Biologically-driven rock weathering: roles of bacterial carbonic anhydrase in arable soils, and mycorrhiza and roots in a floristically diverse grassland.

Atmospheric carbon dioxide (CO2) removal strategies are needed to limit global temperature rise to 1.5 °C above pre-industrial levels. Enhanced rock weathering (ERW), which uses silicate rock dust spread onto agricultural soils to accelerate the inorganic carbon cycle, can theoretically absorb gigatonnes of CO2 globally. Rates of weathering are affected by soil biology, but the importance of the enzyme carbonic anhydrase (CA), bacteria, fungi, and plants, remain unresolved, and ERW potential in grasslands needs to be investigated.
To address these issues, this thesis investigated the potential to accelerate ERW rates by CA and soil organisms. CA’s role in bacterial weathering was studied by growing Burkholderia thailandensis strains with different CA genes inactivated, in a basalt supplemented liquid medium. One of these B. thailandensis mutants had reduced weathering under certain conditions. However, metagenomic analysis of CA abundance in a long-term ERW trial, on North American maize-soybean cropland, found no evidence of selective pressure for CA genes in basalt-treated soils in which ERW was occurring, suggesting it does not play a major role in mineral dissolution in situ.
The first grassland field trial of ERW was conducted on a floristically diverse mildly acidic upland UK site. Basalt amendment increased soil pH and exchangeable cations, indicative of CO2 removal, and raised phosphorus, magnesium, and sodium concentrations in hay, without adversely impacting hay yields or floristic diversity. By combining a novel magnetic rock extraction method with mesh-walled biological simplification columns incubated over the growing season in this grassland, mesh permitting non-saprotrophic filamentous fungal colonisation such as by mycorrhizas appeared to enhance mineral weathering rates in situ, increasing soil concentrations of calcium, strontium, titanium, and iron, whereas plant roots appeared to reduce weathering.
The thesis highlights the complexity of biota in weathering in soils, demonstrating how their contributions can be identified at various scales, and explores the potential of floristically diverse hay meadows for ERW using basalt additions.