Concrete as a soil amendment for carbon capture: learnings from year one of an enhanced weathering field trial in County Wexford, Ireland.

In this study, we investigate the use of milled returned concrete as an enhanced weathering soil amendment, on two arable fields in County Wexford, Ireland. The applied concrete consists of portlandite (Ca(OH)2) cement with limestone (CaCO3) aggregate. The high cation concentration and rapid weathering kinetics of both components indicate good potential for carbonic acid neutralisation and atmospheric CO2 removal as soil-water dissolved bicarbonate (HCO3-). In spring 2023, prior to crop planting (oats and barley), both trial fields were divided into two sections. Milled returned concrete was applied to a ‘treatment’ section while no concrete was applied to a ‘control’ section. All other farming practices (ploughing, tilling, sowing and fertilisation) were equivalent across control and treatment. Twelve suction-cup lysimeters were installed in each field (6 control and 6 treatment) to collect soil-water samples across the growing season and the concentrations of bicarbonate, major cations and anions were measured to assess carbon removal. Preliminary results indicate that where nitrate (NO3-) levels are low in concrete amended sites, bicarbonate concentrations are elevated above control. However, where soil-nitrate levels are high, weathering liberated cations are balanced by nitrate, and bicarbonate production is suppressed. Our findings highlight the importance of fertiliser management for optimising CO2 removal outcomes of enhanced weathering.