MarkSoil organic carbon debt is the difference between soil organic carbon capacity and the actual and typically follows an exponential decline under most agricultural practices. So how did this come about?
In what the populist media call the caveman days—defined here as the time when humans were foragers and not growers of their food—the estimates are that less than 10 million humans were alive at any one time. This is roughly the population of modern-day Sweden or Portugal.
Ten million is the best guess based on archaeological and palaeoecological data.
It may be an uncertain estimate, but with more robust evidence from historical records, we know that the global population through the middle ages was less than 300 million.
Compare that to today.
In late 2022, census data collected by almost all countries show that the global population ticked over 8 billion humans alive.
The human population has exploded.
But here is the thing—the earth’s land area stays the same, and there is 104 million km2 of habitable land on earth.
So in the middle ages, each square kilometre supported three people. Although there were humans everywhere by this time, vast tracts of land were as they had always been. Ecological footprints were small except around a handful of larger settlements.
In 2022, approximately 11 million km2 of land was used by humans for crops and another 40 million km2 for livestock. This is roughly half the livable land area, and there are signs of human habitation throughout most of the remaining land.
Each square kilometre of habitable land now supports 77 people or 157 people if you count the agricultural land.
Most of this increase has happened in the last 100 years.
How was this population explosion even possible?
Elephants exist on African savannas at roughly one animal per square kilometre. This means a massive consumption of food and the production of about 150 kg fresh dung or 30 kg dry weight dung per day.
Imagine that number increasing by 25x to 3.7 tonnes of manure per day. Not possible. There needs to be more food to feed that many animals.
Food is energy, so the answer to how the human population explosion happened is because of external energy. We could not support this many people on the natural vegetation. We had to subsidise photosynthesis and channel it into crops and livestock.
Apologies for repeating this ‘population because of energy’ reality again, but it is essential for everything FED, especially the topic of this post, soil organic carbon debt.
We have already explained that there is more carbon in the soil than in the atmosphere, and there are good reasons why we love soil carbon, not least because fixing climate with soil carbon has a big bonus.
But first, let’s see what happened to soil carbon during the fossil-fueled human population explosion.
Soil organic carbon debt
Sustainably FED is all about the consequences of global changes and what they mean for how to feed everyone well. A dip into our sustainable food, food ecology and sustainable diet categories will reveal juicy morsels on what has happened and what could happen next.
One consequence we have not covered in detail is the loss of carbon from the top 2 m of soil across the planet—the soil organic carbon debt.
Since the beginning of the Industrial Revolution, land-use change and soil cultivation have contributed roughly 136 Gigatons of carbon (Gt C) to the atmosphere from changes in biomass carbon, with depletion of soil organic carbon (SOC) accounting for an additional 78 Gt C.
Combined 214 Gt C emissions from changes to ecology from land use compared to the historical carbon source of 270 Gt C from fossil fuel combustion.
This massive volume of carbon is the difference between the original pre-agriculture and the current stocks of SOC in exploited soils. It represents a 30–50% loss of SOC in agricultural mineral soils relative to the degree of intensity and duration of soil exploitation.
Here is what that looks like at any given location.
Loss of soil carbon along the curve above happens for three main reasons.
- Natural vegetation is removed or altered to allow for agricultural production, causing an immediate loss of carbon as trees and shrubs are removed or burnt, and plants that no longer add dead leaves, twigs and branches to the soil.
- Vegetation provides cover to the soil that shades and helps retain moisture, so the loss of cover exposes soil to wind and sun, increasing the mineralisation (loss) of soil carbon.
- When the crops or the pasture are grown, a significant proportion of that production is taken away to be eaten by humans somewhere else, meaning that less plant production decays or is cycled locally.
These three effects result in soil carbon loss to create a debt, more carbon lost than gained, hence the decline in the graph.
Soil organic carbon debt is the difference between soil organic carbon capacity and the actual and typically follows an exponential decline under most agricultural practices.
the world’s cultivated soils have lost between 50 to 70 percent of their original carbon stock, which has been released into the atmosphere in the form of CO2, mainly due to unsustainable management practices resulting in land degradation and amplifying global warming. Land degradation lowers the soil’s ability to maintain and store carbon, contributing to global threats such as climate change, and costing trillions of dollars per year
UN Food & Agriculture Organisation
Loss of carbon depletes the capacity of soil to support plant production. No matter that agriculture is a net gain in output as crops and livestock. It is a channelling of primary production into human foods with the consequence that soil carbon is lost relative to the levels before agriculture.
Repaying the soil carbon debt.
Humans have a peculiar relationship with debt.
We know that it represents an obligation that requires one party, the debtor, to pay money or other agreed-upon value to another party, the creditor. But we also see debt as a mechanism to get what we want without waiting for it and extend that idea to a virtue we call development.
Soil organic carbon debt should fall into the obligation category.
The creditor in this instance of debt is our future selves and, should we wish to place a value on nature, all future organisms on the planet.
Fortunately, it is possible to pay back some of the soil’s organic carbon debt.
True to the peculiarities of human logic, a perverse idea has emerged to repay this debt. Land, where management has depleted soil organic carbon, is now a potential carbon sink.
We will return to why this is perverse shortly, but the basic logic holds. If management actions deplete soil organic carbon (SOC), then alternative activities should be able to restore carbon levels through two mechanisms.
- Retain more of the products of photosynthesis in the soil.
- Add carbon to the soil.
Plants fix atmospheric CO2 into the soil via plant residues, root exudates, and other organic solids to form the products of photosynthesis. This organic carbon stimulates soil biology, helps retain moisture, and carbon is stored as part of the soil humus.
Recent research means that the science of this complex dynamic built around soil biodiversity is better understood than before. While SOC stocks can increase via plants, there are management options through external inputs such as manure, crop residues, and biochar.
Before we get too carried away, there is an obvious elephant in the room.
If agriculture, the process that generates food to feed everyone, depleted the SOC, how will a change to agriculture continue to feed everyone while replacing the carbon?
Whatever the land management changes are, they must be compatible with food production and competitive with other land demands. It is impossible to stop growing food or even to slow production because of the 22 trillion a day challenge. Cultivating dedicated biomass for SOC gains is unlikely, given there are already demands for growing biofuel, rewilding, and vegetation for ecosystem services.
An alternative is to use marginal land unsuitable for food production for SOC gains. There are two types of unsuitable: 1) land that was never productive in the first place or 2) land that was under production but is now abandoned.
The FAO classification of low productivity soils (e.g. natural high salinity soils or heathlands such as the Mediterranean garrigue) includes too cold (polar/boreal), alluvial soil in deserts, too dry, steep lands (dominant slope > 30%), shallow lands, poorly drained, coarse texture, vertisols, infertile (e.g. nutrient-poor), saline/sodic, acid sulphate, and peats (organic soils).
Degraded or abandoned agricultural lands occur due to biophysical constraints but also by reasons of farm structure, agrarian viability, as well as to changing population, political regimes, nature conservation and other regional contexts.
Recent estimates using global land use comparisons identified 2,714 million ha of marginal land, including bare land (74.49%), sparsely vegetated areas (25.39%), and nearly 4 million ha of abandoned agricultural land (0.14%).
In summary, the soil carbon debt could be repaid, at least in part and in instalments, but plenty of demands on soil conflict with this outcome.
Repayments on the soil organic carbon debt
Sequestering organic carbon in soil may potentially, and in a technically feasible manner, remove between 0.79 and 1.54 Gt C per year from the atmosphere. The FAO suggests a higher figure for sequestration potential at 2.45 Gt C per year, more than the yearly CO2 emissions from the aviation sector.
Accumulating soil organic carbon (SOC) will take effort for several reasons:
- SOC sequestration takes time, with the outcome of carbon gain only appearing from one to 20 years, depending on the organic matter fraction measured
- Local climate conditions heavily influence the rate of soil sequestration—if it rains, it helps.
- Soil type interacts with climate.
- Land management has a considerable influence on the amount and rate with the general requirement for keeping ground cover and retaining some of the primary production on the fields.
- The adoption of SOC-centered activities is rarely common practice.
There is uncertainty too. Heated debates are ongoing about how several issues will impact the sequestration rate and the permanence of the added soil carbon. They include
- Temperature dependence and dynamics of soil organic matter in differently managed soils.
- Practical challenges that priming may pose to efforts to store more carbon in soils.
- Impact of inorganic carbon compounds on the dynamics of soil carbon storage.
- Relative potential of various agricultural or forest management practices to preserve or even increase carbon stocks in the future.
And we could go on. The reality is that whilst the premise of returning carbon to soil is a good one and is likely in many degraded soils, the absolute amount, rates and permanence remain uncertain.
What sustainably FED suggests
Given considerable effort and will, 2 Gt C per year could be drawn down from the atmosphere into the soil to repay the soil organic carbon debt.
Easy to say, easy to justify, hard to do.
We must try hard to implement soil organic carbon repayments in all farms.
Even if there was no climate mitigation benefit, regaining SOC positively affects soil structure, water retention, and nutrient supply to plants and is crucial to sustaining ecosystem services and agricultural productivity. We will need this production resilience more than the mitigation.
Science sources
Albers, A., Avadí, A., & Hamelin, L. (2022). A generalizable framework for spatially explicit exploration of soil organic carbon sequestration on global marginal land. Scientific Reports, 12(1), 1-12.
Amelung, W., Bossio, D., de Vries, W., Kögel-Knabner, I., Lehmann, J., Amundson, R., … & Chabbi, A. (2020). Towards a global-scale soil climate mitigation strategy. Nature communications, 11(1), 1-10.
Chabbi, A., Rumpel, C., Hagedorn, F., Schrumpf, M., & Baveye, P. C. (2022). Carbon storage in agricultural and forest soils. Frontiers in Environmental Science, 153.
FAO. (2019). Recarbonization of global soils: a dynamic response to offset global emissions.
Georgiou, K., Jackson, R. B., Vindušková, O., Abramoff, R. Z., Ahlström, A., Feng, W., … & Torn, M. S. (2022). Global stocks and capacity of mineral-associated soil organic carbon. Nature Communications, 13(1), 1-12.
Zomer, R. J., Bossio, D. A., Sommer, R., & Verchot, L. V. (2017). Global sequestration potential of increased organic carbon in cropland soils. Scientific Reports, 7(1), 1-8.
Hero image from photo by Martin Dörsch on Unsplash
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