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Research Questions/Objectives: The objective of this project is to experimentally determine the effects of soil acidity-related variables and how they may impact carbon dioxide removal through the enhanced weathering of alkaline minerals within the soil. These variables include a) the pH of the soil, b) the stored acidity within the soil, and c) soil processes that produce or consume acidity.
Brief Description of the Project: The enhanced weathering (EW) of alkaline silicate minerals within the soil environment presents an opportunity to use a common geological process to facilitate the removal of carbon dioxide from the atmosphere at a higher rate than is achieved naturally. Tropical agricultural soils are a suitable medium for enhanced weathering deployment as weathering of added minerals will be fast due to high temperature and humidity. The strongly acidic nature of tropical agricultural soils also contributes strongly to fast weathering rates found in these soils. However, recent models have found that the efficacy of carbon capture within acidic environments (pH<4.5) is low, which presents the need to investigate how soil acidity and the components that contribute to it, impact carbon dioxide removal effectiveness.
Background and Significance of the Research Question to drought risk, vulnerability, preparedness, or resilience: The risk of drought has been exacerbated by rising temperatures caused by global warming. Continued warming up to 1.5-2°C above pre-industrial levels increases the likelihood of more intense and more frequent droughts to impact the tropical North Queensland region. According to the IPCC report released in 2018, successfully limiting global warming to 1.5°C is expected to significantly reduce the probability of extreme drought and the risks associated with reduced water availability (Hoegh-Guldberg, 2018). However, the combined deployment of carbon dioxide removal strategies and reduced emissions is a necessity to restrict warming to 1.5°C above pre-industrial levels with limited overshoot.
Enhanced weathering presents an opportunity for farmers to contribute to the removal of carbon dioxide from the atmosphere whilst simultaneously improving the quality of their soil. Traditionally, liming agents are used to treat soil acidity which contributes to carbon dioxide emissions from the agricultural sector (West & McBride, 2005). However, incubation experiments testing the effectiveness of olivine as an alternative liming agent found that while the mineral had an overall weaker impact on increasing soil pH compared to traditional lime, it was able to decrease aluminium availability to a level suitable for plant growth in a relatively short time (Dietzen et al., 2018).
As a result, large scale deployment of enhanced weathering as a carbon dioxide removal strategy may leave farmers in tropical regions with the most to gain. Before such deployments are possible, there is a need to better quantify how soil acidity-related variables may impact carbon dioxide removal rates. Such information will aid future programs in determining the best land management practices to use when amending soils in order to derive the largest benefits for the farmers and the soil.
Hoegh-Guldberg, O., D. Jacob, M. Taylor, M. Bindi, S. Brown, I. Camilloni, A. Diedhiou, R. Djalante, K.L. Ebi, F. Engelbrecht, J. Guiot, Y. Hijioka, S. Mehrotra, A. Payne, S.I. Seneviratne, A. Thomas, R. Warren, and G. Zhou, 2018: Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 175-312, doi:10.1017/9781009157940.005.
West, T. O., & McBride, A. C. (2005). The contribution of agricultural lime to carbon dioxide emissions in the United States: Dissolution, transport, and net emissions. Agriculture, Ecosystems and Environment, 108(2), 145–154. https://doi.org/10.1016/j.agee.2005.01.002
Dietzen, C., Harrison, R., & Michelsen-Correa, S. (2018). Effectiveness of enhanced mineral weathering as a carbon sequestration tool and alternative to agricultural lime: An incubation experiment. International Journal of Greenhouse Gas Control, 74, 251–258. https://doi.org/10.1016/j.ijggc.2018.05.007
Academic and research experience relevant to the honours project: I completed my Bachelor of Science at James Cook University in 2023, majoring in chemistry and achieving an overall GPA of 6.40. During my studies, I enrolled in several classes that deepened my knowledge and understanding about the environment within the context of chemistry. EA3207: Soil Properties and Processes was a class responsible for introducing soil science as a niche yet critical area of study and helped me to better understand how the conditions of soils critically impact all other environmental systems. EA2404 From Icehouse to Greenhouse outlined the history of the Earth’s climate and the drivers behind the changes from formation to today, helping to better understand the processes that may impact the current climate. While this is my first research project I possess experience within the field of soil analysis. For Work Integrated Learning during my degree, I partnered with a local company in Townsville, during their experiment in treating high conductivity dredge soil with organic matter. My role in this project was to analyse the samples collected from the site over the course of the experiment and provide accurate data that mapped the changes in soil pH and salinity that occurred.
Principal Supervisor’s skills and experience in relation to this project topic: Paul Nelson is a soil scientist with a 25-year record of research project leadership, often in collaboration with industry. He has led projects on carbon cycling in soils and catchments, nutrient cycling and management, GHG emissions, and soil chemistry in tropical agriculture in Queensland, Papua New Guinea and Indonesia, including the project in collaboration with the Leverhulme centre for Climate Change Mitigation that led to this proposal. Since joining academia in 2004 he has supervised 10 PhD (4 as Primary Advisor), 7 MPhil (3 as Primary Advisor) and 11 Honours candidates to completion. He is currently leading a large research project on enhanced weathering, including field trials on 3 farms, and is supervising 2 PhD projects on the topic.
I grew up primarily in a remote town in the Northern Territory, surrounded by an environment that was breathtakingly beautiful and uniquely Australian. Already naturally curious, this lived experience instilled within me a deep love for the natural world and a desire to gain a deeper understanding of how environments function and thrive.
As a Chemistry major, I spent my undergraduate years pursuing this knowledge and understanding, completing elective classes that explored the complex systems that govern different environments, leading to an introduction to soil science. Since commencing my experiments in soil science, I have found that the discipline provides an encompassing perspective of the complexities and variabilities of our entire environment delivered in one small slice of the planet.
My projects have been focussed on testing possible solutions to the ongoing issues arising as the climate continues to change, experimenting with the concept that soil can facilitate the sequestration of CO2 from our atmosphere to create bicarbonate ions that our planet can store in the ocean. This experimental concept does provide challenges in my area of research, however gaining an understanding of climate science makes the subject of soil science wholly rewarding and satisfying to study, especially as I learn about hopeful solutions for a better future.
Future Career Goals:
After completing my Honours project, I would like to use my expertise to successfully manage and restore agricultural environments and to seek further opportunities to continue learning the intricacies of soil science.
Background
Enhanced rock weathering (ERW) is being pursued as a potential solution to mitigate global warming by reducing the concentration of carbon dioxide in the atmosphere. By limiting global warming to 1.5°C above pre-industrial levels, extreme droughts are less likely to occur (Hoegh-Guldberg, 2018) however, this requires the deployment carbon dioxide removal strategies and emission reduction. Enhanced rock weathering may contribute to both. ERW harnesses natural weathering reactions that have influenced atmospheric carbon dioxide over geological time scales through the breakdown of alkaline silicate minerals (Schuiling & Krijgsman, 2006) to reduce carbon dioxide flux from the soil into the atmosphere. Agricultural soils provide a logistical advantage for ERW compared to forested areas as they are accessible and already have the infrastructure required for spreading (Moosdorf et.al., 2014; Strefler et.al., 2018). Furthermore, targeting agricultural soils provides the co-benefit of countering soil acidification and increasing the yield of the crops grown (Kelland et.al., 2020; ten Berge et.al., 2012). Geochemical modelling indicates tropical regions to be the most effective locations for ERW due to their high temperatures and rainfall throughout the year (Baek et.al., 2023), placing tropical North Queensland farmers at the forefront for potential benefits from ERW.
Experiment Overview
The 10 soils chosen for this study are representative of the sugarcane growing areas of North Queensland. Two soil incubation experiments were completed, one to address the effects of active acidity on carbon capture and the other for the effect of reserve acidity and other soil variables. To quantify carbon capture within the soils, half of the samples were incubated with an addition of Ca(OH)2 and the other half without an addition. An additional experiment was conducted to test the effect of moisture content on carbon capture. Carbon capture was calculated as the difference in extractable bicarbonate between soils with and without added Ca(OH)2. The total carbon and nitrogen contents were measured by Nutrient Advantage. Linear regression analysis was used to test the dependence of carbon capture on soil pH. Spearman’s correlation analysis was used to assess significance of correlations within Experiment 2.
Key Findings
The combined data set for Experiment 1 and 2 indicated a positive relationship between carbon capture via weathering and soil pH (Figure 1). Carbon capture values in Experiment 2 were found to be significantly negatively correlated with total carbon, and total nitrogen contents of the soil. The lowered moisture content in the treated soil samples in significant decreases in carbon capture (by 2.1 mg kg-1) compared to the samples at 100% field capacity (Table 1)
Figure 2: Carbon capture against soil total carbon and total nitrogen %
Conclusion
The relationship provides experimental evidence confirming the recent geochemical modelling that suggested higher pH levels are more conducive to carbonic acid weathering and carbon sequestration (Bertagni & Porporato, 2022; Green et al., 2024). These results indicate that achieving high levels of carbon capture with enhanced rock weathering may only be possible in soils with near neutral pH levels as well as a constant high moisture content. This information may be useful to land managers when deciding which soils may be suitable for enhanced rock weathering projects.
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