Enhanced weathering in agriculture: impacts on organic carbon, soil fertility, and greenhouse gas dynamics – Thesis

Climate change constitutes one of the most urgent global challenges, driven predominantly by rising atmospheric concentrations of greenhouse gases. To remain within the temperature limits stipulated by the Paris Agreement, scenario analyses increasingly indicate that substantial atmospheric carbon dioxide removal will be required alongside rapid emission reductions. Within this context, enhanced weathering (EW) of silicate minerals has emerged as a promising carbon dioxide removal (CDR) strategy. By accelerating natural silicate dissolution, EW removes atmospheric CO₂. However, despite accelerating interest in EW, significant uncertainties persist regarding its interactions with the complex biogeochemical processes of agricultural soils, most notably soil organic carbon (SOC) dynamics, nitrogen cycling (with particular attention to nitrous oxide (N₂O) emissions), and crop performance. This dissertation addresses these uncertainties through controlled mesocosm experiments and a field trial in Westmalle, Belgium, employing both natural silicates (basalt, tephrite) and steel slags derived from industrial steel production. Across all experiments, EW consistently elevated soil pH and base cation availability, confirming its capacity to counteract soil acidification. In contrast, crop biomass, nutrient uptake, and broader agronomic performance exhibited only minimal responses, suggesting that agronomic co-benefits are likely limited in already relatively fertile agricultural systems. This thesis also shows that EW affects SOC cycling, with responses strongly shaped by biotic interactions: basalt reduced soil organic matter decomposition only in plant-free mesocosms, indicating SOC stabilization. In planted treatments, basalt increased rhizosphere activity, underscoring the central role of biological processes in governing EW outcomes. EW also influenced nitrogen cycling, with steel slags treatments increasing N₂O emissions, thereby revealing a risk associated with EW deployment. Heavy metal mobilisation due to silicate application remained generally low, yet measurable increases in nickel highlight the need for continued environmental monitoring to ensure safe and sustainable application. Across the thesis, mineralogical differences among feedstocks drove variability in agronomic responses and greenhouse gas fluxes, emphasizing the critical importance of feedstock selection. Overall, the findings demonstrate that EW can contribute to SOC stabilization and mitigate soil acidification, yet its net climate effect depends on interactions among feedstock mineralogy, soil properties, and biological processes. These results highlight the necessity for monitoring, reporting, and verification frameworks to account not only for inorganic carbon sequestration but also for organic carbon cycling and emissions of non-CO₂ greenhouse gases such as N₂O. This dissertation provides a more comprehensive understanding of EW and an empirical foundation for its responsible implementation in agricultural landscapes.