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Tutton, Charlotte Eliza (2024) Investigating the use of torrefied biochar in agriculture for carbon sequestration. EngD thesis, University of Nottingham.

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Abstract

Multiple different approaches are needed globally to mitigate the effects of climate change and reach carbon reduction targets. The Royal Society and Royal Academy of Engineering integrated assessment models indicate that Greenhouse Gas (GHG) removal by 2100 must equate to 230 giga tonnes of carbon, which corresponds to around a 0.4oC temperature reduction. The UK has set a target to be net (carbon) zero by 2050 which means substantial changes to industries must take place, in addition to utilising a range of GHG removal technologies. A key mechanism for capturing and storing CO2, is a process known as carbon sequestration. The key challenge for this is cost of capture, but biochar production is one method of carbon sequestration that has the potential to be economically viable due to the wide range of applications for the carbon-rich, stable material.



The overall aim of this study was to focus on the potential of biochar as an agricultural amendment. This technology has the added advantage of potentially helping the agricultural sector to achieve the 64% reduction in GHG emissions by 2050 outlined by the Committee on Climate Change. As an agricultural amendment, biochar has already been shown to improve crop yields, soil health and to reduce reliance on chemical fertilisers. However, published data are variable and depend on feedstock used for biochar production, pyrolysis temperature and soil type, with most positive results obtained when biochar is applied to nutrient-poor, acidic tropical soils. There is a lack of data on the effects of biochar when applied to temperate neutral and alkaline soils. The aim of this study was to address that gap and use three bespoke biochars as amendments to temperate soils to determine whether this method of carbon sequestration is beneficial under these circumstances, or whether unintended consequences occur. The three biochars were produced by torrefaction, which uses lower temperatures and produces higher yields than more commonly used pyrolysis methods.

The objectives of the work were to quantify how alterations in production conditions affect the characteristics of biochar, to evaluate the effect of biochar application on alkaline agricultural soils and associated plants and to quantify the benefits of using biochar during compost production and subsequent use as a fertiliser.

Characterisation of the three biochars showed that small changes in production conditions resulted in significant differences in specific surface area (SSA), pH and total and extractable elements. Increasing degree of torrefaction was associated with decreased concentrations of extractable elements including heavy metals, and although changes in physiochemical characteristics were not uniform, the most torrefied biochar had the most desirable characteristics in terms of stability. A key finding was that whilst total nutrients increased with degree of torrefaction, their availability decreased, highlighting a need to optimise production conditions depending on the intended use of the biochar. The characteristics were also compared to European and International guideline standards for biochar and all met the criteria for use in agriculture.

The effect of the three biochars on soil chemistry and plant growth in an alkaline soil was studied using kale as a model plant. Biochar application rates of 2.5%, 5% and 10%, each at two particle sizes < 2 mm and > 2 mm were tested against each other and a control. No significant effects on plant biomass were observed, although plant growth when application rates of >5% were applied were slightly lower. Both soil pH and soil nutrient content significantly changed with biochar application, but these did not affect kale yield. Most field-scale applications of biochar are in the region of 10-20 tonnes ha-1, which is considerably lower than the 5% biochar highlighted here. This suggests that applying biochar to alkaline soils at field-realistic concentrations, is unlikely to lead to negative unintended consequences, with the understanding that extrapolating from pot to field is challenging.



Several experiments were carried out where biochar was mixed into an established farmyard manure compost and also added to greenwaste at the start of the composting process. Once mature, the fertiliser capability of the composts (± biochar) was tested on cress plants grown in an alkaline agricultural field soil and an acidic woodland soil from the same series. Plant biomass was increased in the presence of each compost compared with the unamended controls. Growth was only marginally enhanced further by biochar amendment of the composts, and this depended on whether the compost was applied by weight or by crop nitrogen requirement. If the latter, a greater quantity was applied to adjust for the dilution effect of adding biochar and plants responded more favourably to this application method if greenwaste was used. In contrast, plants responded better to the farmyard manure compost if added on a weight basis rather than adjusting for crop nitrogen requirements. A key observation was that the farmyard manure and the greenwaste composts had different pH values (pH 7.2-6.4 ± biochar and 9.8-10.1± biochar respectively) and plants generally performed better in the presence of the farmyard manure than the greenwaste manure. Biochar addition to the composts lowered the availability of several macro- and micro-nutrient which might explain why plants did not respond favourably, or responded conservatively, to the biochar/compost mixtures.



A further composting experiment was undertaken to enable processes to be monitored regularly for 12 weeks. Chicken litter was amended with biochar and incubated at a constant temperature, and during that time biochar amendment resulted in a net decrease in ammonia and GHG emissions from the litter. Biochar amendment increased the aromaticity and maturity of the compost (measured by Fourier Transform Infrared Spectroscopy) and increased the pH and nutrients.

Overall, this study demonstrated that biochar has potential for use as a soil amendment on temperate agricultural soils as a means of sequestering carbon because no unintended negative consequences were observed with application rates similar to those used in field trials. Furthermore, co-composting biochar with organic residues minimised GHG losses from the process. If biochar were applied to the six million hectares of arable and temporary grassland in the UK at a rate of 10 tonnes per hectare, it would amount to approximately 50 million tonnes of carbon, which, although represents a small contribution to the 35 Mt carbon per year annual target of the Royal Society, other benefits from application add to the carbon sequestration benefits, such as enhanced water holding capacity, which is increasingly important in light of climate change.

The next stage would be to scale-up the trials to field-scale and quantify the effects in a real-world environment.

Item Type:	Thesis (University of Nottingham only) (EngD)
Supervisors:	West, Helen Snape, Colin
Keywords:	Biochar; Agricultural amendment; Temperate soils; Carbon sequestration; Alkaline soils; Fertiliser
Subjects:	T Technology > TP Chemical technology
Faculties/Schools:	UK Campuses > Faculty of Engineering UK Campuses > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID:	77966
Depositing User:	Tutton, Charlotte
Date Deposited:	18 Jul 2024 04:40
Last Modified:	18 Jul 2024 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/77966

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