Detection and isotopic characterisation of methane emissions:An integrated approach developing and applying mobile methods and stationary measurements

Methane (CH4) is a powerful greenhouse gas which is formed through both microbial and geological processes. Emissions of CH4 originate from natural sources, such as wetlands, and from anthropogenic sources, such as agriculture, fossil fuel infrastructure, and landfills. Due to anthropogenic activity, atmospheric concentrations of CH4 have increased by 160% since preindustrial times and are responsible for around 20% of total radiative forcing. Considerable uncertainties over the contribution of different sources remain, due to large spatial and temporal variability and because emission sources are often co-located in the landscape, hindering accurate attribution of emissions. Thus, understanding regional CH4 sources is important for reducing fugitive emissions and to better constrain atmospheric CH4 budgets. Stable isotope analysis is a powerful method for constraining methane budgets as source categories differ in their isotopic signatures. My aim in this thesis was to develop and apply isotope-based methods to characterise, and attribute CH4 emissions. I developed a system for mobile isotopic CH4 measurements and used instrument comparisons and model simulations to evaluate system performance. My findings have implications for the interpretation and comparability of data and provide a framework for optimising sampling strategies (Chapter 2). I used dual-isotope sampling and mobile measurements to characterise and identify emission sources in North West England prior to the start of shale gas exploration. My results show that dual isotope analysis can distinguish between microbial emission sources in the region and provide evidence for offshore emissions. Mobile measurements identified fugitive emissions from landfills and gas pipelines (Chapter 3). To investigate seasonal variations in wetland isotopic signatures, I performed a 2.5-year monitoring study at an ombrotrophic peat bog. Despite large changes in emission fluxes over time and, I found constant isotopic signatures throughout the sampling period (Chapter 4). The work presented in this thesis provides insights for evaluating novel methods for CH4 emission measurements and contribute to the understanding of emission sources needed to effectively constrain CH4 budgets and reduce emissions

Klimatiltak for å redusere klimagassutslipp fra drenert organisk jordbruksjord

Rapporten er utarbeidet på oppdrag for Miljødirektoratet. Den gir en oversikt over kunnskapsgrunnlag for mulige tiltak for å redusere utslipp av klimagasser fra drenert organisk jordbruksjord i Norge, både ved restaurering og ved fortsatt jordbruksdrift. Vurdering av egnete arealer for tiltak, muligheter for bokføring i det nasjonale klimagassregnskapet, positive og negative effekter av tiltakene inngår også.Klimatiltak for å redusere klimagassutslipp fra drenert organisk jordbruksjordpublishedVersio

Mobile methane measurements: effects of instrument specifications on data interpretation, reproducibility, and isotopic precision

Recent research has used mobile methane (CH4) measurements to identify and quantify emissions, but the effect of instrument response time on concentration measurements is rarely considered. Furthermore, stable isotope ratios are increasingly used in mobile measurements to attribute sources, but the precision of mobile isotopic measurements depend on a combination of instrument and measurement conditions. Here we tested the effect of instrument speed on concentration measurements by outfitting a vehicle with isotopic and concentration-only gas analysers with different response times and conducting multiple mobile surveys. Additionally, we performed a sensitivity analysis for the isotopic precision achievable under different conditions by programming a physical model, validated with empirical data from our mobile surveys. We found that slower response time led to a greater underestimation of measured CH4 concentration, during both driving and stationary measurements, while the area under peaks in concentration is consistent and provides a robust means of comparing data between instruments. We also explore the use of an algorithm to improve instrument response. Our sensitivity analysis showed that the precision of isotopic measurements increases with the concentration range and the duration of the measurement following a power law. Our findings have important implications for the reporting and comparability of results between surveys with different instrumental setups and provide a framework for optimising sampling strategies under given objectives, conditions, and instrument capabilities

Soil organic matter quality exerts a stronger control than stoichiometry on microbial substrate use efficiency along a latitudinal transect

A substantial portion of soil organic matter (SOM) is of microbial origin. The efficiency with which soil mi-croorganisms can convert their substrate carbon (C) into biomass, compared to how much is lost as respiration, thus co-determines the carbon storage potential of soils. Despite increasing insight into soil microbial C cycling, empirical measurements of microbial C processing across biomes and across soil horizons remain sparse. The theory of ecological stoichiometry predicts that microbial carbon use efficiency (CUE), i.e. growth over uptake of organic C, strongly depends on the relative availability of C and nutrients, particularly N, as microorganisms will either respire excess C or conserve C while mineralising excess nutrients. Microbial CUE is thus expected to increase from high to low latitudes and from topsoil to subsoil as the soil C:N and the stoichiometric imbalance between SOM and the microbial biomass decrease. To test these hypotheses, we collected soil samples from the organic topsoil, mineral topsoil, and mineral subsoil of seven sites along a 1500-km latitudinal transect in Western Siberia. As a proxy for CUE, we measured the microbial substrate use efficiency (SUE) of added sub-strates by incubating soil samples with a mixture of 13 C labelled sugars, amino sugars, amino acids, and organic acids and tracing 13 C into microbial biomass and released CO 2 . In addition to soil and microbial C:N stoichio-metry, we also determined the potential extracellular enzyme activities of cellobiohydrolase (CBH) and phenol oxidase (POX) and used the CBH:POX ratio as an indicator of SOM substrate quality. We found an overall decrease of SUE with latitude, corresponding to a decrease in mean annual temperature, in mineral soil horizons. SUE decreased with decreasing stoichiometric imbalance in the organic and mineral topsoil, while a relationship of SUE with soil C:N was only found in the mineral topsoil. However, contrary to our hypothesis, SUE did not increase with soil depth and mineral subsoils displayed lower average SUE than mineral topsoils. Both within individual horizons and across all horizons SUE was strongly correlated with CBH:POX ratio as well as with climate variables. Since enzyme activities likely reflect the chemical properties of SOM, our results indicate that SOM quality exerts a stronger control on SUE than SOM stoichiometry, particularly in subsoils were SOM has been turned over repeatedly and there is little variation in SOM elemental ratios

Effects of soil organic matter properties and microbial community composition on enzyme activities in cryoturbated arctic soils

Enzyme-mediated decomposition of soil organic matter (SOM) is controlled, amongst other factors, by organic matter properties and by the microbial decomposer community present. Since microbial community composition and SOM properties are often interrelated and both change with soil depth, the drivers of enzymatic decomposition are hard to dissect. We investigated soils from three regions in the Siberian Arctic, where carbon rich topsoil material has been incorporated into the subsoil (cryoturbation). We took advantage of this subduction to test if SOM properties shape microbial community composition, and to identify controls of both on enzyme activities. We found that microbial community composition (estimated by phospholipid fatty acid analysis), was similar in cryoturbated material and in surrounding subsoil, although carbon and nitrogen contents were similar in cryoturbated material and topsoils. This suggests that the microbial community in cryoturbated material was not well adapted to SOM properties. We also measured three potential enzyme activities (cellobiohydrolase, leucine-amino-peptidase and phenoloxidase) and used structural equation models (SEMs) to identify direct and indirect drivers of the three enzyme activities. The models included microbial community composition, carbon and nitrogen contents, clay content, water content, and pH. Models for regular horizons, excluding cryoturbated material, showed that all enzyme activities were mainly controlled by carbon or nitrogen. Microbial community composition had no effect. In contrast, models for cryoturbated material showed that enzyme activities were also related to microbial community composition. The additional control of microbial community composition could have restrained enzyme activities and furthermore decomposition in general. The functional decoupling of SOM properties and microbial community composition might thus be one of the reasons for low decomposition rates and the persistence of 400 Gt carbon stored in cryoturbated material

Microbial carbon use efficiency along a latitudinal transect through West Siberia

Böden stellen den größten terrestrischen Kohlenstoffspeicher dar. Dabei ist alleine in den Böden arktischer und subarktischer Ökosysteme mehr Kohelnstoff gespeichert als in der gesamten Atmosphäre. Langfristig stabile organische Substanz in Böden entsteht großteils aus den Überresten mikrobieller Biomasse. Daher ist die Partitionierung von aufgenommenem Kohlenstoff in Respiration und Wachstum durch Mikroorganismen bestimmend für das Potential von Böden Kohlenstoff zu speichern. Um den Effekt von Stickstoffverfügbarkeit auf den Kohlenstoffumsatz von Mikroorganismen zu untersuchen, wurden in dieser Studie Proben der von drei Bodenhorizonten in sechs Ökosystemen entlang eines 1.500 km langen Nord-Süd Transekts in Westsibirien genommen. An den Proben wurde die mikrobielle Kohlenstoffnutzungseffizienz (CUE) mittels Inkubation mit 13C markiertem Substrat gemessen. Des Weiteren Wurde die Aktivität extrazelluärer Enzyme bestimmt. Die Ergebnisse zeigten, dass die CUE im organischen Oberbonden und dem obersten mineralischen Horizont ähnlich wahr, aber im unteren mineralischen Horizont tiefere Werte annahm. Dieses Ergebnis stand im Gegensatz zu der Erwartung, das aufgrund erhöhter Stischstoff Verfügbarkeit die CUE mit der Horizonttiefe zunehmen würde. Die potentielle Enzymaktivität oxidativer Enzyme nahm mit der Bodentiefe zu. Dies deutet darauf hin, dass die Substratqualität und/oder Sbustratverfügbarkeit mit zunehmender Bodentiefe abnimmt. Innerhalb der Bodenhorizonte bestand eine negative korrelation zwischen der CUE und der Aktivität der oxidativen Enzyme. Diese Ergebnisse zeigen, dass die Substratqualiät in allen Horizonten einen wichtigen Einfluss auf die CUE hat. Des weiteren führen sie zu dem Schluss, dass Mikroorganismen in tieferen Bodenhorizonten in ihrem Wachstum durch Substratmangel und die chemische Komplexität des Substrats gehemmt sind.Soils represent the largest terrestrial pools of organic carbon (C), and arctic and sub-arctic ecosystems, where decomposition is thought to be mainly limited by climate and low nitrogen (N) availability, store more C in their soils than the whole atmosphere. As stable soil organic matter is largely derived from microbial compounds, the partitioning of C uptake by microorganisms into growth and respiration determined the C storage potential in soils. To investigate the effect of nitrogen availability on soil microbial C cycling we established at 1,500 km latitudinal transect through West Siberia and measured microbial carbon use efficiency (CUE), as well as C and N pools and extracellular enzyme activities in the top three horizons of seven sites along the transect. We found that while C:N ratios decreased with soil depth, CUE was similar in the organic topsoil and upper mineral horizon, but lower in the deeper mineral horizon, which is counter to an expected increase predicted by stoichiometric theory. Potential oxidative enzyme activities increased with soil depth, indicating reduced substrate quality and/or accessibility in lower horizons. Within horizons, CUE was always negatively related to oxidative enzyme activity, as well as to dissolved and total C in the organic horizons and to C:N ratios in the upper mineral horizons. We conclude that substrate quality is an important control on CUE in all soil horizons. We further conclude that microorganisms in deeper soil horizons are limited in their growth by substrate limitation and chemical complexity

Microbial physiology and soil CO2 efflux after 9 years of soil warming in a temperate forest - no indications for thermal adaptations

Thermal adaptations of soil microorganisms could mitigate or facilitate global warming effects on soil organic matter (SOM) decomposition and soil CO2 efflux. We incubated soil from warmed and control subplots of a forest soil warming experiment to assess if 9 years of soil warming affected the rates and the temperature sensitivity of the soil CO2 efflux, extracellular enzyme activities, microbial efficiency and gross N mineralization. Mineral soil (0-10 cm depth) was incubated at temperatures ranging from 3 - 23 °C. No adaptations to long-term warming were observed regarding the heterotrophic soil CO2 efflux (R10 warmed: 2.31 ± 0.15 μmol m(-2) s(-1) , control: 2.34 ± 0.29 μmol m(-2) s(-1) ; Q10 warmed: 2.45 ± 0.06, control: 2.45 ± 0.04). Potential enzyme activities increased with incubation temperature but the temperature sensitivity of the enzymes did not differ between the warmed and the control soil. The ratio of C:N acquiring enzyme activities was significantly higher in the warmed soil. Microbial biomass specific respiration rates increased with incubation temperature, but the rates and the temperature sensitivity (Q10 warmed: 2.54 ± 0.23, control 2.75 ±0.17) did not differ between warmed and control soil. Microbial substrate use efficiency (SUE) declined with increasing incubation temperature in both, warmed and control soil. SUE and its temperature sensitivity (Q10 warmed: 0.84 ± 0.03, control: 0.88 ± 0.01) did not differ between warmed and control soil either. Gross N mineralization was invariant to incubation temperature and was not affected by long-term soil warming. Our results indicate that thermal adaptations of the microbial decomposer community are unlikely to occur in C-rich calcareous temperate forest soils. This article is protected by copyright. All rights reserved