During the Fukushima Daiichi power plant incident sea water was used in an attempt to cool reactor Unit 3. Since sea water contains an excessive amount of chloride, 36Cl has likely been formed and spread in the environment. Because of the long residence time and the presumed high mobility in water there is an increased interest to learn more about the biogeochemical cycle of chlorine from a radiation risk assessment perspective. Chlorine occurs in inorganic form as chloride (Clin) or bound to organic matter as organic chlorine (Clorg) and is commonly found in the environment due to both anthropogenic and natural processes. Though there are still uncertainties regarding all of the components of the chlorine cycle in soil, the chlorination of organic matter has been exemplified by research. The reverse process, Clorg mineralizing into Clin, has however not been thoroughly investigated. For this study the objective was to observe at what rate Clorg mineralizes into Clin, this by using 36Cl as a tracer in forest soil. 36Cl was added to the soil and 36Clorg was formed over a period of approximately 100 days. After chlorination the samples were incubated in different conditions and the amount of 36Clorg was observed over a period of time (180 days). The result showed no evident dechlorination during the experiment period which indicates that Clorg can be stable in the organic horizon in forest soil.

Most undisturbed peatlands sequester carbon, and rising levels of atmospheric nitrogen deposition may have the potential to destabilize this function, possibly resulting in an increased release of carbon dioxide into the atmosphere.  It is therefore of vital importance to investigate further the link between atmospheric nitrogen deposition and carbon dynamics in exposed ecosystems such as peatlands. The work described in this thesis aimed to elucidate the impact of increasing nitrogen on aspects of carbon turnover in peatlands.  Using a long-term field-based experiment, I tested the effects of 4 years of ammonium and nitrate addition (8, 24 and 56 kg N ha^-1 y^-1) on the fate of newly photosynthesised carbon by plants and the turnover of labile and recalcitrant carbon.  A second set of experiments undertaken in the laboratory assessed the use of plant wax analysis as potential biomarkers of past changes in vegetation and carbon status in peat. Overall, this work has shown that the form of nitrogen (ammonium versus nitrate) is a crucial component of atmospheric pollution and must be taken into consideration when investigating or predicting effects of reactive nitrogen on peatlands.  In addition, nitrogen addition affected the fate of newly synthesised carbon differently in <i>Eriophorum vaginatum </i>and <i>Calluna vulgaris, </i>revealing the importance of considering plant traits when investigating environmental changes in terrestrial ecosystems.  Furthermore, it has led to the development of an investigative tool for further exploration of the historical effects of atmospheric nitrogen deposition on vegetation an carbon content in peatlands.

To determine the maximum carbon dioxide (CO2) emissions consistent with a given global warming threshold, the scientific community must robustly quantify what proportion of human emitted CO2 will be taken up by the terrestrial and marine carbon reservoirs. The North Atlantic Ocean is a region of intense uptake of atmospheric CO2. To assess how the North Atlantic CO2 sink has evolved over the past decades and understand the mechanisms involved in that uptake, observations and models are used. To appreciate the strengths and limitations of observation-based and modelled products, I explore the sources of uncertain- ties of two widely-used biogeochemical observational products (GLODAP and SOCAT), and carefully evaluate the latest generation of Earth System Models (ESMs) (i.e. the CMIP5 models) against these data. The lack of robust uncertainties on observation-based estimates of the North Atlantic CO2 uptake has so far limited the community’s ability to use observed trends to evaluate CO2 uptake behaviour simulated by the models. Here, by making use of the strengths of observation-based and modelled products, a novel gap-filling and uncertainty assessment method is developed to (1) robustly quantify the recent change in the basin-wide North Atlantic CO2 sink, and (2) evaluate simulations of the recent uptake in ESMs. Through the assessment of robust interpolation uncertainties on the annually-varying North Atlantic CO2 uptake and on the resulting trends over the period 1992-2014, I find that (1) the North Atlantic CO2 uptake increased at a rate of 0.081 ± 0.012 PgC/yr/decade from 1992- 2014, corresponding to an additional uptake of 2.2 PgC over this interval relative the flux in the 1992, and (2) state-of-the-art ESMs are consistently biased to lower trend values, with a mean that is about three times smaller than the observation-based trend, equating to an additional uptake of only 0.72 ± 0.40 PgC over the period 1992-2014. I further show that the inability of these models to capture the observed increase in CO2 uptake is due primarily to biases in modelled ocean biogeochemistry, which I explore through comparison with observations. Our current understanding of the ocean carbon-cycle, as synthesised by ESMs, cannot explain the recent behaviour of the North Atlantic CO2 sink. Current projections may therefore underestimate the contribution of the North Atlantic to mitigating increasing future atmospheric CO2 concentrations.

One central goal in ecology is to understand how biodiversity, and key organismal traits, interact with ecosystem properties and processes, and ultimately to understand and predict how these interactions will be affected by rapid environmental change. Thus, global change experiments and observational gradients in diversity provide the opportunity to examine and test hypotheses about how organismal traits, multiple dimensions of biodiversity, and ecosystem function will respond to environmental change. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying rapid warming is thought to significantly alter plant community composition and ecosystem function. The following four chapters examine hypotheses about the responses of species’ traits, multiple dimensions of biodiversity, and ecosystem function to the effects Arctic warming. Chapter 1 examines plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming-induced fertilization, demonstrating declines in plant diversity and an increase in the capacity for ecosystem carbon uptake at the highest level of fertilization. Chapter 2 examines a set of physiological and functional leaf traits across the same N and P gradient in order to evaluate the possible physiological mechanisms underlying community and ecosystem responses, highlighting the effects of increasing nutrient availability for deciduous shrub species. Chapter 3 found that single-dose, long-term nutrient addition (i.e., > 20 years) led to significant declines in multiple dimensions of diversity (taxonomic, functional and phylogenetic), and that these effects persist through time, increasing for dimensions that capture organismal traits (functional and phylogenetic). Finally, Chapter 4 examined the relationship between multidimensional diversity and ecosystem function across a natural gradient of diversity, and found that taxonomic diversity and functional diversity were significantly and positively related to whole ecosystem productivity, and, conversely, functional evenness and dispersion were significantly and negatively related to ecosystem productivity. Cumulatively, these four chapters advance our understanding of the connections between communities and ecosystems in a rapidly changing ecosystem.

Many onshore windfarms in Scotland are being built on peatlands. As a consequence, the impacts of development activities on the peatland and downstream environments are an important emerging issue. For example, a reduction of the quantity of carbon (C) stored in the peatland and increased phosphorus (P) and nitrogen (N) concentrations in streams may cause changes in the peatland carbon sink strength and aquatic carbon emissions, and exceedance of EU Water Framework Directive (WFD) P threshold values, respectively. To further assess the impacts from peatland-based windfarm development, the following four research questions were posed: 1. Which specific development activities have the greatest impact on concentrations of C, N and P in streams? How long does it take for C, N and P concentrations to return to baseline conditions? 2. Which specific windfarm development activities caused the greatest aquatic export of carbon and nutrients? 3. What are the controls on concentration and export of aquatic C, N and P in streams draining peatland developments? 4. How effective are site-derived adsorbent materials and an iron ochre at preventing phosphorus from entering upland streams on peatland? Presented in this thesis, are the results from 3-weekly spatial streamwater sampling of 18 sub-catchments (ranging 0.03 - 12 km2) – from October 2011 to March 2013 – which covers the majority of the peatland-based development and one year beyond, of the Whitelee windfarm Phase 2 extension 16 km south of Glasgow, Scotland. Dissolved and particulate organic carbon (DOC/POC), soluble reactive phosphorus (SRP) and total oxidised nitrogen (TON) concentrations were measured and exports were calculated from stream gauging. The spatial catchment controls on stream water chemistry were investigated through a multiple linear regression analysis. Fraction of sub-catchment area that was subject to forest felling was found to be significantly positively correlated with concentrations and exports of DOC, POC, SRP and TON; with TON specifically by felling that had occurred more than one year previously. Density of tracks / cable trenches / turbines, and fraction non-forested area in general exerted a negative control on exports. Evidence of SRP attenuation processes in addition to dilution were found in some streams. While neither river sediment nor track or cable trench gravels were very effective at removing SRP, from the results of batch adsorption experiments, a sample of mineral subsoil was. SRP was attenuated in areas where there was a greater proportion of peaty gleys, in the riparian zone along the main stream, and the outcome of the batch adsorption experiments points to attenuation of SRP onto mineral subsoils, where present. Comparing long-term (seven year) time series of the original Whitelee windfarm directly north of the site, with the extension site (this study) it was possible to estimate the time required for recovery to pre-development concentrations. Mean annual [DOC] increased to 31.4 mg l-1 during development of the extension site. While it took five years for the DOC to recover at the original site, in the year following development at the extension site, mean annual [DOC] was still above the baseline. Mean annual [POC] increased to a maximum of 2.6 mg l-1 during development of the extension site, and as with the original site it returned to the baseline the year following development. The mean annual [SRP] increased to a maximum of 50.5 μg l-1 during development, leading to a reduction in water quality from ‘good’ to ‘moderate’ based on comparison with Environmental Quality Standards (EQS). It took four years for the SRP to recover at the original site, and mean annual [SRP] was still above baseline at the extension site in the year following development. The streamwater C, N and P has not been measured as close to, and for such a range of, windfarm development activities. This research has demonstrated links between windfarm development, forest felling in particular, and increased concentrations and exports of DOC, POC, SRP and TON in streams. While the increases were localised within the site itself, investigation of attenuation processes point to peaty gley soils along the main stream, and tracks, cable trenches and turbines – and their associated settling lagoons and silt fences – as potentially mitigating the increased dissolved carbon and nutrients. Outcomes of this research are suggested improvements to the guidance for developing peatlands, especially with regards to monitoring streamwater carbon for a sufficient period post-development, and for the removal of brash on peat soils. Further research could investigate the use of adsorption materials to mitigate phosphorus mobilisation from brash sources to streams, over a time scale of at least two years at the field scale. Laboratory-based environmental fate studies of the increased streamwater carbon could investigate the rates of CO2 efflux with time and with varying concentrations of nutrients, which would help to improve the Scottish Government's carbon calculator estimates of carbon loss from peatland-based windfarm development.

Le cycle global du carbone est fortement lié au bilan entre l’enfouissement en profondeur du carbone dans les zones de subduction, et les émissions de CO2 dans l'atmosphère par dégazage volcanique et métamorphique. Dans la zone d’avant arc (75-100 km en profondeur), les réactions de volatilisation et la dissolution des carbonates induite par l'infiltration des fluides aqueux sont les processus à l'origine de la production de fluides de composition C-O-H. Le carbone initialement piégé sous forme minéral dans les roches peut donc être mobilisé et transporté par ces fluides vers le manteau ou la croûte lithosphérique. Des estimations récentes prévoient que, compte tenu de l'ensemble des processus qui ont lieu dans les zones de subduction (volatilisation, dissolution, mais aussi bien le magmatisme et la formation de diapirs de metasediments), presque la totalité du carbone enfoui serait mobilisé et transféré en phase fluide dans la croûte ou dans le manteau.La percolation de fluides COH à travers des roches de la plaque plongeante et du manteau n'est pas seulement critique pour le recyclage du carbone, mais elle joue aussi en rôle sur le contrôle de l'état d’oxydoréduction du manteau, sur la mobilisation des éléments non volatils, ainsi que sur la rhéologie de ces roches. Cependant, les connaissances sur l'évolution de ces fluides à hautes pressions sont très limitées. Cette étude est centrée sur la caractérisation pétrologique, géochimique et isotopique des échantillons naturels de roches métasomatiques carbonatées de l'unité en facies lawsonite-eclogite de la Corse Alpine (France). Ces roches métasomatiques se localisent sur plusieurs kilomètres le long des contacts lithosphériques majeurs hérités de la plaque océanique subductée, et peuvent révéler des informations importantes sur l'évolution des fluides COH en condition de haute pression pendant la subduction. Dans ce travail, il sera démontré que l'interaction des fluides COH avec des roches silicatées à hautes pressions (entre 2-2.3 GPa et 490-530 ° C) peut causer la dissolution des silicates et la précipitation de carbonates, processus défini comme carbonatation à haute pression. Une caractérisation pétrologique et géochimique détaillée des échantillons, couplée à une étude systématique des isotopes de l'oxygène, du carbone et du strontium-néodyme sera utilisée pour déduire la composition et l'origine multi-source des fluides impliqués. Les implications géochimiques des interactions fluide-roche seront quantifiées par des calculs de bilan de masse et de flux de fluides intégrés dans le temps. Cette étude met en évidence l'importance de la remonté des fluides COH le long des gradients en pression et température pour le stockage du carbone dans les zones de subduction. / The balance between the carbon input in subduction zone, mainly by carbonate mineral-bearing rock subduction, and the output of CO2 to the atmosphere by volcanic and metamorphic degassing is critical to the carbon cycle. At fore arc-subarc conditions (75-100 km), carbon is thought to be released from the subducting rocks by devolatilization reactions and by fluid-induced dissolution of carbonate minerals. All together, devolatilization, dissolution, coupled with other processes like decarbonation melting and diapirism, are thought to be responsible for the complete transfer of the subducted carbon into the crust and lithospheric mantle during subduction metamorphism. Carbon-bearing fluids will form after devolatilization and dissolution reactions. The percolation of these fluids through the slab- and mantle-forming rocks is not only critical to carbon cycling, but also for non-volatile element mass transfer, slab and mantle RedOx conditions, as well as slab- and mantle-rock rheology. The evolution of such fluids through interactions with rocks at high-pressure conditions is, however, poorly constrained. This study focuses on the petrological, geochemical and isotopic characteristic of carbonated-metasomatic rocks from the lawsonite-eclogite unit in Alpine Corsica (France). The study rocks are found along major, inherited lithospheric lithological boundaries of the subducted oceanic-to-transitional plate and can inform on the evolution of carbon-bearing high-pressure fluids during subduction. In this work, it will be demonstrated that the interaction of carbon-bearing fluids with slab lithologies can lead to high-pressure carbonation (modeled conditions: 2 to 2.3 GPa and 490-530°C), characterized by silicate dissolution and Ca-carbonate mineral precipitation. A detailed petrological and geochemical characterization of selected samples, coupled with oxygen, carbon and strontium, neodymium isotopic systematic will be used to infer composition and multi-source origin of the fluids involved. Geochemical fluid-rock interactions will be quantified by mass balance and time-integrated fluid fluxes estimations. This study highlights the importance of carbonate-bearing fluids decompressing along down-T paths, such as along slab-parallel lithological boundaries, for the sequestration of carbon in subduction zones. Moreover, rock-carbonation by fluid-rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Lastly, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

No description available.

<p>Recent research in microbial ecology has focused on how aquatic bacterial communities are assembled. Only a few of these studies follow a “Gleasonian” approach where the roles of single bacterial populations are in focus. In this thesis, novel molecular tools were used to describe the distribution and evolutionary relationships of microbes in productive aquatic environments. Many new phylogenetic groups of bacteria were identified, likely representing bacterial populations restricted to productive freshwaters. I also addressed the dynamics and functional role of individual bacterial populations in eutrophic lakes and brackish environments with a focus on either biogeochemically significant or potentially pathogenic representatives. <i>Flavobacteria</i> blooms were observed, on occasions characterized by high heterotrophic production. In addition to high temporal dynamics microbial community composition and function differed on the spatial scale, as exemplified by free-living and <i>Cyanobacteria</i>-associated habitats. At the community scale, microbial processes, such as biomass production and substrate uptake could be predicted from the presence and absence of individual bacterial populations. I also studied the niches of potentially pathogenic <i>Vibrio </i>populations in various coastal waters. Using a novel culture-independent method, a <i>V. cholerae</i> population was detected along the entire Swedish coastline. Results from an environmental survey and a laboratory mesocosm experiment reveal that phytoplankton-derived dissolved organic matter enhance the growth of <i>V. cholerae</i> and other <i>Vibrio</i> spp. and hence create a largely overlooked niche for these heterotrophic bacteria. This thesis and future work on the role of individual bacterial populations will facilitate predictions of biogeochemical cycles and the distribution of bacteria in the context of global climate change and local eutrophication.</p>

Carbon and energy fluxes were measured with the eddy covariance technique above two semi-arid ecosystems, ponderosa pine and juniper/sagebrush, located in central Oregon. The two ecosystems have low LAI and a very open canopy structure. The energy closure was ~70-80% at both ecosystems, equivalent to an imbalance of 150-250 W m⁻² on cloudless summer days, when net radiation (R[subscript n]) was ~600-700 W m⁻². The lack of closure cannot be explained by the uncertainty of an estimate of available energy due to a single R[subscript n] sensor location. At the more open juniper/sagebrush ecosystem, a numerical model showed that spatial variation in R[subscript n], even for large differences in surface radiation temperature and reflection coefficient between ecosystem components (soil and vegetation), is less than 10% of measured R[subscript n]. The uncertainty in R[subscript n] at the two-layered ponderosa pine ecosystem with patches of young and old-growth trees is expected to be smaller than at the juniper ecosystem. Net carbon exchange (NEE) at the pine site strongly depends on environmental factors effecting carbon assimilation (A[subscript c]) and ecosystem respiration (R[subscript e]). A more detailed analysis of the carbon budget showed a strong negative response of carbon uptake to large vapor pressure deficits (VPD), whereas water vapor exchange (LE) was less affected. At large VPD the vegetation maintains a sustainable water flow through the soil-plant system by stomatal control of transpiration. The stomatal closure leads to limitation in A[subscript c], but LE is subject to a positive feedback from higher evaporative demand. Annual NEE of the ponderosa pine forest (200-300 gC m⁻²) was in the mid-range of reported NEE of temperate forest ecosystems, though, unusually, much of the annual carbon gain occurred during the fall through spring, because the relatively mild winters allowed carbon assimilation to occur and R[subscript e] rates were low. The information gathered at our ponderosa pine site during two years with contrasting climate suggests that the carbon uptake of the ponderosa pine ecosystem will be more sensitive to global climate change than the water vapor exchange. / Graduation date: 2000

Ryegrass residue consists of three main C fractions: readily available soluble C, intermediately available cellulose and hemicellulose, and slowly available lignin. Changes in chemical composition during decomposition influence rate of degradation as well as composition of the microbial community involved. Use of ����C-labeled plant material coupled with analysis of phospholipid fatty acids (PLEA) by isotope ratio mass spectrometry results in a powerful tool for linking microbial community structure and C cycling processes during decomposition. The objective was to investigate the role of soluble C in the decomposition of ryegrass straw. We wanted to determine (i) if the presence or absence of labile C in straw affects C mineralization by the microbial community, (ii) if community structure would differ based on the presence of labile C, and (iii) if community structure would shift as decomposition progressed. Residue was added to soil microcosms at rates that reflect field loads. Treatments were unleached straw (US), leached straw (LS), and leachate (L), plus an unamended control (C). Added substrates had ������C values between 120% and 180% the native soil signature was 26%. Respiration was measured every 4 to 6 hours for the first 5 d, and weekly thereafter. Destructive sampling took place after 0.6, 1 .6, 1 5, 1 8. 50, and 80 d of incubation and microbial biomass '��C (MBC) and PLFAs were analyzed. The soluble component of ryegrass straw strongly influenced C mineralization and assimilation, as well as microbial community composition and dynamics. CO2 evolution rates and ����C signatures were similar in US and L during the first 3 d of incubation. Most soluble C from leachate was consumed during that time, indicated by the rapid decrease in ������C value of CO2 evolved from L treatment. Substrate-derived C moved quickly into and through the microbial biomass. Distinct temporal shifts occurred in community composition. Early communities in amended soils were dominated by short and branched-chain PLFAs such as 15:Oa. Later samples contained more complex and longer PLFAs. 19:Ocy was an indicator for late succession communities in US and L, and 18:2w6,9 characterized late samples in LS. Soluble C affected when the temporal shift occurred in LS and L, communities shifted earlier than in US. Lipids were differentially enriched with ����C. Fungi, as indicated by 18:2w6,9, were more effective at incorporating substrate C into cellular lipids, as this was the most highly labeled of all PLFAs. / Graduation date: 2004