Adventures in carbon neutral farming: mitigating potent greenhouse gas emissions from soils with rock dust

1. Background. Agriculture is one of the dominant managed landscapes worldwide with potential for being part of the solution to climate change as well as being part of the problem. Land-based enhanced rock weathering (ERW), the process of applying ground silicate rock to soils, is a UN-recognized strategy for atmospheric carbon dioxide removal (CDR) at large-scale applicable to managed croplands (Beerling et al., 2018). However, increases in soil pH following basalt addition may also exert a 'liming effect' and reduce soil N2O emissions, delivering added climate change mitigation potential per unit crop yield. Per molecule basis, N2O has almost 300 times the warming potential of CO2 over a 100-year time horizon.
On-going field trials with fertile, organic matter-rich US corn-belt soils support this hypothesis (DeLucia et al. 2019; Geophys Res. Abstr. 21, EGU2019-4500). However, it is also possible that reductions in N2O fluxes are associated with changes in other greenhouse gas (GHG) fluxes (CO2 and CH4) and the atmospheric pollutants HONO, NO and NH3 emissions but this remains largely unassessed. Loss of N from soil to the atmosphere and waters represents loss of plant-essential nitrogen from the soil system, potentially decreasing yields, and perpetuating a cycle in which farmers compensate for this loss with nitrogen fertilizers, resulting in further emissions. Reduced emissions of N2O from agricultural soils could increase the nitrogen-use efficiency (NUE) of crop production.

2. Objectives. The project will investigate the exciting climate change mitigation hypothesis that agricultural soils amended with basalt, at rates proposed for CDR by ERW, reduce soil N2O fluxes for UK arable crops and pasture. Importantly, they will assess the responses of other important GHG fluxes (CO2, CH4) and soil nitrogen trace gas emissions (HONO, NO and NH3) to this treatment. The project will address these key issues by undertaking bench-scale experimental work using established protocols and numerical modelling with approaches familiar to the team.

2.1 Experiments. Replicated mesocosm soil incubation studies planted with wheat will be conducted to investigate the impact of basalt amendment, crop, soil type and changes in soil moisture, incl. drought and flooding events, on soil GHG, N leaching (nitrate, dissolved organic nitrogen), yield and NUE. A subsection of the soils will be analysed for the more difficult-to-measure atmospheric pollutants NO, HONO, and NH3. Soils will be collected from a targeted set of land uses (cereals, grasslands) with varying pH values and contrasting soil types.

2.2 Modelling: In phase two of the project, experimental results will be used to parameterize and calibrate the process-based biogeochemical model DAYCENT. This will allow us to understand key controls on N2O, and other GHG and nitrogen trace gas emissions, for basalt-amended soils. The objective is to rigorously upscale our findings to predict how ERW with basalt might affect these GHG and trace gas emissions from UK agricultural lands under current and future climate change scenarios.

3. Novelty
As far as we are aware, our on-going field trials are the first to report reduced soil N2O fluxes from agricultural soils amended with crushed basalt dust, with the potential to improve the greenhouse gas balance of intensive agriculture. This could substantially lower the adverse impacts of agriculture on climate per unit yield, amplifying climate change mitigation potential of ERW. But we now need to understand urgently the generality of these findings and their relevant to UK/European agricultural lands.

4. Timeliness. Mitigating global climate change is one of the greatest scientific challenges facing humanity. The IPCC 1.5 Degree Special Report concluded that avoiding 'dangerous' climate change means deployment of CDR techniques is now essential. This project addresses new N-cycle co-benefits of a significant CDR strategy.