There is the potential to store much more carbon in soils, drawing it down from the atmosphere. A recent estimate based on a model of soil organic carbon sequestration suggested 3.5 Gigatonnes of carbon dioxide a year(1) could be sequestered in the soil. (A Gigatonne [Gt] is 1 billion tonnes.) Total global anthropogenic emissions are 37 Gigatonnes of carbon dioxide equivalent a year.

Resilient food production. Resilience refers to the ability of a system to ‘bounce back’ after a stress on the system, to resume its former function. In the present context, we might consider the likely stressors to be war and climate change. War may bring soil compaction, burning and toxic pollutants to soil. Climate change will bring storms, flooding, drought, wildfire and may bring economic and social collapse. A resilient food system would be able to continue production through such shocks, resume production soon after a time-limited shock, and adapt to changing factors affecting food production, such as climate and sea level rise. Its goal would be food security for all.

Food security as defined by the United Nations’ Committee on World Food Security, is the condition in which all people, at all times, have physical, social and economic access to sufficient safe and nutritious food that meets their dietary needs and food preferences for an active and healthy life. Food production will need to increase dramatically to provide for projected population levels mid-century. Fertile soils with ideal levels of soil organic carbon are needed to accomplish that.

It is worth noting that currently, the leading cause of global hunger, according to the United Nations Food and Agriculture Organization (FAO) has become climate change, with its attendant extreme weather events, land degradation, desertification, water scarcity and rising sea levels.(2) This cause leads violent conflict and economic depression as longstanding causes of malnutrition.

Soil carbon sequestration is any process that brings carbon from the atmosphere (in the form of greenhouse gas carbon dioxide) to increase the amount of carbon held in the soil. This includes all stages of decomposition of organic matter on the way to becoming stable humus, insoluble carbonates as a result of rock weathering, or charcoal.

The problems

1. Soil health status

The UN Food and Agriculture Organization (FAO) in a 2015 document, The Status of the World’s Soil Resources, says ‘the majority of the world’s soil resources are in only fair, poor or very poor condition. Today, 33 percent of land is moderately to highly degraded due to the erosion, salinization, compaction, acidification and chemical pollution of soils. Further loss of productive soils would severely damage food production and food security, amplify food-price volatility, and potentially plunge millions of people into hunger and poverty.’

FAO has produced a remarkable map showing the levels of soil organic carbon in all terrestrial areas.(3) It has been estimated that soil has lost over 50% of its former carbon content, both by natural and human processes.(4)

We need to consider two processes — physical soil loss and qualitative soil degradation.

Soil loss

Without the presence of humans, there is a continual process of soil formation and soil loss. Soil is formed by the physical weathering of the rocks that form the Earth’s geosphere, as they crumble on the surface into tiny pieces under the influence of wind and water. This occurs at the rate of about a centimetre every few hundred years. The wind and water also cause the loss of soil as it then gets swept away by water and transferred to the sea or blown away by winds to the sea or a different land location. Vegetation coverage of land halts or slows this process.

Humans have immensely accelerated the process of soil loss, especially by changing the use of the land to agricultural and pastoral uses. Use of the land that involves stripping it bare of vegetation and ploughing it to expose its below-surface structure to oxygen, sun, wind and water greatly speeds soil loss, particularly on sloping land.

A thousand years ago, humans used relatively little of the Earth’s land. Pasture and cropland occupied 1-2% of the Earth’s ice-free surface. Now in most countries almost all the land with suitable soils and climate is changed for human use. This has entailed large physical losses of soil. Other losses are incurred by urbanization. Human settlements were built where people could derive a living from agriculture on fertile land. As settlements grew, more and more of the fertile land was taken for buildings, roads and other infrastructure. Now that more than half of humanity lives in urban settings, the amount of land covered and removed from natural processes has become significant. Furthermore, modern cities cover the land with impermeable substances like cement and asphalt, called ‘soil-sealing’. Life in the soil beneath these surfaces has been extinguished permanently.

Climate change is causing more soil loss. More violent winds and rainstorms increase losses. Sea level rise from climate change is inexorably erasing coastlines, and salinizing fertile coastal plains. These physical losses are occurring at the same time the human population is rising and requiring more food production.

Loss of soil life

These processes also contribute to the loss of the vibrant web of soil life. The most important of these is changing the use of land from natural forest or grassland to cropland.(5)

This results in losses of 30-50% of soil organic carbon. Ploughing exposes soil carbon to oxidation and moisture loss and to further loss of soil life. It also disrupts the enormous and complex web of fungal roots (mycelia or mycorrhizae) that are part of mutual exchanges in supporting plant growth and the total soil food web. The use of inorganic fertilizers to add nitrogen, phosphorus and potassium to soil will indeed nourish the crop but tends to kill the soil life, as will herbicides and pesticides. Compaction of soil by heavy machinery and by poorly managed grazing will cause degradation of soil structure and soil life.

This loss of soil fertility is sometimes put into dramatic form by statements such as ‘We have only 60 harvests left before fertility gives out’. While it is hard to be so precise, the message is valid — soil, the basis for our lives and all other land species, is disappearing and degrading. Soil life is the basis of soil resilience. The complex community of living organisms in soil may be expected to slowly change as the climate changes. Such changes depend on an initial biodiversity. With speedier shocks to soil health, such as in flooding or drought, recovery of soil health also depends on the ability of a residual, diverse community to repopulate the soil.

2. Soil health and climate change

Loss of soil carbon contributes to climate change

Emissions due to land use change include those by deforestation, biomass burning, conversion of natural to agricultural ecosystems, drainage of wetlands and soil cultivation. The process of transfer of soil carbon to atmospheric carbon began 10,000 years ago as humans learned to clear and till the land for crops. This conversion of forest to cropland transfers 30-50% of soil carbon to the atmosphere. For most of human history, the gross quantities transferred were insignificant. For the last 150 years, the era of exponentially rising anthropogenic carbon emissions, those due to land use and changes of land cover made up 30% of the total emissions.(6)

It is also possible, but hard to measure, that some of the losses of soil carbon from water and wind erosion are deposited in bodies of water and thus sequestered from the atmosphere. However, with population growth, mechanization and then industrialization of agriculture the gross quantities have become a highly significant part of the climate crisis.

Soil emissions of nitrous oxide

Nitrous oxide is a long-lasting greenhouse gas around 300 times more potent in its greenhouse effect than carbon dioxide. It is emitted from agricultural soils to which inorganic nitrogen has been applied, and from pastoral soils from the urination spots of ruminants.

Soil methane

When biological matter decomposes in the presence of oxygen, carbon dioxide is formed. Without the presence of oxygen, methane is formed. Methane is a short-lived greenhouse gas, 34 times more potent than carbon dioxide in its first 100 years in the atmosphere(7), while it slowly converts to carbon dioxide.

Rice cultivation is responsible for between 9 and 19% of global methane emissions.(8) The flooded paddies lead to anaerobic decomposition of organic matter, producing methane.

Climate change affects soil health

As discussed above, extreme weather with flooding and high winds is likely to increase the loss of topsoil. Climate change induced drought will result in loss of soil moisture and therefore fertility.

We are accustomed to hearing about positive (vicious) feedback cycles in relation to climate change. There is one negative (benign) feedback cycle in relation to the impact of climate change on soil health. Higher levels of carbon dioxide in the air increase plant growth, absorbing more carbon. This in turn is assumed to lead to more root exudates, supporting more soil microlife, increasing fertility and sequestering more soil carbon. This relationship is speculative.

3. Food security and climate change

Food security depends on the availability of food, the ability of people to acquire it and use it, and the stability of this process over time. Here we will deal only with the availability of food from the land and the impact of climate change.

The relevant factors are increased average temperature, changes in rainfall and soil moisture, changes in weather variability, and extreme weather events.

In tropical areas, available human labour to tend the land will be affected. Labour output decreases with increasing temperature (9). This factor and diminishing productivity of some crops (10) affects not only food availability, but also access to food by the millions of people dependent on agricultural livelihoods.

Wild foods, especially important to those on the edge of food insecurity, are particularly vulnerable to changes in temperature and rainfall caused by climate change, and are predicted to markedly drop in availability.(11)

With the current level of commitments under the Paris Agreement, global temperatures may stabilize at an average of 3 degrees above preindustrial levels. At this level, productivity of all crops will be reduced. At a lower temperature increase, crops in temperate regions may be more productive, and those in tropical regions less so. Those most vulnerable to the impacts of climate change on food security are the populations already experiencing inadequate nutrition – usually in areas where both crop productivity and livelihoods are threatened. These are also the areas of highest population growth.

Solutions

We must reverse the human impact on soil loss and degradation (where it is reversible) and increase the capacity of soil to sequester carbon, while at the same time producing food to eventually support 10 billion people. Is this possible? It is quite unclear that this can be done. However we do understand the processes that will take us in this direction.

1. Stabilize population growth as soon as possible

The conversion of forest to cropland for food, fodder and fuel to accommodate the needs of an expanding human population is a major driver of climate change, Women in many countries lack access to reproductive health services. These are the very populations most vulnerable to climate impacts on food availability. Perhaps there could be a stream in the voluntary carbon offsetting system that flows to support reproductive health clinics as an emissions reducing (and soil health promoting) measure.

2. Make a phased transition to a circular economy oriented to equitable human well-being rather than to economic growth, and operating within Earth’s biophysical limits.

Land use change is partly driven by the impulsion toward economic growth, largely benefitting the already wealthy.

3. Cut deforestation, support afforestation

The carbon sequestration protection or gain is in both the wood biomass and in the soil carbon of the forest. This is particularly pertinent to tropical rainforest. Currently the Brazilian Amazon appears to be at great risk.

4. Reduce soil loss

• There is little likelihood of being able to prevent the loss of large areas of fertile land to sea level rise. It has been suggested that good topsoil might be removed from areas where loss from sea level rise is imminent and certain.
• Avoid soil disturbance on steep slopes. Where crops are planted on slopes, skillful terracing can minimize soil loss.
• The methods of conservation farming (see below) serve to prevent soil loss as well as to sequester carbon.(12)
• Agroforestry (see below) also prevents soil loss and sequesters carbon.

5. Increase soil carbon

The FAO speaks of significant ‘opportunities for soil carbon sequestration across all climatic zones and a wide range of cropping, grazing and forestry land use systems’.(13) They point out that increasing the carbon content of soils could reverse soil degradation and desertification, enhance productivity, increase water retention, increase the resilience of communities to climate change, especially drought, enhance biodiversity, reduce fossil fuel use in agriculture and reduce agricultural emissions of nitrous oxide, carbon dioxide and methane.

Accomplishing this entails major changes to agriculture systems, some of which are :

Organic farming

There is a large overlap between methods of soil carbon sequestration and of organic farming, which, in contrast to conventional farming does not use inorganic fertilizers, pesticides and herbicides. Conventional farming supplies crops with the essential nutrients (nitrogen, potassium and phosphorus), but not carbon, which plants derive from carbon dioxide in the air. Organic farming methods involve the use of compost and manure to supply these essential nutrients. Because these plant-based materials contain a lot of organic carbon, some of this will be turned into humus and sequestered.

Regenerative farming and Biological Farming

These more recent movements in agriculture demand whole systems thinking of the farmer, including soil health and biodiversity below and above ground. Both movements include the methods below.

Conservation tillage or ‘no-till’ methods

These methods leave undisturbed the structure of the soil with a slightly compacted protective upper layer. This protects the soil against the impact of wind, raindrops, light and oxygen. The undisturbed soil has better earthworm, fungal and microbial activity. It greatly reduces water runoff and therefore erosion by wind and water, and increases water retention. The latter may be the cause of increased resilience of no-till farming in situations of drought and high temperatures.(14)

In no-till methods, the carbon-rich humus is less exposed to the air. This reduces its oxidation and therefore its carbon dioxide emissions.

Mulching is the practice of covering bare soil to protect it against water run-off, wind, sunlight and heat, and to prevent weed growth. A great variety of materials may be used for this task, ranging from crop residues straw and bark to black plastic sheets. Clearly the purposes on which we are currently focussed strongly favour mulching materials that are high in carbon. These materials break down eventually, and some of their carbonaceous material goes to form humus, a relatively stable carbon store.

Cover crops. These are crops grown in between main crops for the purpose of maintaining the soil cover and producing crop residue to continue the mulch process.

Nutrient management. Carbon-rich materials supplying essential nutrients to plants will favour carbon sequestration. Compost and manure are a vital part of this cycle. To prevent nitrogen leakage into waterways, it is important to give the plants what they need in fertilization, and no more. The excess is likely to turn into inorganic compounds and to leach into waterways and emit nitrous oxide into the overburdened atmosphere.

Crop rotation and crop diversity encourages diversity of soil life.

Agroforestry is a collective name for land-use systems and technologies where woody perennials (trees, shrubs, palms, bamboos, etc.) are deliberately used on the same land-management units as agricultural crops and/or animals, in some form of spatial arrangement or temporal sequence. Soil carbon is enriched by the root mass of the trees or other woody perennials. (In addition of course, carbon is sequestered in the wood of the above-ground parts of trees.) Benefits of agroforestry according to FAO are:(15)

• Increased crop and livestock production and economic gain
• Increased soil moisture
• Soil conservation from wind erosion and improved soil quality
• Sequestration of atmospheric carbon
• Increased biodiversity by creating new habitat
• More humane conditions for animals (and farm labourers)
• Increased snow protection for crops
• Decreased heating costs when planted fairly close to a house.

Experimental methods of sequestering carbon in cropland are:

Enhanced weathering.(16) The slow natural process of chemical weathering of rocks incorporating silicates, eg basalt, involves extraction of carbon dioxide from the air. The resulting compounds alkalinize the soil and eventually run to the sea, countering ocean acidity. There is experimental work on mechanical pulverization of basalt, which is then spread on cropland fields. The hope is that the resulting reactions, enhanced by a great increase of surface area of exposed rock dust, will increase plant growth, counter ocean acidity and sequester carbon. All this remains to be demonstrated. The process is energy intensive and fairly costly.

Biochar application. Charcoal produced by anaerobic combustion of biomass provides habitat for soil life, and increased water retention. It is a particularly long lived form of sequestration. No-tillage subsurface application of biochar-water slurries minimizes disruption of soil root zone microlife and allows earthworms to slowly mix biochar into topsoils.(17)

Management of pastures and rangeland

About a third of global terrestrial carbon is stored in grasslands. Most of this is underground, in the soil life, grassroots and mycorrhizae. Most grasslands are subject to some form of human management. Historically it appears that grassland soil has lost a large proportion of its carbon to the atmosphere, especially during conversion of grassland to cultivation of crops. A meta-analysis of such conversions showed a loss of 59% of soil carbon.(18)

Remaining grassland is mainly used for grazing large herbivores for human use. This land is subject to erosion by wind and water and to overgrazing. There is potential to store more carbon in this ‘sink’, drawing down from atmospheric carbon. This can be done by:(19) (20)

• Fertilization
• Irrigation
• Sowing legumes or improved grasses
• Management of the numbers of animals per hectare
• Agroforestry, especially shelter belts to protect from erosion
• Enclosure of degraded areas from livestock
• Managed grazing(21) This is an important and growing pastoral practice. Herbivores are rotated through pasture paddocks on a schedule that allows regeneration of above and below-ground parts of the grassplants. At the time of grazing of the grass blades, much of the root mass is shed, enriching the soil with decomposing carbon compounds which will eventually form humus. This cycle repeats with every grazing rotation.

Other experimental initiatives to increase grassland soil carbon are:

• Regenerating the circumpolar steppes toward its former grassland-herbivore system.(22) This is a return to the natural model of interval grazing as described above. A very large-scale experiment is proceeding on the Siberian steppes where dangerous permafrost thawing has begun. By the reintroduction of herbivores it is intended that northern boreal forest will be converted back to grasslands. When snow-covered, this will increase the reflectivity of the area. Where herbivores scrape away snow to feed on frozen grass, they remove the insulation of snow on the permafrost, and its temperature decreases, preventing thawing. The potential impact of this experiment, if successful, is huge.
• The introduction of biochar. This has the important advantage of decreasing nitrous oxide emissions(23), but is currently seen as too costly for use at this scale.
• Pasture cropping.(24) Annual crops are seed-drilled into perennial pasture when domestic animals have been rotated out of these fields. After the harvest, the fields are again grazed. No-till, biological methods are used. Soil carbon rises.

6. Reduce emissions of soil nitrous oxide and methane

Nitrous oxide. Reduce use of inorganic nitrogen fertilizer. Apply nitrogen fertilizer with precision in timing and application to achieve maximum uptake by plant, with little left for emissions or leaching. Reduction of cattle herd numbers will also reduce this source of a potent greenhouse gas.

Soil Methane. New ways of managing rice production will result in less methane emissions from this source. Avoid draining wetlands and peatlands.

Impact of these measures on greenhouse gas emissions and on food productivity

Scientists associated with the Drawdown Project(25) have done calculations for some of the measures above, estimating the extent of possible adoption of each measure between 2020 and 2050, and calculating the carbon dioxide equivalent of saved emissions. The current global emission level is about 37 Gt a year. If all the soil carbon improving measures listed in the Drawdown list of 100 (largely corresponding to the lists above) are added together, they total about 150 Gt saved over 30 years, or 5 per year. While this is a very rough calculation, it serves to show that the impact of soil health improving measures on climate change is a substantial one.

The impact on food productivity is not calculable, but in each case it would be expected to be positive.

Impact of these measures on biodiversity

For each measure the biodiversity of the soil is protected or improved. This is very important. The ecosystems under our feet are unseen, highly complex, and our lives depend on them. Those strategies that involve ceasing the use of herbicides, pesticides and fungicides also protect above-ground diversity, particularly of insects, some of which are pollinators. Some measures, eg forest protection, reafforestation and agroforestry, protect or increase habitat for a myriad creatures — insects, birds, reptiles and mammals.

Problems with the solutions

Nitrogen leaching

Application of organic materials (compost, mulch) to enrich soil carbon will also increase levels of organic nitrogen. This is an essential nutrient for plants, and higher levels may increase food production. However, if nitrogen inputs are higher than can be absorbed into plant growth, they may convert into inorganic soluble forms of nitrogen and leach into waterways.(26) (27) Higher levels of soil carbon mitigate this effect. This is also a significant problem in the use of inorganic nitrogen fertilizer when applied in excess of plant needs. Nitrogen leaching is a serious pollutant of waterways, and thus an important factor in loss of biodiversity of freshwater systems and ocean ecosystems.

Albedo effects of biochar

Charcoal applied to the upper layer of topsoil darkens the earth. This may be used to advantage to warm the soil and enhance plant growth. New Zealand Maori applied this technology to their kumara gardens. However, over large areas, this would decrease the reflectivity of the Earth’s surface and increase at least local warming. This is an example of what is known as the ‘albedo effect’.

Limits of sequestration

There are limits to how much carbon a soil can hold in relatively inactive form. We may be able to sequester at a high rate for 30-50 years(28) before this carbon sink is satiated. That carbon will remain sequestered only while restorative farming practices continue.

Who is working on this?

4 per 1000. The ‘4 per mille Soils for Food Security and Climate’ was launched at the COP21 in Paris in 2015 with an aspiration to increase global soil organic matter stocks by 4 per 1000 (or 0.4 %) per year to sequester carbon.(29)

World Soil Charter. A new World Soil Charter(30) was adopted by the members of the UN Food and Agriculture Organisation in 2015. It states that

The overarching goal for all parties is to ensure that soils are managed sustainably and that degraded soils are rehabilitated or restored.

Good soil governance requires that actions at all levels — from States, and, to the extent that they are able, other public authorities, international organizations, individuals, groups, and corporations – be informed by the principles of sustainable soil management and contribute to the achievement of a land-degradation neutral world in the context of sustainable development.

The adoption of the World Soil Charter and the 4 per 1000 target would be a useful basis for action in developing supporting policy.

The UK government is in the process of introducing an agriculture bill that pays attention to soil health and incentivizes farmers to do so. It sets a goal of restoring all degraded soils by 2030.(31)

What can the ordinary citizen do?

For climate change the data is clear. We need policies that will protect soils from loss, degradation, sealing, physical disruption, and unskillful nitrogen application. We have global statements. We know the strategies. We need citizens who understand the need and push policies at all levels. We need urbanites who understand the enormous importance of soil health and who will demand policies that enhance it.
External Link
For food production, the situation looks difficult and involves tough choices between the soil ecosystem service of biomass production and all the other soil ecosystem services, eg water provision, biodiversity habitat. In biomass production, we need to choose between biofuel, with little return of residues to the soil, and food. With food production, we need to choose between meat and dairy and food crops. Do we have enough land to support population sizes ahead? This is not clear. We must avoid further conversion of natural ecosystems to food production. We probably need to convert biofuel production areas and animal fodder production to human food production. Biofuel to food conversion would gain soil carbon because of crop residue return. Animals could still be raised as totally pasture-fed on land unsuitable for crops or by silvopasture methods on partially forested land. We must avoid any further deforestation and we need to increase forest area.

The ordinary citizen can ensure that their diet is consistent with maintenance of good health, with minimal carbon emissions, and from food systems that are sustainable. This means a plant-based diet with no or small amounts of meat and dairy foods. The EAT-Lancet commission on healthy diets from sustainable food calls this The Great Food Transformation, and asserts that it is an essential component of mitigation of climate breakdown.(32)

At a household level, anyone can compost organic waste. In household gardens, the practices of no-till, compost application, mulching, crop rotation and high diversity will benefit soil carbon content and also productivity.

Farmers need incentives to change practices. Observation of increased yields with some practices may be sufficient. However it would be helpful to be able to incentivize farmers for increasing their soil levels of organic carbon. For this, reliable measurement is necessary, and this has been a problem. However satellite and drone-based technologies are emerging and this may facilitate incentivizing strategies.(33)

Conclusion

Soil conservation and soil carbon sequestration are methods of mitigation of climate crisis that can be instituted rapidly and produce results beginning within a year. Many elements of implementation are low cost . They work with natural processes. They are decentralized and can be done at any scale. They have the valuable benefit of enhancing food production. These merits contrast markedly with other methods of carbon sequestration such as carbon capture and storage. They should feature far more prominently in policy development at all levels.

Bibliography

FAO Soils Portal. http://www.fao.org/soils-portal/soil-management/soil-carbon-sequestration/en/

Acknowledgements

Thanks to Adrian Myers, farmer and scholar, Don Graves, Katerina Seligman and Jack Santa Barbara for helpful comments on the manuscript.

References for this article can be seen at the Footnotes 3 page on this website (link will open in a new page).