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The increasing demand for sustainably produced timber and the expansion of land-based greenhouse gas removal technologies is driving the growth of the forestry sector in the U.K. This is creating increased competition for the finite land resource. Historic planting on nutrient limited upland soils, coupled with open nutrient cycles due to high intensity silvicultural systems, is creating a productivity crisis across upland plantation forestry. Climate change is expected to further impact the productivity of U.K forests and managing soil health is vital in ensuring crop resilience to our changing climate. Biochar is an established greenhouse gas removal technology (GGR) that can increase the carbon storage capacity of land and enhance the efficacy of other land based GGR technologies, such as woodland creation. The agronomic benefit of biochar is well understood, however, there has been minimal research thus far on the potential application of biochar in productive silvicultural systems as a novel method of forest fertilisation. This thesis explores the potential utilisation of targeted biochar application to improve seedling growth on nutrient poor soils during the establishment phase of forest cycles. A selection of distinct biochar types, ranging from high to low available phosphorus concentration, were tested in a controlled environment growth experiment on Scots pine (Pinus sylvestris L.), Sitka spruce (Picea sitchensis (Bong.) Carr.) and Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) along with conventional phosphorus fertilisation intervention. A specified biochar was also developed to address the primary soil limitations on seedling establishment in phosphorus limited soils. The vascular cambial zone of roundwood is relatively high in phosphorus and is a readily available sawmill co-product produced during ring debarking. This feedstock was used to create VCZ biochar. The physical and chemical properties of this specified biochar address the limitations of low phosphorus upland forest soils. Biochar amendment was further tested in a large-scale field experiment on a typical phosphorus limited restock establishment site. The effects of biochar on seedling growth and morphology were variable, according to biochar properties, dose and seedling species. The moderate native biochar phosphorus content of VCZ biochar, coupled with high porosity and internal surface area, improved above ground biomass growth, stimulated root proliferation and enhanced mycorrhizal abundance. Optical and SEM microscopy was used to investigate the importance of root and hyphal interaction with biochar structure and the accessibility of available nutrients within biochar particles in relation to seedling nutrient uptake strategies. High phosphorus fertilisers and high phosphorus biochar were found to limit the development of below ground biomass and suppress mycorrhizal fungi, in turn impacting above ground biomass development. The mechanisms which contribute to the effect of specified biochar on seedlings was explored. Biochar is a direct source of micro and macro-nutrients and can also improve localised soil nutrient availability and accessibility through sorption, pH and biofertilisation. Foliar analysis showed that biochar can be used to alleviate seedling phosphorus deficiency. The effect of biochar on soil water holding capacity (WHC) was also found to be a critical factor in improving establishment and increasing drought resilience. This thesis details the impact of biochar on forest carbon. The implication of forest establishment practices, including biochar amendment, on carbon balance and year of net carbon storage was modelled to assess the optimal scenarios for carbon storage on nutrient poor soils. Early growth improvements from low dose targeted biochar amendment have the potential to reduce the time to net carbon storage by three to six years on low yield organomineral planting sites. It can be concluded that the targeted application of specified biochar can improve early-stage seedling growth and enhance the development of the rhizosphere, providing greater silvicultural benefit compared to conventional forest fertilisation. This in turn can create longer term benefits to forest health, crop stability and future rotation yields. Enhancing the localised soil conditions through specified biochar amendment should be considered as a viable management intervention to increase forest resilience, species suitability, carbon sequestration and close nutrient cycles.

Over decades anaerobic digestion (AD) has been successfully established as technology to treat organic wastes. The perspective of turning organic wastes into biogas, a source of renewable energy and profit, through a low-cost process, has gained increased interest around this technology. The AD as biological process is strongly dependent on the environmental conditions such as temperature, pH, presence of inhibitors, and nutrients content, that, in particularly unfavourable situations, can attribute to undesirable drop in performance and even for detrimental failures. Therefore, several strategies such as feedstock pre-treatment and process optimization have been developed to secure AD performance. However, these approaches do not remove inhibitor from the process, which may result in accumulation of the inhibitor and further destabilization of the AD system. From this point of view, it is beneficial to develop methods that remove and/or reduce the mobility and bioavailability the inhibitor within the digestion process. In this study, char derived from lignocellulosic wastes was added to anaerobic digestion to elucidate their roles on process performance and efficiency. The results of this analysis suggest that changes of AD performance (i.e. biogas production and intermediates concentrations) are strongly correlated to biochemical characteristics of char materials. Furthermore, the impact of changes in inhibitor concentrations on the digester's performance was observed, and found that inhibition tolerance of AD was improved by the presence of selected char materials. However, contrary to the initial hypothesis based on previous studies in related hydrochar application, the addition of RH hydrochar led to reduction in biogas production but an enhanced methane content in biogas was observed. Except from batch AD tests, a larger-scale AD system (semi-continuous stirring tank reactor) was used to determine the possibility of biochar on enhancing digestate structure and reduce the sensitivities towards environmental factors such as pH and other inhibitors. The further outcome from scale-up study revealed that biochar had improved ammonium tolerance (1.59 mg NH4-N/kg) by recovering 12-16.4% biogas production. This study also investigated effects of iron impregnated-thermal activation on physiochemical properties of the resultant biochar. Effects of these modified biochars on gas production and volatile fatty acids concentration was also measured and significant changes was observed. Overall, char material had a positive effect on AD system stability and promoted bio-methanation during the long run operations.

Intensification of agricultural practices is necessary to support a growing global population. Water movement and storage in soils are very crucial for successful intensification of agriculture. They are important for nutrient delivery to plant and overall crop productivity. Climate change and population growth have been predicted to limit water supply especially in arid regions. Therefore, there is an urgent need to proffer solutions that would help maintain or even increase soil moisture retention. The use of biochar for improving soil hydraulic properties is a current and growing area of research. Biochar is a stable form of charcoal gotten from heating natural organic materials in a high temperature and low oxygen process known as pyrolysis. Biochar physical and chemical properties vary due to the pyrolysis process conditions, the type of feedstock, and the ratios of lignin, cellulose, and hemicellulose in the biomass. These change the structure of the biochar and will invariably affect to what extent it can improve soil water retention. To understand how biochar affects soil water properties we must understand the specific characteristics of biochar that influence these changes. Understanding the mechanisms is important for easy prediction of when and by how much biochar will improve soil water properties. The aim of this thesis is to provide new and extensive insight into 4D structural changes of biochar produced from different feedstock and pyrolysis conditions and how these affects soil moisture characteristics of different soil textures. The first aspect of the thesis was to carry out a meta-analysis of the available literature to quantify the effect of biochar on soil hydraulic properties. To enable matching of biochar to soil constraints and application needs, a thorough understanding of the impact of biochar properties on relevant soil parameters is necessary. This meta- analysis of the available literature for the first time quantitatively assessed the effect of not just biochar application, but different biochar properties on the full sets of key soil hydraulic parameters. The review shows that the key factors influencing biochar performance were particle size, specific surface area and porosity indicating that both soil-biochar inter-particle and biochar intra-particle pores are important factors. Next, the role of biochar particle size and hydrophobicity in controlling soil water movement and retention was assessed. Softwood pellet biochar in five particle size ranges was used for the experiment. These particle sizes were tested on two soil types at four different application rates in the laboratory. The results clearly show that both biochar intraporosity (pores inside biochar particles) and interporosity (pore spaces between biochar and soil particles) are important factors affecting amended soil hydraulic properties. Biochar interpores affect mainly hydraulic conductivity; both interpores and intrapores control soil water retention properties. Our results suggest that for a more effective increase in soil water retention of coarse soils, the use of hydrophilic biochar with high intraporosity is recommended. The third aspect of the thesis was to assess the 4D structural changes and pore network model of standard biomass feedstock during pyrolysis. Biochar properties are highly variable and are dependent on the biomass feedstock and production conditions. Despite these variabilities, there has been no study that uses a single sample to study the development of microstructural properties of biomass and biochar through the full range of pyrolysis temperatures. In this study, synchrotron x-ray micro-tomography (SµCT) was used to visualize the internal structure and characterise pore structure of biochar from several feedstock during pyrolysis (50 - 800 °C). The results show a wide range of variation in the pore structural characteristics of the biochar depending on feedstock and pyrolysis temperature, with observed porosity in the range of 7.41 - 60.56 %. The results from this study are not only important for the use of biochar as soil amendments but also a range of other applications relying on biochar pore characteristics, such as biochar as habitat for microbes, water, and wastewater treatments, as an absorbent for the removal of acid gases, or an additive in construction or engineering materials. In addition, the in-depth insights into changes in biomass structure during heating are valuable for research and applications related to fire safety. Finally, the data set from the SµCT was used to produce statistical models enabling the prediction of the effect of biochar on soil water holding capacity from biochar microstructural properties. The results of this thesis, therefore, clearly establish the mechanisms with which biochar improves soil hydraulic properties. It also demonstrates that by appropriately matching microstructural properties of biochar to those of the target soil, it is possible to achieve considerable improvement in soil properties using relatively low application rates of biochar. The relationship between biochar structural properties and its effect on soil moisture characteristics of specific soil texture, is an attractive pathway towards development of precisely tailored biochars aimed to enhance water use efficiency and provide low-dose, high-efficiency benefits.

Carbon dioxide removal technologies present an opportunity to alleviate some of the worst consequences of climate change by actively removing CO2 from the atmosphere while the decarbonization of our economies progresses. Biomass valorization through the production of biochar is a unique carbon negative technology as it comprises benefits beyond carbon sequestration, from the utilization of agricultural waste, the co-production of renewable energy to the use as an adsorbent. However, widespread adoption of biochar is still lacking due to economic obstacles in monetizing its various environmental benefits and high production prices compared to fossil materials. I address these obstacles by evaluating current biochar systems and developing an alternative concept to increase the economic competitiveness of biochar. I argue that the price of biochar can be significantly reduced by changing the perception of biochar as a product. Utilization systems specifically designed to monetize biochar's multifunctional properties are capable to reduce its price without requiring lower production costs. The developed concept of sequential biochar systems is based on the perception of biochar as a carrier for environmental services rather than a product. Within this service-oriented concept, biochar is rented out and recycled rather than sold, enabling an iterative valorization of its properties while retaining its carbon sequestration value. This approach centers around the novel concept of biochar recycling to effectively decouple the price of biochar from its production costs by distributing these costs over several applications. To evaluate the prospects of sequential biochar systems, I conducted an experimental proof-of-concept trial assessing a sequence of three applications connected by two recycling steps. Engineered biochars were designed with a parallel focus on increased application capacity and recycling performance to provide synergies throughout the sequence. The applications targeted the use of biochar in wastewater and gas filtration as high value applications with emerging use scenarios for biochar. Recycled biochars showed comparable performance to commercial materials, and recycling displayed environmental advantages to current methods by retaining carbon throughout the sequence. The exploratory work in this thesis provides various starting points for further research on effective biochar utilization strategies, which are urgently needed to accelerate the development of biochar into an economically self-sustaining carbon dioxide removal technology.