Soil processes govern alkalinity and cation retention in enhanced weathering for carbon dioxide removal

Abstract. Avoiding the most damaging consequences of climate change will almost certainly require pairing rapid emission cuts with large‑scale carbon dioxide removal (CDR). Among the proposed CDR pathways, enhanced weathering (EW) accelerates natural mineral dissolution to convert atmospheric CO₂ into long‑lived bicarbonate and carbonate reservoirs. Despite the many reported data from EW experiments, large uncertainty remains about the realisable CDR potential of applying rock materials to agricultural land. One of the relevant sinks for CO₂ is the transfer to bicarbonate alkalinity, and various EW studies have reported a wide range of results for this process. Intercomparison of these data is problematic due to the different experimental set-ups, environmental conditions as well as combinations of rock materials and soil types. In order to assess and compare the realisable CDR potential of various EW combinations, a large greenhouse experiment was set up in which 4 different soil types (7 different soil batches) were treated with 13 different feedstock materials. The experiment included growing perennial ryegrass (Lolium perenne) and was conducted over two years with high irrigation rates (> 2000 mm a-1) and elevated temperatures (>19 °C) to speed up the weathering process. Alkalinity production was highly variable among the treatments and some even showed a loss of alkalinity compared to their controls. Consistent with expected dissolution kinetics, alkalinity production rates followed the trend: steel slag > limestone / carbonate-rich metabasalt > peridotite > basanite. Matrix analyses of soil properties versus feedstock revealed that alkalinity production from acidic soils was highest. At higher pH-levels (> 7 pH), carbonate mineral saturation likely constrains further dissolution, potentially favouring carbonate formation. Detailed analyses of cation pools (exchangeable, carbonates, oxides and clay) revealed large changes within the first year where 10–50 times more cations were retained than exported via leachate, making the realised CDR potential as alkalinity relatively small compared to the CDR potential of cations retained. Understanding the dynamics of transfers between cation pools and their potential saturation are important to develop models and enable projections. Data reported from EW studies so far are insufficient to enable calibration of models, specifically if projections in CDR-realisations should span decades.