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Between visible and invisible: pluralising groundwater knowledges of Maharashtra, India
No full textGroundwater plays an important role in India’s water story as it supports millions of smallholder farmers for their agriculture-based livelihoods, ensures access to domestic water and safeguards a range of ecosystem services like sustaining river flows. These are all threatened by groundwater depletion, which has triggered a series of state-led responses in the form of policies and programmes. While recognising the role of knowledge in shaping these responses, scholars and practitioners have questioned the pre-eminence of techno-managerial approach for understanding groundwater that has led to a ‘prescriptive paradigm’ of governance that promotes certain ways of knowing groundwater over other ways of knowing ‘invisible’ water. This thesis develops a research approach to pluralise ways of knowing groundwater with an intent to contribute towards efforts to seek just governance transformations. It brings together work in Critical Political Ecology on politics environmental knowledge, and Science and Technology Studies work on practices of mapping and monitoring the subsurface, to explore the politics and processes of groundwater knowledge production, application, and (re)circulation in the western Indian state of Maharashtra.
The thesis is based on a 9 month long multi-sited ethnographic fieldwork conducted with farming communities of rural Maharashtra and officials, practitioners, scientists, and consultants involved in the state’s groundwater agency, NGOs and international organisations. The thesis enhances understanding of groundwater governance in three ways: first, it unpacks scalar politics of mobilising watersheds as sites of groundwater management and knowledge production and how particular international discourses shape national and regional strategies of governance; second, it reveals socially situated practices of groundwater monitoring by state officials and how it moves beyond hydrogeology to integrate bureaucratic, programmatic and other concerns to inform everyday governance; third, it shows how predominant narratives of water scarcity and drought overlook the politics of ‘excess water’ by attending farmer’s practices of managing waterlogged lands that came to be problematised through an irrigation ontology of Green Revolution coupled with discourse of wastelands and land development.
Together, these three insights advance the critique of techno-managerial water management paradigms and open new areas of inquiry at the intersection of groundwater governance, droughts, and politics of excess and waste amid scarcity. It troubles the radical separations of scientific-unscientific, local-expert, excess-scarcity, and surface-subsurface and contributes towards recent calls for critically examining and challenging forms of segregation and hierarchy in knowing groundwater, thus making a case for pluralising knowledges, a key step in efforts seeking just governance transformations
Balanced decisions: exploring sports practitioners' theoretical and applied methods in teaching and coaching
No full textGiven the importance of physical literacy in boosting health, well-being and developing the basis for lifelong physical activity, sports practitioners' (i.e., teachers/coaches) ultimate goal should be improving learners' biopsychosocial outcomes through effective practice, focused on the optimisation of physical literacy in age-appropriate fashion. Underpinning these face valid goals, learning theories provide basic principles for understanding the mechanisms through which people learn, based on a combination of field or laboratory studies. Unfortunately, however, there are several clear conflicts between theories (i.e., the cognitive and ecological dynamics perspectives) and consequent methods in practice (i.e., teaching/coaching). As such, key challenges among sports practitioners relate to knowing which theoretical approach to adopt and, therefore, methods to apply. This contradiction is even more confusing since some arguments from each approach are coherent with current practice, whilst others are either inconsistent, unclear or even counter to established teaching views.
As a general rule, and according to different skill acquisition theories, top-down/bottom-up processes will differentially impact motor skill improvements and should therefore inform the pedagogic approach adopted by sports practitioners. Despite these predictions, however, a wide range of ‘advice’ is offered, but often with little direct comparison of the efficacy of different approaches from these perspectives to determine which, if any, are most appropriate and under what circumstances. Thus, whilst professionalism requires that teaching/coaching methods should be selected to optimise each learner's learning, guidance on optimal teaching/coaching methods is limited. Consequently, sports practitioners lack the necessary guidance as to the ‘what, how and why’ underpinning various teaching/coaching methods, both in common current practice and as proposed.
In an attempt to offer evidence-informed advice, a pragmatic philosophy was seen as appropriate for this thesis. The thesis encompassed one desk-based study and three empirical studies, investigating a range of skill development contexts in Kuwait. The literature-based desktop study explored the differences in theoretical perspectives and proposed a need for multiple approaches that can integrate different teaching/coaching methods to address the complexity of teaching/coaching sports skills. Following, factors influencing sports practitioners' choice of teaching/coaching methods when teaching/coaching specific sports skills to particular learners were examined (e.g., training goal, participants age and stage, etc.). Finally, the effectiveness of different teaching/coaching methods informed by different skill acquisition approaches was assessed in two different applied environments. These discriminated between the need for early success, as typified by younger students, against the need for longer-term development in older pupils.
Taken together, these findings reveal the following: i) sports practitioners employ different teaching/coaching methods and face challenges in their teaching/coaching roles; ii) different teaching/coaching methods may generate different learning outcomes and understanding in learners depending on various contextual and age-related factors; iii) a more prescriptive method can support early success and lead to improved understanding and movement smoothness.
Consequently, a set of guidelines based on the principles of the Professional Judgement and Decision Making approach are proposed to provide clearer guidance for sports practitioners. This approach promotes an environment in which learners are differentially supported to improve the overall quality of their education, from both a motoric, psychological and social perspective. The thesis concludes with recommendations for future research, intending to provide some direction to promote better translational research. In short, sports practitioners should adopt an ‘it depends’ approach to their research and practice
Black hole growth at the dawn of the universe: insights from cosmological simulations
Get PDFObservations of supermassive black holes within the first billion years after the Big Bang challenge standard growth models. They appear too massive to have grown at Eddington-limited rates, giving rise to three broad categories of explanation: earlier formation, larger initial seed masses, or periods of super-Eddington accretion. Some recent observations support the latter, but the conditions required to enter this regime and its duration remain unclear. This thesis explores sustained super-Eddington accretion using high-resolution cosmological simulations of small-scale gas dynamics in pre-galactic environments during the Cosmic Dawn. Simulations were performed with the {\tt{Enzo}} code, which I modified to resolve scales relevant for the widely used Bondi-Hoyle-Lyttleton accretion model.
Comparisons between this model and direct inflow measurements under no-feedback conditions show that small black hole seeds (11 \msun) struggle to grow, while larger seeds (270 \msun) exhibit more stable, convergent growth histories as resolution increases. Black hole thermal feedback was then introduced at varying intensities and resolutions once these black holes had developed a pc-scale disc structure. Super-Eddington growth persisted with certain parameter configurations, establishing that black holes with initial masses of 1000 \msun can still accrete efficiently over timescales 1 \Myr under effective thermal feedback. The resolution of this subset of super-Eddington simulations was then lowered, and they accreted for 10 \Myr with thermal feedback activated. Their growth was quickly curtailed, suggesting that either early-stage accretion was resolution-dependent, feedback required longer to become fully disruptive, or the mini-halo failed to accumulate a sufficient gas reservoir for long-term black hole growth. I conclude that black holes with initial masses of 1000 \msun can achieve super-Eddington accretion, but long-term sustainability is constrained by feedback strength and the host halo’s ability to retain gas. Overall, my results suggest that both larger-than-stellar seed masses and efficient accretion are necessary to form supermassive black holes in the early Universe. High-resolution cosmological simulations that capture the complex interplay between accretion, feedback, and large-scale gas dynamics are critical for extending this work
GPR56/ADGRG1 activity facilitates the propagation and persistence of immature haematopoietic populations
Get PDFIn Acute Myeloid Leukaemia (AML), high expression of G-protein coupled receptor 56
(GPR56/ADGRG1) is associated with adverse outcomes including reduced disease
free and overall survival. High receptor expression is postulated to be a feature of
the leukaemic stem cell (LSC) compartment, a subpopulation of cancer cells which
maintain behaviours and abilities similar to native haematopoietic stem cells (HSCs).
GPR56/ADGRG1 expression is a feature of early HSCs within the embryonic aorta and
persists in adult HSCs and immature haematopoietic progenitors, with loss of
expression occurring as cells differentiate towards definitive lineages.
GPR56/ADGRG1 is also an essential feature of the engrafting compartment in
xenotransplantation studies, with non-expressing CD34+ cells showing reduced
engraftment potential.
Previously, there have been challenges in attempts to study the specific effects of
GPR56/ADGRG1. Models of knock-down and silencing were hampered by the
persistence of multiple splice forms and by the functional redundancy of
GPR56/ADGRG1 with GPR97/ADGRG3. The GPR56/ADGRG1 receptor can be also
activated in distinct ways, notably Stachel-dependent versus -independent, with the
mechanism of activation appearing to influence downstream effects. Finally, data
has been published suggesting that some effects e.g., migration and adhesion may
even vary according to the system or cell type being studied. To better understand
GPR56/ADGRG1 Stachel-dependent activation in haematopoietic cells, we have
utilised an SCFi55 induced pluripotent stem cell (iPSC) line which had been modified
to express a constitutively active form of GPR56/ADGRG1 (GPR56ca) in response to
doxycycline. After differentiating this line towards haematopoietic type cells, we
were able to study the effect of GPR56ca in vitro on apoptosis, cell cycle, cell
migration and adhesion and differentiation, and in vivo effects on HSC function in
xenotransplantation experiments. We further validated our functional experiments
by performing bulk RNA sequencing to examine the transcriptomic profile of GPR56ca
expressing cells versus controls.
Our results indicate that high expression of GPR56ca leads to an expansion of the
immature HSC like CD34+CD38- compartment following haematopoietic
differentiation and alters cell cycle progression. GPR56ca expression also exerts an
anti-apoptotic effect, both at baseline and in response to adverse stimuli including
chemotherapeutic agents and radiation and influences in vitro haematopoietic
differentiation of both expressing and adjacent non-expressing cells in a temporally
dependent manner. In primary transplantations, our data suggests that GPR56ca
high-expressing cells are capable of haematopoietic engraftment and generating
multilineage progeny in haematopoietic organs. Of note, GPR56ca expression did not
appear to drive clonal expansion within the recipients, suggesting that high levels of
Stachel-dependent receptor activity alone are insufficient to drive a leukaemic
phenotype. These functional results are supported by RNA sequencing, which
indicates upregulation of pathways related to stem cell functions.
Our results are of relevance in understanding how GPR56/ADGRG1 influences HSClike
behaviours in iPSC-derived haematopoietic cells and suggests mechanisms by
which GPR56/ADGRG1 may sustain or expand the HSC compartment in
developmental and non-disease settings. In the context of AML, our results also
suggest mechanisms for the observed adverse outcomes in GPR56/ADGRG1 high
expressing patients, including the propagation and persistence of cancer
subpopulations via expansion of immature cell compartments and anti-apoptotic
mediated mechanisms of therapeutic resistance
(Im)possible to dream: a (self-)censored rhizomatic exploration into Hongkonger becoming-identity
No full textThis thesis is a rhizomatic exploration into Hongkonger becoming-identity grounded in Deleuze and Guattari’s central philosophy of becoming, rhizome and assemblage. Beginning with a menacing nightmare which evokes strong emotions in me connecting to my (un)familiar homeland – Hong Kong, my research journey unfolds through a series of affective encounters, dreams, dialogues, and reflections situated within the socio-political context. Writing as a nomadic subject (Braidotti 1993), I embark on this (non-)methodological rhizomatic process, and plug into (Deleuze and Guattari 1988) different assemblages including psychoanalysis, schizoanalysis, psycho-social thinking, Social Dreaming (SD) and nomadic/sedentary spatial relating. This rhizomatic cartography engages with one of the main becoming-questions (Jackson and Mazzei 2023) that emerge: How do I relate to my Hongkonger identity and how is it becoming along the process of this work? Emphasising dynamicity, fluidity and complexity, the work conceptualises identity not as a fixed entity but as a dynamic process that de-re-territorialises. Along the becoming line of flight, elements emerge including fear, anxiety, sense of loss, loneliness, disconnection, panopticism, internalised surveillance, (self-)censorship and performative terror, as well as, on the contrary, the desire for movement and a sense of hope. Being caught up in-between being possible and impossible to be a Hongkonger, this thesis makes political, ethical, academic and psychotherapeutic contributions by troubling and rethinking with the political structures, bodies and their potential, SD matrix and – as I offer a post-structuralist way to see the matrix, expanding on Julian Manley’s (2009, 2018, 2019, 2020, 2024) Deleuzian perspective on SD as it deterritorialises along the process – the more-than-matrix. It employs a rhizomatic and nomadic ontology to reimagine subjectivity, identity, home, boundaries within the HK socio-political landscape, while also bringing to life the nomadic mode of spatial relating in the therapeutic setting. This thesis is not a linear account but always in the middle of becoming, an open invitation to think with and move with a practically Spinozian ontological question: What can our bodies do
Protein-functionalized DNA-based Materials
Get PDFDeoxyribonucleic acid (DNA) stands out among polymers due to its exceptional
specificity, enabled by Watson-Crick base pairing. This unique feature allows for
precise control over molecular interactions, making DNA an ideal building block
for creating smart biomaterials. Its versatility has spurred interest in applications
ranging from energy storage and diagnostic devices to the development of
advanced hydrogels for biomedical purposes.
In this thesis, we explore hydrogels constructed from star-shaped DNA nanostructures
known as DNA nanostars (DNAns). These nanostars offer a promising
platform for engineering materials that can mimic the mechanical properties of
natural tissues and support cell growth. By leveraging the programmability of
DNA, we are able to tailor the stiffness of these hydrogels through careful design of
the nanostar architecture—adjusting parameters such as arm length and network
connectivity. Our findings reveal that the macroscopic mechanical properties of
these materials are intricately linked to their microscopic structure, providing a
new avenue for designing materials with customizable viscoelastic behaviour.
Furthermore, we demonstrate that the mechanical properties of DNAns hydrogels
can be modulated using enzymes and DNA-binding proteins. For instance, DNA-cutting
enzymes can trigger controlled degradation of the network, while proteins
that bind and bend DNA can trigger conformational changes altering network
permeability. Such dynamic responses open the door to responsive materials
that adapt to their environment, making them highly suitable for applications in
tissue engineering, 3D cell culture, and drug delivery.
Lastly, since producing gels solely from DNA on a large scale is prohibitively
expensive, we explore methods to functionalize conventional polymers with
small amounts of DNA. This hybrid approach seeks to combine the low
cost of polymers—which make up most of the material—with the unique
programmability of DNA. This has the potential to offer an affordable alternative
to standard cell culture media like Matrigel, which lacks precise control over
stiffness, porosity, and mesh size.
In summary, this thesis shows that DNA-based hydrogels are tunable, versatile
and capable of dynamic adaptation, making them promising candidates for nextgeneration
biomaterials. These findings open the door to further research on
smart, responsive materials for a range of biomedical applications
Essays on constrained decision-making and strategic interactions
Get PDFThis dissertation examines decision-making under cognitive constraints and strategic interactions
between agents in competitive environment. The first chapter establishes a micro labour market
model that moves beyond traditional assumptions of perfect rationality by exploring the impact of
limited attention on workers’ job search process and the resulting market outcome. I build on existing
literature by allowing inattentive workers to have diverse priors and heterogeneous attention costs. The
model reveals that labour market mismatch can stem from biases in workers’ default search strategies,
and heterogeneous attention costs can contribute to greater variability in equilibrium outcomes. I
also identify equilibrium multiplicity, which was not adequately addressed for in previous studies, and
found that equilibria where workers adopt different application strategies may generate both higher
market efficiency and lower monopsony power as compared to scenarios where workers use the same
application strategies. The second chapter investigates how experience, serves as heuristic for future
choices, shapes workers’ adaptive learning behaviour in job applications and affects coordination in
the labour market. I analyse workers’ search behaviour when they do not observe wages and can
only learn from feedback, as well as when wages are posted first, and workers’ choices depend on
both the observed wages and their past experiences. I show that in presence of multiple equilibria,
experience-based learning generally leads to more efficient outcome, where workers coordinate on
applying with high probability to different firms, and this can be locally asymptotically stable. The
final chapter explores strategic interactions among synthetic agents represented by various large
language models (LLMs) in beauty contest games. By drawing parallels between experiments with
LLM-based agents and human subjects, I suggest LLMs may be used as complements to human
participants in future experiments and as simulation tools to mirror human-like complexities. I show
that LLM-based agents exhibit reasoning depths ranging from level-0 to 1, which fall below that
observed in human experiments, but these agents display similar convergence patterns toward Nash
Equilibrium in repeated setting. I also demonstrate that environment with lower strategic uncertainty
enhances convergence for LLM-based agents, and environments with mixed strategic types could
accelerate convergence for all subjects
Acute heart failure: Scottish temporal trends in hospitalisation, outcomes and clinical application of natriuretic peptides
No full textAcute heart failure (AHF) is common and is a major cause of morbidity and mortality in the UK. The prevalence of heart failure is estimated at 1-2% and is increasing with our ageing population. Available data from several countries suggests there may have been a decline in both the incidence of and mortality from AHF in recent years. However, these trends have not been universally identified and there has been relatively sparse reporting of trends across key subgroups as well as outcomes data. Timely diagnosis of acute heart failure is essential to improving outcomes and natriuretic peptides (NPs) are the recommended biomarkers for diagnosis of AHF. NP concentrations are known to be altered by patient factors, such as age, renal impairment and arrhythmia, which are common in populations with heart failure. The impact of these patient factors on the performance of guideline-recommended thresholds is unknown. Whilst having reasonable rule-out value, current guideline thresholds have limited specificity suggesting substantial potential for over- investigation, misdiagnosis and inappropriate initiation of treatment.
The principal aims of my thesis were three-fold. First, to examine the national temporal trends in the incidence and outcomes for AHF hospitalisations in Scotland, including important sub-groups of interest. Second, to evaluate the performance of the current guideline-recommended natriuretic peptides thresholds (N-terminal-pro-B-type natriuretic peptide (NT-proBNP) and B-type natriuretic peptide (BNP)) in suspected AHF, including key sub-groups. Lastly, to explore the potential to improve the diagnostic performance of NT-proBNP by applying novel variable thresholds across different subgroups.
Our analysis of temporal trends (1990-2014) of the Scottish population for incident hospitalised AHF using a national individual-level linkage approach showed that incident acute hospitalised heart failure has reduced over the study period. This was apparent across all age groups, both sexes and all socioeconomic groups. This reduction was most marked in the oldest age group and lowest socioeconomic group. The case fatality rate following acute heart failure declined modestly over the 25-year study period but remained higher in socially deprived groups and there was a slight rise in the incidence of rehospitalisation with heart failure. The reduction in mortality rate over time in older patients was similar to that observed in younger patients.
Our individual patient-level meta-analysis showed that the current thresholds to rule out AHF perform fairly well for NT-proBNP and BNP, and are largely in keeping with reported meta-estimates in the literature. However, the overall negative predictive value (NPV) thresholds masks major heterogeneity across important subgroups, such as older age, history of ischaemic heart disease, atrial fibrillation, renal impairment and, most of all, prior heart failure. The rule-in threshold performance for both NT- proBNP and BNP using a single threshold is modest at best, with a false positive rate of between 30 and 40%. Applying higher separate rule in threshold results in 20% indeterminate results (AHF diagnosis uncertain) but with less false positive tests, only 25%. Most notably, patients without prior heart failure had a positive predictive value (PPV) of only around 50% with a single guideline-recommended threshold (>300pg/mL) approach with NTproBNP. The PPV in this group improved incrementally with the single optimised and the separate rule in optimised threshold, to 65% and 73% respectively. Patients without a history of heart failure stand to gain most from diagnosis and treatment of heart failure, but current thresholds appear to underserve arguably the important subgroup. Therefore, deriving distinct optimal thresholds for patients with and without a prior diagnosis of heart failure - and potentially other key subgroups - has the potential to improve the practical clinical utility of NT-proBNP testing.
Overall, my thesis findings suggest that improvements in the prevention and treatment of heart failure have had progressive and sustained effects on the incident AHF hospitalisation and outcomes at the population and subgroup level although the overall prognosis remains poor. Whilst these temporal trends are informative, more granular data, such as prescribing data, will be key to understand the cause for the disparity in outcomes across key groups. However, this analysis demonstrates that using Scottish health data, we can appreciate longitudinal trends in the whole population and sub- groups but doing so as close to real-time as possible with more granular data on treatments would yield analysis that has the potential to influence and improve outcomes.
We carried out the first individual patient level meta-analysis of NP testings in AHF to assess optimal thresholds to rule-in and rule-out AHF across key subgroups. This showed that whilst having overall reasonable performance, there is significant heterogeneity in subgroups. Our analysis of the sub-population of patients with and without heart failure history with NT-proBNP suggests that there is scope to optimise the rule-out and rule-in thresholds in patients without a prior heart failure history. Our subsequent derivation and publication of our decision tool for AHF suggests there may be additional gains over and above applying an optimal rule-in and rule-out NP threshold alone. However, these findings need real-world evaluation to support its entry into clinical practice and we have proposed a study to progress this further
Extreme star formation modes in interacting dwarf galaxies
Get PDFIn the Local Universe, the vast majority of dwarf galaxies (M < 10⁹M⊙)
are either satellites of a more massive host, or isolated. Dwarf pairs are
exceedingly rare, being less than 5% among the low-mass systems. While
infrequent at z = 0, the hierarchical nature of galaxy formation implies
dwarf-dwarf interactions to be very common at early times. Therefore,
interacting dwarf galaxies in the nearby Universe open a unique window
to the star formation processes that operate at early epochs, as they
resemble the physical conditions of primordial galaxies, but with a detail
and sensitivity unmatched by high redshift observations.
In this thesis, I aim to advance our understanding of the star formation
processes that operate in the early Universe. To do that, I carry out
a detailed multiwavelength study of a nearby (Dₗ =23Mpc) interacting
dwarf galaxy pair, namely UGC 5205 and CGCG007-025. The strong
interaction has triggered nearly coeval galaxy-scale starbursts, with a delay
time of approximately 500 Myr between them. It has also altered their
stellar structure and content, as indicated by the presence of prominent
tidal features. Using broadband photometry from the Wide Field Camera
3 at Hubble Space Telescope (WFC3/HST) and integral field spectroscopy
from the Multi Unit Spectroscopic Explorer at the Very Large Telescope
(MUSE/VLT), I characterise the stellar populations and ionised gas properties
of these systems and discuss their resemblance with early star formation.
The more massive companion, UGC 5205, is a post-starburst dwarf with
an extended tidal tail. The interaction-induced high pressure of the starforming
event has created a large population of young massive clusters
(YMCs). In the first chapter, I catalogue the YMCs and estimate their
physical properties –such as mass and age– using SED fitting. Then, I derive
the cluster mass function as a function of the galaxy environment, finding
evidence that the truncation mass is not universal. Exploiting MUSE data,
I study the properties of the host galaxy, namely the star formation history
(SFH) and the total stellar mass formed in the last burst. Finally, I compare
this total mass to the mass enclosed in the clusters and calculate the cluster
formation efficiency, Γ. A value of Γ ≈ 15% suggests the YMCs population
in UGC 5205 has not experienced significant disruption events.
In the second chapter of the thesis I study the less massive companion,
CGCG 007-025. This galaxy features a strong off-centre starburst with
giant, metal-poor Hii regions. However, the MUSE data reveals an
underlying old stellar population. The SFH is compatible with a long
quiescent period until very recently, with the newest starbursting episode
being 5 Myr old, dated by the presence of Wolf-Rayet stars. Opposite
to the other galaxy, the star-forming event has not been efficient enough
to form bounded star clusters. Furthermore, the optical spectrum of this
galaxy is predominantly composed of strong emission lines. Using the
emission line fluxes, I derive the physical properties of the ionised gas and
the star formation rate (SFR) surface density, which displays a clumpy
distribution. Together with photoionisation modelling, I estimate the gas-phase
metallicity, that shows an anti-correlation with the SFR surface
density. All together, this suggests the accretion of pristine gas is responsible
for the rejuvenation of the galaxy, likely fostered by the interaction.
In the last chapter, I further investigate the complex structure of the
emission line profiles in the brightest star-forming region of CGCG 007-
025. To do so, I acquired high-resolution echelle spectroscopy using
Magellan/MIKE, whose wide wavelength coverage permits a detailed
chemodynamical study of the region. Using the fluxes of 30 emission
lines simultaneously, I derive the electron temperature, Te, electron density,
ne, and chemical abundances using the direct method. The high spectral
resolution (R∼40,000) allows us to resolve kinematic components not visible
in previous datasets. Together with the detection of the He ii emission line,
I found evidence of shocks superimposed with stellar photoionisation as the
main ionising mechanisms of this starbursting region.
In summary, this thesis aims to investigate the star-forming processes that
operate at high redshift by studying the resolved physical and chemical
properties of a pair of local dwarf galaxies in interaction. The interaction has
led to star forming episodes on galactic scales in both galaxies, favouring the
formation of massive stellar clusters. Similar environments at high redshift
could be the mechanisms which explain the origin of the globular cluster we
see now
Assessment of biochar longevity and carbon dioxide removal (CDR) durability using modelling: implications for agricultural carbon management and global climate strategies
No full textIntensified human activities have led to a dramatic growth of emissions of greenhouse
gases (GHG), especially carbon dioxide (CO₂), causing significant global warming.
To mitigate climate change, the Intergovernmental Panel on Climate Change (IPCC)
has set a target of net zero emissions by 2050 and keeping global warming below 2℃
by 2100. However, to achieve this target, it is not enough to solely rely on reducing
CO₂ emissions, but it also requires other ways for neutralization of emissions such as
carbon dioxide removal (CDR). CDR technologies or approaches aim to remove and
durably store atmospheric CO₂, and are considered to have significant potential for
achieving net-zero or even negative emissions.
In the current technological system dominated by land-based CDR, biochar technology
is one of the most noteworthy. Biochar, a solid by-product produced from the pyrolysis
of biomass residues under oxygen-limited conditions, features a highly porous
structure and is rich in stable aromatic carbon. Biochar is not only a material for carbon
storage, but can also enhance the sequestration of largest terrestrial pool of organic
carbon—soil organic carbon (SOC) when applied as a soil amendment, with many
additional advantages such as improving nutrient availability and promoting crop
growth. As such, biochar application to soil has been considered as a cost-effective
and competitive CDR option with multiple benefits. However, the durability of biochar
for CDR is determined by the time that biochar can persist or biochar absolute
longevity in soil. The accuracy of biochar longevity prediction in soil can directly
affect assessment of biochar CDR durability and contribution potential, and thus
influence the confidence of market investment and development of CDR strategy. This
thesis aims to analyse and reduce the uncertainty in predicting biochar longevity in
order to enable a more robust evaluation of biochar CDR durability.
Due to the highly physico-chemical stability, biochar can persist in soil for centuries
to millennia. It is impractical to determine the absolute longevity exactly by
monitoring natural decomposition process from experiments. Currently, the most
widely used method for estimating biochar longevity involves developing kinetic
models based on short-term decomposition data from incubation experiments.
However, this approach introduces significant uncertainty and often results in
substantial discrepancies compared to black carbon—a natural analogue of biochar,
due to the need to extrapolate over much longer timescales. Assessing the possibility
of reducing the uncertainty by improving the extrapolation from kinetic models was
the first objective of this thesis. Since two carbon components (pools) with different
decomposition rates are detected in most experimental observations, a dual
exponential model is often adopted to describe the short-term decomposition kinetics.
This model was applied to fit decomposition data from 29 biochar incubation
experiments to derive kinetic parameters. By analysing the distributions, correlations
and predictabilities of these kinetic parameters, the possibility of inferring the
decomposition kinetics of more stable pools to improve model extrapolation and
reduce uncertainty was evaluated. The results showed that the kinetic parameters
exhibited high variability, low correlations, and limited predictability, indicating a
constrained potential to inform on more stable carbon pools or to reduce the
uncertainty of kinetic models derived from short-term incubation data.
Field experiments share similar limitations, but they can potentially be used to
calibrate kinetic models and improve their responses to environmental conditions—at
least theoretically in the short term under field settings. However, such calibration
depends on a robust kinetic foundation, which in turn requires substantial statistical
power from tightly controlled and standardised experimental systems. The second
objective aimed to assess whether explainable decomposition kinetics can be derived
under complex field conditions to enable the parameterisation of environmental factors
and the calibration of kinetic models. By assuming biochar carbon to behave as a
permanent, inert pool within the Rothamsted Carbon Model (RothC, Version 26.3),
deviations between model-predicted and measured total organic carbon (TOC)
following biochar application were analysed using data from 17 published field
experiments lasting over three years. If the deviations of TOC stocks between field
measurements and RothC model simulations can be interpreted from a kinetic
perspective, the measurement data may be used to calibrate and enhance kinetic
models for more accurate predictions of longevity and CDR durability. However, the
results revealed highly visible and variable deviations caused by model-related and
data-related uncertainties across 17 field experiments, highlighting the limitations of
these field experiments in calibrating kinetic model and parameterising environmental
responses to biochar decomposition.
According to these limitations of current modelling methods, the third objective of this
thesis was to explore an alternative approach for longevity modelling. Biochar carbon
pools of varying stability were re-defined based on the aromaticity represented by the
hydrogen-to-carbon (H:C) molar ratio. Using existing H₂O₂ oxidation data, complete
kinetic curves of biochar decomposition under accelerated oxidation were generated,
and subsequently calibrated to the natural timescale by the surface oxygen-to-carbon
(O:C) molar ratio, which represents the ageing degree, with reference to known ages
of black carbon. Based on the kinetic characteristics of each pool estimated, a new
model for predicting absolute longevity was developed, described by a triple
exponential function. Compared to other studies, this new kinetic model provides a
different longevity prediction. The accuracy of new model prediction is highly
dependent on the reliability of pool partitioning and kinetic calibration. Therefore, the
effectiveness of using the H:C and O:C ratios to characterise biochar kinetics needs
further validation with a broader range of biochars and black carbon samples.
At the regional level, differences in predicted biochar longevity can inevitably affect
the assessment of its CDR contribution potential, but longevity is not the only
influential factor. Therefore, it is essential to quantify the extent to which uncertainty
in longevity predictions affects the assessment of biochar CDR contribution potential
on a global scale. This was the fourth objective of this thesis. Based on a global dataset
of cereal crop residues production and usage, the availability of residues for biochar
production and potential yield of biochar were estimated. Under a generalized
simulation framework, biochar CDR contribution potential under different scenarios
were assessed using different kinetic models integrated into RothC. The results showed
that the CDR contribution potential of biochar estimated by different kinetic models
were similar in most regions within a short timescale (75 years), but the discrepancies
between predictions from different models may become significant in more regions
with the timescale extending and climate changing.
In conclusion, this thesis explored an alternative approach to modelling biochar
longevity in response to the demonstrated limitations of existing methods, due to the
constrained potential for reducing the extrapolation uncertainty in kinetic model
developed from incubation experiments and low efficacy of current field experiments
in calibrating environmental responses. The new model offers an alternative reference
for carbon markets and policy makers in predicting biochar longevity and assessing
CDR durability and contribution potential, thereby increasing market confidence and
enhancing the credibility and practical utility of biochar within the broader CDR
landscape. Further improvements are needed, particularly in the following areas:
identification of the molecular characteristics of decomposed components in
incubation experiments; robustness analysis of pool partitioning and kinetic calibration
indicators; definition of standard decomposition environments and parameterisation of
environmental effects; establishment of systematic benchmark sites for field
experiments; and the development of more comprehensive datasets for regional CDR
assessments
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