Biogas Digester Hydraulic Retention Time Affects Oxygen Consumption Patterns and Greenhouse Gas Emissions after Application of Digestate to Soil

Knowledge about environmental impacts associated with the application of anaerobic digestion residue to agricultural land is of interest owing to the rapid proliferation of biogas plants worldwide. However, virtually no information exists concerning how soil-emitted NO is affected by the feedstock hydraulic retention time (HRT) in the biogas digester. Here, the O planar optode technique was used to visualize soil O dynamics following the surface application of digestates of the codigestion of pig slurry and agro-industrial waste. We also used NO isotopomer analysis of soil-emitted NO to determine the NO production pathways, i.e., nitrification or denitrification. Two-dimensional images of soil O indicated that anoxic and hypoxic conditions developed at 2.0- and 1.5-cm soil depth for soil amended with the digestate produced with 15-d (PO15) and 30-d (PO30) retention time, respectively. Total NO emissions were significantly lower for PO15 than PO30 due to the greater expansion of the anoxic zone, which enhanced NO reduction via complete denitrification. However, cumulative CO emissions were not significantly different between PO15 and PO30 for the entire incubation period. During incubation, NO emissions came from both nitrification and denitrification in amended soils. Increasing the HRT of the biogas digester appears to induce significant NO emissions, but it is unlikely to affect the NO production pathways after application to soil.

Stable Isotopes Reveal Rapid Cycling of Soil Nitrogen after Manure Application

Understanding the fate of applied nitrogen (N) in agricultural soils is important for agronomic, environmental, and human health reasons, but it is methodologically difficult to study at the field scale. Natural abundance stable isotope measurements (δN) were used in this field study with micrometeorological measurements of nitrous oxide (NO) emissions to identify the biogeochemical processes responsible for rapid N transformations immediately after application of liquid dairy manure. Fifteen samplings occurred between 16 Mar. 2012 and 5 Apr. 2013, with a focus on spring manure application (before and after) and a winter snowmelt period. Concentrations and δN values of ammonium (NH), nitrate (NO), NO, and total N were measured throughout the year. Approximately 56 (±7)% of the NH-N applied in the spring could not be accounted for 3 d after manure application and was presumably lost by ammonia volatilization before it was tilled into the soil and/or removed from the inorganic N pool by microbial assimilation. Almost all of the remaining manure-NH (95 ± 1.1%) was converted within 3 wk to NO and NO by nitrification and nitrifier-denitrification, respectively. The in situ N isotope effect for nitrification (ε) was calculated to be -32.0 (±5.3)‰. Overall, field-scale measurements of δN at natural abundance provided valuable information that was used to distinguish sources of NH (manure vs. soil organic N) and to follow the production and consumption of NO and the pathways of NO production in soil.

Isotopocule analysis of biologically produced nitrous oxide in various environments

Natural abundance ratios of isotopocules, molecules that have the same chemical constitution and configuration, but that only differ in isotope substitution, retain a record of a compound's origin and reactions. A method to measure isotopocule ratios of nitrous oxide (N₂ O) has been established by using mass analysis of molecular ions and fragment ions. The method has been applied widely to environmental samples from the atmosphere, ocean, fresh water, soils, and laboratory-simulation experiments. Results show that isotopocule ratios, particularly the ¹⁵ N-site preference (difference between isotopocule ratios ¹⁴ N¹⁵ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O and ¹⁵ N¹⁴ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O), have a wide range that depends on their production and consumption processes. Observational and laboratory studies of N₂ O related to biological processes are reviewed and discussed to elucidate complex material cycles of this trace gas, which causes global warming and stratospheric ozone depletion. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:135-160, 2017.

From the ground up: global nitrous oxide sources are constrained by stable isotope values

Rising concentrations of nitrous oxide (N₂O) in the atmosphere are causing widespread concern because this trace gas plays a key role in the destruction of stratospheric ozone and it is a strong greenhouse gas. The successful mitigation of N₂O emissions requires a solid understanding of the relative importance of all N₂O sources and sinks. Stable isotope ratio measurements (δ¹⁵N-N₂O and δ¹⁸O-N₂O), including the intramolecular distribution of ¹⁵N (site preference), are one way to track different sources if they are isotopically distinct. ‘Top-down’ isotope mass-balance studies have had limited success balancing the global N₂O budget thus far because the isotopic signatures of soil, freshwater, and marine sources are poorly constrained and a comprehensive analysis of global N₂O stable isotope measurements has not been done. Here we used a robust analysis of all available in situ measurements to define key global N₂O sources. We showed that the marine source is isotopically distinct from soil and freshwater N₂O (the continental source). Further, the global average source (sum of all natural and anthropogenic sources) is largely controlled by soils and freshwaters. These findings substantiate past modelling studies that relied on several assumptions about the global N₂O cycle. Finally, a two-box-model and a Bayesian isotope mixing model revealed marine and continental N₂O sources have relative contributions of 24–26% and 74–76% to the total, respectively. Further, the Bayesian modeling exercise indicated the N₂O flux from freshwaters may be much larger than currently thought.

Proper interpretation of dissolved nitrous oxide isotopes, production pathways, and emissions requires a modelling approach

Stable isotopes ([Formula: see text]15N and [Formula: see text]18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the [Formula: see text] values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N2O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N2O production rate, and the production-weighted [Formula: see text] values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.

Isotopologue enrichment factors of N(2)O reduction in soils

Isotopic signatures can be used to study sink and source processes of N(2)O, but the success of this approach is limited by insufficient knowledge on the isotope fractionation factors of the various reaction pathways. We investigated isotope enrichment factors of the N(2)O-to-N(2) step of denitrification (epsilon) in two arable soils, a silt-loam Haplic Luvisol and a sandy Gleyic Podzol. In addition to the epsilon of (18)O (epsilon(18O)) and of average (15)N (epsilon(bulk)), the epsilon of the (15)N site preference within the linear N(2)O molecule (epsilon(SP)) was also determined. Soils were anaerobically incubated in gas-tight bottles with N(2)O added to the headspace to induce N(2)O reduction. Pre-treatment included the removal of NO(3) (-) to prevent N(2)O production. Gas samples were collected regularly to determine the dynamics of N(2)O reduction, the time course of the isotopic signatures of residual N(2)O, and the associated isotope enrichment factors. To vary reduction rates and associated fractionation factors, several treatments were established including two levels of initial N(2)O concentration and anaerobic pre-incubation with or without addition of N(2)O. N(2)O reduction rates were affected by the soil type and initial N(2)O concentration. The epsilon(18O) and epsilon(bulk) ranged between -13 and -20 per thousand, and between -5 and -9 per thousand, respectively. Both quantities were more negative in the Gleyic Podzol. The epsilon of the central N position (epsilon(alpha)) was always larger than that of the peripheral N-position (epsilon(beta)), giving epsilon(SP) of -4 to -8 per thousand. The ranges and variation patterns of epsilon were comparable with those from previous static incubation studies with soils. Moreover, we found a relatively constant ratio between epsilon(18O) and epsilon(bulk) which is close to the default ratio of 2.5 that had been previously suggested. The fact that different soils exhibited comparable epsilon under certain conditions suggests that these values could serve to identify N(2)O reduction from the isotopic fingerprints of N(2)O emitted from any soil.

N(2)O concentration and isotope signature along profiles provide deeper insight into the fate of N(2)O in soils

Nitrous oxide is an important greenhouse gas and its origin and fate are thus of broad interest. Most studies on emissions of nitrous oxide from soils focused on fluxes between soil and atmosphere and hence represent an integration of physical and biological processes at different depths of a soil profile. Analysis of N(2)O concentration and isotope signature along soil profiles was suggested to improve the localisation of sources and sinks in soils as well as underlying processes and could therefore extend our knowledge on processes affecting surface N(2)O fluxes. Such a mechanistic understanding would be desirable to improve N(2)O mitigation strategies and global N(2)O budgets. To investigate N(2)O dynamics within soil profiles of two contrasting (semi)natural ecosystem types (a temperate acidic fen and a Norway spruce forest), soil gas samplers were constructed to meet the different requirements of a water-saturated and an unsaturated soil, respectively. The samplers were installed in three replicates and allowed soil gas sampling from six different soil depths. We analysed soil air for N(2)O concentration and isotope composition and calculated N(2)O net turnover using a mass balance approach and considering diffusive fluxes. At the fen site, N(2)O was mainly produced in 30-50 cm soil depth. Diffusion to adjacent layers above and below indicated N(2)O consumption. Values of delta(15)N and delta(18)O of N(2)O in the fen soil were always linearly correlated and their qualitative changes within the profile corresponded with the calculated turnover processes, suggesting further reduction of N(2)O. In the spruce forest, highest N(2)O production occurred in the topsoil, but there was also notable production occurring in the subsoil at a depth of 70 cm. Changes in N(2)O isotope composition as to be expected from local production and consumption processes within the soil profile did hardly occur, though. This was presumably caused by high diffusive fluxes and comparatively low net turnover, as isotope signatures approached values measured for ambient N(2)O towards the topsoil. Our results demonstrate a highly variable influence of diffusive versus production/consumption processes on N(2)O concentration and isotope composition, depending on the type of ecosystem. This finding indicates the necessity of further N(2)O concentration and isotope profile investigations in different types of natural and anthropogenic ecosystems in order to generalise our mechanistic understanding of N(2)O exchange between soil and atmosphere.

Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions

Nitrous oxide (N2O), a greenhouse gas, is mainly emitted from soils during the nitrification and denitrification processes. N2O stable isotope investigations can help to characterize the N2O sources and N2O production mechanisms. N2O isotope measurements have been conducted for different types of global terrestrial ecosystems. However, no isotopic data of N2O emitted from Antarctic tundra ecosystems have been reported although the coastal ice-free tundra around Antarctic continent is the largest sea animal colony on the global scale. Here, we report for the first time stable isotope composition of N2O emitted from Antarctic sea animal colonies (including penguin, seal and skua colonies) and normal tundra soils using in situ field observations and laboratory incubations, and we have analyzed the effects of sea animal excrement depositions on stable isotope natural abundance of N2O. For all the field sites, the soil-emitted N2O was 15N- and 18O-depleted compared with N2O in local ambient air. The mean delta values of the soil-emitted N2O were delta15N = -13.5 +/- 3.2 per thousand and delta18O = 26.2 +/- 1.4 per thousand for the penguin colony, delta15N = -11.5 +/- 5.1 per thousand and delta18O = 26.4 +/- 3.5 per thousand for the skua colony and delta15N = -18.9 +/- 0.7 per thousand and delta18O = 28.8 +/- 1.3 per thousand for the seal colony. In the soil incubations, the isotopic composition of N2O was measured under N2 and under ambient air conditions. The soils incubated under the ambient air emitted very little N2O (2.93 microg N2O--N kg(-1)). Under N2 conditions, much more N2O was formed (9.74 microg N2O--N kg(-1)), and the mean delta15N and delta18O values of N2O were -19.1 +/- 8.0 per thousand and 21.3 +/- 4.3 per thousand, respectively, from penguin colony soils, and -17.0 +/- 4.2 per thousand and 20.6 +/- 3.5 per thousand, respectively, from seal colony soils. The data from in situ field observations and laboratory experiments point to denitrification as the predominant N2O source from Antarctic sea animal colonies.

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.

Gas chromatography/isotope-ratio mass spectrometry method for high-precision position-dependent 15N and 18O measurements of atmospheric nitrous oxide

We describe an automated gas chromatography/isotope-ratio mass spectrometry (GC/IRMS) method for the determination of the (18)O and position-resolved (15)N content of nitrous oxide at natural isotope abundance. The position information is obtained from successive measurement of the isotopic composition of the N(2)O(+) ion at m/z 44, 45, 46 and the NO(+) fragment ion at m/z 30, 31. The fragment ion analysis is complicated by a non-linearity in the mass spectrometer that has to be taken into account. Evaluation of the absolute peak areas allows for a simultaneous determination of the N(2)O mixing ratio for atmospheric samples. Samples with mixing ratios ranging from a few nmol/mol up to the percent level can be analyzed using different sample inlet systems. The high concentration inlet system provides an easy and quick method to carry out various diagnostic tests, in particular to perform realistic linearity tests. A gas chromatographic set-up with a split column and a backflush possibility improves analytical precision and excludes interferences by substances with long retention times from preceding runs. We also describe a new open split interface that uses only a single transfer capillary to the mass spectrometer for sample and reference gas.

Diurnal fluxes and the isotopomer ratios of N(2)O in a temperate grassland following urine amendment

There is an urgent need to provide an accurate, up-to-date estimate of N(2)O fluxes in order that national policies can be developed to reduce emissions of N(2)O from soils. There are only limited data on temporal and diurnal patterns of N(2)O fluxes to the atmosphere, mainly due to constraints in the measurement techniques. In this paper we present the first terrestrial source values of N(2)O isotopomers and have measured and quantified the temporal and diurnal variability in N(2)O fluxes following urine addition to a grassland system in the UK. The experiment was carried out over a 2-week period on an artificially drained grassland system at the Institute of Grassland and Environmental Research (IGER), North Wyke, UK. Duplicate samples of urine, each of 2 L, were collected from dairy cows and applied to chambers (of area 0.16 m(2)). The N(2)O diurnal fluxes from urine and control (no urine) plots were measured by an automatic closed chamber technique. The isotopomers of N(2)O were obtained by analysing the gas samples collected during a peak emission phase. Soil and meteorological data were also collected. The results showed strong diurnal variations in N(2)O fluxes with minimum fluxes generally occurring between 7:00 and 14:00 hrs. The total cumulative flux of N(2)O for the whole experimental period was higher by a factor of >2 compared with estimates based on the daytime (between 10.00-16.00 hrs) measurements only. Therefore, measurements of N(2)O fluxes based on daily single exposure and expressed on a 24-h basis could impose a considerable bias and inaccuracy to the emission estimates, depending on when it was taken. The measured site preference values (difference between the centre (delta(15)Nalpha) and the end (delta(15)Nbeta) N atom of the N(2)O molecule) for soil-emitted N(2)O measured during our study were always lower than the tropospheric value. This work confirms that the enhanced tropospheric N(2)O site preference value could be the result of the back injection from the stratosphere. The intramolecular isotope ratios of nitrogen (delta(15)N) and oxygen (delta(18)O) and the site preference of the emitted N(2)O indicated that there was a shift of processes during the measurement period.