The potential of enhanced weathering as a CO2 removal technique in UK agricultural soils
Keywords: carbon dioxide removal
terrestrial weathering
liming
negative emissions
enhanced weathering
basalt weathering
industrial silicates

Enhanced weathering (EW) is increasingly proposed as a promising negative emission technology that sequesters atmospheric carbon dioxide without substantially changing established agricultural practices. Current estimates suggest enhanced weathering could remove 0.5-4 GtCO2 yr-1 globally by the end of this century (Smith et al., 2015) which equates to a substantial fraction of global anthropogenic emissions (49 GtCO2eq yr-1; IPCC, 2014). However, these estimates are based on limited experimental assessment of the complexities of the soil environment which inhibit alkalinity release, and existing pot and core studies do not consider the influence of natural hydrological conditions on dissolution rate and mineral saturation. In this thesis, I used two long-term, fully replicated soil core studies to comprehensively understand the geochemical dynamics of dissolution and carbon dioxide removal (CDR) in a soil environment. I used multiple, natural soil cores extracted from a typical UK agricultural site exposed to seasonal changes in weather and temperature to closely simulate field conditions.
I applied crushed basalt to agricultural soil cores at 100 t ha-1 in a 14-month preliminary study (Chapter 3) and developed an innovative experimental method of extracting soil solution for geochemical analysis whilst minimising disruption to natural hydrology (Chapter 2). Assessment of soil solution sampled over five sampling events provided an initial insight into basalt dissolution and CO2 drawdown in a soil environment. The experimental method established in this study formed the premise of a 16-month soil core study used to assess the dissolution of a range of proposed treatments applied at 50 t ha-1 (Chapter 4). Treatments included naturally occurring silicates (crushed olivine, crushed basalt, and volcanic ash), silicates produced from industrial processes (cement kiln dust, crushed steel slag) and agricultural lime (aglime).
High-resolution sampling provided a first look into the pathway of dissolution products through the soil-water system. Treatment dissolution elevated the pH of soil treated with basalt and steel slag, and the alkalinity of soil solutions increased following addition of all treatments, except olivine. Chemical changes to the soil-water system were most marked at the top of treated cores. The surface-area normalised ion-release rate varied from 10 -13.96 ± 0.03 mol(Ca) cm-2 s-1 (aglime) to 10 -18.99 ± 0.01 mol(Ca) cm-2 s-1 (basalt). This difference is partly attributed to the fast dissolution rate of Ca-carbonate relative to silicate minerals and rocks. The introduction of micro-porosity during crushing is also likely to have artificially elevated the BET surface area of crushed basalt, and, in turn, reduced the surface-area normalised ion release rate of basalt relative to uncrushed treatments. Soil processes, such as exchange and secondary mineral formation (measured with XRD and XRF, and modelled with PHREEQC), reduced the flux of alkalinity into solution, particularly in olivine-treated cores.
The carbon dioxide removal potential after a single application at 50 t ha-1 was, in decreasing order: steel slag (20 ± 3 kgCO2 ha-1 yr-1) > cement kiln dust (CKD) (16 ± 2 kgCO2 ha-1 yr-1) > basalt (5.0 ± 0.7 kgCO2 ha-1 yr-1) > volcanic ash (2.7 ± 0.4 kgCO2 ha-1 yr-1) > aglime (2.2 ± 0.2 kgCO2 ha-1 yr-1) > olivine (0.0 ± 0.2 kgCO2 ha-1 yr-1). Industrial silicates were shown to be an effective source of alkalinity; however heavy metal toxicity may limit addition of steel slag to arable soils, and availability of CKD will limit large-scale application. These findings demonstrate dissolution of aglime in alkaline soils is a previously unquantified negative flux of CO2 that could influence national carbon accounting. Of the six treatments assessed, this research suggests basalt is the most promising treatment for nationwide enhanced weathering.
Scaling the CDR potential of basalt over UK cropland suggests basalt application will remove 1.2-1.3 MtCO2 yr-1. This value takes into account hydrological variations, which this research indicates are a critical control on CO2 removal potential. The resulting flux is equivalent to <3% of UK agricultural emissions, and is 5- to 25-fold lower than previous modelled estimates (Kantzas et al., 2022); likely due to complexities of soil systems and to water limitation on alkalinity release. Further research is needed to fully understand the impact of water flux on the efficacy of enhanced weathering in a real-world setting across a range of hydrological and soil environments.