Quantifying potential carbon dioxide removal via enhanced weathering using porewater from a field trial in Scotland

Enhanced weathering (EW) is cited as a promising carbon dioxide removal (CDR) strategy, and is being rapidly commercialized. Rigorous monitoring, reporting and verification (MRV) are essential to ensure carbon claims are accurate and carbon credits are not mis-sold. MRV protocols incorporate multiple approaches, including soil and porewater sampling. This paper calculates potential CDR from porewater, via an alkalinity estimation calculated from charge balance, and from soil samples, via the accumulation of exchangeable cations on soil exchange sites. These potential CDR estimations are then compared to the maximum theoretical CDR potential. The data were collected from a 1.5 year field trial, situated in south-east Scotland. Crushed basalt was surface-applied to plots at rates of 0 (control), 23, 78 and 126 t ha⁻¹. To calculate direct-measured potential CDR (direct pCDR) from porewater, ion concentrations of porewater samples extracted from a depth of 5 and 10 cm were integrated with precipitation surplus to estimate the flux of cations leaching from each depth over c. two week periods, as water budgets allowed. Generalized linear model results identified a significant effect of treatment as an explanatory variable for potential CDR, both at 5 and 10 cm depth. Significant potential CO2 removal ranging from 0.33 to 0.53 tCO2 ha-1 after c. 1.5 years of weathering was calculated in the 5 cm depth treatment in the 78 and 126 t ha-1 application treatment relative to the control. No significant difference was observed between the control and the 23 t ha-1 treatment at 5 cm depth, nor were there any significant differences in the 10 cm treatments when evaluated relative to the control. Carbonate precipitation was also assessed, but remained below the detection limit. Potential CDR (inferred pCDR) calculated from the exchangeable cation pool (ammonium acetate extractable pool) in 30 cm-deep soil samples revealed no significant inferred pCDR, possibly as a result of experimental design and sampling density. Overall, when direct pCDR is normalized to mass of rock applied and duration of weathering (e.g. mass-time-normalized-pCDR), the values fall within the mid-range of values published from other field studies.