Magical rockdust

Soil is brilliant at capturing carbon dioxide and keeping it out of the atmosphere. But what if we could make it do an even better job?

On a farm overlooking the broad River Tay in Perthshire they've sprinkled the fields with the waste product from quarrying. Nature does the rest- using the rockdust to pull carbon dioxide from the air and store it in the soil.

With the help of Rachael James from Southampton University, Tom Heap and Tamsin Edwards check out a technique that could be applied to millions of hectares of the world's farmland.

Listen now on BBC Radio 4

What our experts say

We asked Society Fellows Professor Heather Viles from the University of Oxford, Professor Larissa Naylor and Dr Adrian Bass from the University of Glasgow and Dr Phil Renforth from Herriot-Watt University to offer some observations on the potential of processes to accelerate the chemical breakdown of rocks in reducing carbon emissions. Their points take some of the themes of the programme a step further.

Larissa Naylor and Adrian Bass

This type of dust could be applied beyond traditional agricultural settings, such as in the large increase in urban farming and urban greening measures, as well as earth moving/replanting associated with linear infrastructure networks (e.g. roads, rail) and forestry settings.

Modelling studies could be usefully added to this science to explore what the longer-term impacts of using enhanced rock weathering (ERW) materials (rock dust) would be at the farm scale, for carbon dioxide (CO2) mitigation and also for the wider landscape impacts of widespread application of the use of rock dust. Studying how effective (or not) ERW is in a range of climate and soil types would also be highly beneficial to allow this technique to be targeted in locations where a carbon sink is likely to be created and avoided in places where the opposite is likely to occur. These experiments and forecasts would extend beyond recommendations (Beerling et al. 2018, Beerling and Long, 2018), and allow us to forecast the longer-term impacts.

What are the limiting factors?

Prof Larissa Naylor

Social acceptance and the need for subsidies/incentives are cited by Beerling et al. (2018) as key to increasing the use of rock dust in small holder farming communities in rural regions, like China and India. They found that farming practices adopted for increasing sustainable productivity have increased profits by US$12.2 billion over a decade, by involving local scientists in conducting research into its effectiveness and safety to build trust and engagement with stakeholders, bringing smallholders out of extreme poverty and restore highly degraded soils. Research we carried out in rural China identified that there is less research in China on environmental science knowledge exchange practices than elsewhere in the world.

Understanding the local social context is important to identify what key social learning and policy processes can help identify and overcome obstacles to implementing farming practice changes, such as ERW via rock dust. Combining human geography research and practice with ERW science and involving practitioners in the research process can help us better co-identify pathways to gather evidence of how ERW works in practice and on how to implement agricultural practices that can potentially meet climate change mitigation targets.

Dr Adrian Bass

Logistical considerations are potentially key, and this can include the suitability of the process vs. other soil carbon storage techniques. For example, organic carbon storage in stable forms (e.g., pyrolysed carbon - biochar) may be of more practical benefit as it also presents a disposal route for agricultural organic waste, reapplied to the soil in stable form. This gains further significance as application of these amendments per hectare is generally expensive, though this may be mitigated with scale. The effectiveness of the different techniques, as well as any mutual benefits or inhibitions, need to be well established.

A whole system approach is necessary beyond the field. Studies recognise the potential impact on the drainage network, primarily in the form of additional dissolved inorganic carbon, but the direct effect is still to be constrained. This includes quantification of downstream effects on carbon fluxes (surface water degassing vs. incorporation into biomass vs. burial etc) and how this effects estimates of net carbon (net-C) capture. 

Prof Heather Viles

Hydrology, slope angle and topography may complicate reaction rates and soil dynamics. Storm events may remove basalt dust before it can be effective.

The wind regime may result in removal of basalt dust if applied during dry periods before it can be effective.

What are the co-benefits?

Dr Phil Renforth

• Increased yield

• Reduction in nitrous oxide emissions

• Replacement of material lost through soil erosion

• Possible positive impact on receiving water/coastal water pH

Prof Larissa Naylor

Recent research has suggested that ERW could allow for reduced use of fertilisers and water for irrigation/improving hydrology through altered plant characteristics. These wider carbon and ecosystem benefits of ERW could be carefully evaluated and costed as part of a cost-benefit analysis. If fertiliser use is reduced and less irrigation is required, due to improved soil conditions for plants, then there are potentially wider environmental (including carbon mitigation) benefits of adopting ERW practices. A detailed, multi benefits assessment would aid identification of these wider co-benefits to strengthen the business case for the use of ERW. 

Prof Heather Viles

Basalt dust can also act as a fertiliser, improving soil health, encouraging microbial growth and improving crop yields. It may also enhance soil animal activities (ants and worms for example) which may also contribute to carbon sequestration (or perhaps release).

Potential negative impacts of this idea?

Phil Renforth

These remain speculative, however, there are potential social and environmental impacts of increased quarry operations. Moreover, there may be some dust generation either during crushing or spreading.

Larissa Naylor

The carbon costs of producing, transporting and applying the rockdust would need to be carefully evaluated in comparison to the potential benefits of adding rockdust to the soil. Beerling et al. 2020 explore how re-use of industrial waste, such as mining and/or industrial by-products, could potentially be used instead of new mining/grinding of rocks to help offset this potential negative impact. 

This is an ‘unknown’ rather than a co-benefit or a negative: research has shown that adding rock minerals to soil alters microbiological communities. In terms of carbon sequestration potential, what is not known is if/how these microbial communities interact with the rockdust over longer time periods especially. Similarly, the effects of elevating the CO2 of the soil on microbial processes (such as decomposition rates by microbes) could lead to a net carbon source rather than sink.

And the long-term impacts of manipulating soils in this way? Can we model what these impacts (or benefits might be) to help weigh up large scale application of this technique compared to (or alongside) other measures, such as nature-based solutions, that can absorb and store excess atmospheric carbon (e.g. seagrass beds)? Involving environmental economists to weigh up the relative benefits/disbenefits different options to sequester carbon through alterations to landscapes may be a useful means of helping international bodies identify which suite of techniques could lead to the greatest range of multifunctional benefits, and also those which have the most likelihood of widespread implementation so that the potential carbon (and wider) benefits can be gained sooner. A 'windows of opportunity' approach (Brown et al. 2017 and Rose et al. 2020 ) might help to identify which policy/incentive/social windows can be harnessed to accelerate implementation of techniques, such as ERW/rocks for soils.

Adrian Bass

What are the impacts on the wider aquatic system? Much of the carbon initially drawn down will not stay on site and the effects of its subsequent lateral transport away from the site and into the water network needs study. This is not a negative, but a current unknown.

Heather Viles

• Changing soil characteristics could have some unanticipated consequences, perhaps producing non-linear ecological responses.

• Basalt dust might influence the surface albedo in ways which could be unhelpful.

• Silicate dust is known to cause health hazards once entrained in the air, so care in applying the dust would be needed to reduce any deleterious health impacts

Further reading

• Beerling, D. et al. (2018), Farming with crops and rocks to address global climate, food and soil security, Nature Plants, 4, 138-147

• Beerling, D. (2018), Guest post: How ‘enhanced weathering’ could slow climate change and boost crop yields, Carbon Brief

• Beerling, D. et al. (2020), Potential for large-scale CO2 removal via enhanced rock weathering with croplands, Nature, 583, 242-248

• Carney, K. et al. (2007), Altered soil microbial community at elevated CO2 leads to loss of soil carbon, PNAS, 104 (12), 4990-4995

• Carrington, D. (2020), Spreading rock dust on fields could remove vast amounts of CO2 from air, The Guardian

• Carson, J. (2007), Altering the mineral composition of soil causes a shift in microbial community structure, FEMS Microbiology Ecology, 61 (3), 414-423

• Edwards et al. (2017), Climate change mitigation: potential benefits and pitfalls of enhanced rock weathering in tropical Agriculture, The Royal Society

• EPSRC (2014), Sustainable Urban Carbon Capture: Engineering Soils for Climate Change (SUCCESS), Newcastle University

• Green Minerals

• Hartmann, J. (2013), Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification, Revies of Geophysics, 51, 113-149

• Hensel, J. and Schacher, J. (1894), Bread from Stones, a new rational system of land fertilization and physical regeneration, Environmental History

• Minx, J. (2017), Fast growing research on negative emissions, Environmental Research Letter, 12 (30)

• Renforth, P. (2012), The potential of enhanced weathering in the UK, International Journal of Greenhouse Gas Control, 10, 229-243

• Renforth, P. (2017), Assessing ocean alkalinity for carbon sequestration, Reviews of Geophysics

• Oliver, D. et al. (2020), How does smallholder farming practice and environmental awareness vary across village communities in the karst terrain of southwest China? Agriculture, Ecosystems and Environment, 288

• Project Vesta

• Smith et al. (2019), Land-Management options for greenhouse gas removal and their impacts on ecosystem services and the sustainable development goals, Annual Review of Environment and Resources, 44, 255-286

• The Future Forest Company

About the series

39 Ways to Save the Planet is a new radio series by BBC Radio 4 developed in partnership with the Society and broadcast in 2021. It showcases 39 ideas to relieve the stress that climate change is placing on the Earth. In each 15 minute episode Tom Heap and Dr Tamsin Edwards meet the people behind a fresh and fascinating idea to cut the carbon.

Programme website

Over the course of 2021, the Society will be producing events and digital content to accompany the series.