Amending soils with basalt: evidence for rock weathering, mineral precipitation, and soil organic carbon stabilisation

Enhanced rock weathering (ERW) is a CO2 removal (CDR) technology
which utilises one of Earth’s natural CO2 regulators to remove CO2 from the
atmosphere via silicate weathering. One ERW method aims to accelerate rock
weathering by applying finely crushed, fast weathering silicate rocks to cropland
soils. Basalt: an abundant, fast-weathering rock readily available in a finely
crushed form as a by-product of the aggregate industry as ‘basaltic quarry fines’
is a prime candidate for ERW.
There is limited, direct evidence for understanding how basalt grains
weather and the fate of Al, Fe and Mn released during weathering when basalt is
applied to soils. These weathering products could stabilise organic carbon (OC) in
soils, preventing its release into the atmosphere as CO2, therefore increasing
basalt’s CDR potential via ERW. The interaction of basalt’s weathering products
with OC during ERW is also unknown. Broadly, the aims of this thesis were to:
(1) characterise different basaltic quarry fines and assess their CDR potential via
weathering, (2) investigate how basalt grains weather in soil solution, (3) assess
changes to Al, Fe and Mn soil sinks after basalt amendment and (4) assess
evidence for OC stabilisation after basalt amendment into cropland soils. Using
six commercially available basaltic quarry fines and samples from two ERW
experiments, a range of techniques including: geochemical modelling, chemical
extractions, mineralogical characterisation, surface area characterisation and
elemental analysis were used to investigate these aims.
The thesis identified a large range in CDR potential between the quarry
fines and chemical analysis identified the leaching of alkaline and alkaline-Earth
metals during weathering. Several increases in Al, Fe and Mn were also identified
as a result of basalt amendment and the use of either conventional or organic
fertiliser. However, there was limited evidence to suggest that basalt amendment
significantly affected OC that had interacted with Al, Fe or Mn ions/precipitates.