What do we know about Carbon in soil?
Published on: May 22, 2020
Fact and fiction on carbon in soils
Many of us will have heard about the potential of soils to store carbon. But can soils really sequester a significant amount of carbon, contributing to climate change mitigation? And, for how long can carbon be locked in the soil? And … what does agroforestry have to do with it? This article explores the potential of soils to store carbon, contribute to tackling climate change, and what the role of Regenerative Agroforestry could be.
How great would it be: halting climate change whilst increasing agricultural production. Some think this is not a dream, but that this could genuinely be a feasible reality. How? By storing carbon in soils.
The international initiative “4 per 1000“, launched at the COP21 aims to demonstrate that agricultural soils can play a crucial role at the crossroads of climate change and food security. The initiative is based around the idea that an annual growth rate of 0.4% in the soil organic carbon (SOC) stocks (or 4‰ per year, hence the name of the initiative) in the first 30-40 cm of soil would significantly reduce the CO2 concentration in the atmosphere.
As such, the initiative wants to show that even a small increase in the soil carbon stock can contribute to achieving the long-term objective of limiting the temperature increase to the 2°C threshold.
However, the initiative has also received criticism. This includes arguments such as: carbon sequestration in soils risks being a temporary but not definite solution; some soils have insufficient nitrogen and phosphorus available, which limits the potential for carbon sequestration; there is a need for more comprehensive greenhouse gas (GHG) accounting, also considering emissions of methane (CH4) and nitrous oxide (N2O) from soils. In addition, case studies (1,2) have pointed out that the goal of increasing SOC 4‰ per year is not always realistically attainable.
All in all, the initiative has received criticism for having a simplistic view on soils, and for maintaining an unrealistic goal. Indeed, soils are complex systems, and the potential of soils to sequester carbon is limited by biophysical, socio-economic and political barriers, demanding region-specific approaches to carbon storage in soils. And, despite the fact that the goal of 4‰ per year is perhaps not attainable everywhere, the simplicity it communicates did encourage widespread participation and adoption by many.
It is clear that carbon sequestration in soils can be quite complex. What is our take on carbon sequestration in soils?
Where do emissions go?
Burning fossil fuels and other human activities such as farming causes emissions of large amounts of GHGs such as carbon dioxide (CO2) as well as methane (CH4) and nitrous oxide (N2O). These gases should absolutely be considered in addition to CO2 since, over a period of 100 years, one unit of CH4 and N2O are 21 and 310 times more potent respectively in regards to global warming.
The GHGs released by human activities firstly spread throughout our atmosphere, but not all of them will remain there. They spread across multiple places on Earth, like for example, CO2.
An important sink of CO2 is the ocean. Between the beginning of the industrial revolution and the mid-1990, the ocean has absorbed roughly 30% of the global human-induced CO2 emissions. Despite having a buffering effect, this absorption has also led to ocean acidification, detrimental for sea life such as corals. Another import sink of CO2 are plants. Between 2007 and 2016, plants absorbed 30% of the global human-induced CO2 emissions through photosynthesis.
When those plants die and decompose, the living organisms of the soil, such as bacteria, fungi or earthworms, transform plant residues into carbon-rich organic matter. As such, the world’s soils contain more carbon than its vegetation and the atmosphere combined. In places where decomposition is slow, such as wetlands, peatlands, and permafrost soils, large amounts of stable carbon build up. Together, these soils comprise extremely large stocks of carbon globally.
The FAO’s Global Soil Organic Carbon Map (Version 1.0) shows where the world’s soil carbon is stored, and for which climates, soil types, and land cover.
The capture and long-term storage of CO2 from the atmosphere is what’s called carbon sequestration. A forest, for instance, has the ability to sequester carbon by moving it from short-lived labile pools – such as leaves and hummus – to long-lived stable pools with slow turnover times – such as standing biomass or recalcitrant (hard to further decompose) organic matter in soils.
Even though an ecosystem such as a forest can be a sink for carbon in one year and a source in another, it only really sequesters carbon if it remains a sink over long timescales. Depending on a range of factors, including management of the land, carbon can stay in soils for decades, centuries, or even millenia – if managed wisely, soils have the potential to sequester large amounts of carbon, contributing to climate change mitigation and adaptation.
Carbon farming and the climate: How much can soils contribute?
Bringing carbon back to soils is clearly among the strategies to successfully take it out of the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) estimates (with high confidence) that soil carbon sequestration in croplands and grasslands lands can store between 0.4 to 8.6 gigatonnes of CO2 equivalents per year by 2050. In 2016, global emissions from agriculture were 5.3 gigatonnes of CO2 equivalents.
With these numbers, increasing soil organic matter content is mentioned among the options that have a very high potential to mitigate climate change. Do note that the estimated potentials as mentioned in the IPCC report are the technical potential: this means that social, economic, or other barriers to implementation are not taken into account.
Increasing soil organic carbon in agricultural lands is not the only way in which soils can help to mitigate climate change – it is also the management of agricultural resources related to soils that can contribute.
For instance, in cropping systems, options with large potential for climate change mitigation not only include soil carbon sequestration, but also reductions in N2O emissions from fertilisers, reductions in CH4 emissions from paddy rice, and bridging of yield gaps (the gap between the actual yield and the theoretically attainable yield). And in livestock systems, options include better grazing land management, with increased net primary production and soil carbon stocks, and improved manure management.
All in all, what is needed is an increase of soil carbon as well as a reduction of GHG emissions (CO2, CH4, N2O) from soils. Hence, not only increasing the soil organic matter content but also better management of crop residues are mentioned among the options that have a very high potential to mitigate climate change.
Taking the potential of improving crop and livestock activities and implementing agroforestry together, the IPCC estimates (with medium confidence) that the technical mitigation potential ranges between 2.3 to 9.6 gigatonnes of CO2 equivalents per year by 2050.
To put that into perspective: between 2007 and 2016, the average global emissions directly caused by agriculture (crop and livestock production) were 6.2 gigatonnes of CO2 equivalents per year. Adding to this the emissions indirectly caused by agriculture (deforestation and peatland degradation) this number becomes 11.1 gigatonnes of CO2 equivalents per year.
Clearly, if we stop deforestation and peatland degradation and change the way we farm (improving crop- and livestock management and implementing agroforestry), we will start making large progress in moving towards climate-neutral farming – and perhaps even climate-positive farming (!) – on a global scale.
Sink or source: How you farm is essential
It is clear that how you farm is essential to the potential of the soil to either act as a sink or as a source of GHGs (as also discussed in our article on agroforestry as a promising climate change solution).
The IPCC estimates (with medium confidence) that cropland soils have lost 20–60% of their organic carbon content prior to cultivation, and that soils under conventional agriculture continue to be a large source of GHGs.
The good news is: we can turn this around. With different agricultural management, we can make sure that soils are carbon sinks instead of carbon sources.
Alternative agricultural management options that the IPCC lists to reduce GHG emissions from soils (or perhaps, to even transform soils from GHG-sources into GHG-sinks) include zero tillage, perennial crops, erosion control, agroforestry, controlled grazing, and rangeland management.
All in all, the IPCC estimates (with medium confidence) that better management of soils can offset between 5 and 20% of current global human-induced GHG emissions: just managing soils better by farming better could help us fight climate change.
Crucial considerations: Preventing non-permanency, accounting for saturation
One important thing to consider when estimating the potential of soils for mitigating climate change is the issue of non-permanency. Non-permanency refers to the release of previously sequestered carbon, which negates some or all of the benefits from sequestration that has occurred in previous years.
Various types of carbon sinks, including soils and trees, have an inherent risk of future reversals. Natural events such as drought or fire may in some cases cause profound changes in the ability of an agricultural system’s ability to store carbon.
Another important thing to consider is the issue of saturation. Saturation means that carbon sequestration in soils or vegetation cannot continue indefinitely. The carbon stored in soils and vegetation reaches a new equilibrium as the trees mature or as the soil carbon stock saturates, and the annual carbon removal (sometimes referred to as the sink strength) decreases until it becomes zero.
However, an alternative view is that saturation does not occur, with studies from old-growth forests (like this one for example) showing that these ecosystems can continue to sequester carbon in soils even if the increment in the net living biomass is near zero (in other words, even if the trees are fully matured).
What can we learn from these considerations? First, we learned that we have to prevent reversibility of carbon storage by maintaining the better farming system that can be created. Second, we learned that there might be a ceiling to how much carbon better farming systems might store, yet that some systems – old-growth forests – could continue to store carbon even if the living biomass has ceased to net increase.
Beyond the climate: Soil and food security
Remembering the goal of the 4 per 1000 initiative, increasing soil carbon stocks does not only contribute to climate change mitigation – it also greatly contributes to strengthening food security.
Soil organic matter (SOM), of which soil organic carbon (SOC) is the major constituent, has several direct benefits for plant productivity. It improves the soil’s capacity to hold moisture, increases plant nutrients and improves plants’ ability to take up these nutrients, and improves the soil’s structure and aeration. Furthermore, it increases the soil’s resistance to erosion, and stimulates an active, healthy, and biodiverse soil life, maintaining overall soil functioning.
Therefore, increasing carbon in the soil leads to improved productivity, a decreased need for external inputs, and increased agricultural stability and resilience.
Farmers can take many actions to maintain, improve, and rebuild their soils, especially soils that have been under cultivation for a long time. The key is to maximize the retention and recycling of organic matter and plant nutrients and to minimize the losses of these soil components caused by leaching, runoff, and erosion.
Practices that increase SOM include compost, cover crops/green manure crops, crop rotation, perennial forage crops, zero or reduced tillage, and agroforestry – where of course, different approaches are required for different soils and climate conditions.
Carbon farming: Healthy soils for a better world
Carbon farming describes how better the management of agricultural soils can help farmers to mitigate and adapt to climate change. To scale up carbon farming, firstly the benefits of healthy soils should be understood well by farmers, consumers, industry, and government. Secondly, these actors need to effectively collaborate, where all actors throughout food chains are rewarded for better soil management.
For instance, The North Sea Region (NSR) Carbon Farming project has identified four business model types in which carbon farming could become a rewarding endeavor for farmers and other actors. For instance, companies within the agri-food chain could work together with farmers and advertise their use of carbon farming on product packaging, whereas companies outside the agri-food chain could buy carbon farming certificates to compensate for their own emissions.
Healthy soils are crucial for our climate and our food security. Carbon farming can help to mitigate and adapt to climate change and boost and stabilize food production. Practices like regenerative agriculture play a key role.
Do you want to share the importance of soils with others? Share this article or share the following video: The Soil Story – it effectively communicates the importance of soils for our climate and the potential of regenerative agriculture to contribute.
