16.5: Soil Carbon Recovery and Sequestration

While emissions reduction is a critical step for slowing the climate change crisis, emissions reduction alone is no longer sufficient to solve the problem. The issue is clearly stated in the following quote from the Intergovernmental Panel on Climate Change Report (2014):

A large fraction of anthropogenic climate change resulting from CO₂ emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO₂ from the atmosphere over a sustained period. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO₂ emissions. Due to the long time scales of heat transfer from the ocean surface to depth, ocean warming will continue for centuries. Depending on the scenario, about 15 to 40% of emitted CO₂ will remain in the atmosphere longer than 1,000 years.

The difference between emissions reduction alone and the combination of emissions reduction with CO₂ removal is illustrated in Figure 16.5.1. In this figure a hypothetical emissions reduction scenario shows a slower but still increasing trend of atmospheric CO₂ concentrations. What is needed to change this trend? We need to bend the curve! The best way to bend the curve is to combine emissions reduction with CO₂ removal from the atmosphere.

Figure 16.5.1 Hypothetical patterns in atmospheric CO₂ concentrations from 2018 to 2060 as a result of different climate change mitigation scenarios. The business-as-usual scenario is depicted in blue with a sold black trend line. A hypothetical emissions reduction scenario, even when optimistic, will still only slow the rate of increase of CO₂ in the atmosphere (dotted black line). Combining emissions reduction with CO₂ removal will help lower atmospheric CO₂ concentrations and bend the curve (dotted red line). (ppmv = parts per million by volume). Image by Whendee L. Silver.

Land-based solutions

Land-based solutions hold considerable promise to help bend the curve, particularly through organic carbon capture, recovery, and sequestration in soils (Table 16.5.1). Well-established agricultural management approaches that have been shown to increase carbon stocks include the following:

• Reduced or no tillage
• Improved grazing regimes
• Fire management
• Use of cover crops
• Use of plant species with high root allocation
• Conversion from annual to perennial crops
• Crop rotation involving perennials
• Agroforestry
• Wetland restoration
• Fertilization
• Irrigation
• Organic matter amendments

Table 16.5.1 Potential global soil carbon sequestration with improved agricultural land management

Management	Land Use	Mean Sequestration Potential Pg∙C∙yr^-1	High Range Pg∙C∙yr^-1	Low Range Pg∙C∙yr^-1
Biochar^a	All	1.05 ± 0.75	1.10 ± 0.70	0.59 ± 0.41
Nutrient, tillage, irrigation^b	Cropland	0.56 ± 0.14	1.01 ± 0.33	0.48 ± 0.17
Grazing^c	Rangeland	0.26 ± 0.05	0.77 ± 0.31	0.26 ± 0.11
Combined potential (excluding biochar)^d	All	0.83 ± 0.11	1.78 ± 0.32	0.74 ± 0.14
Combined potential (including biochar)^e	All	1.88 ± 0.35	2.89 ± 0.42	1.32 ± 0.13

Source: Adapted from Mayer et al. 2018.

High and low estimates are the mean values of the lower and upper limits of potential sequestration given in the associated references. Values are means plus and minus standard errors. (Pg∙C∙yr^-1 = petagrams of carbon per year)

^a Griscom et al. 2017, Woolf et al. 2010;

^b Griscom et al. 2017, Lal 2010, Paustian et al. 2016, Smith et al. 2008, Zomer et al. 2017;

^c Griscom et al. 2017, Henderson et al. 2015, Lal 2010, Paustian et al. 2016, Smith et al. 2008;

^d Griscom et al. 2017, Henderson et al. 2015, Lal 2010, Paustian et al. 2016, Smith et al. 2008, Zomer et al. 2017;

^e Griscom et al. 2017, Henderson et al. 2015, Lal 2010, Paustian et al. 2016, Smith et al. 2008, Zomer et al. 2017, Woolf et al. 2010

Land-based solutions that can help mitigate climate change fall into three general categories. The first category includes practices where the primary goal is to slow SOC losses. The second category includes the manipulation of plant species composition to increase SOC capture and associated storage. The third category, using natural and working lands, is the use of soil amendments. Below, we cover each of these in more detail.

Slowing carbon losses in agriculture

Some land use practices result in the loss of carbon from plants and soils. When carbon losses are slowed, emissions are reduced, and carbon storage is enhanced. Examples of land use practices that can slow the loss of carbon from ecosystems include reduced or no tillage, improved grazing regimes, and fire management.

Tillage is the manual or mechanical practice of turning and agitating soils before planting. It is commonly used in agriculture to reduce weeds, mix soils, and incorporate dead plant material left over from the previous harvest. Tillage also breaks up soil aggregates, exposing soil organic matter to decomposition; tilled fields tend to have lower surface SOC than nontilled fields. Reducing tillage, or doing away with it altogether, decreases the rate of decomposition of soil organic matter and can lead to soil carbon sequestration. Rates of carbon gain tend to be slow, particularly if decreasing the amount of tillage slows the rate of plant growth. Slower plant growth can present challenges to farmers. Furthermore, low- or no-till soils have a greater potential to produce nitrous oxide. From a climate change perspective, the ultimate benefit of reduced- or no-till practices will be a function of the rate of new plant carbon inputs relative to the rate of soil organic matter decomposition and physical soil carbon losses, as well as overall greenhouse gas emissions. Improved tillage practices associated with maintaining soil cover have the potential to save 0.3 to 0.5 petagrams CO₂ per year.

Livestock grazing is another practice that can result in significant SOC losses. Grazing practices vary widely, from a few animals grazed for short periods to continuous grazing of large herds. Overgrazing can decrease the ability of plants to recover and can degrade soil resources, akin to overharvesting of an agricultural crop. Overgrazing can also compact soils and lead to SOC losses via erosion. Changes in grazing regimes that provide opportunities for plant regrowth can decrease the rate of SOC losses. Similarly, restricting herd size and movement during periods when soils are vulnerable (for example, when soils are very wet and thus easily compressed) can reduce SOC losses. Plant regrowth not only increases carbon capture, but also provides additional forage for livestock. Root regrowth can also help hold soil in place and limit erosional losses. Together, improved grazing practices are estimated to have the potential to save 0.2 to 0.7 petagrams CO₂e per year.

Fire leads to rapid carbon losses through the oxidation of plant biomass and surface organic material. By oxidation we are referring to the conversion of solid carbon to CO₂ and other carbon gases during burning. Fire management is challenging. Attempts to suppress fire through management in regions where fire is a natural part of the landscape can have devastating results. Fuels, in the form of dead organic matter or standing biomass, can accumulate in the absence of fire, leading to hotter, more severe, uncontrolled fire events such as wildfires. Fuel management can reduce the chance of wildfire. Examples of fuel management include forest thinning, removal of downed wood, and grazing or mowing of grasslands and woodlands to remove residual dead plant material. Fire management not only decreases the amount of carbon loss via severe fire events, but also facilitates plant growth and associated CO₂ capture. The potential of fire management to lower greenhouse gas emissions and increase soil carbon storage is poorly understood at a global scale. Preliminary estimates suggest that carbon savings of 0.2 to 0.4 petagrams CO₂e are possible.

Plant species–based approaches

The carbon-friendly management of plant species and communities can take many forms, but some of the best-documented examples include the planting of cover crops, use of plant species with typically high root biomass, conversion from annual to perennial crops, crop rotation using perennial plant species, agroforestry, and wetland restoration.

Cover crops can increase carbon capture by increasing the length of time that plants are active in an ecosystem. In some forms of crop agriculture, soil is left bare during the fallow periods in between growing seasons. Bare soil is vulnerable to erosion and associated carbon losses, and the lack of live plants means that CO₂ is not being captured. Nitrogen-fixing cover crops can increase the nitrogen content of the soil, reducing the need for inorganic fertilizer, as mentioned above. Species that tend to build large root systems are particularly helpful for sequestering SOC, as most SOC is thought to be dominantly derived from root biomass.

Similarly, the use of perennial crops, alone or in crop rotations or with agroforestry, can increase carbon capture and sequestration. Annual crops live out their entire life cycle within a single year. Examples of important annual crops include corn, wheat, rice, and soy. Annual plants must grow from seed each year, establishing new root systems and aboveground plant parts. Perennial plants such as alfalfa, grapes, artichokes, asparagus, and tree crops persist for multiple years. In the case of tree crops, the below- and aboveground plant parts remain on the landscape. In the case of nonwoody plants like alfalfa, the aboveground plant parts may die back or be harvested, while the root system remains and becomes reactivated during the next growing season. There are many advantages of perennial crops from a carbon perspective. Perennial crops often have a longer growing season, as they have greater access to soil resources. A longer growing season translates into greater potential for carbon capture from the atmosphere and storage in soil, even past the time when the fruit or vegetable is harvested. Deeper, more extensive root systems can access water and nutrients not available to annual species. The maintenance of perennial root systems helps hold the soil in place, limiting erosion and associated carbon losses. Perennial species are often used to rehabilitate degraded, overgrazed, or over-harvested lands. Although carbon sequestration rates can be slow, owing to lack of nutrient and water resources, the use of perennials can be an effective climate change mitigation approach for degraded soils. Crop selection and conservation agriculture techniques (particularly the use of cover crops) combined can save 0.3 to 1 petagrams CO₂e per year.

Wetland plant species can also contribute to climate change mitigation. Wetlands in some regions have been drained for agriculture because the underlying peat soil is often rich in organic matter and nutrients. However, when peat soils are exposed to the atmosphere and become aerated, the organic matter decomposes rapidly. In the Sacramento–San Joaquin Delta of California, the biggest freshwater wetland in the western US, wetland drainage has led to the loss of approximately 1 petagram of carbon to the atmosphere. The loss of SOC has contributed to land subsidence. In some areas, the land surface has dropped 10 meters or more (Figure 16.5.2).

Figure 16.5.2 Soil subsidence in the Sacramento–San Joaquin Delta of California. Wetland drainage led to the decomposition of soil organic matter, leading the land surface to drop over time. Reproduced from USGS.

Wetland restoration has the potential to sequester carbon by restoring peat soils. Estuarine, swamp, and marsh wetlands are among the most productive ecosystems in the world, meaning that they have the highest rates of CO₂ capture and conversion, globally. Plant growth in these wetlands benefits from high soil moisture year round and the near-constant input of nutrients leached from upslope sources. Wetland restoration could potentially save 0.3 to 1.3 petagrams CO₂e per year. Wetland flooding helps create anaerobic conditions in soils that slow organic matter decomposition. Thus, high rates of carbon capture by wetland plants coupled with low decomposition rates in wetland soils lead to rapid rates of organic carbon accumulation in soils. The anaerobic conditions in wetland soils can also lead to methane production and emissions. The net benefit of wetland restoration for climate change mitigation must carefully consider the balance between carbon sequestration and methane emissions. Wetland restoration also provides many co-benefits, including reduced flood risk and increased downstream water quality.

Soil amendments

Irrigation is also used in agricultural ecosystems to enhance plant growth. Irrigation helps land managers maintain optimal soil moisture conditions for plant productivity. Periodic drought associated with natural climate patterns (that is, annual dry seasons) can limit plant growth to the rainy months of the year. Irrigation can be used to lengthen the growing season and to minimize the impacts of rainfall variability. Approximately 89 million acre-feet of water (1 acre-foot = 326,000 gallons) was applied to farmland in the US in 2013. Over 82% of that water was applied to farms in the western US alone, because of the strong rainfall seasonality in the region. Where irrigation increases plant growth, it has the potential to also increase soil carbon sequestration. However, increasing soil moisture can also stimulate microbial decomposition and the loss of SOC from soils. Careful management of the timing and amount of irrigation, as well as the way it is applied (subsurface, drip, or sprinkler systems), can help limit SOC losses. Overwatering can lead to soil water saturation, the development of anaerobic conditions, and the emissions of nitrous oxide and methane.

Transporting and applying water represent some of the carbon costs of irrigation, although the actual emissions are very difficult to quantify. Transport-related greenhouse gas emissions from irrigation depend upon the distance the water has to travel and the change in elevation required to bring water to the site of delivery. Climate change–related increases in drought frequency and severity, particularly in regions like the western US, are likely to result in additional greenhouse gas emissions from water transportation. Estimates of the potential CO₂e costs and savings from improved irrigation are lacking at a global scale. Organic matter amendments are another land use practice that has potential to help mitigate climate change.

Organic matter amendments can take many forms, from residual plant material not utilized from a harvest, such as corn stover, to livestock manure, composted urban or agricultural organic waste, and biochar produced by burning organic materials. Adding organic matter to soils is thought to increase the chances for soil organic matter formation. However, the application of fresh plant material (called green waste) to the soil surface often leads to higher emissions of CO₂ due to the stimulation of microbial activity. This stimulation, call the priming effect, can lead to the loss of some of the existing SOC stock, although the duration and amount of SOC loss is variable and dependent upon a suite of environmental factors. The application of raw animal wastes has been shown to increase SOC storage but also leads to emissions of nitrous oxide. As nitrous oxide is a much more potent greenhouse gas than CO₂ with regard to atmospheric warming potential, the net benefit of soil amendments of livestock manures can be low or even negative—leading the ecosystem to become a net contributor to climate change.

Composting organic material before land application can significantly decrease the rate of decomposition and greenhouse gas emissions and lead to net carbon sequestration in soils. Composting organic waste also removes the waste from high-emitting sources such as landfills and manure ponds, leading to large greenhouse gas savings. For example, composted green waste applied to just 5% of California’s grasslands resulted in a net savings of 28 million metric tons of CO₂e over 3 years. Much of this savings came from reducing methane and nitrous oxide emissions from waste management, while the remainder came from the new carbon added to soil via enhanced plant growth and the additional storage of the compost carbon added as the amendment. Preliminary estimates based on the generation of organic waste suggest that composted organic amendments could save on the order of 2 petagrams CO₂e per year globally.

Biochar is an amendment that is produced from the burning of organic residues and waste. Under some conditions, biochar has the potential to remain for years, decades, or longer, although the actual decomposition rate is dependent upon the chemical and physical characteristics of the biochar, as well as the climate and soil characteristics of where it is applied. Biochar can act as a slow-release fertilizer similar to compost and can improve other soil chemical and physical characteristics, including soil aeration and drainage. This can lead to enhanced plant growth and associated carbon capture and storage. Estimates of the carbon savings from biochar amendments vary widely, as this is a relatively new approach for climate change mitigation in the agricultural sector. Scientists estimate that carbon savings range from less than 1 to over 2.5 petagrams of carbon per year.