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Rapid anthropogenic climate change is one of the greatest challenges that human civilisation will face in the 21st century. A 25-180 % increase in atmospheric carbon dioxide content since the early 1800’s and a predicted increase of 2-3% each year will lead to a 2-6°C rise in tropospheric temperatures. The consequences of increased atmospheric temperatures are profound and would put unsustainable strain on human infrastructure, which was conservatively estimated in the Stern Review (2006) to cost approximately 20% of GDP. Given the political, technical, economic and social barriers preventing the transition to a low carbon economy, there is an unequivocal need to research ‘geoengineering’ technologies that can bridge the gap between carbon emission reduction targets and actual emissions. Soil mineral carbonation is one such technology. The atmosphere is one of the smallest carbon pools at the Earth’s Surface (depending on how each pool is demarcated). Soils turn over the quantity of carbon in the atmosphere in under a decade and collectively form one of the largest carbon pools (3-4 times the quantity of carbon in the atmosphere). Land use change since the agricultural revolution has released 256 GtC (40 % of anthropogenic emissions). Research investigating the potential for carbon accumulation in soils is primarily focused on restoring organic carbon concentration to pre-agricultural values through modification of farming practices. The research presented in this thesis is the first that explores the potential of increasing the inorganic carbon pool as an emissions mitigation technology. Inorganic carbon accumulation is promoted by introducing divalent cation rich (predominantly calcium and magnesium) silicate and hydroxide minerals into the soil, which weather and supersaturate the soil solution with respect to carbonate minerals (predominantly calcite, aragonite, magnesite and dolomite). The carbon in the resultant precipitate is derived from the atmosphere. This is analogous to mineral carbonation technologies which induce carbonate precipitation from silicate weathering in industrial scale reactors at elevated temperatures and pressures. However, carbonation in soil exploits natural weathering processes to the same effect with minimal energy and infrastructure input. The research presented in this thesis broadly investigates soil mineral carbonation by contributing work towards the fundamental issues associated with application of soil mineral carbonation technology. Research activity described herein covers a range of laboratory batch weathering experiments, field work, geochemical modelling, plant growth trials, soil microcosm experiments and literature reviews. While eclectic, all work packages contribute to the same goal of describing the efficacy, effectiveness and potential impacts of soil mineral carbonation. The efficacy of mineral carbonation technology is primarily limited by the availability of appropriate silicate bearing material. A literature search suggests that approximately 15-16 Gt a- 1 of silicate rich ‘waste’ materials are produced as a consequence of human activity. This has a carbon capture potential between 190 and 332 MtC a-1, which is equivalent to other emissions mitigation strategies. Quarrying silicate specifically for carbonation is a suggested strategy that may be able to store on the order of 102 GtC a-1 (based on two sites in the US). Therefore, mineral carbonation may form part of global mitigation strategies collectively equivalent to 14 GtC a-1 to stabilise the CO2 concentration of the atmosphere at 500 parts per million by volume. Considering that the potential capacity of soil mineral carbonation is sufficient to act as a substantial emissions mitigation strategy it was appropriate to investigate issues associated with the application of such a technology. In the first instance, sites known to contain silicates were investigated. These include soils developed on natural silicates (on the Whin Sill in Northumberland), construction and demolition waste (at a brownfield site and waste transfer stations) and slag (at a former steelworks). Interpretation of fieldwork results suggests that inorganic carbon accumulation is rapid (up to 38 gC kg-1(soil) a-1), and is orders of magnitude xxv greater than organic carbon accumulation in natural soils. The average concentration of inorganic carbon (20-30 Kg m-3) is equivalent to organic carbon in natural soils. The unusually light carbon and oxygen isotope ratios of the carbonate (-3.1 ‰ and -27.5 ‰ for δ13C and -3.9 ‰ and -20.9 ‰ for δ18O) were used to determine that up to 55% of the carbon was derived from the atmosphere. The rate of carbon capture, which is the same as the precipitation rate of carbonate, is a function of solution chemistry. The more supersaturated a solution is with respect to a carbonate mineral, the more rapid the precipitation rate. Saturation of a solution is a function of divalent cation and carbonate anion concentration. Therefore, the supply of each of these components was investigated in laboratory experiments. Batch weathering experiments were used to investigate the supply of calcium from artificial silicates (hydrated cement gel). Up to 70-80 % of the calcium contained in the mineral was removed, which is consistent with efficiencies reported for conventional mineral carbonation. The log rate of weathering was between -10.66 and -6.86 mol Ca cm-2 sec-1, which is several orders of magnitude greater than that usually reported for natural silicates. Microcosm experiments were conducted to investigate the rate of supply of carbonate from the organic carbon mineralisation in high pH solutions. The research clearly demonstrates that high pH solutions inhibit the breakdown of organic carbon as a function of nutrient supply. Where organic carbon was successfully mineralised the log rates (-3.4 mmol g-1(field moist soil) sec-1) were equivalent to that found in previous studies. While the influx of dissolved carbonate mineral components into the soil solution is the primary controlling step in the rate of carbon accumulation, there is a complex relationship between soil physical properties and geochemistry. This was highlighted in a numerical model that was constructed for this thesis, which suggests that soil pore volume and particle size distribution are important variables. An additional numerical model was constructed to investigate the transportation of silicate material to the application site. This model suggests that an economics of soil mineral carbonation is a function of transport costs, the value of the silicate material and the price of carbon. Field observations, growth trials, microcosm experiments and previous research suggest a complex interaction between biology, weathering and carbonate precipitation. Additional work is required to investigate carbonate precipitation mediated by plant and microorganism activity and the degree to which soil mixed with silicates impact on ecosystem functioning. This research has demonstrated that mineral carbonation in soils could form a substantial emissions mitigation strategy, but additional work is required in a number of areas to which this thesis provides a suitable foundation.

The objective of this thesis was to better understand the mechanistic control of carbon (C) cycling in two terrestrial ecosystems (agricultural grasslands and Arctic tundra), with an aim to identify the contribution of microbial respiration to below-ground C cycling. Firstly, I evaluated different techniques for measuring CO2 evolution from soil. I found that different in-situ chamber-based CO2 gas analyzers gave comparable results across contrasting ecosystems. However, the addition of collars to the CO2 chamber induces variable flux estimates due to the disturbance created upon collar insertion, severing root and mycorrhizal networks. In subsequent studies, I showed that microbial breakdown of individual dissolved organic C (DOC) components demonstrated good reproducibility when performed under either in-situ and ex-situ conditions. After validating the experimental techniques, they were then used to study C turnover in two plant-soil systems. In Arctic tundra, soil temperature was identified as the key driver initiating microbial and vegetation response to snow melt, thereby driving early season CO2 efflux. However, as the growing season progressed, soil water content was hypothesized to become a more important regulator of C turnover with older C compounds becoming more susceptible to decomposition as soil water content increases. In a grassland soil I found that soil microbial community composition does not correlate with increased rates of mineralization across a wide pH gradient. This suggests that abiotic drivers of respiration may directly influence microbial metabolic processes independent of community structure. Further research involving advanced molecular techniques (metabolomics, proteomics, transcriptomics) will help disseminate how metabolic processes are being influenced by different respiration drivers. The application of mathematical models to respiration data provides a more quantitative and mechanistic understanding of processes involved in soil C cycling. I found the fitting of exponential models to respiration data is a reliable proxy for describing substrate mineralization; however, the correct choice of model is critically dependent on the number of measurement points and length of experiment. The modelling approach was subsequently used to quantify the turnover of functional microbial C pools. By combining modelling with experimental measures of soil solution C concentration, we estimated that the microbial contribution to total soil respiration is ea. 18%. This research provides a more detailed understanding of how C constituents are processed by the microbial decomposer community to drive soil respiration. This is crucial to accurately model global terrestrial C fluxes in different ecosystems and to predict how these fluxes are likely to respond to future changes from both natural (e.g. climate change) and anthropogenic (e.g. land-use change) sources.

Northern temperate forests have been identified as major contributors 10 the terrestrial C sink. Among the different land uses, afforestation and reforestation have been recommended as practices to mitigate climate change by promoting C storage in both soils and biomass but the main factors affecting long-term C stabilization in soils remain uncertain. This research investigated how different soil types and tree species affect the C sequestration potential of forest soils with respect to soil C quality and how soil properties and the activities of key enzymes may influence soil carbon transformation processes in topsoil (A and E) and subsoil (B) horizons.

Temporal variations in primary production have important consequences for carbon transfer, both through the air-sea interface and through pelagic food webs. Coastal waters account for ca. 30% of global primary productivity and long-term biogeochemical time series have greatly improved our understanding of the environmental and phytoplankton community factors that control carbon fixation. For the first time, the temporal and depth-dependent variations in photosynthetic parameters were investigated at station L4 in the western English Channel. From 2009-2010, the highest productivity (1856 mg C m-2 d-l) occurred during August 2009 during a bloom of dinoflagellate Karenia mikimotoi which impacted upon oxygen concentrations. Harmful algal species, K mikimotoi and Phaeocystis poucbetii were shown to account for substantial proportions of the total phytoplankton carbon from 1992-2009 during summer and spring respectively. Persistent summertime rainfall and low-salinity riverine pulses were identified as the key drivers of K mikimotoi blooms, whilst Phaeocystis blooms were related to the North Atlantic Oscillation. An empirical bio-optical model of surface primary production based on phytoplankton light absorption was constructed and validated for use in coastal waters. The model performed better than standard chlorophyll-based algorithms (RMSE < 8%). This approach was successfully extended throughout the water column using characteristic depth-profiles to derive depth-integrated primary production, and subsequently extended to two phytoplankton size groups. Size-fractionated production data revealed that although microplankton contributed much of the total carbon fIxation, smaller phytoplankton had a higher photosynthetic efficiency. The model was further applied to an 8-year time series of absorption measurements, to investigate longer-term temporal dynamics and drivers of productivity at L4. Nano- and picoplankton were found to contribute 48% of primary production at L4. Different driving mechanisms of temporal variability were determined for instantaneous production and seasonal budgets.

Due to changes in our climate and environment, partly caused by human behaviour, it is becoming increasingly important to understand the processes involved in Earth systems, such as the carbon cycle. There are many models that attempt to describe the dynamical behaviour of carbon stocks and stores, but, despite their complexity in attempting to describe all crucial processes, significant uncertainties remain. Our aim is to look at the qualitative behaviour of one of the simplest carbon cycle models, the Data Assimilation Linked Ecosystem Carbon (DALEC) model, and consider in detail the processes involved. DALEC is a simple vegetation model of processes involved in the carbon cycle of forests. Our analysis shows that the dynamics of both evergreen and deciduous forests in DALEC are dependent on a few key parameters and it is possible to find a tipping point at which stable sustainable behaviour of a forest gives way to widespread mortality. We also study and simplify the Gross Primary Production (GPP), a complex photosynthesis function at the heart of DALEC, and create annual maps of the five carbon pools involved, using the simplified and averaged GPP. These results are then used to examine the effects of moisture shocks on the tipping point, as well as to examine parameter sensitivity from both a numerical and analytical point of view. The net ecosystem exchange (NEE) is an important measure of whether a forest is a CO2 sink or source and also serves as a means to find parameters in the model. During the process of examining parameter sensitivity we simplify the Net Ecosystem Exchange (NEE) equation in various ways, depending on which time period is considered. We find that we can identify the sensitivity of NEE to the parameters and furthermore, we find that the NEE becomes more sensitive to some parameters over time and less to others. Furthermore we find that during a certain important time period NEE can be expressed in terms of the annual mean GPP. These results give important insights into what affects the NEE.

For the past two decades, the spread of angiosperm trees in the Cretaceous and Palaeogene has been thought to have enhanced the weathering of silicate minerals resulting in increased fluxes of Ca and Mg to the oceans, drawing down atmospheric CO2 and ultimately sequestering it in marine carbonate sediments. In this thesis, I present an alternative hypothesis: the spread of ectomycorrhizal fungi since the Cre- taceous has been more important for global weathering and drawdown of CO2 from the atmosphere than the spread of angiosperms. These fungi act as biosensors, seek- ing out nutrient-bearing minerals and releasing weathering agents such as protons and organic acids at the scale of individual mineral grains. By contrast, fine roots have diameters several orders of magnitude larger than the fungi and are therefore less able to target mineral grains smaller than themselves. I developed deterministic process-based models of silicate weathering mediated by roots and mycorrhizal fungi with the aim of examining their influence on delivery of Ca and Mg to river waters. My zero-dimensional global model results show that EM fungi could have accounted for the CO2 drawdown previously attributed to angiosperm trees, but results from my two-dimensional spatially-resolved model indicate a more nuanced picture, with AM and EM angiosperms playing an important role due to their high net primary productivity. This model also predicts that falling CO2 causes weathering rates to decline, supporting a lower limit for CO2 drawdown. Overall, the models indicate that biological weathering is a complex interplay between climate, lithology, and plant and mycorrhizal functional types

Increases in human population and activities have lead to significantly enhanced inputs of carbon and nitrogen fiom both point and difiuse sources to rivers, estuaries, and coastal seas, altering the global carbon and nitrogen biogeochemical cycles. This increased load has had a marked ecological impact globally, with detrimental changes to primary production, conununity stricture and water quality. Understandmg the nature and cycling of dissolved organic carbon (DOC) and nitrogen (DON), significant components of the global carbon and nitrogen cycles, in estuaries will provide better estimates of C and N transport to coastal and ocean waters. This study involves: (1) optimisation of the h i ^ temperature catalytic oxidation (HTCO) analytical method used for reliable analysis of DOC and total, dissolved nitrogen (TDN), (2) investigation of spatial and temporal variations of DOC and DON, and other reliated determinands, in two contrasting estuarine environments, the Yealm and Plym, in south-west England, (3) investigation of DON bioavailability using a bacterial incubation experiment, and (4) preliminary work on the use of stable nitrogen isotope ratios to identify DON sources. The sampling and analytical protocols required for rapid, precise and reliable determinations of DOC and DON, using the coupled HTCO-chemiluminescence technique, are described ui this study. This metiiod gave detection limits of approxunately 6.2 \iM C and 0.46 | iM N , and precisions of < 2-3 % and < 3-5 % (n=3-5) for DOC and TDN, respectively. The mean DOC and TDN of the CRMs analysed, over a period of 2 years, were 48 ± 3.9 | iM C and 20 ± 1.5 nM N that were close to the certified values of 44 - 45 pM C and 21 | iM N , respectively. Concentrations of DOC ranged from 61 | iM C at the seaward end to 335 pM C at the fresh water end for the Yealm, and 71 - 290 |j,M C for the Plym. DON concenfrations were mamly m the range of 1.8 - 62 pM N for the Yeahn, and 4-94 \iM N for the Plym. The enhanced DON concenfrations in the Yealm might be the result of sewage discharges and agricultural run off, while in the Plym they may be due to sewage discharge, run off from the Chelson Meadow landfill and other anthropogenic activities within the urban Plym catchment. Except during a limited numbers of surveys, DOC and DON generally behaved in a non conservative manner in these estuaries. Nifrate and filterable reactive phosphate (FRP) behaved relatively conservatively m the Yealm, but were more non-conservative in the Plym. The spatial distribution of DOC and DON concenfrations in the Plym Estuary appeared much more mfluenced by anthropogenic uiputs relative to distributions in the Yealm Estuary. The seasonal variation of DOC and DON was characterised by lower concenfrations during winter and a slight increase in spring and suimner followed by highest concenfrations during late smmner and auturrm, suggesting a sfrong link to seasonally variable phytoplankton production. The confribution of DON to die TDN pool ranged between 4 and 79 % for the Yealm, and 3.5 - 84 % for the Plym. Higher values (53 - 79 %) were observed during late summer, emphasising the important contributions of DON to TDN- pool. Incubation experiments usmg the Plym Estuary water mdicated that 30 - 58 % of DON was bioavailable for heterofrophic bacterial utilisation; at the same time nitrate concentrations increased by 9 - 35 %, presumably through mineralization of DON. From the studies undertaken in the present work, it may be concluded that the omission of DON in estuarine and coastal water studies will result in underestimation of the total nifrogen load. As a significant part of the DON appeared to be bioavailable, ignoring this fraction will result in an underestimation of eutrophication pressures on coastal and ocean waters.

CO2 Storage is one of the key technologies to mitigate climate change at a large scale and ensuring that the injected CO2 stays trapped underground is one of the main challenges. It is critical to develop fast and more physically accurate methods for CO2 storage simulations, otherwise computation times become prohibitive, especially when geological uncertainty is large, as in deep saline aquifers. Injection strategies and geological uncertainty have an impact on how much CO2 can be trapped as residual saturation. Fast and accurate simulators such as the one in this work are necessary to run the large number of simulations used in optimising CO2 sequestration. Our existing research streamline simulator has been extended with two improved thermodynamic models to maintain thermodynamic equilibrium along the streamlines. This minimises time-step size dependence and convergence errors. 1D simulation along the streamline was compared against analytical solution. Models were validated on 2D and 3D sections of the SPE 10th Comparative Model using water alternating gas (WAG) injection followed by chase brine. Results show that both new thermodynamic algorithms are faster (lower CPU cost) and have a faster convergence of results than the previous algorithm. Based on the validated model, we run 3D simulations for a single well strategy for the stage 2 CO2CRC Otway Project to test residual trapping. Simulation results were compared to TOUGH2 (finite-difference simulator) simulation results to study numerical dispersion, convergence of results and CPU times. Streamline simulations decreased computational time by a factor of five but results were not in agreement. Streamline simulations simulate advection processes accurately. However, there are other non-advective processes, such as diffusion, dispersion and buoyancy effects, which streamlines cannot simulate properly. This could cause the differences between streamline and TOUGH2 simulation results. Incompressibility was one of the main assumptions of the streamline-based simulator and this could pose some challenges when trying to simulate CO2 sequestration projects where injection strategy is complex. The CO2 streamline code was extended to add compressibility. Supercritical CO2 is slightly compressible so including compressibility in the streamline code is important to be able to model the physics more accurately. Streamlines can now end anywhere in the reservoir. Expansion or contraction of fluids can create source or sink cells which act as injection/production cells. Initially the pressure profile obtained numerically was compared to the analytical solution for radial single-phase flow and 1D simulations were run to study the effect of compressibility on the saturation profile. 2D simulations of a slightly compressible case on the SPE10 geological model were compared to ECLIPSE simulations, resulting in good matching. Then, the 3D Otway field case was re-simulated using the compressible code and results were compared to those obtained by TOUGH2 without obtaining a good agreement due to the complexity of the case. With most of the storage potential being in geological formations which are poorly characterised, monitoring will be a central part of any CO2 storage project. We have adapted a new approach for streamline-based history matching which exploits the analogy between the propagation of a wave front and the pressure front in the reservoir. This approach uses diffusive time-of-flight which determines the velocity at which pressure propagates as a function of static and fluid properties. This tool enables us to reconcile response data with static geological data at an earlier time, improving the management of the project. This approach has been applied to drawndown-buildup well test for a 2D synthetic case and a 3D real field case. Results for both cases were satisfactory, showing a clear improvement in the pressure matching after the 10th iteration in most cases.

Over half of the world's calcification is carried out by algae or by organisms which harbour them, such as coccol ithoph ores, foraminiferans, coralline seaweeds and reef-building corals. Calcification acts as a sink for inorganic carbon and although rather little is known about the precise mechanisms of biological CaC03 formation, the process as a whole is thought to be under threat from atmosphericC02 rise. This study examined the response of a reef-building coral, Montipora digitata and a coralline seaweed, Corallina officinalis to the main factors which influence calcification, namely light, dissolved inorganic carbon (DIC), pH, nitrate and calcium. In contrast to the commonly held view, this study demonstrates that both photosynthesis and calcification were carbon limited in seawater. Since the degree of stimulation by DIC in the light was different for each process, and dark calcification also increased with added DIC, it is clear that photosynthesis and calcification are only loosely coupled. Simultaneous pH measurements were made on the surface of the epithelium and at the site of calcification in the coral Galaxea fascicularis using pH microelectrodes, and demonstrated for the first time that pH at the site of calcification is not a simple response to seawater pH. 2 In this study, nitrate inhibition of calcification was shown to be more powerful in the dark than in the light, indicating that daylength may be a more significant factor in coral biology than previously realised. The currently-accepted hypothesis that biological calcification rates are a simple function of seawater CaC03saturation state was tested experimentally. Results from both Corallina officinalis and Montipora digitata reveal that: a) calcification is far more responsive to changes in inorganic carbon than to calcium concentrations; and b) when [C03 2-] is kept constant, increases in [HC03-1 cause dramatic increases in calcification rates, even at reduced pH. All of these data suggest that calcification in M. digitata and C. officinalis is a strongly biologically controlled process, influenced principally by the seawater bicarbonate concentration and pH, but strongly mediated by light and combined nitrogen.