrogen transformations tracked by natural abundance isotope studies and microbiological methods: A review

Nitrogen is an essential nutrient in the environment that exists in multiple oxidation states in nature. Numerous microbial processes are involved in its transformation. Knowledge about very complex N cycling has been growing rapidly in recent years, with new information about associated isotope effects and about the microbes involved in particular processes. Furthermore, molecular methods that are able to detect and quantify particular processes are being developed, applied and combined with other analytical approaches, which opens up new opportunities to enhance understanding of nitrogen transformation pathways. This review presents a summary of the microbial nitrogen transformation, including the respective isotope effects of nitrogen and oxygen on different nitrogen-bearing compounds (including nitrates, nitrites, ammonia and nitrous oxide), and the microbiological characteristics of these processes. It is supplemented by an overview of molecular methods applied for detecting and quantifying the activity of particular enzymes involved in N transformation pathways. This summary should help in the planning and interpretation of complex research studies applying isotope analyses of different N compounds and combining microbiological and isotopic methods in tracking complex N cycling, and in the integration of these results in modelling approaches.

Effect of increased carbon load on denitrification efficiency and nitrate isotope enrichment factors in subsurface wastewater infiltration system

Denitrification plays a dominant role in nitrate removal in subsurface wastewater infiltration system (SWIS). However, the effect of increased carbon (C) load on denitrification efficiency in the SWIS remain unclear. In this study, we used analyses of stable isotopes of nitrogen (N) and oxygen (O) in nitrate to investigate the N and O isotope enrichment factors (¹⁵ ε and ¹⁸ ε) and quantified N losses via denitrification in SWIS. The results demonstrated that an increase in C loads positively affected the pollutant removal performance of SWIS. The natural abundance of ¹⁵ N and ¹⁸ O increased with decreasing nitrate concentration from 12.5 to 7.3 mg/L, accompanied by increased ¹⁵ ε and ¹⁸ ε from -8.7‰ to -10.6‰ and -5.9‰ to -8.2‰, respectively, as the C load increased from 18 to 36 g/(m²  d). The contribution of denitrification to nitrate removal was 62%, 71%, and 77% when C loads were 18, 27, and 36 g/(m²  d), respectively, indicating that increased C loads could improve the nitrate removal through denitrification in SWIS. PRACTITIONER POINTS: Increasing C loads positively affected the nitrate removal performance of SWIS. N and O isotope enrichment factors of nitrate increased with the enhancement of influent C load. A C load of 36 g/(m²  d) is recommended in SWIS to improve the N removal performance and denitrification efficiency.

Sources of nitrous oxide emissions from hydroponic tomato cultivation: Evidence from stable isotope analyses

Introduction

Hydroponic vegetable cultivation is characterized by high intensity and frequent nitrogen fertilizer application, which is related to greenhouse gas emissions, especially in the form of nitrous oxide (N₂O). So far, there is little knowledge about the sources of N₂O emissions from hydroponic systems, with the few studies indicating that denitrification could play a major role.

Methods

Here, we use evidence from an experiment with tomato plants (Solanum lycopersicum) grown in a hydroponic greenhouse setup to further shed light into the process of N₂O production based on the N₂O isotopocule method and the ¹⁵N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after ¹⁵N labeling at two occasions using the closed chamber method and analyzed by gas chromatography and stable isotope ratio mass spectrometry.

Results

The isotopocule analyses revealed that either heterotrophic bacterial denitrification (bD) or nitrifier denitrification (nD) was the major source of N₂O emissions, when a typical nutrient solution with a low ammonium concentration (1-6 mg L^-1) was applied. Furthermore, the isotopic shift in ¹⁵N site preference and in δ¹⁸O values indicated that approximately 80-90% of the N₂O produced were already reduced to N₂ by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the ¹⁵N labeling (30-60 mg L^-1), results from the ¹⁵N tracing approach showed that N₂O mainly originated from bD. Both, ¹⁵N label supplied in the form of ammonium and ¹⁵N label supplied in the form of nitrate, increased the ¹⁵N enrichment of N₂O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of ¹⁵N-labeled ammonium into nitrate.

Discussion/Conclusion

Comparing the results from N₂O isotopocule analyses and the ¹⁵N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N₂O emissions observed in this study. Overall, our findings help to better understand the processes underlying N₂O and N₂ emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N₂O mitigation measures.

Mechanism of biochar on nitrification and denitrification to N2O emissions based on isotope characteristic values

To clarify the mechanism of biochar on nitrification and denitrification to N₂O emissions in farmland soil, the effects of combined application of biochar and different nitrogen sources on the contributions of nitrification and denitrification to N₂O emissions were studied using isotope characteristic values. The results showed that the soil N₂O emissions from ammonium nitrogen fertilizer treatments were significantly higher than that from nitrate nitrogen fertilizer treatments. The biochar combined with ammonium nitrogen fertilizer reduced soil N₂O emissions by 31.0%-30.8%, and biochar combined with nitrate nitrogen fertilizer reduced soil N₂O emissions by 70.6%-63.0%. The isotope model showed that the application of ammonium nitrogen fertilizer was more favorable for soil nitrification in the early stage of the experiment (0-2 d), and more favorable for denitrification in the middle and later stages of the experiment (3-17 d). Application of nitrate nitrogen fertilizer enhanced the nitrification of soil nitrifying bacteria in the early and middle stages of the experiment (0-8 d), and the denitrification of soil denitrifying bacteria in the later stage of the experiment (9-17 d). The effects of biochar on N₂O emissions were mainly in the middle and later stages of the experiment by promoting the nitrification of nitrifying bacteria and inhibiting denitrification of denitrifying bacteria, so as to reduce N₂O emission in soil. These results may help to understand the mitigation mechanism of biochar on N₂O emission in upland soil.

Effect of S2O32--S addition on Anammox coupling sulfur autotrophic denitrification and mechanism analysis using N and O dual isotope effects

Anaerobic ammonia oxidation (Anammox) coupling sulfur autotrophic denitrification is an effective method for the advanced nitrogen removal from the wastewater with limited carbon source. The influence of S₂O₃^2--S addition on Anammox coupling sulfur autotrophic denitrification was investigated by adding different concentrations of S₂O₃^2--S (0, 39, 78, 156 and 312 mg/L) to the Anammox system. The contribution of sulfur autotrophic denitrification and Anammox to nitrogen removal at S₂O₃^2--S concentrations of 156 mg/L was 75% ∼83% and 17%∼25%, respectively, and the mixed system achieved completely nitrogen removal. However, Anammox bioactivity was completely inhibited at S₂O₃^2--S concentrations up to 312 mg/L, and only sulfur autotrophic denitrification occurred. The isotopic effects of NO₂^--N (δ¹⁵N_NO2 and δ¹⁸O_NO2) and NO₃^--N (δ¹⁵N_NO3 and δ¹⁸O_NO3) during Anammox coupling sulfur autotrophic denitrification showed a gradual decrease trend with the increase of S₂O₃^2--S addition. The ratios of δ¹⁵N_NO2:δ¹⁸O_NO2 and δ¹⁵N_NO3:δ¹⁸O_NO3 was maintained at 1.30-2.41 and 1.36-2.52, respectively, which revealed that Anammox was dominant nitrogen removal pathway at S₂O₃^2--S concentrations less than 156 mg/L. Microbial diversity gradually decreased with the increase of S₂O₃^2--S. The S₂O₃^2--S addition enhanced the S₂O₃^2--driven autotrophic denitrification and weakened the Anammox, leading to a gradually decreasing trend of the proportion of Candidatus Brocadia as Anammox bacteria from the initial 27% to 4% (S₂O₃^2--S of 156 mg/L). Yet Norank-f-Hydrogenophilaceae (more than 50%) and Thiobacillus (54%) as functional bacteria of autotrophic denitrification obviously increased. The appropriate amount of S₂O₃^2--S addition promoted the performance of Anammox coupling sulfur autotrophic denitrification achieved completely nitrogen removal.

The influence of soil acidification on N2O emissions derived from fungal and bacterial denitrification using dual isotopocule mapping and acetylene inhibition

Denitrification, as both origins and sinks of N₂O, occurs extensively, and is of critical importance for regulating N₂O emissions in acidified soils. However, whether soil acidification stimulates N₂O emissions, and if so for what reason contributes to stimulate the emissions is uncertain and how the N₂O fractions from fungal (f_fD) and bacterial (f_bD) denitrification change with soil pH is unclear. Thus, a pH gradient (6.2, 7.1, 8.7) was set via manipulating cropland soils (initial pH 8.7) in North China to illustrate the effect of soil acidification on fungal and bacterial denitrification after the addition of KNO₃ and glucose. For source partitioning, we used and compared SP/δ¹⁸O mapping approach (SP/δ¹⁸O MAP) and acetylene inhibition technique combined isotope two endmember mixing model (AIT-IEM). The results showed significantly higher N₂O emissions in the acidified soils (pH 6.2 and pH 7.1) compared with the initial soil (pH 8.7). The cumulative N₂O emissions during the whole incubation period (15 days) ranged from 7.1 mg N kg^-1 for pH 8.7-18.9 mg N kg^-1 for pH 6.2. With the addition of glucose, relative to treatments without glucose, this emission also increased with the decrement of pH values, and were significantly stimulated. Similarly, the highest N₂O emissions and N₂O/(N₂O + N₂) ratios (rN₂O) were observed in the pH 6.2 treatment. But the difference was the highest cumulative N₂O + N₂ emissions, which were recorded in the pH 7.1 treatment based on SP/δ¹⁸O MAP. Based on both approaches, f_fD values slightly increased with the acidification of soil, and bacterial denitrification was the dominant pathway in all treatments. The SP/δ¹⁸O MAP data indicated that both the rN₂O and f_fD were lower compared to AIT-IEM. It has been known for long that low pH may lead to high rN₂O of denitrification and f_fD, but our documentation of a pervasive pH-control of rN₂O and f_fD by utilizing combined SP/δ¹⁸O MAP and AIT-IEM is new. The results of the evaluated N₂O emissions by acidified soils are finely explained by high rN₂O and enhanced f_fD. We argue that soil pH management should be high on the agenda for mitigating N₂O emissions in the future, particularly for regions where long-term excessive nitrogen fertilizer is likely to acidify the soils.

What can we learn from N2 O isotope data? - Analytics, processes and modelling

The isotopic composition of nitrous oxide (N2 O) provides useful information for evaluating N2 O sources and budgets. Due to the co-occurrence of multiple N2 O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N2 O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope-ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N2 O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site-specific analyses. When reviewing the link between the N2 O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot-to-global scales, mixing of N2 O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi-quantitative attribution of co-occurring N2 O production and reduction processes. More recently, process-based N2 O isotopic models have been developed for natural abundance and 15 N-tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter-laboratory comparisons, further exploration of N2 O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N2 O isotope community will continue to advance our understanding of N2 O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets.

Evaluation of N2O sources after fertilizers application in vegetable soil by dual isotopocule plots approach

Nitrogen (N) fertilizer is the major deriver of nitrous oxide (N₂O) emissions in agricultural soil. In the vegetable fields in China both inorganic and organic fertilizers are largely applied as basic sources of nitrogen. Identifying the effects of fertilizer type on soil microbial activities involved in N₂O emissions would be of great help for future development of N₂O reduction strategies. N₂O isotopocule deltas, including δ¹⁵N^bulk, δ¹⁸O and SP (the ¹⁵N site preference in N₂O), have been used to analyze microbial pathways of N₂O production under different treatments, including bio-organic fertilizer treatment, half bio-organic fertilizer and half urea (mixed fertilizer) treatment, urea treatment and no fertilizer treatment. We measured environmental factors, N₂O fluxes and N₂O isotopocule deltas to evaluate the dynamics of N₂O emissions and constructed the dual isotopocule plots (δ¹⁵N^bulk vs. SP and δ¹⁸O vs. SP) of the main N₂O emission phases to assess contribution of the involved microbial processes (bacterial nitrification, bacterial denitrification, nitrifier denitrification and fungal denitrification). According to the results of the main N₂O emission phases, we found that bio-organic fertilizer and mix fertilizer treatments had significantly lower N₂O emissions compared to urea treatment, with average N₂O fluxes of 1477 ± 204, 1243 ± 187 and 1941 ± 164 μg m^-3 h^-1, respectively, but there were no significant effects on mineral N and cabbage yield. In addition, the urea treatment and the mixed fertilizer treatment had close and higher nitrogen use efficiency. Furthermore, the δ¹⁸O vs. SP plot was useful for providing insight into microbial processes, showing that fungal denitrification/bacterial nitrification was the dominant microbial pathway and bio-organic fertilizer and mix fertilizer treatments had higher denitrification and N₂O reduction compared to urea treatment. Those findings demonstrated that the partial replacement of urea with bio-organic fertilizer was a better choice, by means of enhancing denitrification to reduce N₂O emissions and also guaranteeing the nitrogen use efficiency and the cabbage yield.

Quantifying N2O reduction to N2 during denitrification in soils via isotopic mapping approach: Model evaluation and uncertainty analysis

The last step of denitrification, i.e. the reduction of N₂O to N₂, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to establish mitigation strategies for N₂O. However, assessing N₂ production via denitrification at large spatial scales is still not possible due to lack of reliable quantitative approaches. Here, we present a novel numerical "mapping approach" model using the δ¹⁵N^sp/δ¹⁸O slope that has been proposed to potentially be used to indirectly quantify N₂O reduction to N₂ at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He-O₂ atmosphere. Furthermore, we analyse the contribution of different parameters to the uncertainty of the model. The model performance strongly differed between studies and incubation conditions. Re-evaluation of the previous data set demonstrated that using soils-specific instead of default endmember values could largely improve model performance. Since the uncertainty of modelled N₂O reduction was relatively high, further improvements to estimate model parameters to obtain more precise estimations remain an on-going matter, e.g. by determination of soil-specific isotope fractionation factors and isotopocule endmember values of N₂O production processes using controlled laboratory incubations. The applicability of the mapping approach model is promising with an increasing availability of real-time and field based analysis of N₂O isotope signatures.

Constraining N cycling in the ecosystem model LandscapeDNDC with the stable isotope model SIMONE

The isotopic composition (ic) of soil nitrogen (N) and, more recently, the intramolecular distribution of ¹⁵ N in the N₂ O molecule (site preference, SP) are powerful instruments to identify dominant N turnover processes, and to attribute N₂ O emissions to their source processes. Despite the process information contained in the ic of N species and the associated potential for model validation, the implementation of isotopes in ecosystem models has lagged behind. To foster the validation of ecosystem models based on the ic of N species, we developed the stable isotope model for nutrient cycles (SIMONE). SIMONE uses fluxes between ecosystem N pools (soil organic N, mineral N, plants, microbes) calculated by biogeochemical models, and literature isotope effects for these processes to calculate the ic of N species. Here, we present the concept of SIMONE, apply it to simulations of the biogeochemical model LandscapeDNDC, and assess the capability of ¹⁵ N-N₂ O and, to our knowledge for the first time, SP, to constrain simulated N fluxes by LandscapeDNDC. LandscapeDNDC successfully simulated N₂ O emission, soil nitrate, and ammonium, as well as soil environmental conditions of an intensively managed grassland site in Switzerland. Accordingly, the dynamics of ¹⁵ N-N₂ O and SP of soil N₂ O fluxes as simulated by SIMONE agreed well with measurements, though ¹⁵ N-N₂ O was on average underestimated and SP overestimated (root-mean-square error [RMSE] of 8.4‰ and 7.3‰, respectively). Although ¹⁵ N-N₂ O could not constrain the N cycling process descriptions of LandscapeDNDC, the overestimation of SP indicated an overestimation of simulated nitrification rates by 10-59% at low water content, suggesting the revision of the corresponding model parameterization. Our findings show that N isotope modeling in combination with only recently available high- frequency measurements of the N₂ O ic are promising tools to identify and address weaknesses in N cycling of ecosystem models. This will finally contribute to augmenting the development of model-based strategies for mitigating N pollution.

Improved isotopic model based on 15 N tracing and Rayleigh-type isotope fractionation for simulating differential sources of N2 O emissions in a clay grassland soil

RATIONALE

Isotopic signatures of N2 O can help distinguish between two sources (fertiliser N or endogenous soil N) of N2 O emissions. The contribution of each source to N2 O emissions after N-application is difficult to determine. Here, isotopologue signatures of emitted N2 O are used in an improved isotopic model based on Rayleigh-type equations.

METHODS

The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N2 O, N2 and CO2 over a 12-day incubation period. To determine the source of N2 O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2 O and those from the 15 N-tracing technique.

RESULTS

The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The 15 N-enrichment analysis shows that initially ~70% of the emitted N2 O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N-pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh-type model applied to N2 O isotopocules data (δ15 Nbulk -N2 O values) shows poor agreement with the measurements for the original one-pool model for treatment 1c; the two-pool models gives better results when using a third-order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches.

CONCLUSIONS

The importance of N2 O emissions from different N-pools in soil for the interpretation of N2 O isotopocules data was demonstrated using a Rayleigh-type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N-pool sizes.

Evaluation of nitrate sources and the percent contribution of bacterial denitrification in hyporheic zone using isotope fractionation technique and multi-linear regression analysis

Denitrification has documented as a promising pathway to permanently remove nitrate from a system. Numerous studies have used the isotope fractionation technique (IFT) to evaluate the denitrification rate in the constructed wetlands (CWs), but the potential of IFT method to quantify the denitrification rate in hyporheic zone (saturated sediments beneath a stream) is still challenging. Thus, more studies are required to investigate that if measurements of the natural abundance of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- (IFT) can be employed to calculate the fate of nitrate in hyporheic zone. Therefore, in this study, the possibility of the IFT to quantify the hyporheic-denitrification rate was investigated. Then, the results were verified by the combined application of the pre-established net Sediment N₂ flux and multi-linear regression analysis (p < 0.01). Finally, the groundwater bacterial groups (Fecal coliform (FC) and Escherichia coli (EC)), and the mass balance isotope mixing model were used to investigate the dominant sources of hyporheic-nitrate. The IFT reveals that denitrification contributes 74.1% and 29.1% of the hyporheic-nitrate removal during dry and wet seasons, respectively. The multi-linear regression analysis, considering at 99% confidence interval (R² = 92.1%; n = 44; p < 0.01), slightly overestimates the rate and the percent contribution of denitrification in the dry season (475.15 ± 101.18 μmol/m2d; 80.7%) and underestimates it during the wet season (205.072 ± 35.39 μmol/m2d; 24.01%). The analysis of EC and FC demonstrates that manure (41.9 ± 4.2%) and sewage (54.1 ± 8.9%) are the dominant contributors of the hyporheic-nitrate load. In addition, the results achieved by the analysis of the fecal bacterial indicators (EC and FC) were confirmed by NO₃^-/Cl^- vs Cl^- diagram. This study provides an alternative-initiative framework to accurately quantify the spatio-seasonal variations in the hyporheic-nitrate sources and hyporheic-denitrification rate that enables decision-makers to apply appropriate and targeted strategies to regulate nitrate load in river-aquifer systems.

High nitrogen isotope fractionation of nitrate during denitrification in four forest soils and its implications for denitrification rate estimates

Denitrification is a major process contributing to the removal of nitrogen (N) from ecosystems, but its rate is difficult to quantify. The natural abundance of isotopes can be used to identify the occurrence of denitrification and has recently been used to quantify denitrification rates at the ecosystem level. However, the technique requires an understanding of the isotopic enrichment factor associated with denitrification, which few studies have investigated in forest soils. Here, soils collected from two tropical and two temperate forests in China were incubated under anaerobic or aerobic laboratory conditions for two weeks to determine the N and oxygen (O) isotope enrichment factors during denitrification. We found that at room temperature (20°C), NO₃^- was reduced at a rate of 0.17 to 0.35μgNg^-1h^-1, accompanied by the isotope fractionation of N (¹⁵ε) and O (¹⁸ε) of 31‰ to 65‰ (48.3±2.0‰ on average) and 11‰ to 39‰ (18.9±1.7‰ on average), respectively. The N isotope effects were, unexpectedly, much higher than reported in the literature for heterotrophic denitrification (typically ranging from 5‰ to 30‰) and in other environmental settings (e.g., groundwater, marine sediments and agricultural soils). In addition, the ratios of Δδ¹⁸O:Δδ¹⁵N ranged from 0.28 to 0.60 (0.38±0.02 on average), which were lower than the canonical ratios of 0.5 to 1 for denitrification reported in other terrestrial and freshwater systems. We suggest that the isotope effects of denitrification for soils may vary greatly among regions and soil types and that gaseous N losses may have been overestimated for terrestrial ecosystems in previous studies in which lower fractionation factors were applied.

Estimating N2 O processes during grassland renewal and grassland conversion to maize cropping using N2 O isotopocules

RATIONALE

Enhanced nitrous oxide (N₂ O) emissions can occur following grassland break-up for renewal or conversion to maize cropping, but knowledge about N₂ O production pathways and N₂ O reduction to N₂ is very limited. A promising tool to address this is the combination of mass spectrometric analysis of N₂ O isotopocules and an enhanced approach for data interpretation.

METHODS

The isotopocule mapping approach was applied to field data using a δ¹⁵ N^sp_N2O and δ¹⁸ O_N2O map to simultaneously determine N₂ O production pathways contribution and N₂ O reduction for the first time. Based on the isotopic composition of N₂ O produced and literature values for specific N₂ O pathways, it was possible to distinguish: (i) heterotrophic bacterial denitrification and/or nitrifier denitrification and (ii) nitrification and/or fungal denitrification and the contribution of N₂ O reduction.

RESULTS

The isotopic composition of soil-emitted N₂ O largely resembled the known end-member values for bacterial denitrification. The isotopocule mapping approach indicated different effects of N₂ O reduction on the isotopic composition of soil-emitted N₂ O for the two soils under study. Differing N₂ O production pathways in different seasons were not observed, but management events and soil conditions had a significant impact on pathway contribution and N₂ O reduction. N₂ O reduction data were compared with a parallel ¹⁵ N-labelling experiment.

CONCLUSIONS

The field application of the isotopocule mapping approach opens up new prospects for studying N₂ O production and consumption of N₂ O in soil simultaneously based on mass spectrometric analysis of natural abundance N₂ O. However, further studies are needed in order to properly validate the isotopocule mapping approach.

Relative Contribution of nirK- and nirS- Bacterial Denitrifiers as Well as Fungal Denitrifiers to Nitrous Oxide Production from Dairy Manure Compost

The relative contribution of fungi, bacteria, and nirS and nirK denirifiers to nitrous oxide (N₂O) emission with unknown isotopic signature from dairy manure compost was examined by selective inhibition techniques. Chloramphenicol (CHP), cycloheximide (CYH), and diethyl dithiocarbamate (DDTC) were used to suppress the activity of bacteria, fungi, and nirK-possessing denitrifiers, respectively. Produced N₂O were surveyed to isotopocule analysis, and its ¹⁵N site preference (SP) and δ¹⁸O values were compared. Bacteria, fungi, nirS, and nirK gene abundances were compared by qPCR. The results showed that N₂O production was strongly inhibited by CHP addition in surface pile samples (82.2%) as well as in nitrite-amended core samples (98.4%), while CYH addition did not inhibit the N₂O production. N₂O with unknown isotopic signature (SP = 15.3-16.2‰), accompanied by δ¹⁸O (19.0-26.8‰) values which were close to bacterial denitrification, was also suppressed by CHP and DDTC addition (95.3%) indicating that nirK denitrifiers were responsible for this N₂O production despite being less abundant than nirS denitrifiers. Altogether, our results suggest that bacteria are important for N₂O production with different SP values both from compost surface and pile core. However, further work is required to decipher whether N₂O with unknown isotopic signature is mostly due to nirK denitrifiers that are taxonomically different from the SP-characterized strains and therefore have different SP values rather than also being interwoven with the contribution of the NO-detoxifying pathway and/or of co-denitrification.

Use of oxygen isotopes to differentiate between nitrous oxide produced by fungi or bacteria during denitrification

RATIONALE

Fungal denitrifiers can contribute substantially to N₂ O emissions from arable soil and show a distinct site preference for N₂ O (SP(N₂ O)). This study sought to identify another process-specific isotopic tool to improve precise identification of N₂ O of fungal origin by mass spectrometric analysis of the N₂ O produced.

METHODS

Three pure bacterial and three fungal species were incubated under denitrifying conditions in treatments with natural abundance and stable isotope labelling to analyse the N₂ O produced. Combining different applications of isotope ratio mass spectrometry enabled us to estimate the oxygen (O) exchange accelerated by denitrifying enzymes and the ongoing microbial pathway in parallel. This experimental set-up allowed the determination of δ¹⁸ O(N₂ O) values and isotopic fractionation of O, as well as SP(N₂ O) values, as a perspective to differentiate between microbial denitrifiers.

RESULTS

Oxygen exchange during N₂ O production was lower for bacteria than for fungi, differed between species, and depended also on incubation time. Apparent O isotopic fractionation during denitrification was in a similar range for bacteria and fungi, but application of the fractionation model indicated that different enzymes in bacteria and fungi were responsible for O exchange. This difference was associated with different isotopic fractionation for bacteria and fungi.

CONCLUSIONS

δ¹⁸ O(N₂ O) values depend on isotopic fractionation and isotopic fractionation may differ between processes and organism groups. By comparing SP(N₂ O) values, O exchange and the isotopic signature of precursors, we propose here a novel tool for differentiating between different sources of N₂ O.

Influence of Lumbricus terrestris and Folsomia candida on N2 O formation pathways in two different soils - with particular focus on N2 emissions

RATIONALE

The gaseous N losses mediated by soil denitrifiers are generally inferred by measuring N₂ O fluxes, but should include associated N₂ emissions, which may be affected by abiotic soil characteristics and biotic interactions. Soil fauna, particularly anecic earthworms and euedaphic collembola, alter the activity of denitrifiers, creating hotspots for denitrification. These soil fauna are abundant in perennial agroecosystems intended to contribute to more sustainable production of bioenergy.

METHODS

Two microcosm experiments were designed to evaluate gaseous N emissions from a silty loam and a sandy soil, both provided with litter from the bioenergy crop Silphium perfoliatum (cup-plant) and inoculated with an anecic earthworm (Lumbricus terrestris), which was added alone or together with an euedaphic collembola (Folsomia candida). In experiment 1, litter-derived N flux was determined by adding ¹⁵ N-labelled litter, followed by mass spectrometric analysis of N₂ and N₂ O isotopologues. In experiment 2, the δ¹⁸ O values and ¹⁵ N site preference of N₂ O were determined by isotope ratio mass spectrometry to reveal underlying N₂ O formation pathways.

RESULTS

Lumbricus terrestris significantly increased litter-derived N₂ emissions in the loamy soil, from 174.5 to 1019.3 μg N₂ -N kg^-1 soil, but not in the sandy soil (non-significant change from 944.7 to 1054.7 μg N₂ -N kg^-1 soil). Earthworm feeding on plant litter resulted in elevated N₂ O emissions in both soils, derived mainly from turnover of the soil mineral N pool during denitrification. Folsomia candida did not affect N losses but showed a tendency to redirect N₂ O formation pathways from fungal to bacterial denitrification. The N₂ O/(N₂  + N₂ O) product ratio was predominantly affected by abiotic soil characteristics (loamy soil: 0.14, sandy soil: 0.26).

CONCLUSIONS

When feeding on S. perfoliatum litter, the anecic L. terrestris, but not the euedaphic F. candida, has the potential to cause substantial N losses. Biotic interactions between the species are not influential, but abiotic soil characteristics have an effect. The coarse-textured sandy soil had lower gaseous N losses attributable to anecic earthworms. Copyright © 2016 John Wiley & Sons, Ltd.

N2O source partitioning in soils using (15)N site preference values corrected for the N2O reduction effect

RATIONALE

The aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP-based N2O source partitioning.

METHODS

The N2O SP values (n = 431) were derived from six soil incubation studies in N2-free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N2 were compensated for using three different approaches: a closed-system model, an open-system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end-member isotopic mass balance.

RESULTS

The average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed-system model, resulting in -2.6 ‰ (±9.5), while the open-system model and the dynamic apparent NIE model gave average SP0 values of 2.9 ‰ (±6.3) and 1.7 ‰ (±6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (±21.0) according to the closed-system model, while the open-system model and the dynamic apparent NIE function resulted in values of 31.0% (±14.0) and 28.3% (±14.0), respectively.

CONCLUSIONS

Using a closed-system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account.