Amundson et al. (2015). Soil and human security in the 21st century

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REVIEW

◥

SOIL SCIENCE

Soil and human security in the
21st century

Ronald Amundson,

1

*

Asmeret Asefaw Berhe,

2

Jan W. Hopmans,

3

Carolyn Olson,

4

A. Ester Sztein,

5

Donald L. Sparks

6

Human security has and will continue to rely on Earth

’

s diverse soil resources. Yet we have

now exploited the planet

’

s most productive soils. Soil erosion greatly exceeds rates of

production in many agricultural regions. Nitrogen produced by fossil fuel and geological
reservoirs of other fertilizers are headed toward possible scarcity, increased cost, and/or
geopolitical conflict. Climate change is accelerating the microbial release of greenhouse
gases from soil organic matter and will likely play a large role in our near-term climate
future. In this Review, we highlight challenges facing Earth

’

s soil resources in the coming

century. The direct and indirect response of soils to past and future human activities will
play a major role in human prosperity and survival.

S

oil is the living epidermis of the planet (

1

Globally, soil is the medium through which
a number of atmospheric gases are biolog-
ically cycled and through which waters are
filtered and stored as they pass through

the global hydrological cycle (

2

). Soil is a large

and dynamic reservoir of carbon and the phys-
ical substrate for most of our food production.
Profound changes are on the horizon for these
interconnected functions

—

particularly sparked

by changes to climate and food production

—

that

will likely reverberate through society this cen-
tury. Ultimately, the way in which we directly and
indirectly manage our planet

’

s soil will be inter-

woven within our future success as a species.

Soil is commonly thought of as the ~1-m-thick

layer of biogeochemically altered rock or sediment
at Earth

’

s surface that has acquired numerous

qualities during its exposure to the atmosphere that
greatly distinguish it from its geological sources (

3

Soil-forming chemical reactions create micrometer-
sized electrically negative clay minerals that im-
part soil with plant nutrient retention capabilities
(

4

). The electrical charge characteristics of soil,

combined with its small particle size and high
surface area, allow it to temporarily store rain and
snow melt for plant use and provide sufficient
residence time for a multitude of chemical reac-
tions to occur that may remove or reduce the

toxicity of contaminants. The water stored in
soil

—

termed green water (

5

—

serves as the source

for 90% of the world

’

s agricultural production

and represents ~65% of global fresh water (

5

Last, the intimate intermingling of life

—

plant,

animal, and microbial

—

within the soil matrix

drives redox reactions that control many elemen-
tal cycles (

6

) and creates a reservoir of organic

C that greatly exceeds the C in the global atmo-
sphere and biosphere (

7

). The microbial com-

munities that mediate these redox reactions are
now believed to represent much of Earth

’

s total

biodiversity (

8

), but the nature, function, and

economic potential of this soil biosphere is only
beginning to be probed (

6

Soil, due to global variations in climate, geol-

ogy, and biota (

3

), has tremendous spatial diver-

sity. More than 20,000 soil types (or soil series)
have been identified and mapped in the United
States alone (

9

), and the number identified in-

creases as land area investigated increases. If the
soil series

–

to

–

land area relationship (

10

) is extrap-

olated to global ice-free land area, the results
suggest that there are more than 300,000 series
on the planet. The response of these soils to per-
turbations can be extremely varied because of their
diverse chemical, physical, and biological char-
acteristics, suggesting the importance, as a sim-
ple precautionary principle, of maintaining segments
of this diversity for the stability and resilience of
global biogeochemical systems in the face of an-
thropogenic disturbances.

Human Imprint on Soil

Humans altered the ecosystems they encoun-
tered as they began their spread across the globe.
However, the most momentous development in
human landscape change occurred with the in-
vention and adoption of agriculture (

11

). Most

agricultural practices involve the removal of the
natural flora, the simplification of biodiversity to
favor monocultures, and the physical disruption

of the soil. Since the Industrial Revolution, ex-
panding populations have relied on the exploi-
tation of more and more soil for a corresponding
growth in food production. Today, ~12% of ice-
free land is in cropland, and 38% is used for com-
bined cropping and grazing (

12

), an area roughly

equivalent to the land area covered by ice and
scoured or otherwise disturbed during the last
glacial maximum (Fig. 1A). In addition to the sim-
ilarity in area, the agricultural impact on soil
processes rivals or exceeds the effect of those ice
sheets in both rapidity and magnitude.

Undisturbed soils have the characteristic, as

result of a number of feedback mechanisms, of
being able to retain many of their features indef-
initely over time

—

their thickness, C content, and

nutrients, for example

—

a condition that is equat-

able to sustainability (Fig. 2). Cultivated soils are
highly modified forms of their wild predecessors
and may thus be viewed as domesticated soils
(

9

). One key characteristic is that domesticated

soils seldom are able to maintain the qualities of
their original conditions, and these changes
greatly affect their productivity and their impact
on surrounding geochemical cycles. The efforts
to improve the management and conservation of
these domesticated soils, and the preservation of
portions of their remaining wild ancestral stock,
will be among the most important challenges this
century (

9

,

13

). Analyses of the combined agricul-

tural and urban impact on soil series in the United
States, for example, revealed large areas in the
agricultural heartland where more than 50% of
the soil series had been domesticated. Soil diver-
sity, like biodiversity (

14

), provides an array of

human-valued goods and services. Among the most
apparent issues is the ability of soil to provide
sustained agricultural production.

The domesticated soil landscape is one of Earth

’

s

most valuable commodities. For example, nearly
$3816 billion (U.S. dollars) in agricultural pro-
ducts were produced globally in 2012 (

15

). How-

ever, agriculture is competing with increasing
urban and suburban soil demands. The conver-
sion of soil to urban land is largely irreversible on
human time scales. There is uncertainty both in
the present and the future distribution of urban
land on Earth (Fig. 1B). A recent meta-analysis
suggests that between 1970 and 2000, an area
greater than the size of Denmark was urbanized,
and that in the next 20 years, 1.5 million km

2

of

land (the size of Mongolia) will be urbanized (

16

The conversion of farmland to urban areas must
be weighed against the fact that our most pro-
ductive soils have already been exploited and
that demand for food production will continue
to increase.

Soil and Climate Security

A relatively stable climate has been the stage
on which the great human inventions of agri-
culture and industrialization have evolved, and
direct or indirect human impacts on soil C cy-
cling processes will have much to do with atmo-
spheric greenhouse gas concentrations and the
associated climate implications by the end of this
century.

RESEARCH

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1

Department of Environmental Science, Policy, and

Management, University of California, Berkeley, CA 94720,

USA.

2

Life and Environmental Sciences Unit, University of

California, Merced, CA 95343, USA.

3

Land, Air, and Water

Resources, One Shields Avenue, Davis, CA 95616, USA.

4

Climate Change Program Office, Office of the Chief

Economist, U.S. Department of Agriculture (USDA), 14th and

Independence SW, Washington, DC 20013, USA.

5

Board on

International Scientific Organizations, National Academy of

Sciences, 500 Fifth Street NW, Washington, DC 20001, USA.

6

Plant and Soil Science, Chemistry and Biochemistry, Civil

and Environmental Engineering, and Marine Science and

Policy, University of Delaware, Newark, DE 19716, USA.

*Corresponding author. E-mail: earthy@berkeley.edu

Organic C stored in soil is the balance between

plant inputs and microbially mediated metabolic
losses as CO

2

(Fig. 2). In unperturbed conditions,

soils achieve steady-state C pools on time scales
of centuries to a few millennia. The total store of
soil organic carbon is still uncertain, but recent
estimates suggest pools on the order of 2300 giga-
tons (Gt) in the upper 3 m (

7

). Soil cultivation

and clearing has caused a major fraction of total
anthropogenic greenhouse gas emissions since the
19th century (

17

). Cultivation is a major disruption

to the natural C balance in soil, one that alters
the physical and biological structure of soil, effec-
tively igniting, through microbially mediated pro-
cesses, a vast store of labile C that has accumulated
over millennia (

18

). During the first few decades

that soil is cultivated, up to 50% of the carbon
pool is oxidized to CO

2

; eventually, a quasi-steady-

state soil C pool is achieved (

19

). Based on the

global agricultural land area, cultivation has like-
ly released between 50 and 70 Gt of C to the at-

mosphere over the course of human history (

20

),

and the combined cultivation and biomass burn-
ing contributions to atmospheric CO

2

exceeded

that of fossil fuel emissions well into the 20th
century (

17

). However, the agricultural imprint

on atmospheric greenhouse gas concentrations
appeared much earlier in the Holocene (

21

). Early

spikes in atmospheric CO

2

and CH

4

corresponded

to agricultural expansion in Mesopotamia and in
China (

22

). Much of the historical C loss was from

the soils of forests and grasslands of the northern
latitudes. However, today the locus of land alteration
has shifted to the equatorial latitudes, and up to
10% of global anthropogenic CO

2

emissions are from

a combination of biomass burning and soil culti-
vation in the humid and subhumid tropics (

23

).

Under changed management or through land

abandonment, global agricultural soils have the
capacity to reapproach their original C storage
and regain up to a half a decade of present fossil
fuel emissions (over a multidecade period). Bet-

ter stewardship of domesticated soils that leads
to higher organic matter contents is a valuable
practice from an ecological perspective and from
an agronomic point of view (

24

). There is now a

large body of research on the rates of C seques-
tration under differing management practices.
However, there are limits to these practices as a
means of mitigating continued fossil fuel emis-
sions. First, a serious concern with management-
based soil C sequestration strategies is that they
are dependent on restricted management op-
tions in a highly decentralized and economically
driven agricultural sector (

25

). A change in land

ownership, or a change in factors driving agricul-
tural practices, can rapidly release much of regained
C. Second, the effectiveness of soil C sequestration
is time dependent. For example, if all potential
soil sequestration strategies were established,
they would initially serve as a sink of about 1.3 Gt
of C year

–

1

(Table 1) (

26

), but this sink term

would be expected to decline nonlinearly to low

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Fig. 1. The global patterns of soil properties and the human imprint on
them.

Expanding urban centers (

55

) remove soil from other uses, whereas

the creation of cropland (

56

) is one of the major drivers of imbalances in the

soil carbon cycle (

57

) and accelerated rates of soil erosion (

58

). Phosphorus, a

critical plant nutrient, is unevenly distributed and inherently low in warm and
humid climates (

59

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sequestration rates over a period of several dec-
ades as a new soil C steady state is reached (

27

Equally as important is the difficulty of actually
achieving this maximum potential, which involves
multiple governments and millions of individual
land managers. Last, soil management effects on
the global C balance are inherently small relative
to the climate-driven changes to soil C storage
that will occur in this century.

Soil C storage is well documented to decline

with increasing temperature and decreasing soil
moisture, and soil C storage patterns mirror glob-
al climate zones (

28

,

29

), with secondary impacts

by bedrock, topography, and soil age (

20

). Soil C

pools are the balance between plant inputs and
microbial decomposition (Fig. 3), and the responses
of these processes to anthropogenic climate change
are considered to be large, (in the case of inputs)
poorly constrained (

30

), and complicated by tem-

perature and moisture interactions. Anthropo-
genic increases in atmospheric CO

2

may drive

increased net primary production (NPP) as long
as nutrient and water limitations do not occur
(

), which ultimately may have a negative feed-

back on atmospheric CO

2

through increased in-

puts to soil C (Fig. 3). On the other hand, increasing

air temperatures warm soil, melt permafrost, and
stimulate biological metabolism of soil carbon
pools, driving what appears to be a large positive
feedback process (

32

) (Figs. 1C and 3). Based on

current earth system modeling, additions of soil
C by increased NPP (relative to an 1850 BCE ref-
erence date) are projected to be between 160 and
1230 Gt by 2100, whereas C losses by increased
decomposition are projected to be between 104
and 629 Gt (

). Overall, models suggest net soil

C changes from a loss of 72 Gt to gains of 253 Gt
by 2100 (

). However, such exercises include great

uncertainties in both projected gains (by CO

2

-

enhanced photosynthesis) and losses (by soil warm-
ing) and in assumptions about long-term ecosystem
response to ever-increasing CO

2

concentrations.

One important uncertainty is the response of
northern latitude soils to warming, which could
result in net soil C losses between 50 to 150 Gt
(

32

,

33

). Last, the current generation of earth

system models has difficulty in matching present-
day soil carbon storage patterns (

34

), and tuning

the models is challenged by empirical uncertain-
ties in the global soil C pool of more than 770 Gt
(

34

), an uncertainty similar in size to the present

atmospheric C pool.

Still debated is the impact of soil erosion on

the global C cycle. When agricultural soil is lost
by water or wind erosion, the surficial, and most
C-rich, material is preferentially removed, which
accelerates the decline in the soil C pool. Rates of
soil C replacement by crops and plants are rapid
enough in certain situations to maintain soil C
levels at a steady state under the condition of
constant erosion

—

e.g., creating an ongoing sink

(

35

). This sink represents a net reduction in

atmospheric CO

2

only if the eroded C is not re-

oxidized. Because some depositional environments
are conducive to partial preservation of buried
C (lakes, reservoirs, basins, floodplains), the net
effect of accelerated agricultural erosion was first
suggested to be a global C sink of 0.6 to 1.5 Gt
year

−

1

, a rate similar to the total global land sink

(

35

). If the eroded C is largely oxidized, however,

it may result in no net sink (or possibly even a
net source) (

36

). The most recent estimates sug-

gest that agricultural erosion of soil C may be
0.40

T

0.20 Gt of C year

–

1

37

). If we benefit from

an unintended C sink due to soil erosion, any
benefits must be clearly balanced against the
related losses of nutrients and reduction of en-
vironmental quality that require fossil fuel energy
to remediate (

38

).

The global soil C cycle has been greatly per-

turbed by human activity, both directly through
farming and indirectly through anthropogenic
climate change. All projected soil C gains and
losses this century are highly uncertain because
of economic, population, and political influences
(which will largely affect carbon sequestration
efforts) and uncertainties in the magnitude of the
soil response to warming (because of the com-
plexity of the soil C pool structure) (Fig. 3). Hu-
man changes to the global atmosphere and
climate are likely to simultaneously drive both
very large gains and losses of soil C

—

fluxes that

are equivalent to decades of emissions at present

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Table 1. Published estimates of soil C sources and sinks for the 21st century.

Management Type

Maximum Flux

(Gt of C year

−

1

)

Reference

Cumulative Flux

(Gt)

Carbon Sinks

Increased net primary production

(

160 to 1230

Erosion

0.40

37

40

Management

Cropland management

0.36

(

Grazing land management

0.37

(

Restore degraded land

0.18

(

Restore organic soils

0.36

(

Total for management

1.26

16.4*

Carbon Sources

Land clearing

†

250

Soil warming
Boreal regions

32

50

–

270

Globe

34

104

–

629

Total

Net balance

–

188 to +137

*Based on maximum new cropland by 2050 (10 billion ha) (

68

) and assumed loss of 25% of an average C

content of 10 kg m

−

2

(

29

†

Calculated assuming exponential decline to an ultimate landscape

saturation after 50 years.

Fig. 2. Changes in the balance of important soil processes caused by human disturbance.

Many

soil characteristics are the balance or the result of a number of processes that respond to changes in
environmental variables (

3

). However, properties such as hillslope soil thickness, organic carbon storage,

N content, and other features attain steady state in intervals of a few centuries to millennia and appear
capable of regaining stability. Human intervention in soil processes many times exceeds natural per-
turbations and thus exceeds the resiliency of soil to recover to its original condition. Viewed broadly,
steady state is a quantitative measure of soil sustainability.

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rates of fossil fuel consumption. The presently
unknown balance, and most importantly its sign,
between the large fluxes represent considerable
uncertainty for climate security (Table 1).

Soil and Food Security

The late 20th and early 21st centuries have been,
for industrialized countries, an unprecedented
era of increasingly low food prices (

39

). There are

numerous factors that may reverse this trend

—

such as increased global demand; climate change
(

40

); and competition for soil by nonagricultural

uses, such as biofuels or urbanization. Abundant
energy has been the key driving force behind our
ability to maintain food production apace with
an expanding population that is estimated to reach
11 billion by 2100 (

41

). Low-cost energy, which

led to advanced agricultural machinery replacing
human labor, is causing migration to urban cen-
ters. Energy is used to replace the soil nutrients
removed or lost by agricultural perturbations of
soil. Energy transforms atmospheric N

2

to the

bioavailable NH

4

fertilizer through the energy-

intensive Haber-Bosch process

—

constituting the

first and most important green revolution (

42

(Fig. 4A), one that allows us to feed the increas-
ing global population (Fig. 4A). Before the indus-
trial fixation of N, any increase in food production
for a given country was largely due to increased
soil used for production (

43

), and only after the

advent of N fertilizer (Fig. 4A) did yields per area
of major crops begin their upward trajectory (

43

Last, energy is essential to mine and transport
essential plant nutrients, such as P and K, that can
only be accessed from limited geological reservoirs.

Agricultural soil erosion is one of the most de-

structive human perturbations to soil sustain-
ability. Given little opportunity or desirability for
further agricultural expansion, stewardship of our
existing domesticated soil is essential for sus-

tained human prosperity. Yet despite the impor-
tance of soil conservation, the implementation of
practices to minimize soil erosion has not fol-
lowed apace with the severity of the problem. The
most pervasive mechanism of soil erosion is via
water. Before European contact and the removal
of native vegetation by plowing and cultivation,
the geological mechanism of soil erosion on most

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Fig. 3. A cause-effect diagram of the major soil-atmospheric CO

2

feed-

back processes.

The values in parentheses, in the circles, are the approximate

pool sizes (Gt) of C. A solid arrow represents a direct response (e.g., as CO

2

increases, temperature increases); an arrow with a circle indicates an inverse
response (e.g., as temperature increases, soil carbon storage decreases). Dashed
lines are for processes that are less well understood. (

A

) Environments with

adequate moisture. (

B

) Water-limited environments. In (A), the a-b-c loop is a

positive feedback process (even number of inverse relationship arrows), one
especially important in regions of melting permafrost.The d-f-c loop is a negative
feedback, one with less certain feedbacks between vegetation and soil (f-g loop)
and temperature (h-i).The strength of the a-b-c- versus d-f-c loops on soil carbon
pools will likely determine whether soil carbon losses in northern latitudes serve

as a major source of CO

2

and CH

4

this century, a balance that also hinges on the

ability of the soils to supply nutrients to plants (arrow g) in order to respond to the
increases in CO

2

(arrow d). In (B), regions with limited moisture, the strengths of

vegetation response to CO

2

(d) and soil carbon response to temperature (b) may

be weakened (thinner arrow). In addition, the vegetation response to increasing
temperature may become negative. These figures reveal the importance of soil
carbon to the global CO

2

balance this century, as well as the uncertainties in the

strength and direction of important processes. Arrow references are as follows: a,
(

); b, (

); c, CO

2

loss by respiration is the overwhelming pathway of C

removal from soils; d, water efficiency response in (

61

); f, soil C is the balance

between plant inputs and decomposition losses; g, not well constrained, but see
discussion in (

62

); h, (

63

); i, e.g., (

64

).

Fig. 4. The post

–

World War II rise in fertilizer production and cost coincides with the spike in

global human populations.

The growth in world population in the late 20th century (

65

) mirrors the

increasing use of industrially derived N fertilizer. Before N fertilization, for major U.S. crops (

66

), wheat and

other grain yields per acre increased only following World War II, coincident with the rise in the use of N
fertilizer produced by the Haber-Bosch process. Prices have sharply risen (

67

) because global demand is

straining the supplies.

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uplands was by slow, biologically driven creep
(

44

). The removal of plant cover allows the mecha-

nism of erosion to change to raindrop dislodg-
ment of soil particles and their subsequent removal
by overland flow, which is a far more rapid erosional
process. For example, recent analyses of U.S.
erosion rates before European contact place the
average rate at about 21 m My

–

1

(My, million years)

45

). Today, erosion rates in the central United

States can exceed 2000 m My

–

1

(

45

), whereas the

rates of soil erosion in portions of the loess pla-
teau of China approach 10,000 m My

–

1

46

) (Fig.

1D). This eroded sediment is ultimately replaced
by the conversion of the underlying sediment or
rock into new soils with the addition of organic
matter and nutrients through biological mecha-
nisms. Until the past decade, the pace of this re-
placement process was poorly known, and the
acceptable rates of soil erosion on agricultural
lands were placed at 400 m My

–

1

or more (

47

). Nu-

merous studies of natural rates of soil production
now suggest rates between 50 to 200 m My

–

1

for

many environments, indicating that the pace of
erosion in numerous agricultural areas is, or un-
til recently has been, unsustainable (

47

). Not only

does the loss of soil remove nutrients from the
site of agricultural production (

38

), but the sedi-

ment generated adversely affects local drainages,
water bodies, and regional aquatic ecology. Last,
the maintenance (or even the improvement) of
agricultural production in the face of accelerated
rates of soil erosion is energy intensive. Although
microbial symbiosis with plants can fix atmospheric
N to bioavailable forms and can substitute for N
fixed by the Haber-Bosch process, there is no bi-
ological or atmospheric source for rock-derived
nutrients, such as P, K, and Ca.

Although natural processes of soil production

and formation replace or release nutrients, the
paces of these processes are slow relative to our
anthropogenic use rate (Fig. 2). The transport of
crops from the site of production to other loca-
tions remove plant-essential nutrients from the
soil, potentially causing deficiencies that limit
potential production levels (

48

). This further

drives a dependence on the mining and distri-
bution of macronutrients from geological sources,
which can create economic inequalities or geo-
political conflicts between nations (

49

) (Fig. 1E).

The growing demand for P has recently caused
an increase in the cost of rock phosphate from
about $80 per U.S. ton in 1961 to up to $450 per
ton in 2008. Prices since then have fluctuated
but are now at about $700 per ton (

50

) (Fig. 4).

In addition to cost is the related issue of access.
Morocco is estimated to have the world

’

s largest

geological P reserves, much of it in disputed ter-
ritory (

49

). The United States, on the other hand,

has only about 2% of global P resources (

51

). At

current rates of retrieval, the most productive
mine in the United States will be depleted in
20 years (

49

), which will force it to become in-

creasingly reliant on imports to sustain its agri-
cultural and industrial sectors.

Because most other P-reliant countries lack the

geological resources to indefinitely sustain cur-
rent use, the only means to confront the decline

in reserves (other than conversion from domestic
to imported P) is to develop a more coherent and
integrated program of P (and other nutrient) re-
cycling. The loss of nutrients in our human and
animal waste streams is environmentally damag-
ing and economically problematic. Regaining con-
trol of these resources, now largely considered
waste, would go far toward substantially lower-
ing the demand for imported nutrients and other
resources (

52

). In addition to P, other soil nu-

trients appear to be entering periods of limitation
or high demand (Fig. 1F). For example, K (potash)
prices were ~$875 per metric ton in 2009 and are
expected to reach $1500 by 2020.

The 21st Century Challenge

Humans have domesticated our soil resources
and the planet (

12

,

53

). This domestication has

in turn perturbed a number of soil cycles such
that they are no longer in balance, and the im-
balance is changing soil in ways that will affect
future generations and their climate (Fig. 2). Soil
management must be geared toward passing a
habitable, albeit highly altered, landscape to the
generations that follow

—

one where our exploita-

tion of, and impacts on, soil resources is adjusted
to the pace of our planet

’

s renewal. These strat-

egies should focus on regaining a balance in (i)
organic C inputs and losses, (ii) soil erosion and
production, and (iii) release and loss of nutri-
ents. Soil sustainability

—

based on quantitative

principles and measurements of soil erosion and
production, soil nutrient loss and release, and soil
carbon loss and return

—

must be the ultimate goal

for managing the global soil resource and should
serve as the driving principle for soil research
that will support this management.

These are challenging goals that will be diffi-

cult to achieve. The solutions will require an ef-
fort commensurate with the magnitude of the
problems. First, effective solutions to soil sustain-
ability, much like the approaches required to con-
tend with climate change (

54

), must involve highly

multidisciplinary research in novel intellectual
settings or institutions. Second, the ultimate suc-
cess of any innovation requires a dialog and in-
terface with policy makers and public institutions,
the ultimate

“

deciders

”

in broad-scale social change.

These linked efforts will depend on continued,
and arguably much greater, investments in knowl-
edge and innovative knowledge transfer and simply
different ways of conceptualizing and approaching
problems. From our vantage point, the future of
Earth

’

s soil resources is tenuously in our control

or within our ability to sustain it into the future.
Only those on Earth in 2100 will know how well
we succeeded.

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AC K N OW L E D G M E N TS

The paper resulted from discussions within the U.S. National

Committee for Soil Sciences, P. Bertsch, Chair. We thank I. Fung,

C. Koven, and G. Sposito for discussions and P. Gottschalk for

figure data.

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