Soil carbon sequestration not a panacea for agriculture’s climate change mitigation

World Resources Institute’s article  Regenerative Agriculture: Good for Soil Health, but Limited Potential to Mitigate Climate Change, generated a spirited discussion. The Institute’s  Tim Searchinger and Janet Ranganathan  provide further elaboration on that article’s conclusions.

The term “regenerative agriculture” is a broad and not-yet-clearly-defined term. It can refer to a range of practices with both climate and non-climate benefits. Our previous blog post focused specifically on the limited potential to mitigate climate change by removing carbon from the atmosphere and storing it in soils. 

One goal of our post was to shift attention away from soil carbon to other ways regenerative practices can achieve climate benefits. For example, in Africa, practices such as agroforestry offer much promise to boost productivity, increase above-ground vegetative carbon and meet rising food demands without clearing natural forests.

Implemented in the right way, preserving the huge, existing reservoirs of vegetative and soil carbon in the world’s remaining forests and woody savannas by boosting productivity on existing agricultural land (a land sparing strategy) is the largest, potential climate mitigation prize of regenerative and other agricultural practices. Realizing these benefits requires implementing practices in ways that boost productivity and then linking those gains to governance and finance to protect natural ecosystems. In short, “produce, protect and prosper” are the most important opportunities for agriculture.

By contrast, most claims for the climate mitigation role of regenerative practices focus on their potential to build soil carbon. As our blog post noted, we do not consider this potential to be large.  Among the comments we received was a critical letter from one group of scientists (Paustian et al. 2020) and a supportive letter from another group (Powlson et al. 2020)¹. Here we elaborate our thinking:

I. Examining the Practices

In the simplest language, practices that sequester significant carbon at the field level mainly involve taking crops out of production and therefore cannot be easily scaled, given growing needs for food. All else held equal, efforts that reduce or take land out of agricultural production will require plowing up other land elsewhere to replace the forgone production, which releases carbon, offsetting gains at the field level.

The main practices long thought to sequester carbon that maintain crop and grazing production — namely no-till farming and grazing management — are now known to be far less effective or even ineffective, and often face major adoption challenges. Other practices have promise and warrant continued work, such as cover crops, but their total achievable potential to sequester carbon in soil is currently uncertain.

Practices That Increase Soil Carbon at the Field Level, But Do Not Generate Overall Climate Benefits

Increasing soil carbon at the field level does not necessarily generate a global climate mitigation benefit. Climate benefits depend on a full accounting of all greenhouse gas (GHG) effects, including those related to agricultural expansion to make up for any reduced production. Large claims of climate mitigation benefits through soil carbon typically fail to account for all GHG effects. We call this limitation a “GHG accounting error.” Four categories of soil carbon practices are subject to this error:

1. Taking agricultural land out of production: The practices that sequester the most carbon on individual fields are those that take those fields out of agricultural production. These practices, including peatland restoration, play an important role in broadly cited papers such as Paustian et al. (2016) and Smith et al. (2008). These papers are also cited in Paustian et al. (2020), the critical comment sent to WRI. This comment also cites the soil carbon sequestration potential of turning marginal cropland into perennial grasses and trees, and into grassed waterways and buffer strips, both of which fall into this category because the “regenerated” land no longer produces food. Other soil carbon papers cited in Paustian et al. (2020) assign a large role to avoided deforestation (Soussana et al. 2019; Bossio 2020), which does not sequester carbon, but avoids its loss.

We agree on the importance of these measures. But, one, they are not about building soil carbon on working agricultural lands, and therefore are not the focus of our blog post. Two, their climate benefits rely on additional efforts to boost yields or reduce demand.

To reforest lands not just at the field level, but also on a net global basis requires a distinct set of efforts to reduce the need for agricultural land, such as moderating meat demand, reducing food loss and waste, avoiding competition from bioenergy for food crops and land, and boosting crop and pasture yields. Doing so also requires finance and good land use governance. With such efforts, restoration can generate net carbon gains.

Co-mingling such comprehensive land preservation and restoration efforts with soil carbon gains on working lands under a common umbrella term of “regenerative agriculture” sets up high expectations for carbon savings — which predominantly come from landscape protection and restoration rather than agricultural practices. But because it is called regenerative agriculture, it focuses action on the smallest contributors to carbon sequestration — namely, practices on working agricultural lands.

• Turning cropland into grazing land or perennials: Another practice that sequesters soil carbon at the field level is to convert cropland into grazing land. “Integrating livestock” into crop production by rotating crops and grazing in different years is often presented as a core principle of regenerative agriculture.² Cropland can also be converted to grazing permanently.³ 

The opportunity to convert annual cropland into grazing is also limited by the growing need for annual crops. There are places where incorporating a grazing rotation holds promise, such as parts of the United States and Canadian Great Plains, where doing so can break pest cycles and where livestock grazing is still common. But given demands for crops, turning one hectare of crops into grazing will usually require plowing up more grazing or forest land at another place to replace the crops, releasing carbon and likely resulting in no net climate mitigation overall.

And even if overall diet changes or yield gains could free up cropland globally, that land could alternatively sequester carbon through reforestation, so its use for grazing would still have a high opportunity cost.

• Adding manure and organic amendments: Adding manure or other organic amendments can build soil carbon in a field but that does not mean it increases net soil carbon sequestration overall. Because the world generates only so much manure — and even if temporarily stored, nearly all manure is eventually added to some field somewhere — adding more manure to one field mainly involves adding less to another.

Suggestions that composting municipal solid waste could be an alternative source of carbon exaggerate the scale of that resource, which could, at most, add around 11 million tons of carbon to soils in the United States each year, of which only some part would likely remain long-term.⁴ They also overlook that there are other potential uses for municipal solid waste, such as aviation biofuels, and that the globally agreed Sustainable Development Goals have set a target for cutting in half per capita  food waste by 2030.

• Increased fertilization:  In some locations, increasing fertilizer use has led to increased soil carbon (Poulton et al. 2017), which helps to explain soil carbon gains in China. But the pollution and GHG emissions associated with the manufacture and use of that nitrogen can overwhelm any climate mitigation benefits from soil carbon gains (Gao et al. 2018).

Likewise, better nutrient management or more legumes do not generally help build soil carbon (Soussana et al. 2019). Better nutrient management generally means applying less nitrogen to avoid water pollution and emissions. And the appropriate goal of increasing legumes in most places is to replace fertilizer, not to add more. Practices that limit erosion can be beneficial for soil carbon, but they do so by maintaining existing soil carbon, not building more.

No-till and Grazing Land Management

For practices that do not change the uses of cropland or grazing land, the principal large estimates of soil carbon sequestration have relied on no-till farming and changes in grazing management. Whatever the original justification for these large estimates, recent science has greatly reduced estimates of their potential.

No-till: As reflected in Ogle et al. (2005), cited by the Paustian et al. (2020) critique of our blog post, science once supported substantial soil carbon sequestration claims for no-till farming, but scientific understanding of no-till has changed. Since that time scientists realized that what causes soils to retain carbon is extremely complex and little understood. Most significantly, new studies and reanalysis of old data show that once soil carbon is measured properly and below the top-soil layer — even to a medium depth of around 30-35 centimeters — gains in soil carbon at the top are compensated in general by losses below, resulting in no overall carbon gains or only small carbon gains.⁵

Three authors of the Paustian et al. (2020) critique agreed in Soussana et al. (2019) last year that this new science undercuts claims that no-till would build soil carbon: “Meta-analyses conducted in recent years and covering the entire soil column suggested no significant positive difference in change in SOC [soil organic carbon] on average in no-till soils, although some increase in organic matter (and hence C) concentrations in the 15-20 cm layer of top-soil is usually observed” (Soussana et al. [2019]). Griscom et al. (2017), also cited, makes the same point.

Soussana et. al. (2019) also endorses the findings that using no-till for five years or less is likely to increase emissions of nitrous oxide, another potent GHG. As we discuss in our report, in the United States, no-till farmers typically plow up their land more frequently than that, ⁶ and there is broad agreement that doing so eliminates most or all of any soil carbon benefits. This science therefore implies that no-till is not sequestering carbon in the United States, and that it could be increasing GHG emissions through nitrous oxide.

Figure: Direct drill cropping research is beginning to question the techniques value for significantly increasing soil organic carbon despite its co-benefits including soil protection, organic matter retention, improved soil water holding capacity and lower fossil fuel emissions per hectare. Photo: Patrick Francis

Grazing management: Although scientific reviews in the early 2000s were hopeful about soil carbon gains from grazing management “subsequent analyses have shown that the impact of improved rangeland management practices on soil carbon is highly complex, site-specific, and hard to predict.” Consistent with our finding, several authors, including an author of Paustian et al. (2020), published an article called “Soil carbon sequestration in grazing systems: managing expectations” (Godde et al. 2020),⁷ which articulates heavy limitations.

Much of the work prior to 2007 suggesting large soil gain potential on grazing land was led by Richard Conant, a co-author of Paustian et al. (2020). But Conant has also made valuable contributions showing that the potential to do so on working grazing lands is more limited than previously thought. For example, Conant co-authored a report in 2015 (Henderson et al. (2015),⁸ which lowered estimates of global technical potential for carbon sequestration in grazing lands to 300 million tons of CO₂ — one-fifth of previous estimates — despite the vast 3 billion hectares global grazing lands occupy.

This lowered estimate did not address economic potential, which would further reduce the ultimate potential sequestration. While half this potential could come from adding more legumes into some pastures, the paper warned that there is a “high risk of the practices, particularly legumes, increasing soil-based GHGs if applied outside of [a] relatively small effective area.” The paper also warned that scientists lack the tools to clearly identify which lands would gain soil carbon from legumes and which would not.

Similarly, as Conant et. al (2017) summarized persuasively in a review paper, the big soil carbon gains per hectare are from turning cropland into grazing land (which, as we noted previously, is limited by the world’s growing need for crops) or by adding legumes, limited for reasons set forth in the Henderson et al. (2015) paper. The remaining potential gains just from changes in grazing management are much smaller. And as Godde et al. (2020) noted, they are “derived from a limited number of observations and practices occurring in particular contexts and regions and cannot be extrapolated to global grazed area since sequestration rates are highly context-dependent.”

Figure: The impact of improved rangeland management practices on soil carbon is highly complex, site-specific, and hard to predict.

Cover Crops and Agroforestry

This category of practices focuses on those valuable measures that increase the growth of vegetation on existing cropland or pasture, typically through cover crops or by adding trees and shrubs. Despite our enthusiasm for these practices for other potential climate benefits, we do not believe the evidence to date justifies expansive estimates of their soil carbon potential, either in relation to the potential tons sequestered per hectare or to the total number of hectares that could be managed with these practices.

For agroforestry, soil carbon claims are based on limited and in some cases implausible data on soil carbon gains per hectare.⁹ There also are feasibility challenges of practicing agroforestry in mechanized cropping systems. We also consider the focus on unwarranted soil carbon gains potentially distracting given that the predominant and clearer carbon gains occur in the trees and shrubs themselves.

Cover crops are today used on only a small percentage of cropland. Although cover crops are likely to sequester some soil carbon, for a variety of reasons we address in an endnote, we do not consider estimates of the quantity of soil carbon gains per hectare from cover crops reliable at this time.¹⁰ Cover crops may also increase emissions of nitrous oxide, which could offset some or all of carbon sequestration.¹¹ More information is needed about these issues and on cover crop adoption potential and ways to overcome barriers to adoption.¹²

Despite our reluctance to assign a soil carbon “climate wedge” to these practices based on the evidence today, we consider them of exceptional value for water quality and soil erosion control, while agroforestry clearly can sequester carbon in vegetation.

II. Large Global Estimates of Soil Carbon Sequestration Potential

Some responses to our blog post cited large estimates of global soil carbon potential. We have now more closely reexamined these estimates. To evaluate an estimate, it is necessary to know the different levels of soil carbon gains that estimate is assuming for different practices in different countries, types of land and soils, the adoption rates estimated for different types of farming and locations, and the evidentiary justification for those estimates. Unfortunately, when we further searched for this information, we found shortcomings in the documentation behind the estimates, as well as estimates that either rely on taking cropland out of production or on estimates of no-till or grazing benefits that are no longer justified.

IPCC 2007 estimates

By far the most cited estimates of soil carbon sequestration on working agricultural lands come from estimates done for the 2007 IPCC report, which are relied on for example in Paustian et al. (2016). They amount to 3 billion tons of CO₂ per year in technical potential, divided almost equally between cropland and grazing land management. Estimates of economic potential are roughly half. The technical potential was more than half of estimated agricultural mitigation emissions at the time, which may explain why soil carbon sequestration has dominated much thinking for agricultural climate mitigation since then.

However, it is not possible to evaluate the bases for these large estimates because there is no ultimate documentation. Neither the original IPCC report nor subsequent papers that rely on it, such as Paustian et al. (2016), explain these estimates independently, and they instead cite to Smith et al. (2008).  (Technically, the IPCC report cites a version of Smith et al. available in 2007). Smith et al. (2008), in turn, explains that the technical potential estimates for cropland management result from modeling of some kind using a dataset of carbon sequestration gains from Ogle et al. (2005). Yet there is no documentation available for this modeling to understand the quantities of mitigation assumed, adoption rates, or other bases for these estimates. For the technical potential to sequester carbon on existing grazing land, there is no identified source at all, as Ogle et al. (2005) did not address grazing land improvements on existing grazing land.

For the global economic potential (1.6 billion tons of CO₂ per year), Smith et al. (2008) identifies a 2006 report of the U.S. Environmental Protection Agency (EPA) as the source of the estimates.¹⁴ But that EPA report only addresses non-CO₂ mitigation, such as methane and nitrous oxide, not the carbon sequestered in soils. Through email correspondence, we confirmed that economic estimates were ultimately derived from a rarely used, global version of the U.S. land use and emissions model FASOM-GHG. Further personal correspondence with the lead author of Smith et al. (2008) and with the two lead FASOM modelers indicate that no document exists that explains the bases for these global model results.

Despite these limitations, the sources identified as influencing these global estimates make clear that the vast majority of soil carbon sequestration from cropland management was supposed to derive from the adoption of no-till farming, which, according to these estimates, is low-cost.¹⁵ As discussed above, new science undercuts these earlier claims.

The assumptions for the grazing management estimates are even harder to trace because not only does Ogle et al. (2005) not address them, but the U.S. version of the FASOM-GHG model declined to incorporate carbon sequestration on pasture into its estimates on the grounds that “limited data are available on the cost of adopting practices and corresponding carbon and other GHG effects.”¹⁶ From other evidence,¹⁷ we infer that the authors must have made simple assumptions about carbon gains and costs per hectare of grazing land globally despite this lack of data. Whatever the basis, as discussed above, these estimates are no longer consistent with the science of the limited, uncertain gains from changes in grazing management on existing grazing lands.

Other Large Global Estimates

Other large global estimates are no more convincing. For example, the major global initiative to build soil carbon today is the “4 per mille” initiative, which claims a potential to sequester a stunning 9 gigatons or more of carbon dioxide per year. The principal paper justifying this estimated potential (Minasny et al. 2017) lists case studies in which practices have sequestered carbon. But almost two-thirds of these case studies rely on adding manure or other soil amendments or on reducing crop area, practices that increase carbon in a field but, as noted above, but do not lead to net climate mitigation benefits.

Moreover, the paper only assumes large global gains if these practices are broadly applied at some unspecified levels and in unspecified ways. That soils can store more carbon and that practices exist to sequester carbon in individual fields do not tell us how feasible it is to expand those practices and by how much, or whether they even result in net climate mitigation benefits after all land and GHG effects are taken into account.

We have also discovered that old large global estimates continue to be cited even by those disagreeing with them. For example, even though Soussana et al. (2019) appears to endorse the new understanding of no-till, it also continues to cite large 2007 IPCC estimates of sequestration from cropland management that were based on no-till.¹⁸ Godde et al. (2020) cites large estimates of grazing management from the IPCC even while explaining that the science on which they are based is not valid.

Feasibility

Simple estimates of technical potential are valuable as preliminary bounding exercises, but they cannot be properly used as estimates of feasible potential. Many typical estimates, such as those used in Paustian et al. (2016) and Griscom et al. (2017) for cover crops, multiply an estimate of average carbon gains per hectare for a practice by large numbers of hectares to simulate a scenario of broad adoption. These estimates are seductive. If there is a large technical potential, say 1 billion tons of CO₂, the intuition may be that the world can achieve at least some meaningful fraction of that, say 300 million tons.

But the concept of technical potential is too malleable to justify that latter assumption. For example, the technical potential exists to eliminate all emissions from cars if people replaced all driving with walking, but that is not a meaningful estimate because doing so is not feasible. To be meaningful, the analyzed technical potential must also be feasible.

Large estimates of soil carbon potential have also not addressed the challenge of persistence. Unlike carbon in trees, which will tend to persist on its own, soil carbon is not something farmers can add to their soils once and leave alone. Soil carbon is constantly at war with bacteria, fungi and other microbes that eat organic matter and return carbon to the atmosphere.

To maintain soil carbon, farmers must generally maintain the practices that built it. Soil carbon in agricultural lands is therefore significantly more unstable than soil carbon in the forest because farmers often need to change what and how they farm with changing economics. A proper feasibility analysis needs to factor the risk of those changes into its projections, but we cannot find such analyses. No-till is an example. The great majority of U.S. farmers — either because of weeds, soil compaction or changing crops — feel that they need to regularly plow up their no-till, losing most or all of any previous carbon gains even if they occur. We have not seen any study that attempts to estimate adoption of indefinite, continuous no-till in a way that reflects these constraints.

Why Does This Matter?

Properly estimating potential soil carbon gains matter, we believe, for three reasons:

First, we are concerned that overly optimistic expectations around the potential to sequester carbon in working agricultural lands risks diverting effort from more promising opportunities for reducing GHG emissions from agriculture. These alternatives may include the use of regenerative practices such as improving grazing, which can protect forests and their carbon, but often only if linked to governance that protects forest — the “produce, protect and prosper” strategy.

Other promising mitigation opportunities include practices beyond common definitions of regenerative agriculture, such as restoring peatlands, improving manure management, reducing “enteric” emissions from belching cows through feed improvements, and increasing efficiency in the uses of nitrogen and other chemicals.

Second, practices that do not account for all GHG effects when calculating mitigation benefits could take the world in the wrong direction. For example, some propose that converting cropland to grazing land can make beef “carbon neutral,” which if true would make eating beef better for the climate than eating lentils. These claims are based on the theory that beef’s higher emissions are offset by soil carbon gains.²⁰ This claim overlooks the fact that converting cropland to grazing land requires replacing the crop production elsewhere. Others have argued for “lighter grazing” to make beef production carbon neutral (Soussana et al. 2014), but such efforts also require replacing the reduced meat or milk production somewhere else.

These approaches also undermine the critical need to increase output of milk and meat per hectare to meet rising food needs without deforestation and, ideally, free up land for reforestation (see Griscom et al. 2017 and our own report, Creating a Sustainable Food Future).

Third, overestimating the potential to sequester soil carbon based on what the world currently knows could undercut the need for research and development of new perennial systems or other innovative measures that might actually sequester soil carbon in more feasible ways. But why bother to invest in their research if we already claim to know how to increase soil carbon at scale?

Given the 11-gigaton mitigation gap between expected agricultural emissions in 2050 and those needed to hold global warming below 2 degrees C (3.6 degrees F), it is critical to focus on actions with the greatest mitigation promise. Given the challenges discussed above, the realistic ability to sequester additional carbon in working agricultural soils is limited. Because what causes carbon to remain in soils is not well understood, further research is needed, and our views may change as new science emerges. Soil carbon research should continue.

But today’s policy and actions must be based on what we know now. Based on that, some regenerative practices can help stabilize existing soil carbon and build resilience to climate change. They may even contribute to climate mitigation where they boost yields or increase vegetative carbon (as in trees). But these practices should not be relied upon for large-scale mitigation through soil carbon gains. Instead we should pursue a multi-pronged approach, such as the one laid out by WRI and others in Creating a Sustainable Food Future.

Find out more

For full article visit World Resources Institute 24 August 2020 INSIDER: Further Explanation on the Potential Contribution of Soil Carbon Sequestration on Working Agricultural Lands to Climate Change Mitigation | World Resources Institute

On ways to reduce agriculture’s GHG impact:

6 Ways the US Can Curb Climate Change and Grow More Food.

---