Soil core study indicates limited CO2 removal by enhanced weathering in dry croplands in the UK

Abstract

The application of crushed silicate minerals to agricultural soils has been suggested as a route to enhance weathering rates and increase CO2 drawdown. Laboratory studies have attempted to evaluate the potential of enhanced weathering as a CO2 removal technique but do not simulate the geochemical complexity of soil environments, and studies in the field are limited in the nature of data they can collect. To overcome these limitations, this study uses an experimental set-up which fully encapsulates field conditions in a controlled setting using soil cores removed from UK cropland and treated with crushed basalt. Cores were exposed to natural weather conditions throughout a 14-month time series, and soil solution was sampled in 10–20 cm intervals in the core to provide insight into the fate of dissolution products with soil depth.This study assessed the rate and chemistry of basalt dissolution 8 months after addition at a high application rate (100 t basalt ha−1) using direct measurements from a UK soil. Assuming conclusions drawn from this study are representative of field-scale enhanced weathering, findings indicate that a set application of basalt to lime-rich, unirrigated UK soils releases alkalinity at a rate of 310 ± 30 eq ha−1 yr−1 and could remove 10.2 ± 0.8 kgCO2 ha−1 yr−1. Accumulation of undissolved basalt may also lead to large and irreversible changes to soil compositions following repeated application. When considering variation in hydrology around the UK, we assess the drawdown potential of application of basalt to all UK arable land as 1.3 ± 0.1 MtCO2 yr−1 which is equivalent to 3% of current UK agricultural CO2 emissions. This is 5- to 25- fold slower than previous modelled assessments, likely due to complexities of soil systems and to water limitation on alkalinity release. Further research is needed to fully assess controls on the potential of enhanced weathering in the real-world environment, across a range of hydrological and soil environments, before the approach is substantively scaled-up for CO2 removal.