Amelung et al. (2020). Towards a global-scale soil climate mitigation strategy

, J. Leifeld

11

,

B. Minasny

12

, G. Pan

13

, K. Paustian

14

, C. Rumpel

15

, J. Sanderman

16

,

J. W. van Groenigen

17

, S. Mooney

18

, B. van Wesemael

19

, M. Wander

20

&

A. Chabbi

21,22,23

✉

Sustainable soil carbon sequestration practices need to be rapidly scaled up and implemented

to contribute to climate change mitigation. We highlight that the major potential for carbon

sequestration is in cropland soils, especially those with large yield gaps and/or large historic

soil organic carbon losses. The implementation of soil carbon sequestration measures

requires a diverse set of options, each adapted to local soil conditions and management

opportunities, and accounting for site-speci

ﬁ

c trade-offs. We propose the establishment of a

soil information system containing localised information on soil group, degradation status,

crop yield gap, and the associated carbon-sequestration potentials, as well as the provision of

incentives and policies to translate management options into region- and soil-speci

ﬁ

c

practices.

O

ver the past decade (2009

–

18), the net global increase in anthropogenic CO

2

emissions,

after accounting for ocean and land sinks, was 4.9 Gt C yr

−

1

. It is now widely

recognized that to tackle the resulting climate change, it will be necessary to employ

negative emission technologies in addition to drastically reducing fossil fuel emissions

https://doi.org/10.1038/s41467-020-18887-7

OPEN

1

Institute of Crop Science and Resource Conservation

–

Soil Science and Soil Ecology, University of Bonn, Bonn, Germany.

2

Institute of Bio-and Geosciences,

Agrosphere (IBG3), Forschungszentrum Jülich GmbH, Jülich, Germany.

3

The Nature Conservancy, Arlington, VA, USA.

4

Wageningen, University and

Research, Environmental Research, 6700 AA Wageningen, The Netherlands.

5

Chair of Soil Science, Department of Ecology and Ecosystem Management and

Institute of Advanced Study (TUM-IAS), Technische Universität München, München, Germany.

6

Soil and Crop Science, School of Integrative Plant Science

Cornell University, Ithaca, NY, USA.

7

Institute of Advanced Study (TUM-IAS), Technical University Munich, Garching, Germany.

8

Department of

Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA.

9

Department of Geography and Environmental Science,

University of Reading, Reading, UK.

10

Carbon Management and Sequestration Center, CFAES/SENR, The Ohio State University, Columbus, OH 43210, USA.

11

Agroscope, Climate and Agriculture Group, 8046 Zurich, Switzerland.

12

School of Life and Environmental Sciences, Sydney Institute of Agriculture, The

University of Sydney, Camperdown, NSW 2006, Australia.

13

Institute of Resources, Ecosystem and Environment of Agriculture, Nanjing Agricultural

University, Nanjing 210095, China.

14

Department of Soil and Crop Sciences and Natural Resource Ecology Lab, Colorado State University, Fort Collins, CO,

USA.

15

CNRS, Institute for Ecology and Environmental Sciences (IEES) Paris, Paris, France.

16

Woodwell Climate Research Center, Falmouth, MA 02540,

USA.

17

Soil Biology Group, Wageningen University, 6700 AA Wageningen, The Netherlands.

18

O

’

Neill School of Public and Environmental Affairs, Indiana

University, Bloomington, IN, USA.

19

Earth and Life Institute, Université catholique de Louvain, Louvain La Neuve, Belgium.

20

Natural Resources and

Environmental Sciences, University of Illinois at Urbana-Champaign College of Agriculture, Consumer and Environmental Sciences, Urbana, IL, USA.

21

Institut

National de Recherche pour l

’

Agriculture, l

’

Alimentation et l

’

Environnement (INRAE) Centre de Recherche Nouvelle-Aquitaine-Poitiers, (URP3F),

Lusignan, France.

22

UMR ECOSYS, Centre INRAE, Versailles-Grignon, Bâtiment EGER, Thiverval-Grignon, France.

23

These authors contributed equally: W.

Amelung, A. Chabbi.

✉

email:

wulf.amelung@uni-bonn.de

;

abad.chabbi@inrae.fr

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123456789

0():,;

Sequestering organic carbon in soil may potentially, and in a
technically feasible manner, remove between 0.79 and 1.54 Gt C
yr

−

1

from the atmosphere

3

[p. 27], recognizing the substantial

potential of soils in stabilizing the climate (e.g., ref.

4

). However,

to accumulate soil organic carbon (SOC) in globally relevant
quantities, the world has to develop the policies and economic
incentives to tap into this potential.

Soils have recently become part of the global carbon agenda for

climate-change mitigation and adaptation through the launch of
three high-level initiatives. These include the

“

4p1000 initiative

”

,

which was launched at COP21 by UNFCC under the framework
of the Lima-Paris Action Plan (LPAP) in Paris on December 1,
2015. The name of the initiative re

ﬂ

ects that a comparatively

small proportional increase (4

‰

) of the global SOC stocks in the

top 0.3

–

0.4 m of all non-permafrost soils would be similar in

magnitude to the annual global net atmospheric CO

2

growth

5

The second initiatives were the Koronivia workshops on agri-
culture, which included soils and SOC for climate-change miti-
gation and were initiated at COP23 in 2018. Finally last year, the
FAO launched RECSOIL, a program for the recarbonization of
soils

6

. The message of all three initiatives is complementary and

simple: increasing SOC can partly mitigate carbon emissions and
is, at the same time, indispensable for the adaptation of agri-
cultural systems to climate change due to the numerous co-
bene

ﬁ

ts it offers. SOC has positive effects on soil structure, water

retention, and nutrient supply, and is crucial to sustain ecosystem
services and agricultural productivity.

Political and market support are needed to motivate farmers to

adopt sustainable agricultural practices on a scale large enough to
result in the transformation of agricultural production systems.
Messages that are easy to convey are needed to engage with
policymakers and practitioners. Thus, the intent of the 4p1000
initiative was to be simple and easy to communicate. The goal is
aspirational in that it is not viable for all land uses, soils, and all
regions

8

. It is also an inspirational target, designed to raise

awareness of the need to improve soil health and food security
with opportunities for climate-change mitigation

.

The implementation of SOC sequestration on a large-scale is

complex, as it involves different soil groups (de

ﬁ

ned by an IUSS

Working Group

) and their speci

ﬁ

c management in different

climate regions of the world. It will therefore need diverse tailored
approaches. To achieve the required major changes in land-use
practices, actions have to be supported by strong scienti

ﬁ

c, edu-

cational,

political,

and

social

programs

that

rely

on

multistakeholder interactions and transdisciplinary collabora-
tion

. What those are, and how they would be instituted, remains

the critical issue moving forward.

We identify region-speci

ﬁ

c opportunities for C sequestration as

linked to both restoration of degraded soils and related
improvement of crop yields. To gain and maintain SOC under
climate change, we have to increase C inputs. We highlight that
this is most easily communicated at sites where soils have both
the largest C debt and where yield gap is high. At sites with low C
debt, organic matter probably plays only a minor role in closing
yield gaps. The identi

ﬁ

cation of priority regions is supported by

soil group as a basic mapping unit, which integrates relevant
properties controlling yield and C storage, whereas the C-rich
organic soils must remain protected. Hence, this paper offers a
soil-speci

ﬁ

c perspective on feasible C sequestration and some of

its trade-offs, which both depend on regional soil conditions,
regional biomass availability, and, importantly, on regional social,
economic, and political constraints.

Linking soil C sequestration to food security as the way
ahead

Any ton of CO

2

that plants assimilate and that is subsequently

sequestered in soil has been removed directly from the atmo-
sphere and will thus help to mitigate climate change (Fig.

). The

science of CO

2

sequestration in soils is currently advanced

enough to inform the creation of policy and incentive programs
despite some uncertainty in the absolute sequestration rates of
particular practices in speci

ﬁ

c places

11

. To be successfully

implemented at a global scale, appropriate SOC sequestration
management strategies are likely to be adopted faster if SOC is
considered not only as a means for mitigating climate change but
also as a contributor to soil health, increased food security, and
other sustainable development goals

14

16

.

Currently, 33% of the global soils have been degraded

6

and

have lost much of their SOC through the historical expansion of
agriculture and pastoralism

17

and subsequent land-use conver-

sion from native ecosystems (e.g., peatlands, forests, grasslands)
to arable land (Fig.

). This has resulted in a decline in soil

structural stability, increased erosion risks, and reduced water
storage and nutrient supplies. Soil degradation has thus become a
major threat to food security, especially in developing countries

18

.

Soil degradation can proceed when intensifying agriculture
without additional C input. Soil degradation can be stopped with
the maintenance of SOC stocks at good agricultural practice

Forest

Grassland

Peat-
land

Arable land

C storage

Arable land, e.g.

-

GAP, with Crop
rotations

-

Fertilization

Arable land, e.g.

-

Deficit fertilization

-

Bare fallow

-

Drainage

Arable land, e.g.,

-

BMP, with

-

Increased C inputs

-

Conservation
measures

Input

Output

Input

Output

Input

≥

≤

Loss

Sequestration

C

Maintenance

Site-specific
incentives

Output

Fig. 1 Conceptualization of C sequestration potentials in arable land.

Usually C is lost after land-use conversion from native ecosystems (e.g., peatlands,

forests, grasslands) to arable land. Future C storage in agricultural

ﬁ

elds then depends on agricultural management practices, with options to regain C by

increasing the organic matter input relative to ongoing CO

2

release at best management practice options (BMP), to maintain C stocks by continued good

agricultural practice (GAP), or to lose additional C by intensifying agriculture without additional C input, usually followed by soil degradation.

PERSPECTIVE

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(GAP). However, by increasing the organic matter input relative
to ongoing CO

2

release at best-management practice (BMP)

options as, e.g., outlined in the 4p1000

ﬂ

yer, soil degradation can

be reversed by increasing SOC stocks

12

(Fig.

). The related soil-

health bene

ﬁ

ts from sequestering carbon may then help to close

yield gaps in arable soils due to associated improvements in
nutrient supplies, water-holding capacity, and soil structural
stability

19

20

. Old

ﬁ

eld et al.

21

reported that building SOC has the

potential to close 32% of the global yield gap for maize and 66%
of that for wheat, while also reducing fertilizer needs by 5

–

7%,

respectively. Closing the yield gap would also reduce the need for
further agricultural expansion and associated potential SOC
loss

12

. To achieve these bene

ﬁ

ts, priority for the transformation of

agricultural systems to increase SOC sequestration should be
given to regions with large yield gaps, e.g., up to 90%, sub-
Saharan Africa, and South and West Asia [

www.yieldgap.org

]

22

.

Priority for the transformation of agricultural systems to

increase SOC sequestration should also be given to regions with
low SOC contents caused by large historic SOC losses

17

. Unfor-

tunately, the total area of degraded soil, ranging from 1000 to
6000 M ha

−

1

, is not well-de

ﬁ

ned globally

23

, thus impairing a

global agenda that can target land restoration and thereby sup-
port climate mitigation.

Yield gaps and historical SOC losses vary across regions,

therefore biophysical sequestration potentials cannot be achieved
to the same degree on all soils in all ecoregions. This is also due to
site-speci

ﬁ

c nutrient requirements, which limit C sequestration in

non-agricultural systems

8

, and which may cause trade-offs with

nitrate release and particularly nitrous oxide emissions

24

26

. The

analysis of yield gaps and soil C debt in tropical and temperate
soils (424 sites from 38 countries) shows that there is no direct
relation between both parameters (Supplementary Fig. 1 and
Supplementary Data 1), re

ﬂ

ecting that the limitations of water

and nutrients, in addition to SOC loss, are the major yield-
limiting factors.

Options for measurement and veri

ﬁ

cation

Soil management is highly decentralized, and to a large extent,
under the control of individual landowners. Additional research
is needed to accurately predict C sequestration potentials at farm-
scale resolution and for different soil groups, while also
accounting for historical land management and past sequestra-
tion success

27

. In particular, it is not currently feasible to verify

sequestration rates that increase total SOC stocks by <1% on an
annual basis using direct soil measurements

15

. Considering the

general agreement that the most effective way to accumulate SOC
is to increase C inputs

(Fig.

), we may possibly overcome the

desire of continuous assessment of changes in labile or stable SOC
pools

31

. In other words, soils can remain at high organic carbon

levels as long as improved management and C input are retained,
irrespective of which form this carbon exists and how it is sta-
bilized. We can thus promote shifts in management toward
higher C inputs because the additional net amount of SOC
sequestered will reside in soil until a new equilibrium is reached,
which can take a few decades

32

It remains dif

ﬁ

cult to evaluate what quantitative contribution

sequestered C

ﬁ

nally makes to improving crop yields, and

therefore to persuade regional policymakers to implement
incentives if they cannot control the success of these measures

In the search for a solution, some considerations exist to couple
policy making with measurement and monitoring technologies
on broader aggregate regional or global scales

33

35

, and to reward

the C storage achieved to date, through so-called moving base-
lines

36

. Analytical tools to assess C storage exist and have been

extensively discussed in the literature

,

. In particular

when evaluated regionally against benchmark sites, such analy-
tical tools provide clues to the potential long-term success of C-
sequestration measures in the different soils of the world.

Region-speci

ﬁ

c potentials and opportunities for soil carbon

sequestration

Potentials to sequester C in soil show substantial variation from
one region to another, even under the same type of management,
due to variations and gaps in current and potential SOC
levels

41

42

. Variations in C sequestration potentials increase with

differences in climate, soil groups, cropping systems, and avail-
able technologies as well as with different yield gaps (

www.

yieldgap.org

) and soil-speci

ﬁ

c, historical C losses

43

. This unfor-

tunate reality can be a barrier to the global implementation of a
soil carbon climate-mitigation initiative, which will thus need a
coordinated effort at regional scales adapted to these variations.
Putting such region-speci

ﬁ

c potentials for C sequestration and/or

loss into action requires actions with regard to

ﬁ

nalizing yield gap

maps and land-degradation maps and providing them at high
resolution, obtaining relevant soil information for many regions
of the world, and validating methodologies for measuring SOC
and its effect on soil functions across farming systems, ecoregions,
and country borders.

The most pressing need is the development of an agenda that

includes (i) information on region-speci

ﬁ

c soil distribution and

degradation status, (ii) matching of sustainable management
practices to soil group and its degradation status, and (iii) stop-
ping the C loss from speci

ﬁ

c soils that have the potential to

signi

ﬁ

cantly affect the global C balance, e.g., peatlands under

drainage. Currently, only a few countries have robust monitoring,
reporting, and veri

ﬁ

cation systems, but there are ongoing

research efforts to expand these capabilities

http://www.fao.org/global-soil-partnership/en/

36

. Global harmo-

nization of data acquisition as already initiated by FAO

’

s Global

Soil Partnership (

,

e.g., Pillar four) and data sharing needs to be urgently intensi

ﬁ

ed

to provide science-based information on soil status and its
response to climate change.

For (i): A soil group as de

ﬁ

ned in the World Reference base

already integrates information on basic properties relevant for soil
fertility, such as pH, texture, cation exchange capacity, or hard-
pans. Different Reference Soil Groups also have different C sto-
rage potentials

44

, due to soil group-speci

ﬁ

c mechanisms of SOC

stabilization

45

. This information should be utilized to guide the

selection of priority areas for C sequestration and respective
management.

In some west European countries, soils are already mapped at

high resolution (e.g., 1:5000 for many areas of Germany, 1:20.000
in Belgium, 1:50.000 in the Netherlands and in France), and
digital soil mapping should improve coverage. In these areas,
SOC levels and soil degradation status are usually known; yet, the
challenge is that this information cannot be utilized for a cen-
tralized climate-mitigation strategy due to restricted data access.
Political will is needed to overcome such data protection issues
when implementing large-scale SOC sequestration programs.

In developing countries, in contrast, such maps are frequently

lacking or at a scale too coarse (e.g., 1:1,000,000 for Zambia) to
infer site-speci

ﬁ

c management options from regional-scale map

grids. The frequent lack of reliable, detailed maps of the state of
soil degradation in these areas

23

makes it dif

ﬁ

cult to manage the

link between food security and soil restoration through C
sequestration. However, this link can be established if it is sup-
ported by local incentives for farmers and stakeholders, who are
usually well informed about the status of their soils.

For (ii): matching sustainable practices to soil group and

degradation status, two general soil categories have to be

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3

distinguished: mineral soils containing only a few percent of
organic C, but which cover more than 90% of the landsurface,
and organic soils that are rich in organic C, such as peatlands and
wetlands, but which cover only 3% of the landsurface

46

but store

more than 20% of all soil organic C

47

. The management practices

to be applied to these two categories are expressly different

. In

general, actions to increase SOC sequestration are focusing
mainly on mineral soils, while the objective for organic soils is to
reduce SOC loss.

Practices that retain and increase SOC stocks in agricultural

soils are well established

, but may require more action to

implement them. Many of these practices relate to known best-
management practices to improve food security

48

49

. In China, for

example, available best-management practices could attain net
SOC sequestration equivalent to one-third of the potential for
agricultural soils

50

,

51

. The proposed measures will, in particular,

sequester C where the soils have lost SOC in the past. To gain and

maintain SOC over time, we thus have to increase and maintain C
input (Fig.

), at best via increased crop-residue return and

maintenance of increased yield.

The practices to be employed should be soil group-speci

ﬁ

c,

accounting for their actual degradation status, as both aspects
affect C storage potentials

44

45

. The contrasting physicochemical

properties of these soils can be utilized to de

ﬁ

ne management

needs for SOC sequestration in the respective regions where
these soil groups dominate. Examples of adequate practices
for target major reference soil groups are given by Driessen
et al.

52

and outlined in Box

, thus also highlighting target regions

where these soils are most abundant (see Box

and Supple-

mentary Information for more details). To restore degraded
soils, speci

ﬁ

c management options usually have to be combined.

When soils are little if at all degraded and yield is already at
optimum, the potential to sequester additional SOC sustainably is
limited. It should be kept in mind that higher inputs might be

Box 1: | Soil-speci

ﬁ

c options for carbon sequestration (exemplarily)

Measure

Potential effect

Major target soil group for C sequestration

Regions with a high
abundance of target
soils (examples)

Fertilizer and organic residue
management

Improved fertility and related crop
yield and crop-residue inputs of soils
that are:
- Poor in nutrients,
- Poor in SOC, or
- Saline

Soils at subsistence farms, especially in low crop
yield regions, e.g.,
- Highly weathered soils (Acrisols, Ferralsols,

Lixisols)

- Sandy soils (Arenosols)
- Semiarid soils (Calcisols, Gypsisols, Solonetz,

Solonchak)

- Subsaharan Africa
- Brazil
- Australia
- NW-China

Liming

Improved fertility and crop-residue
return in acidi

ﬁ

ed soil; reduction of

N

2

O release

Common soils under the agricultural practice that
are acidic in the topsoil, e.g., Cambisol, Luvisol,
Phaeozem, Acrisol, Ferralsol, Andosol

- China: acidi

ﬁ

ed

areas under
intensive N
fertilization

- Subsaharan

Africa, Brazil

Biochar application

Improved soil physicochemical
conditions
Reduced N

2

O and NH

4

emissions

Highly weathered soils (Acrisols, Ferralsols,
Lixisols, Podzols), and tropical Arenosols
Waterlogged soils under rice cultivation

- Brazil, Columbia,
- S- and E-Asia

Mulching (e.g., with no-
tillage practice) and cover
cropping

Enhanced SOC input
Cooling, moisture preservation,
erosion protection, prolongation of
cropping period in the tropics and
subtropics

Soils with large C debt
Soils affected by heat, e.g., Acrisols, Ferralsols,
Lixisols, Nitisols of the tropics and subtropics

- Ethiopia, N- and

NW-China

Soil management, e.g, bed
and furrow
management, and
reduced or no-tillage

Less SOC decomposition by
reducing aeration and protecting the
soil structure

Soils affected by
waterlogging (Vertisols), and
erosion (e.g., Phaozems, Chernozems, Ferralsols,
Acrisols; sloping soils)

- India, Ethiopia,

USA, Russia,
Brazil,
Zaire, China

Deep soil loosening, deep soil
inversion, clay devolving

Subsoil C incorporation with
enhanced yield

hardpans (by management: Anthrosols,
Luvisols, Durisols)

- E-Europe
- New Zealand,

Australia

Crop systems
management, e.g.,
precision farming, use of crop
mixtures, cover crops,
combined farming like rice-
shrimp

Optimization of resource use and SOC
sequestration in spatially
heterogeneous landscapes, or at
temporally varying water supply

Abundant arable soil groups
- (e.g., Luvisols, Cambisols, Acrisols)
- Fluvisols

- W-Europe,

Australia

- Vietnam,

S-China

Water management, e.g.,
- Flooding

Reduced SOC decomposition by
rewetting and avoiding drainage

Temporarily

ﬂ

ooded soils, such as paddy rice

systems (Anthrosols) or near rivers (Fluvisols);
peatlands and organic soils (Histosols), soils with
groundwater (Gleysols) or stagnant water
(Stagnosols, Planosols)

SE Asia,
central Africa

- Irrigation

Increase yields in dry areas, manage
salinity when drained

Fertile semiarid soils (e.g., Kastanozems,
Lixisols), salt-affected soils (Calcisols, Solenetz)

Southern USA,
South Africa
Australia

Reasoning given in Supplementary Information soils classi

ﬁ

ed according to Word Reference Base

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needed in these soils under climate change to maintain actual C
contents.

Even with soil-speci

ﬁ

c management, global yields and thus the

closure of yield gaps for sequestering C in soil rely on the addition
of fertilizers, particularly of N. Linking C sequestration to food
security thus requires sustainable management of N by relying,
e.g., on site-speci

ﬁ

c fertilizer recommendations, as well as on

other means such as organic residue return and the use of legume
plants to

ﬁ

x atmospheric N wherever possible. A soil group-

speci

ﬁ

c evaluation of these chances and risks is still lacking.

Measures that are less dependent on additional N storage are

options of using biochar within the value chain for sequestering C
and improving yields (e.g., refs.

53

54

, with potential co-bene

ﬁ

ts

for, e.g., N ef

ﬁ

ciency (see Supplementary Information). Yet, also

these options are likely soil and crop and possibly even man-
agement speci

ﬁ

c

54

. Hence, for biochar application as well as for

other options indicated above (Box

), soil-speci

ﬁ

c differences

require local implementation advice.

Due to the size and fragility of the carbon stock in wetlands

and peatlands, (iii) SOC losses must be stopped from these soils,
i.e., their management merits separate attention in carbon-
sequestration-focused efforts

55

57

. Between 1850 and 2015, c. 50

Mha of peatlands have been drained, half of it for agriculture,
with the released 80 Gt CO

2

-eq. Towards the end of the century,

cumulative emissions from drained peatlands without manage-
ment change may reach 250 Gt CO

2

-eq

58

. Whereas historically,

most emissions were derived from the temperate zone, drained
tropical peatlands, particularly in SE Asia, contribute the major
part of current emissions

59

Upon rewetting, however, these emissions can be substantially

lowered

60

and eventually even reversed

61

. Rewetting might occur

at the expense of producing food,

ﬁ

ber, and bioenergy crops and

may require enhanced food production in other areas. Yet, the
area of organic soils currently used for agriculture of c. 25 Mha is
<1.5% of the global cropland and managed grassland area. Taking
managed peatlands out of production might be a viable approach
to meet multiple targets of climate mitigation, combating biodi-
versity loss, and restoring regional hydrology. Sparing peatlands
for natural regeneration could be compensated by closing yield
gaps

62

Economic and political opportunities to achieve widespread
adoption of soil carbon-promoting management measures

Financing gaps exist to sequester climatically signi

ﬁ

cant amounts

of C in soils, especially in developing countries. Even if funds can
be provided, they must be accompanied by the development of
institutions and processes that can support such investments.
This is especially relevant in countries that are politically unstable
and lack robust

ﬁ

nancial and regulatory institutions, such as some

regions of the tropic and subtropics, where the need for yield
increase with related soil C sequestration is greatest. In North
America, Australia, and Europe, such institutions exist, but
despite this, these regions have not sequestered climatically sig-
ni

ﬁ

cant quantities of C to date.

An additional issue is estimating the change in the value of co-

bene

ﬁ

ts, such as the promotion of biodiversity or regulation of

the water cycle, erosion reduction, or other societal bene

ﬁ

ts as a

result of soil management changes

63

65

. These co-bene

ﬁ

ts also

vary spatially and temporally and generate a range of private
values for individual farmers who create them and for society as a
whole that also obtains public value from these bene

ﬁ

ts.

At present, SOC has not been successfully featured into

market-based policies, for two overarching reasons: (1) payments
for ecosystem services (PES), including C sequestration in soil, are
rarely concrete as the bene

ﬁ

ts are dif

ﬁ

cult to measure and not

standardized, thus requiring mediation between global bene-

ﬁ

ciaries and local and regional service providers. (2) Individual

land managers do not focus on sequestering C but on agricultural
production. Therefore, it is necessary to create additional incen-
tives for farmers to sequester additional SOC, such as identifying
enhancements in productivity, superior market access, or

ﬁ

nan-

cial returns to carbon assets

66

.

Net cost estimates for changing management practices to

increase SOC range from $3/ton CO

2

to $130/ton CO

2

67

. They

are in

ﬂ

uenced by soil-speci

ﬁ

c management change and related

ability to increase SOC at a given site (Box

), i.e., these costs vary

considerably across regional scales. Nevertheless, incentives to
adopt management changes that sequester additional C have a
history of some success, either created by the public or private
sectors (or both), for example, in Australia

66

. Potential incentives

include subsidies, taxes, and market-based payments for carbon
or cap and trade systems, the right choice depending on regional
or national politics, societal preferences, and implementation
costs

35

. Each of these options deserves further scrutiny for their

suitability to lead to large-scale SOC sequestration.

Social norms as well as psychological and behavioral factors

need to be considered for widespread adoption of soil carbon-
promoting management measures

68

. These uncertainties and

complexities make a regional and particularly a national soil
management strategy for carbon sequestration a so-called wicked
policy issue, with multiple potential avenues

69

. As a wicked

problem, it cannot be solved with single policy action. In this
context, it is crucial to identify strategies that meet multiple goals
by linking soil C sequestration and greenhouse gas-emission
reductions to food security, biodiversity, and environmental
quality. The solutions that are achievable are likely to be diverse
and incremental. There will be no single global

“

silver bullet

”

, but

rather a vast array of small, diverse, and hopefully inter-
connecting

“

silver buckshot

”

policies

70

.

Tasks and agenda for implementation

There is a general agreement that the most effective way to
accumulate SOC is to increase C inputs (Fig.

(eg., refs.

organic substrates ful

ﬁ

ll this purpose only partly if at all, because

meaningful increases of carbon sequestration at one farm or
region must occur without simultaneous reductions in SOC at
another location from where this material is transported from.
Hence, organic C inputs into soil must be produced on-site, i.e.,
by enhancing crop production and green manure

71

Based on the challenges to develop a global agenda, we suggest

to focus on seven main points of research and development
(R&D) that can support local or region-speci

ﬁ

c SOC sequestra-

tion schemes (Figure for Box

; see

“

need to know

”

criteria), with

six additional research foci that would help to further advance the
agenda of a global soil

–

climate-mitigation strategy (

ﬁ

gure for

Box

;

“

nice to have

”

criteria, see Supplementary Information for

more detailed reasoning). Focusing on yield gaps is one option to
make implementation feasible because it is transparent and
accepted by farmers. However, it may not be the tool to maximize
overall C sequestration, due to weak direct global correlations
with the yield gap (Supplementary Fig. 1 and Supplementary
Information, see also

21

. For the latter, we have to consider overall

and soil-group-speci

ﬁ

c C sequestration potentials (

ﬁ

gure for

Box

, right), as well as alignments to other co-bene

ﬁ

ts, such as

biodiversity, regulation of the water cycle, and speci

ﬁ

c country

needs, e.g., biowaste recycling in China.

In as much as yield gaps relate to soil degradation, any means

to restore soils will increase yields, crop-residue return, and
therewith contribute to sequestering CO

2

from the atmosphere.

These links, though logically well understood, are dif

ﬁ

cult to

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5

quantify at high spatial resolution. We identi

ﬁ

ed the following

seven urgent needs to make a global soil mitigation strategy
successful (

ﬁ

gure for Box

):

i.

Improve soil and yield gap information systems for different
regions of the world:

Currently, soil degradation maps are

not reliable

23

and frequently not available at a regional

scale. The yield gap atlas (

www.yieldgap.org

) does not yet

include all countries. Soil maps, when existing, are
frequently provided at a resolution too coarse and thus
not good enough for regional soil management and for
informing farmers about how to optimize SOC manage-
ment at the local level, especially in many tropical and
subtropical countries such as sub-Saharan Africa.

ii.

Reliable predictions of local and regional yield development
per tons sequestered C

: The main potential for signi

ﬁ

cant

carbon sequestration lies in the world

’

s arable soils with

large yield gaps due to low availability of nutrients and/or
organic matter. To help farmers adopt SOC sequestration
practices, information on the response to yield enhance-
ment through increased SOC in different regions of the
world is crucial. Improved agricultural yields due to
increased SOC were

ﬁ

rst reported by Lal

72

and Pan

et al.

20

. Recent meta-analyses suggest that yield increases

ﬂ

atten out at 2% SOC

21

. Global maps with region-speci

ﬁ

c

yield responses related to SOC increases therefore will
greatly support the ef

ﬁ

cient implementation of sustainable

SOC sequestration practices.

iii.

Additional fertilizer requirements for sustainable C seques-
tration

:

Soil

organic

carbon

sequestration

fre-

quently requires large amounts of mineral fertilizers

, in

particular to replace nutrient removal with harvest and/or
to increase the fertility of degraded land

73

. As fertilizer

production is energy consuming, however, we should never

aim at maximizing C sequestration by maximizing N or
other nutrient inputs. Instead, fertilizer input should be
synchronized to plant uptake and site-speci

ﬁ

c, pedoclimatic

conditions to prevent losses into the gas phase or
contamination of water bodies

74

75

, as well as to

reduce costs.

iv.

Full life-cycle greenhouse gas accounting within C sequester-
ing farming systems:

To be able to evaluate the ef

ﬁ

ciency of

soil C sequestration measures, their impact on other
greenhouse gas emissions has to be taken into account.
This may concern machinery use and transport as well as
other greenhouse gas emissions during, e.g., fertilizer
production. Life-cycle analyses have been applied for
biochar use as a negative emission strategy

53

. They are

particularly necessary with regards to N fertilizer use, which
inevitably generates N

2

O emissions with a global warming

potential exceeding that of CO

2

by a factor of almost 300,

depending on the time horizon under consideration

26

76

.

Relatively modest increases in N

2

O (or CH

4

) emissions

could partly, or even completely, offset any reductions in
atmospheric CO

2

resulting from increased soil C storage

24

.

v.

Assessment and regional mapping of soil C sequestration:

While incentives for farmers may promote C sequestration
at farm scale, policy advice at regional or national scale
requires

larger scale predictions

of possible

SOC-

sequestration potentials by simple analytical tools or
modeling approaches

vi.

Accounting

“

off-site

”

transfers of organic amendments, e.g.,

manure, compost, biochar, to the soil at a state or country
level:

A meaningful increase of C sequestration at one farm

or region must occur without simultaneous reductions in
SOC at another location. In the carbon market, this option
is known as

“

carbon leakage

”

, and its consideration is an

Box 2 | Science R&D crucial to support the global implementation of C sequestration (need to know) and to further advance the
agenda (nice to have)

Need to know

ð

supportive science

Þ

Agenda for priority site selection

1. Detailed soil information systems for different regions of the world that

include assessments of yield gaps and soil degradation status

2. Reliable predictions of local and regional yield development per ton

sequestered C

3. Additional fertilizer requirements for sustainable C sequestration.
4. Full life-cycle greenhouse gas accounting in C sequestering farming

systems

5. Detailed maps of regional and national soil C sequestration potential.
6. Accounting for transfers of organic material that may reduce stored

carbon elsewhere

7. Broad ensemble of policies and bottom-up approaches including farmers

’

incentives, societal standards and actions to scale up adoption of C
sequestering practices

Nice to have

1. A priori assessment of regional C sequestration potentials
2. Quantitative estimates on the persistence of sequestered soil C in different

regions

3. Evaluation of subsoil storage options for additional C
4. Closing gaps in terrestrial soil C modeling, e.g., by including erosion or fate

of inorganic C and dynamics of peatland carbon in natural and
drained state

5. Harmonized, simple analytical tools for a priori assessment of legacy-

driven C losses and C sequestration potentials

6. Documentation of C sequestration success stories for different soils and

climatic regions, including monitoring of ecosystem services and societal
bene

ﬁ

ts

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6

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important pillar of mitigation policy. This could be the case,
e.g., by the use of manure or compost that is bought and
transported from another region where it is not in excess
and also needed for maintaining or improving C storage
and soil fertility. Data collection could be done via
declaration forms; for instance, German farmers complete
such a form when they apply farmyard manure, including
the origin of the manure used.

vii.

The Broad ensemble of policies and bottom-up approaches
including farmers

’

incentives, societal standards, and actions

to scale up adoption of C sequestering practices

: The overall

social, economic, and cultural challenges of changing
management toward soil C sequestration should be
addressed through a diverse set of incentives and measures.
They must take into account region-speci

ﬁ

c barriers that

may hinder the implementation of C-sequestration prac-
tices in soil, such as security of tenure, lack of

ﬁ

nancial

resources, or gender equality

78

.

Government farm subsidies represent a signi

ﬁ

cant source of

support for global farming, conservation, and other related
activities, currently totaling an estimated US$445 billion per
year

79

. Some of these expenditures could be focused on sites and

activities most bene

ﬁ

cial for climate-change mitigation. We sug-

gest to start identifying priority sites based on indicators for C
sequestration potentials and then comparing these with yield gap
analyses or vice versa (

ﬁ

gure for Box

, right scheme), followed by

soil-speci

ﬁ

c amelioration measures. Yet, low yield gaps should

not necessarily prevent action, because there may also be high C
sequestration potentials on fertile lands if the actual yield gap is
low, and because the achievements of co-bene

ﬁ

ts like biodiversity,

water storage and resilience may change priority setting (see also
Supplementary Table 1, Supplementary Information, for an
example of a more detailed site prioritization).

A related scheme for site prioritization could potentially be

applied to any country, focusing on the speci

ﬁ

c soils and yield

gaps of region-speci

ﬁ

c crops of the region, with paddy rice, for

instance, in SE Asia, wheat in Austalia and Europe, or maize in
North America and Africa. As an example, Table

http://www.pmrczambia.com/wp-content/uploads/2015/09/

illustrates

such a case scenario for water-limited yield gaps for maize at
three sites in Zambia (see Supplementary Data 1 for individual
data collection). Consultation from local soil scientists and agri-
cultural of

ﬁ

cers or even targeted

ﬁ

eld surveys may be needed to

derive soil group and speci

ﬁ

c degradation status. A related suc-

cess story is the Farmer Input Support Program of Zambia, where
the government supports ef

ﬁ

cient soil fertilization by farmers via

an E-voucher system, which increases crop-residue return and
may thereby increase SOC, even at small-holder farm level
(

Farmer-Input-Support-Programme-Infographic.pdf

). Soil scien-

tists should document this success and coordinate long-term

ﬁ

eld

experiments across the globe

80

81

. Moreover, soil scientists should

also join forces to harmonize the different initiatives to map yield

gaps, soils, and soil degradation status around the world. Any
success stories, even small and local ones, can serve as templates
for implementation.

It is critical to be aware that soil stewardship cannot be

expected to alleviate all socioeconomic factors that impose a risk
for investment and low yields. However, linkage to yield gaps can
be easily communicated and thus facilitate acceptance by pol-
icymakers and farmers. It may circumvent the problem of per-
suading policymakers to implement incentives for soil C
sequestration if they cannot measure whether it has happened
after a reasonable timescale

https://organicstandard.com/

, e.g., after 1 or 2 years. If not solely

focussing on the amount of SOC sequestered itself but on closing
yield gaps to restore formerly degraded soil, such focus can be
more easily communicated, measured, and even re-

ﬁ

nance

investment schemes. This will lead to more organic C being
introduced into the soil through best-management practices that
are adjusted to the speci

ﬁ

c soil condition. The site-speci

ﬁ

city of

best-management practice justi

ﬁ

es an agenda of diversi

ﬁ

cation.

To achieve this, we need to consider data protection issues in
using localized soil information for climate-mitigation measures
and regulating excessive N inputs in order to reduce trade-offs
from N

2

O release and nitrate pollution.

The greatest current impediments to introducing sustainable

soil management practices are the absence of adoption incen-
tives. However, it is already relatively common, in some nations,
to tie compliance with conservation goals to price support or to
crop insurance payments

82

. This can be (and has been) a suc-

cessful method for improving soil management. Orientating
such policies on a combination of increasing crop yields where
needed and soil-speci

ﬁ

c C sequestration potentials and ameli-

oration measures can break down a global agenda for colla-
borative and successful action to the regional scale. Linking C
sequestration in the soil to programs on food security and
poverty alleviation in rural areas, soil health, and REDD

+

(reducing emissions from deforestation and forest degradation)
and biodiversity might facilitate further policy development and
accelerate implementation. Aligning UN conventions for cli-
mate change, biodiversity, and land-degradation neutrality
would further reduce overlapping organizational efforts and
accelerate the identi

ﬁ

cation of regional priority areas. Moreover,

it could help to bring SOC management closer to the heart of
many important societal issues.

An example demonstrating that joint coordination can facil-

itate success is the organic standard, which refers to worldwide
standards and certi

ﬁ

cation issues in the organic food sector

(

). This standard helped formalize

the science

–

policy dialog, and engaged civil society into a dis-

course on sustainability and well-being

83

. This approach might

also perform well in terms of C sequestration and associated
conservation measures

84

, thus also supporting the achievement of

other sustainable development goals.

Social and economic measures to implement C sequestration

programs quickly and effectively include partnerships between

Table 1 Breaking down a global soil

–

climate-mitigation strategy to regions.

Site

Yield
gap (%)

C debt
(t ha

−

1

)

Soil group

Soil
degradation

Recommended climate
mitigation action

Expected change in economic
revenue

Expected carbon
sequestration

Living stone

84

3

Leptic
Acrisol

Nutrient loss

None

++

−

Kasame

84

16

HaplicAcrisol

Nutrient loss

Liming

+

fertilization; conservation

agriculture

++

Choma

75

22

Leptic
Acrisol

Nutrient loss

Conservation agriculture and
agroforestry

+

+++

Examples from three sites in Zambia (data sources:

www.yieldgap.org

, and ref.

17

showing water-limited yield gap for maize and C-debt analyses.

The number of plus/minus symbols illustrates possible success/failure for yield improvement and C sequestration when investing into soils, respectively.

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business and the NGO sector. Particularly noteworthy are the
activities of several large companies, such as Coca-Cola, Mars,
Repsol, Fronterra, Walmart, or DANONE Inc, who have in
response to consumer demand committed to signi

ﬁ

cant emission

reductions and actions which will lead to increased sustainability
in agricultural systems. DANONE Inc, for example, is embarking
on a program to become C neutral by 2050 (

https://www.danone.

). Due to the

scope and size of the corporation, the company may be able to
in

ﬂ

uence farm management programs for an area about half the

size of Belgium. Even though the programs may not sequester C
at the desired quantity to offset global emissions, they may inspire
further innovations once investments occur and new corporate
policies are implemented by producers. Future policy action
could include innovative structures that recognize the bene

ﬁ

t

from engaging with agribusinesses and related industries with
supply-chains that have a land management component. An
additional advantage of such a structure is the potential for more
effective integration across and beyond geopolitical boundaries.

All the approaches we have suggested rely on multistakeholder

collaboration. Even if orienting on soil-group-speci

ﬁ

c measures,

the steps towards the global-scale soil

–

climate-mitigation strategy

are therefore diverse, imperfect, incremental, and take time.
However, they provide an opportunity because they are based on
the real mechanisms to convert science into practice.

The future gives hope: through the 4p1000 initiative, FAO

RECSOIL, and the Koronivia workshops on agriculture, we have
laid the foundations for moving from discussion to opportunities
to create sustainable solutions. A soil

–

climate-mitigation strategy

will be globally successful if it takes soil-speci

ﬁ

c aspects into

account. We can identify priority areas where soil organic C
storage can improve soil fertility and crop yields to motivate
farmers while excluding regions that can likely be disregarded
because of negative trade-offs. However, future policy measures
should (1) take into account the bene

ﬁ

ts of engagements with

agribusinesses and related supply chain management industries
that have a soil management component, (2) seek to encourage
joint small-scale actions involving local actors from and across
the border regions, and (3) improve local capabilities for sus-
tainable site-speci

ﬁ

c soil management. These efforts are likely to

be able to work more effectively across geopolitical boundaries,
and to tackle the common task of soil

–

climate-change mitigation

on a scale and at a level suited to the complex challenge of land
management.

Received: 8 February 2020; Accepted: 3 September 2020;

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Acknowledgements

This work was supported and bene

ﬁ

ted from INRAE

—“

Agroécosystème

”

department,

INSU

—

CNRS, OSU

—

CNRS, CIRAD, IRD, CLAND Convergence Institute funded

by ANR, Ministère de l

ʼ

Enseignement supérieur, de la Recherche et de l

ʼ

Innovation,

University of Poitiers, AgroParistech, ifP-Energy Nouvelles, IDDRI, SEDE-VEOLIA,
Grand Poitiers, and the Region Nouvelle-Aquitaine. C. Shepande and N. Mutwale are
thanked for feedback on local soil classi

ﬁ

cation in Zambia. R.B. acknowledges Bangor

University for additional support. I.K.-K. acknowledges support from the German
Federal Ministry of Education and Research (BMBF) in the framework of the funding
measure

“

Soil as a Sustainable Resource for the Bioeconomy

—

BonaRes

”

, project

“

BonaRes (Module B): BonaRes Centre for Soil Research

”

(grant 031B0511C), and W.A.

from the Deutsche Forschungsgemeinschaft under Germany

’

s Excellence Strategy, EXC-

2070

–

390732324

–

PhenoRob.

Author contributions

A.C. and W.A. developed the perspective concept. D.B., W.de V., I.K.-K., J.L., and R.A.
discussed the concept, writing, review, and editing. R.B., C.C., R.L., J. L., B.M., G.P., K.P.,

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