Is the spreading of volcanic rock powder on temperate agricultural soils a practice to be encouraged to remove carbon from the atmosphere?

Details

Supervisors

Delmelle, Pierre ; Glorieux, Juliette

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

enTerrestrial Enhanced silicate weathering (ESW) is gaining attention as a climate mitigation strategy for capturing atmospheric CO2 through the enhancement of the natural chemical weathering of silicate rocks. ESW involves applying crushed silicate rock, such as basalt, to soils, like croplands. However, current estimates of its potential for carbon dioxide removal (CDR) vary widely between studies and remain uncertain. This uncertainty primarily stems from the reliance on laboratory-derived kinetics that overestimate field weathering rates. In addition, many studies neglect operational emissions from extraction, grinding, transportation, and spreading of the crushed silicate rock on the field, therefore biasing CDR estimates since the implementation of ESW is not carbon neutral. This research provides new insights into the potential for CDR of ESW as a climate mitigation strategy in temperate agricultural soils, quantifying both its CDR potential and associated operational emissions using a modelling approach. It assesses whether ESW could achieve net CO2 reduction and identify key parameters affecting its effectiveness, including basalt mineralogy, application rate and grain size and soil temperature and CO2 partial pressure (pCO2). Multiple scenarios are simulated using the reactive transport model WITCH, and the resulting CO2 consumption is integrated into a carbon balance that accounts for operational emissions. Our results show grain size has a greater influence on CDR potential than application rate, while also contributing less to operational emissions. This indicates that spreading a low amount (~10 t ha-1) of finely ground basalt (~30 µm) is more effective than using a large quantity (~100 t ha-1) of coarser material (~100 µm). By contrast, operational emissions scale linearly with the application rate, with a high application rate consistently resulting in net CO2 emissions rather than removal. Over 45 years of ESW in a temperate Luvisol, the net CDR ranges from -21 to 4.5 t CO2eq ha-1, with negative values indicating net CO2 emissions. Even at a low application rate (e.g. 10 t ha-1), between 13% and 52% of the theoretical CDR is offset by emissions. Transportation emerged as the dominant contributor to operational emissions (~90%), highlighting that a geochemically suitable basalt may still be unsuitable for ESW if associated with carbon-intensive logistics. Sourcing basalt from more distant quarries appears currently unviable for ESW, unless it involves low-emission transport modes. Repeated annual applications over five years support this conclusion, achieving net CDR only under restrictive conditions, i.e. a low application rate combined with a low-emission transport mode. Finally, our results also highlight that field environmental conditions, characterized by a higher soil pCO2 than theoretical value and lower temperatures in temperate regions, can limit the potential for CDR of ESW compared to more favorable conditions. Overall, our results indicate that expectations regarding ESW’s potential in temperate croplands may need revision. Future work should focus on improving the accuracy of CO2 consumption estimates by improving models and parameters using field data.