The Importance of Soil for Climate Change Mitigation: Soil Health & Carbon Sequestration
This may surprise you: more than 1/3 of all greenhouse gases (GHGs) released into the atmosphere since 1850 is related to land-use changes. That includes natural environments which have been logged or turned into crop lands and pasture lands. Today, between 10% and 14% of total global carbon emissions come from industrial agricultural production. [1]
And all this hinges on one thing: soil. I bet you never thought about the importance of soil for climate change mitigation before! Healthy soil has a surprisingly important role to play when it comes to mitigating climate change. It all has to do with soil organic carbon levels and what makes soil healthy – namely, how to increase soil carbon levels to simultaneously improve the health of degraded soils and draw carbon out of the atmosphere.
In this post, we’ll tell you all about soil health and carbon sequestration – how it works, the advantages, the challenges and why we shouldn’t think of it as a climate change panacea.
Soil Health: What Makes Soil Healthy?
Soil is surprisingly important. In fact, it’s one of the most underrated non-renewable natural resources we have.
Healthy soils are alive with both macro- and microorganisms which help soils to do their job: convey water and essential nutrients to plant roots, and protect them from pests. Optimal soil health means optimal productivity, too, especially in the long term. This isn’t just a plus for the farmer’s bottom line, but also for people’s health.
What exactly do we mean by soil health? It’s basically about the ability of soils to sustain life (plants, of course, but also insects and microorganisms); withstand climate events like droughts, floods and erosion; and provide other ecosystem services.
Soil health has three basic components: physical, chemical and biological. Healthy soil has the right physical qualities (to store water, for instance, and allow air flow), chemical composure (there are 16 elements that plants need which they get from the soil) and biological content (all those microorganisms!) to support plant growth.
Healthy soils also store carbon and other GHGs in soil organic matter (SOM). SOM is rich in carbon, and soil organic carbon (SOC) levels are directly related to the amount of SOM. In fact, the amount of organic matter in soil is often measured by its organic carbon levels, and carbon is an integral part of the soil food web. When carbon inputs from photosynthesis exceed carbon losses through microbial respiration, SOC levels increase over time.
When this soil is disturbed, though, the carbon is released. Intensive land use practices consequently not only add to the problem of global warming, they also degrade the soil. This results in unhealthy, nutrient-depleted soils which require ever-increasing quantities of fertilizers and pesticides in order to be productive.
What Can Damage Soil Health? And How to Fix It
Unfortunately, an estimated 1/3 of the planet’s soils are considered degraded. Unhealthy soils are unproductive and prone to erosion, which makes matters worse.
What’s to blame? In large part, modern industrial agricultural practices, including monocropping, over-tillage and the use of chemicals.
One issue is that chemical fertilizers and pesticides can wipe out beneficial soil organisms, which are necessary for healthy, productive soil. The more you deplete these soil organisms, the less this soil can produce. The resulting unhealthy soil – which is more like dirt than real soil – needs even more chemical inputs in order to achieve the same yields – a vicious cycle!
Another issue is that soils that are low in organic materials have the tendency to crust, becoming hard and impervious on top. When it rains, water can’t filter down into the lower layers of soil, instead running off the top – taking valuable topsoil with it. Losing just a few inches of topsoil could result in an up to 50% lower crop yield, and it can take hundreds of years to build it back.
The only way to bring soil back to health – without applying more and more fertilizers and pesticides – is to increase organic matter content. One experiment in Maryland found that productivity increased by about 80 bushels of corn per acre when organic matter in the soil increased from 0.8% to 2%.
Biologically-rich healthy soils are a good thing for farmers, helping them earn more money by increasing yields while decreasing the need for irrigation and expensive inputs like fertilizers. A diverse community of organisms in healthy soil can also protect against pests and disease, floods and soil erosion.
What’s another benefit of increasing soil health? You guessed it – mitigating climate change.
Soil Health & Carbon Sequestration
According to some researchers, the same practices that improve soil health are also cost-effective ways for the agricultural industry to mitigate their greenhouse gas emissions. This includes managing crop and grazing lands in a regenerative way, and restoring carbon and other organic matter to soils. By restoring carbon levels in the soil through soil carbon sequestration, soils actually have a lot of climate change mitigation potential, while simultaneously enhancing ecosystem services.
As mentioned above, most of the world’s agricultural soils are depleted in carbon (C) compared to their natural carbon content prior to the land being converted to agricultural use. In fact, most cropland has lost 30-50% of the carbon in topsoil, compared to its natural state. [2] This is due to erosion, soil disturbance (i.e., from tilling), nutrient depletion and the harvesting of biomass, which reduces how much organic matter returns to the soil [3] during the soil carbon cycle, as well as deforestation and the conversion of natural environments into agricultural land in the first place. [4]
This process can be reversed by doing the opposite, through things like reforestation or grassland restoration of former cropland. This can reduce the carbon deficit and sequester carbon through higher root productivity.
How Well Can Soil Carbon Sequestration Mitigate Climate Change?
The Intergovernmental Panel on Climate Change (IPCC) has indicated that carbon sequestration will be a critical part of the strategy to fight global warming. [5] When considering the importance of soil for climate change mitigation, consider this: Soil processes store more carbon than trees! In fact, nearly 80% (2500 GT) of all carbon in terrestrial ecosystems is found in soil. Only the ocean has a larger carbon pool!
What is soil carbon sequestration, exactly? It’s when carbon dioxide (CO₂) is removed from the atmosphere and stored in soil, thanks to processes like photosynthesis or the conversion of CO₂ found in the pockets of air in soil into inorganic carbonates.
Depletion of SOC levels has created a soil carbon deficit – and through regenerative land management practices, that deficit can be reversed, possibly leading to the conversion of soil into carbon sinks.
Carbon Sequestration in Soils: The Opportunities and Challenges
There are both tried-and-true carbon soil techniques, and also some innovative ones. Some researchers advocate a two-step approach, incentivizing the adoption of the conventional sequestering techniques while simultaneously continuing to research and develop new approaches that could be applied down the road, in order to maximise the carbon capture benefits.
Some of the conventional approaches include:
improving or conserving the physical structure of the soil through borders or barriers (to prevent erosion) and no-till practices (which don’t disturb the soil);
increasing carbon inputs through additional organic matter (think: compost, manure or biochar);
improve and increase the microbial community in the soil; and
using borders or cover cropping to provide continuous living plant cover, rather than leaving fields fallow when not planted.
Most of these practices fall under the general heading of regenerative agriculture, an approach to farming that is about not just maintaining a certain level of soil health but actually improving it over time.
But there are some challenges with soil carbon sequestration as a tool for climate change mitigation. The greatest concern is putting too much faith in this one approach, at the expense of others, as well as unintended or indirect consequences that could actually increase carbon or other GHG emissions, thereby practically negating any real benefits.
Let’s look at some of these challenges in more detail.
High Entry Costs
For modern industrial farmers, transforming your operations into something more regenerative – or even just adopting one or two practices – can come with an impossibly high price tag. Not only that, but the added administrative costs – like measuring and reporting carbon impacts – can be just as prohibitive.
Luckily, many levels of government offer incentives to help ease the financial burden of adopting regenerative agricultural practices (though there are some challenges with those, too; more on that below).
How Carbon Storage in Soil is Measured
Not only can the cost of measurement be prohibitive for farmers, but there is little consensus about how to properly measure any increase in soil carbon levels.
For instance, it may be more valuable to produce reports that separately measure total GHG emissions, the amount of carbon sequestered in soil, and the amount of carbon permanently sequestered and stored (including geologic storage, which is less vulnerable to re-release than storage in soil), rather than a single net emissions value for the farm as a whole.
And what about the timescale? The amount of carbon that can soils can sequester diminishes over time (see below). So over what time scale should the change in soil carbon levels be measured – 1 year, 5 years, 20? It’s very hard to say, but the impact on our understanding of climate change soil carbon sequestration benefits could be substantial.
The Problem of Diminishing Returns
One of the great things about carbon sequestration as a climate-fighting tool is its dual ability to improve soil health which can combat other global issues, like food security. However, it is a tool with diminishing returns, at least as far as climate change is concerned.
The reason soil carbon sequestration works is due to the fact that unsustainable agricultural practices over decades have dramatically depleted soils of their natural SOC content. There is room to “fill it up” again, so to speak, drawing carbon out of the atmosphere and back into soils. But at a certain point, the soil will be “full”.
When the soil has reached its pre-agricultural levels of carbon, continued “carbon farming” – or following sustainable agricultural practices – can ensure that that carbon doesn’t get re-released back into the atmosphere, but it will stop drawing more out of it.
This means that there’s a hard limit to how much soil can mitigate climate change.
How Much Carbon Can Really Be Sequestered in Soils?
Another factor is how much can really be sequestered. It is estimated that if all cropland in the United States adopted the use of cover crops, total emissions for that country would be reduced by about 100 million metric tonnes (MMT) of CO₂ equivalent per year – about a 1/6 of current total emissions. Nothing to sneeze at, but definitely not enough to be considered a complete solution.
The benefits of other practices like no-till farming can evaporate if the farmer decides to till again – something that seems to happen as much as a third of the time.
Too Much Focus on the Goals – And Not Enough on the Means
Some land management practices can increase nitrogen, which can then be emitted as a GHG or pollute water sources. But unfortunately, most land management strategies focus on the big picture – net carbon capture or emissions – and don’t take enough account of how the carbon is stored (what form), the storage capacity and persistency, or how much nitrogen might be inadvertently produced.
Another thing to think about is how we encourage farmers to adopt regenerative farming practices that will sequester carbon. In many jurisdictions, financial incentives are provided, but their effectiveness or suitability really depend on the type of practice they are meant to incentivize. For instance, does it make sense to pay farmers to adopt no-till practices, when as much as 75% of them would have done so anyway? Compare that to planting cover crops, a practice which has high up-front costs and long-term soil health and carbon sequestration benefits. In this scenario, only 20% of farmers say they would have adopted the practice without a financial incentive.
Are We Overestimating Carbon Sequestration’s Real Impact on Climate Change?
We may also be overestimating the real impact that soil carbon sequestration will have on climate change. Carbon sequestration removes carbon from the atmosphere, but it does nothing to reduce carbon going into it. That is, unless regenerative farming practices are paired with strategies to reduce new emissions, whatever the soil takes out of the atmosphere will be put right back in again.
In the fight against climate change, the value of reducing emissions by 1 T is far more impactful than sequestering 1 T of atmospheric carbon. Not to mention – if we keep emitting carbon, which continues to warm the atmosphere, that will, in turn, warm soils, which could cause the soils to release some of their stored carbon, adding to the problem, rather than helping to solve it.
What Happens When Restored Soils Get Tilled Again for Agriculture?
In addition to the fact that there is a limit to how much CO₂ the soil can hold, there is also the danger of all that stored carbon being released into the atmosphere again if that area is developed. This is a real concern as urban centres encroach on surrounding farmland.
Another potential unintended consequence is “carbon leakage,” where untouched wilderness is converted to new farmland to compensate for potential yield losses on carbon farms. This not only disturbs wildlife, destroys habitat and endangers biodiversity, it also generates new emissions, which may be enough to wipe out any of the carbon sequestration gains on neighbouring regenerative farms, even if regenerative farming practices are also in place there.
The Importance of Soil for Climate Change Mitigation: The Bottom Line
Carbon farming – the adoption of regenerative farming practices that sequester carbon – has a lot going for it. But we shouldn’t put all our eggs in one basket.
Soil carbon sequestration will work best when we recognize both its benefits and its limitations, making sure that we don’t view it as a climate change panacea, but rather as one part of a more comprehensive strategy.
What policymakers can do:
Recognize the limited permanence of soil carbon sequestration and prioritize strategies that sequester carbon permanently (like geological storage);
Focus on incentivizing practices that farmers are unlikely to adopt on their own, like those with high up-front implementation costs;
Avoid incentivizing practices that could lead to lower crop yields, as this could inadvertently increase emissions elsewhere in order to off-set those losses;
Make sure it’s possible to measure, in a comprehensive way, the amount of carbon capture and storage, rather than focusing on net GHG emissions;
Exclude soil carbon sequestration practices from the carbon market and cap-and-trade programs, given its uncertainties;
And, arguably most importantly, avoid equating carbon sequestration with GHG emissions reduction. Think of it this way: if your boat is filling up with water, you need to scoop the water out – but you also have to plug up the leak, or your task will never end.
So let’s not forget the importance of soil for climate change mitigation – but let’s not overestimate it, either.
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About the Author
Sarah has over 10 years’ experience working with indigenous communities in Peru. She travelled to Peru for the first time in 2006, where the culture, the people and the landscapes ignited her passion. Sarah has travelled extensively throughout the country as well as the rest of South America, and is especially familiar with the Cusco and Sacred Valley areas. After completing her law degree at the University of Victoria in 2017, Sarah now splits her time between Cusco, Peru and Kingston, Ontario.
Notes
[1] Paustian, K., Lehmann, J., Ogle, S., Reay, D., Robertson, G. P., and Smith, P. (2016a). Climate-smart soils. Nature 532, 49–57.
[2] Davidson, E. A., and Ackerman, I. L. (1993). Changes in soil carbon inventories following cultivation of previously untilled soils. Biogeochemistry 20, 161–164.
[3] Paustian, K., Andren, O., Janzen, H. H., Lal, R., Smith, P., Tian, G., et al. (1997). Agricultural soils as a sink to mitigate CO2 emissions. Soil Use Manag. 13, 230–244.
[4] Schwartz, Judith D. "Soil as Carbon Storehouse: New Weapon in Climate Fight?" By Judith D. Schwartz: Yale Environment 360. Yale Environment 360, 4 Mar. 2014. Web. 10 Apr. 2016.
[5] IPPC AR5 2014.
