State of the Planet

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Can Soil Help Combat Climate Change?

Photo: NestorT

To avoid the most dangerous effects of climate change, the Paris Accord recommends limiting global warming to less than 2˚ C above pre-industrial levels. Achieving that will likely involve removing carbon dioxide from the atmosphere, according to the Intergovernmental Panel on Climate Change. But strategies like capturing and storing the carbon emissions from biofuel-burning power plants, or planting new forests to absorb carbon, can create their own problems. If used on a scale large enough to be effective, they would require too much land, water, or energy, or are too expensive.

Sequestering carbon in soil, however, is a relatively natural way of removing carbon dioxide from the atmosphere with fewer impacts on land and water, less need for energy, and lower costs. Better land management and agricultural practices could enhance the ability of soils to store carbon and help combat global warming.

The Earth’s soils contain about 2,500 gigatons of carbon—that’s more than three times the amount of carbon in the atmosphere and four times the amount stored in all living plants and animals.

“Thinking about ways to increase soil carbon storage is a really important weapon in the arsenal [against climate change],” said Ben Taylor, an ecosystem ecologist and Ph.D. candidate in Columbia University’s Department of Ecology, Evolution and Environmental Biology. “The carbon in soils is greater than all the carbon in our biomass and the atmosphere combined, so even small changes in that pool are going to have really large effects for us. If we can figure out how to manage that soil carbon pool size, it could be really effective.”

Peatlands in the tundra. Photo: Ben Gaglioti

Currently, soils remove about 25 percent of the world’s fossil fuel emissions each year. Most soil carbon is stored as permafrost and peat in Arctic areas, and in moist regions like the boreal ecosystems of Northern Eurasia and North America. Soils in hot or dry areas store less carbon.

Soils are losing carbon

How much carbon soils can absorb and how long they can store it varies by location and is effectively determined by how the land is managed. Because almost half the land that can support plant life on Earth has been converted to croplands, pastures and rangelands, soils have actually lost 50 to 70 percent of the carbon they once held. This has contributed about a quarter of all the manmade global greenhouse gas emissions that are warming the planet.

Tilling the soil. Photo: KeithWeller USDA

Agricultural practices that disturb the soil—such as tilling, planting mono-crops, removing crop residue, excessive use of fertilizers and pesticides and over-grazing—expose the carbon in the soil to oxygen, allowing it to burn off into the atmosphere. Deforestation, thawing permafrost, and the draining of peatlands also cause soils to release carbon.

How soil stores carbon

Photo: DM

Through photosynthesis, plants absorb carbon dioxide from the atmosphere. They use water and sunlight to turn the carbon into leaves, stems, seeds and roots. As the plants respire, they return some carbon dioxide to the atmosphere and exude some carbon as a sugary substance through their roots. This secretion feeds the microbes (bacteria, fungi, protozoa and nematodes) that live underground. When the plants die, soil microbes break down their carbon compounds and use them for metabolism and growth, respiring some back to the atmosphere.

Because microbial decomposition releases carbon dioxide, the soil can store more carbon when it is protected from microbial activity. One key way that happens is through the formation of soil aggregates. This occurs when tiny particles of soil clump together, sheltering carbon particles inside them. Mycorrhizal fungi, which produce sticky compounds that facilitate soil aggregation, are able to transfer 15 percent more carbon into the soil than other microbes. Soils with high clay content are also able to form chemical bonds that protect carbon from microbes. These aggregates give soil its structure, which is essential for healthy plant growth.

Some carbon, made up mainly of plant residue and the carbon exuded by plant roots, remains in soil only for a few days to a few years. Microbes can easily digest this “fast pool” of carbon, so it emits a great deal of carbon dioxide. The “slow pool,” where carbon can remain for years to decades, is composed of processed plant material, microbial residue from the fast pool and carbon molecules that are protected from microbes. A third “stable pool,” comprised of humus—decomposed organic material—and soil carbon that is well protected from microbes, is found below one meter deep and can retain carbon for centuries to millennia.

Soils can sequester more carbon

A 2017 study estimated that with better management, global croplands have the potential to store an additional 1.85 gigatons carbon each year—as much as the global transportation sector emits annually. Moreover, some scientists believe soils could continue to sequester carbon for 20 to 40 years before they become saturated.

Most crops are annuals, so after harvest, fields are often left bare. Leaving crop residue in the ground or planting cover crops that are not to be harvested, like clover and legumes, can compensate for carbon losses from tillage by putting more carbon into the soil.

Cover crops in a California orchard. Photo: USDA

Crop rotation and the use of diverse crops, especially those with deeper roots such as perennials, add more varied biomass to the soil (some of which might be more resistant to decomposition) and hence more carbon.

When tillage is minimized, soil carbon is not exposed to oxygen and soil aggregates remain intact, sheltering their carbon.

Rotational grazing helps keep carbon in the soil by moving herds to new pastures after grazing, allowing old ones to regrow. In addition, carbon in the form of manure gets spread around.

Manure and compost increase soil productivity and the formation of stable carbon that can remain in the soil for decades. The Marin Carbon Project found that a one-time application of compost increased the soil’s carbon content continually, at a rate comparable to removing 1.5 metric tons of carbon from the atmosphere each year.

“By restoring the soil with natural sources of organics that support beneficial microbes that improve plant growth, the plants will flourish and draw down the carbon from the atmosphere,” O. Roger Anderson, a biologist at the Earth Institute’s Lamont-Doherty Earth Observatory, explained via email. “Theoretically the plants will grow at a more robust rate, drawing down CO2 more rapidly than the relatively lower emissions of CO2 that the metabolism of the microbes produce in a healthy soil ecosystem.”

How will climate change affect soils?

Since temperature and precipitation affect the distribution of organic matter and the amount of carbon in soils, how will climate change alter these carbon reservoirs?

New research suggests that as global warming continues, soils will release more carbon than was previously thought. Earlier studies that heated soils 5 to 20 cm deep found that the soil would release 9 to12 percent more carbon dioxide than normal. But deeper levels of soil contain more than 50 percent of global soil carbon and after heating soils to 100 cm deep, scientists found that 4˚ C of warming could result in soils releasing as much as 37 percent more carbon dioxide than normal.

Mycorrhizal root tips. Photo: Ellen Larsson

As increasing amounts of CO2 in the atmosphere stimulate plant growth and the secretion of root exudates, studies indicate that there will be more carbon for microbes in the fast pool. This will boost microbial decomposition and respiration of CO2. The added root secretions could also “prime” the microbial community to digest soil organic matter that might otherwise not be as available. Some models suggest that, later this century, soils could change from being a carbon sink to a source of carbon due to increased microbial respiration.

Moreover, the amount of carbon that microbes emit might actually increase over time, further intensifying global warming. A 26-year-long soil warming study in a hardwood forest found that warming temperatures could spur recurring pulses of carbon dioxide emissions from soils. For the first 10 years, as microbes in the heated plots decomposed carbon and released it into the atmosphere, CO2 levels spiked. Afterwards, levels fell and remained the same as those of unheated plots for the next eight years. Then CO2 emissions ramped up again for five years followed by another decline. The scientists concluded that the community of microbes changed over the years. After the first wave of microbes that decomposed the easily digestible carbon died off, a new set of microbes evolved that were able to decompose more resistant carbon that contains more minerals or is wood-based. Most global warming studies only calculate the initial rise in carbon emissions; this research suggests that microbes will evolve, resulting in continuing pulses of CO2 into the atmosphere. However, scientists still do not know how much extra carbon might result or how fast it might be released.

No silver bullet

Scientists at the University of California at Irvine think that models may have overestimated soil’s potential to sequester carbon by 40 percent. Using radiocarbon dating and data from 157 soil samples, they found that the average age of soil carbon is much older than earlier estimates. It could take hundreds to thousands of years for soil to absorb large amounts of carbon from the atmosphere. “The soil will eventually be a large carbon sink, but it won’t be present in the next century,” said one of the researchers.

These recent studies suggest that soil carbon storage is not a silver bullet solution to climate change. The continuing debate about its efficacy reflects how complex the system is and how much research still needs to be done.

“Because the [carbon] pool size is so large, there’s a lot of potential there,” said Taylor. “But the fact that we understand so little about what’s going on down there means that we should say ‘hey there’s a lot of potential for that pool to help us, but there’s a lot of potential for it to hurt us too.’ There’s a lot of soil carbon that is vulnerable to being volatilized into the atmosphere in peatlands [and permafrost]. It may really exacerbate climate change in the future.”

Because the soil on the left hasn’t been tilled, it has better biological activity. Photo: USDA

Ultimately, the best way to combat climate change is to reduce our fossil fuel consumption and move to renewable energy sources, but scientists will continue to study how soil carbon storage might help us along the way. In the meantime, the agriculture and land management practices that increase soil carbon also provide other benefits. Fertile soils produce more food, promote biodiversity, hold moisture better, and are less susceptible to erosion, floods, nutrient loss, and desertification. More microbes in the soil enable plants to grow deeper root systems that allow them to tolerate drought better, and be more resistant to pests. Enhanced carbon in soils improves soil and water quality. These are all effects that will help society feed the growing global population and be more resilient to the impacts of climate change.

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Congratulations to our Columbia Climate School MA in Climate & Society Class of 2024! Learn about our May 10 Class Day celebration. #ColumbiaClimate2024

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Iain Climie
Iain Climie
4 years ago

Have a look at Gabe Brown’s stuff in North Dakota for a practical illustration of how well this can work.

Mark
Mark
4 years ago

This is certainly one of the best overviews I’ve read on the merits of soil regeneration.

Thanks

Simon Chaplin
Simon Chaplin
4 years ago

Nitrous oxide is 324 times worse for global warming than CO2 and the soil produces 84% of world NO2. NO2 is created by microbes breaking down organic matter in the soil such as cover crop biomass. Continuous winter sown cereal crops are the least harmful by removing the straw and the seed and re-drilling next cereal crop no open soil is left exposed and nearly all carbon and nitrous oxide problems are taken off the soils. Store the straw carbon in a stack of necessary but please don’t promote nitrous oxide pumping cover crops or we will all fry

Kyle Sterling
Kyle Sterling
Reply to  Simon Chaplin
3 years ago

If you want to be taken seriously, you should first know the difference between nitrous oxide and nitrogen oxides. Nitrifying bacteria create nitrites and nitrates that can be used by plants, but these are not gaseous, and are not the same as laughing gas (nitrous oxide).

Robert Lehmert
Robert Lehmert
Reply to  Kyle Sterling
2 years ago

Kyle, notwithstanding your unfortunate mocking tone, nitrous oxide has a great deal of literature documenting exactly what Simon Chaplin said.

There’s an algorithm for finding these articles called “Google”. You might want to type in “nitrous oxide” and “agriculture”, to see what pops up.

Last edited 2 years ago by Robert Lehmert
Shelia Gibson
Shelia Gibson
3 years ago

Very interesting. I think what we grow can change the soil. I think worms are the secret.

Norm
Norm
3 years ago

One thing that I’ve noticed is that the GMO companies are getting on the band wagon and they are genetically modifying grasses and other plants to take up more carbon and live in drier climates than the native plants. They are going to ruin the environment and make money doing it! Is nothing sacred any more? It seems the world is full of greedy people ready to exploit every opportunity that arises.

Robert Lehmert
Robert Lehmert
3 years ago

This article repeats unfounded objections to increase recalcitrant carbon:

“If used on a scale large enough to be effective, they would require too much land, water, or energy, or are too expensive.”

I’d like to see the underlying study that substantiates that. “Too much land” ignores the vast amount of waste wood, manure, sewage, and other biomass available. For example, in my home state of Vermont, there are over 500,000 tons of wood chips produced by the timber industry. If left unused, these will simply rot uselessly. We do not use large quantities of water to make biochar.

The energy used to make biochar is provided by the volatile gases from the feedstock, and little outside energy is used.

With so many unfounded assumptions, I’d really like to see an LCA on why the author feels this is “too expensive”.

Compared to what?

Robert Lehmert
Robert Lehmert
2 years ago

The article states: “ When the plants die, soil microbes break down their carbon compounds and use them for metabolism and growth, respiring some back to the atmosphere.”

I don’t think describing the respiration/recycling of carbon in terms of “some” gives justice to the fact that almost all — practically all — carbon is consumed by microorganisms and returned to the atmosphere as CO2.

Otherwise, over millennia, our soils would be very deep and carbon rich — which is certainly not the case.

This is one of many advantages to the use of biocarbon (biochar) with its recalcitrant form of carbon and a soil residence measured in centuries or millennia. (See IPCC 2019).

Biochar performs many functions including providing habitat for microorganisms, and plants love to wrap their roots around it. It helps distribute moisture, releasing it when soils are dry and storing it when soils are dry. Its redox qualities help sorb and slowly release nitrogen and phosphorus, and it improves soil tilth and consistency in sandy or clay soils.

So far as replenishing and storing carbon in our soil to increase yields in the face of growing population and climate stress, biochar is remarkable tool, and deserves prominent mention in articles like this one.

Priti Punwani
Priti Punwani
Reply to  Robert Lehmert
1 year ago

Your knowledge is very vast! Amazing points put forth.

M.J. Leiva
M.J. Leiva
1 year ago

Very clear and weell explained. Congratulations!!

Sisay
Sisay
1 year ago

Wow I have seen awesome writing on the way how soil combats climate change! So, is it possible to have the pdf format of the document for further use please!

Last edited 1 year ago by Sisay
Rob Moir
10 months ago

Microbes are the tertiary player in climate change, preceded by Atmospheric carbon dioxide and methane. Mycorrhizal fungi and microbes assisting with soil aggregation are completely good microbes.

Some scientists may believe whatever they like. Soils do not have a carbon saturation point. Plants build soil. The prairies had the most extraordinary soil depth because grasslands sequestered carbon into the ground, creating rich black loom through microbial actions over thousands of years. Little carbon is gassed out of the soil as long as the turf stays intact. Hence regenerative agriculture.

Not to bring up those miscreant microbes again, increased CO2 does not stimulate more plant growth because water and sunlight are the limiting factors, not carbon. Damage to a plant, mowing, or walking on a lawn stimulates plants to grow to repair. A natural lawn cut every two weeks will draw down more carbon annually than the same size lawn mowed once a year.

Hold on. This is a hardwood forest, presumably older than 26 years. Every year plants photosynthesis and store more carbon as carbohydrates in soil. I’ve never heard of microbes preferring younger or older liquid carbon in the ground. But after a while, there was a good mix of all microbes. Microbes that include fungi, nematodes, tardigrades, flagellates, and others do not evolve quickly. Bacteria managed by viruses swap genes and evolve rapidly to meet changing conditions. Wood will always be hard to break down for microbes. Grasslands require hoofed animals to break down the fibers by walking on their toes. Springtails go at it with pinchers and digestion to prepare fibers from microbes- it takes an ecosystem, many communities, to store carbon in the ground.

Surely, you know about humus, the rich black organic matter of the soil. Healthy soils go through a chemical process to become humus. Humus will hold carbon for thousands of years. Humus is known as long-term carbon storage, while wood which takes hundreds of years to break down, is known as short-term. (at least until microbes evolve to speed up their work). We need both storages.

When a ship hits the rocks and is sinking fast, deciding after some deliberation on the best hole to repair and putting all your energies there while others study how the water is rushing in elsewhere without acting is no way to save a sinking ship.

Earth is one giant ecosystem with more interconnections and feedback loops than we can imagine. To save it, we must think systematically and try all approaches simultaneously,

As you point out in the beginning, Soil is the elephant in the climate change room, with 20% of the world’s lands covered by soil and 40% covered by deserts and degraded lands that once had soil. People should not only burn less fossil fuel but also put in more plants, replace impervious surfaces with greens, and slow excess stormwater down with more soil. With more moist soil, plants will draw down more CO2 and direct about ⅔ to biomass and ⅓ to liquid carbon (carbohydrates in the soil.)