Adding crushed rocks to soils absorbs atmospheric carbon and curb global warming

Summary

Enhanced Rock Weathering involves the application of pulverized rocks to soils. It is one of the proposed ways of reducing effects of global warming.

The Paris Agreement on climate change adopted the limiting of the increase in global warming to well below 2oC. Quick and efficient removal of carbon dioxide (CO2) is thus an urgent requirement to achieve the set targets.

The natural chemical weathering of rocks (a process involving the reaction of atmospheric CO2 with water to form a weak carbonic acid (H2CO3), and the attack of silicate and carbonate minerals by the carbonic acid) normally absorbs only 0.3% of the atmospheric CO2. Enhanced rock weathering (ERW) - the modification by acceleration of the removal of atmospheric CO2 by the application of pulverized magnesium-rich silicate rocks (mostly basalts) and calcium-rich rocks (carbonates) - is one of the cost-effective methods of extracting CO2 from the atmosphere.  A recent study has shown that ERW has the potential for net CO2 removal of at least 6 to 30 metric tonnes (Mt) per annum by the year 2050 for the United Kingdom1. Added advantages of ERW include enhanced agricultural productivity, regulation of riverine and oceanic ecosystems, and amelioration of ocean acidification2. 

Pulvarised basalt applied to a field in Illinois
Pulverized basalt applied to an agricultural field in Illinois (Source: e360)

Enhanced agricultural productivity

Silicate and carbonate rocks host chemical elements (nutrients) essential for the plant growth for example calcium (Ca), phosphorus (P), potassium (K), and magnesium (Mg). Thus, the release of these elements by ERW enhances soil fertility3.

ERW also releases silicon (Si) which when it reacts with water forms silicic acid. Tropical crops (e.g., sugarcane, maize, rice and sorghum) require silicon for stronger cell walls which in turn protects them against pests and diseases4. Si is also important for the reduction of leaf transpiration which helps plants to conserve water3. 

Regulation of riverine and oceanic ecosystems

Agricultural  fertilizers contain nitrogen (N) and phosphorus (P). Thus, the erosion of agricultural soils results in the deposition of N and P in rivers and oceans which leads to the high nutrients supply for algal communities (some of the which are toxic) in such environments, and the subsequent algae population increase (algal blooms)5. ERW counters that by increasing the silicon (Si) content which is favourable for diatoms which consume the nutrients which would have been otherwise consumed by algae.

Reduced ocean acidification 

The reaction of atmospheric CO2 with the oceanic water at a high rate results in the formation of carbonic acid which in turn causes oceanic acidification. The hydrogen carbonate (HCO3-) ions produced by the reaction of the carbonic acid and silicate minerals reduces the ocean acidity as these ions react with hydrogen ions (H+) (which increase the acidity) and lowers their concentration.

Potential shortcomings of ERW

Potential pitfalls for ERW (some have not been quantified) include CO2 emission from the machinery used for grinding and pulverisation of silicate and carbonate rocks, the introduction of toxic metals (e.g., nickel and chromium) from silicate minerals which can result in plant and animal poisoning. Additionally, the washing away of unweathered silicate material into rivers during heavy rainfall seasons could result in the increase of water turbidity in rivers which in turn in causes the reduction in the reproduction of fish in riverine systems.

Despite its shortcomings, ERW has a lot of potential in absorbing atmospheric CO2 and amelioration of global warming.


References and further reading

1. Kantzas, E.P., Val Martin, M., Lomas, M.R. et al. Substantial carbon drawdown potential from enhanced rock weathering in the United Kingdom. Nat. Geosci. 15, 382–389 (2022). https://doi.org/10.1038/s41561-022-00925-2

2. Edwards, D.P., Lim, F., James, R.H. et al. Climate change mitigation: potential benefits and pitfalls of enhanced rock weathering in tropical agriculture. Biol. Lett. 13: 20160715 (2017). https://doi.org/10.1098/rsbl.2016.0715

3. Taylor L.L., Quirk J., Thorley R.M.S. et al. Enhanced weathering strategies for stabilizing climate and averting ocean acidification. Nat. Clim. Change 6, 402–406 (2016). https://doi:10.1038/nclimate2882

4. Liang Y., Nikolic M., Be´langer R. et al. Silicon in agriculture: from theory to practice. Amsterdam, The Netherlands: Springer (2015).

5. Michael Beman, J., Arrigo, K. & Matson, P. Agricultural runoff fuels large phytoplankton blooms in vulnerable areas of the ocean. Nature 434, 211–214 (2005). https://doi.org/10.1038/nature03370

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