Can we grow limestone in soil as a CO2 sink?
Tom Goreau
Limestone minerals that grow in soils (caliche, calcrete, or soil carbonate) are ignored in most carbon budgets, but they are a very significant part of soil carbon, so small changes in them have global significance.
It is estimated that authigenic soil carbonates (i.e those that grew out of groundwater solutions, not fragments of limestone physically weathered from outcrops) amount to 940 PgC, and they exchange dynamically with the dissolved soil bicarbonate pool, some 1404 PgC, making soil inorganic carbon more abundant than the 1530 PgC as soil organic carbon (Monger et al, 2015, Sequestration of inorganic carbon in soil and groundwater, Geology, 43:375-378). These inorganic soil carbon pools greatly exceed the carbon in either the atmosphere or in the biosphere. They have generally been ignored in atmosphere CO2 budgets on grounds that 1) they should stay out of sight and out of mind for a long time, and 2) nobody really knows if they are changing.
Soil limestone forms when groundwater evaporates, degassing dissolved CO2, which drives up the pH and causes the remaining bicarbonate to precipitate as calcite, so they are very common in desert environments, but are rarer, and deeper in soil, as rainfall increases. There is a strong soil limestone dissolution and reprecipitation cycle driven by seasonal rainfall, so limestone formed by these cycles results in ephemeral recycling rather than new precipitation, such as that which takes place due to first generation weathering of calcium-containing silicate rocks. The general concern has been that agricultural plowing exposes buried caliche and greatly increases its dissolution, creating a new source of atmospheric CO2. This is especially an issue where dryland soils are irrigated by (fossil-fuel pumped) fossil groundwater in the US, Russia, China, India, the Near East, and Central Asia. Areas in continental interiors where desertification is increasing due to global warming-induced rainfall changes should see gradual increases in soil limestone, with decreases in areas that become wetter.
Acidic wet tropical soils are the last place one would expect limestone to form, but a new biological mechanism has been found which does just that! Certain tropical plants produce high levels of oxalic acid (thought to protect them from herbivores) and grow calcium oxalate crystals in leaves, bark, roots, and stems. Some tropical trees can accumulate up to 15% or more calcium oxalate by weight in Mexico, Haiti, West Africa, and Brazil (Rowley et al., 2017, Plant Soil, 412:465-479; Cailleau et al., 2014, Catena, 116:132-141). In the soil the calcium oxalate is broken down by oxalate metabolizing bacteria, raising soil pH and being replaced by calcium carbonate, precipitating as limestone nodules (Syed et al., 2020, Frontiers in Plant Science, https://doi.org/10.3389/fpls.2020.591297). Astonishingly, these form not only in calcareous soils (with limestone bedrock) but also in highly acidic wet tropical forest soils.
One tree that has been shown to be especially effective at growing limestone is soils is the Central American Breadnut, or “ramon”, Brosimum alicastrum, (Moraceae or Mulberry family) a plant that provides nuts that were the staple crop of the Yucatan Maya, and are now being marketed as a “superfood” due to its highly nutritious composition. Clusters of ramon trees growing in the jungles of Central America are a sure sign that Classical Maya archaeological remains lie beneath the ground (D. E. Puleston, 1968, PhD thesis, University of Pennsylvania). They raise the question whether Indigenous Central Americans created and managed a new pathway of carbon dioxide removal in ancient times, just as ancient Amazonian Indigenous people created Biochar. Limestone soils have excess calcium, but highly infertile Panamian soils that are deficient in calcium but supplemented with basalt powder have been shown to greatly increase both soil calcium and plant growth (Goreau et al., 2014). Basalt powder soil remineralization combined with oxalate producing trees should increase soil carbonate reservoirs significantly and allow more carbon to be stored in soils and biomass. Much more experimental work is needed to see how soil and biomass carbon sinks, both organic and inorganic, can be grown by rock powder fertilization and tree selection.
Thomas J. F. Goreau, PhD
President, Global Coral Reef Alliance
Chief Scientist, Blue Regeneration SL
President, Biorock Technology Inc.
Technical Advisor, Blue Guardians Programme, SIDS DOCK
37 Pleasant Street, Cambridge, MA 02139
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Geotherapy: Innovative Methods of Soil Fertility Restoration, Carbon Sequestration, and Reversing CO2 Increase
http://www.crcpress.com/product/isbn/9781466595392
Innovative Methods of Marine Ecosystem Restoration
http://www.crcpress.com/product/isbn/9781466557734
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Geotherapy: Regenerating ecosystem services to reverse climate change
Jim Baird
Irrigation causes salt to accumulate in the soils, which effectively sterilizes them. This is a common problem in Alberta.
Jim Baird
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Tom Goreau
It’s not a clear tradeoff.
Most open ocean limestone deposition is not preserved, and dissolves in acidic deep bottom waters. That which is in coral reefs or shallow water sediments will basically last one more plate tectonic cycle than continental limestones before being subducted and turned back to CO2, say 100 million years?
Also remember that there is 5 times more Calcium ions than bicarbonate in the ocean, so limestone deposition is carbon limited, but is not calcium limited, so the difference would be small.
Tom Goreau
The lifetime of bicarbonate in the ocean is around 100,000 years, that of calcium is 11 times longer, about 1,100,000 years.