Rock/soil interactions governing alkalinity release and cation retention in greenhouse enhanced weathering experiments

Enhanced weathering (EW) is a promising, scalable approach to carbon dioxide removal (CDR). It involves accelerating the weathering of minerals in soils to convert atmospheric CO₂ into carbonate alkalinity. Despite numerous studies, it remains unclear which soil/feedstock combinations achieve the highest alkalinity export and greatest CDR. This uncertainty remains because results strongly depend on experimental design, environmental conditions and soil/feedstock types.

We systematically investigated alkalinity formation and the fate of released or retained cations in a large-scale greenhouse experiment. Over a period of two years, we operated 10 L lysimeter mesocosms that grew English ryegrass (Lolium perenne) under controlled accelerated weathering conditions (>19 °C and > 2,000 mm irrigation per year). We tested four soil types with thirteen feedstocks and recorded alkalinity and cation export in soil water leachates monthly. For a subset of the samples, we quantified soil cation pools using sequential extractions (exchangeable, carbonate-bound, oxides and clay/silicate fractions) to determine the potential temporary reservoirs and irreversible cation sinks.

Our results show that the amount of weathering products entering the leachate varied substantially: although several treatments showed increased cumulative leachate alkalinity relative to their controls, some soil/feedstock combinations showed no change or even reduced alkalinity export, including for the same feedstock applied to different soils. Alkalinity production generally followed expected dissolution kinetics (steel slag > limestone/carbonate-rich metabasalt > peridotite > basanite). Highest leachate alkalinity export relative to control was observed in acidic soils, whereas little to no relative change observed in more neutral ~pH 7 soils could reflect suppression of mineral dissolution and carbonate saturation and precipitation. Soil cation pool distribution shifted markedly within the first year, and collectively retained 10 to 50 times more cations than exported as alkalinity. This implies that short-term 'realised' CDR as exported carbonate alkalinity can be far lower than the potential unrealised CDR that could be unlocked when cations are released from temporary soil retention pools.

A follow-up greenhouse experiment on 23 soils and 22 feedstocks was commenced in early 2025, spanning more than 300 soil/feedstock combinations. This expanded dataset will enable more robust attribution of controls on EW performance, such as soil pH/buffering capacity, mineralogy and reactive Ca–Mg supply. This setup will also allow identification of soil/feedstock combinations that maximise alkalinity generation under minimal cation retention in non-exportable pools. We will for the first time share early results from this follow-up experiment.

Our results emphasise that robust CDR quantification for EW should consider cation binding dynamics and pool transfers, as well as mineral saturation effects. Leachate-based alkalinity measurements alone provide an incomplete picture of available weathering products, particularly when rapid soil retention dominates early stage dynamics.