Enhanced rock weathering with crops in agricultural soils: Quantifying basalt weathering rates, CO₂ capture and agronomic co-benefits

The impending breach of the 1.5 ºC global warming threshold has catalysed significant recent interest in engineering solutions to climate change. Carbon Dioxide Removal (CDR) technologies are favoured, as they tackle the root cause of climate change, and from within this subset of geoengineering approaches, Enhanced Rock Weathering (ERW) stands out as a potentially scalable, technically-feasible, low-cost and potent strategy. Deployed at scale, ERW aims to increase the rate of global silicate weathering – which in nature acts as a long-term sink for atmospheric carbon dioxide (CO₂) – by spreading pulverised volcanic rocks on agricultural land. Basalt may be the most suitable rock for ERW, due to its ubiquity and low content of potentially toxic trace elements. But whilst basalt does have a long history of use as a supplementary fertiliser – in the developing world and, more recently, the organic agriculture sector – there exists limited evidence for its CO₂ sequestration potential in agricultural soils.

The overarching aim of this thesis was to quantify basalt weathering and CO₂ capture rates in agricultural soils under crop cultivation, and to measure the associated impacts on key soil and plant performance characteristics. This was achieved through a series of mesocosm-scale controlled-environment experiments of increasing complexity and scale, using a variety of soil types and crops. The findings in this thesis offer support for the use of basalt fines for carbon capture in acidic agricultural soils, with reported CO₂ capture estimates in the tonnes-per-hectare range for a single rock application. The use of basalt as a supplementary fertiliser is also justified by the results, as each experiment demonstrated higher yields in rock-amended acidic soils, with additional co-benefits for crop production. However, basalt had no discernible effect in the only alkaline soil included in the trials, leaving open the question of their suitability for ERW.