The enhanced weathering potential of a range of silicate and carbonate additions in a UK agricultural soil

Abstract

Enhanced weathering (EW) is a carbon dioxide removal (CDR) technology which aims to accelerate silicate and/or carbonate weathering in agricultural land. At present, the rate and magnitude of CDR from EW remains uncertain. In this study, soil cores extracted from a typical UK agricultural site in Oxfordshire were used to geochemically assess the efficacy of EW while simulating field conditions. Six material “treatments” were applied to soil cores at a rate equivalent to 50 t ha−1: agricultural lime (aglime), basalt, cement kiln dust (CKD), olivine, steel slag, and volcanic ash. A range of chemical measurements were used to constrain the rate of dissolution, fate of dissolution products, and the CDR potential and environmental impact of treatment. After a single application, the CDR rates were, in decreasing order: steel slag (20 ± 4 kgCO2 ha−1 yr−1) > CKD (16 ± 4 kgCO2 ha−1 yr−1) > basalt (5 ± 3 kgCO2 ha−1 yr−1) > volcanic ash (3 ± 3 kgCO2 ha−1 yr−1) > aglime (2 ± 1 kgCO2 ha−1 yr−1) > olivine (0 ± 2 kgCO2 ha−1 yr−1). Despite its drawdown potential, steel slag addition is not advised because application raised the dissolved concentration of heavy metals in the soil. CKD application will be limited due to availability of this material. Liming of agricultural soil is normally considered a source of CO2, but this study indicates liming could cause CDR in some UK soil conditions. Extrapolating from this site to a wider scale supports the conclusions of recently published research which suggests 10 years of basalt application over UK cropland could remove 1.8 ± 0.9 MtCO2 yr−1. Given the low rates of CDR observed in this study, EW is only likely to be worthwhile, at least in soil and climate conditions common to SE England, where there are co-benefits beyond carbon uptake.