Enhanced weathering of glacial rock flour drives coupled inorganic and organic carbon sequestration in a five-year field experiment

Abstract. Enhanced rock weathering (ERW) has been proposed as a scalable carbon dioxide removal strategy, yet long-term field-based constraints on weathering rates and associated carbon sequestration remain limited. Here, we present a five-year field assessment of Greenlandic glacial rock flour (GRF) applied at 50 t ha^-1 to a sandy agricultural soil in Denmark. Using a soil mass balance approach, we estimate that approximately 56% of the applied GRF dissolved over five years. Initial uptake of CO₂ via ERW is estimated at 3.2-4.9 t CO₂ ha^-1 when accounting for carbonate speciation in the acidic soil solution. If downstream transport of weathering products and subsequent re-equilibration with the more alkaline marine carbonate system is assumed, the estimated long-term atmospheric CO₂ removal increases to 6.39 t CO₂ ha^-1. In addition to inorganic carbon removal, GRF-treated plots exhibited a significant increase in soil organic carbon stocks, equivalent to the storage of an additional 18.3 t CO₂ ha^-1, with gains primarily associated with the mineral-associated organic matter fraction. These results demonstrate substantial weathering and carbon sequestration over five years under field conditions in a temperate climate and suggest that both inorganic and organic carbon dynamics contribute meaningfully to the overall climate impact of ERW of GRF. The findings provide rare multi-year field constraints on ERW performance and underscore the importance of robust measurements for quantifying carbon removal in agricultural systems, representing the first implementation of mass balance methods within the framework of a randomized block design.