Effects of enhanced weathering on soil gas fluxes across UK land uses

Large-scale carbon dioxide removal (CO₂) could be achieved through enhanced weathering (EW) deployment onto agricultural land and forests. Whilst the effects of EW treatment on soil and plant properties have been studied, impacts on soil gas fluxes remain poorly characterised. Soils both emit and uptake a wide range of gases, including greenhouse gases, such as nitrous oxide (N₂O), methane (CH₄) and CO₂, as well as reactive trace gases, such as nitric oxides (NO and NO₂), ammonia (NH₃), hydrogen (H₂) and volatile organic compounds (VOCs) which together influence climate and air quality either directly or indirectly through atmospheric reactions. EW can modify soil properties such as pH, organic carbon and structure, which can affect soil gas fluxes both directly, through physical and chemical processes, and indirectly via impacts on the soil microbiome. Therefore, characterising these responses is critical for determining potential co-benefits and trade-offs associated with large-scale EW deployment and informing monitoring, reporting and verification (MRV) frameworks.

In this laboratory study, fluxes of an extensive range of gases were measured from EW-treated soils collected from existing field trials across the UK on different land uses. Treated and control soils were sampled from arable land, grassland and newly planted mixed-broadleaf and monoculture-Sitka spruce forests. Soil fluxes of N₂O, NO, NO₂, NH₃, carbon monoxide (CO), ozone (O₃) and VOCs were measured online during controlled-temperature incubation experiments (5 to 25 °C with a step of 5 °C) using an advanced dynamic air-through chambers system equipped with high-resolution gas analysers and a proton-transfer-reaction mass spectrometer. CO₂ and CH₄ fluxes were measured online at room temperature using a separate gas analyser, whereas H₂ fluxes were measured offline from chamber headspace samples using gas chromatography.

Overall, we found no consistent pattern of EW effects across gases and land uses. Greenhouse gas fluxes had a land-use dependence, with arable soil showing increased N₂O uptake under EW treatment, CO₂ emissions decreasing in both forest soils, and CH₄ fluxes responding differently across sites, with increased emissions in arable soils but decreased emissions in grassland and increased uptake in broadleaf forest soils. Trace gases generally showed fewer and less systematic responses to EW, with no consistent patterns across land uses. These results suggest that land use and soil properties are important factors in determining soil gas responses to EW and highlight the need for land-use-specific monitoring strategies. Future in situ studies with in-depth soil characterisation will be essential to support robust MRV of EW and to assess potential co-benefits and risks of its large-scale implementation.