Soils and Carbon: quantity

impacts

global implications

David Powlson Lawes Trust Senior Fellow

Department of Sustainable Soils & Grassland Systems

Rothamsted Research, UK

Spot the

Difference!

Contains organic matter

No organic matter

SOM

Soil

quality

Global carbon

cycle

• Food security

• Sustainability

Climate change:

• mitigation

OR

• worsening

• Aggregate formation & stability

• Water infiltration & retention

• Erosion risk

• Nutrients supply: N, P, S

• Nutrient retention (CEC)

• Substrate for microbes and fauna

• …..

SOM influences virtually all soil properties – usually more is better

• Total SOM content is difficult to increase in

arable/horticultural soils

slow; changes over years – decades

• Difficult to measure changes in short term

or even direction of change

• Increasing SOM may not be farmers’

highest priority

cf economics, timing, labour issues

BUT …

Two (complementary) approaches to studying soil organic matter (SOM):

What it IS • Chemical structure

• Chemical/physical interactions with inorganic particles

What it DOES • Quantity in soil

• Influence of management practices

• Influence on soil physical properties

• Root growth, crop yields

• Source of nutrients & CEC

• Biology

Chemistry

Dynamics, Impacts

Formation of lignin - one component of SOM

Powlson, Smith, De Nobili

(in Soil Conditions and Plant Growth,

Eds. PJ Gregory & S Nortcliff, 2013)

Chemical functional groups in soil as revealed by 13C-NMR spectroscopy

Fontaine et al (2007) Nature 450, 277-288

SOM quantity • Inputs

• Outputs

• Equilibrium concept

• Influence of land management and agricultural practices

OM inputs and outputs

Powlson, Smith, De Nobili (in Soil Conditions and Plant Growth,

Eds. PJ Gregory & S Nortcliff, 2013)

Incoming OM processed by fauna and microbes

– a suggestion that SOM derived entirely from microbial metabolites

Agriculture and SOM

• Clearing natural vegetation to create land

for agriculture is almost always bad for soil

organic matter!

• But we do have to eat!

• Soils are part of our ecosystem with

important roles in providing “ecosystem

services”

• “Provisioning services” are among the

ecosystem services – food, fibre, fuel

Limited options for increasing (or even maintaining) SOC

• Land use change

arable → forest, grass, perennial crops

• Inter-crops, cover crops (less bare soil)

• Add organic materials manures, AD digestate, compost, crop residues, …

• Reduced tillage?

• Improved crop growth fertilizers, disease control, irrigation, …

• Crops with larger/deeper roots

• Biochar?

Converting forest to agriculture,

Ethiopia chronosequence

Effects of deforestation and subsequent cultivation on the SOC fraction derived from

natural forest (Cf), and from agricultural crops (Cc) in the 0–10 cm soil layer along a

chronosequence of farm fields converted from tropical dry Afromontane forest.

Leminih et al (2006)

Agriculture, Ecosystems & Environment 109, 9-19

Morrow Plots, Illinois Clearing Prairies (natural grassland) for agriculture

Points: measured Lines: RothC simulation

0

10

20

30

40

50

60

70

80

1860 1880 1900 1920 1940 1960 1980 2000 2020

Year

SOC (t/

ha t

o 1

5cm)

Bluegrass

Border

Continuous

Corn

Corn-oats

-clover

C lost to

atmosphere

Gollany et al (2011) Agronomy Journal 103, 234-246

40

30

20

10

0

1960

90

1940

70

50

80

100

60

20001980

Year

Org

an

ic C

in

so

il, t

ha

-1

SOC changes following land use change, Rothamsted

Started arable

Started grass

Johnston et al (2009) Advances in Agronomy 101, 1-57

Movement towards new equilibrium SOC content

Broadbalk Wilderness

- Woodland reversion

since 1882

Soil pH gradient,

pH 8.3 to 4.0

Hoosefield - Spring barley

since 1852

Broadbalk - Winter wheat

(continuous & rotation)

since 1843

Exhaustion Land since 1856

P & K studies

(rates of run-down and

recovery)

Fosters Ley-arable

(crop rotations)

since 1949

> 250,000 grain, straw, herbage,

soil, fertilizer and manure samples,

some dating back to 1843.

Rothamsted Sample Archive ( Refurbished 2009)

Broadbalk Soil organic C in selected treatments

Points – measured data Lines – RothC simulation

Bellamy et al (2005) Nature 437, 245-248

England & Wales. SOC changes, 1978-2003

England & Wales. SOC changes, 1978-2003

Range of original SOC

g kg-1

% of area

Rate of change of SOC g kg-1 yr-1

0-20 19 0.34

20-30 21 0.13

30-50 28 -0.10

50-100 20 -0.68

100-200 6 -2.18

200-300 2 -4.00

>300 5 -7.37

Bellamy et al (2005) Nature 437, 245-248

-

+ Increased crop yields,

straw return

Changes in soil organic C, Java, 1930 to 2010

Minasny et al (2011) Global Change Biology 17, 1917-1924

Is soil carbon disappearing?

1930-1970: deforestation. Conversion to plantations (tea, rubber), then annual crops

1960-1970: rice, low input

1970 onwards: intensification of cropping. Increased inputs, residue returns

Minasny et al (2011) Global Change Biology 17, 1917-1924

SOM fractionation

• Classic humic and fulvic acids – extraction

with alkali, precipitation with acid.

• Particle size

• Density

• Combined particle size and density

• Biological - microbial biomass

So

il o

rgan

ic C

(%

)

0

1

burnt

incorporated

So

il t

ota

l N

(%

)

0.00

0.05

0.10

Bio

mass C

(kg

ha

-1)

0

50

100

150

200

250

300

350

400

Bio

mass N

(kg

ha-1

)

0

15

30

45

60

75

%C %C %N BC BC BN BN%N

Straw incorporation experiment, spring barley, Denmark

(18 years)

Powlson et al (1987) Soil Biology & Biochemistry 18, 159-164

No measurable effect

on soil total C or N

40% increase in microbial biomass

- “early warning”

Influence of SOM quantity on soil physical

properties

Highfield Reversion Experiment

N

Fallow

Arable

Grass

Acknowledgement:

Chris Watts, Rothamsted Research

1

2

3

4

1945 1955 1965 1975 1985 1995 2005

% o

rgan

ic C

in

so

il

permament grass

arable

bare fallow

Reduction in organic carbon in arable and fallow soil

compared to grassland

The benefits of SOM may not be directly proportional to total content

Soils repeatedly wetted and compressed

Treatment SOC %

Bulk density Mg m-3

Plastic Limit

%

Grass 3.2 1.39 18.2

Arable 1.5 1.71 23.0

Fallow 1.1 1.84 34.4

Highfield Experiment

20 mm aggregates collected from surface 10 cm. Soil: Chromic Luvisol, Batcombe Series.

25% clay, 58% silt, 17% sand

Review of straw experiments

• 25 experiments,

– 6-56 years, Europe, North America, Australia

• All had “straw returned” and “straw removed” treatments.

• Mainly wheat or barley, some maize, sorghum

• Straw removed

– mainly baled, in a few cases burned.

Powlson, D.S., Glendining, M.J., Coleman, K., Whitmore, A.P. (2011)

Agronomy Journal 103, 279 – 287

Total SOC Small changes from straw return or removal.

Only significant in 6 out of 25 experiments (mainly <10%)

Microbial biomass (and other “active fractions”

within total SOC) Changes in response to straw return/removal proportionately much greater than total SOC

Soil physical properties Larger impacts - aggregate stability, penetrometer resistance

- even when no measureable change in total SOC

Similar results with “conservation agriculture” in Southern Africa and South Asia

Results

Influence of SOM on crop yields

After:

Continuous arable

3 year grass ley + N

3 year grass/clover ley

Winter wheat Spring barley

Johnston et al (2009) Advances in Agronomy 101, 1-57

SOM influencing crop yield through N supply

Broadbalk Soil organic C in selected treatments

Points – measured data Lines – RothC simulation

0

1

2

3

4

5

6

7

8

9

10

1840 1860 1880 1900 1920 1940 1960 1980 2000 2020

Gra

in, t/

ha a

t 85 %

dry

matt

er

Cont wheat Unmanured

Cont wheat FYM

Cont wheat N3PK

1st wheat FYM+N2 (+N3 since 2005)

1st wheat Best NPK

Fallowing Liming

Herbicides

Fungicides

Red Rostock Red Club Sq. Master Red Standard Sq. Master

Cappelle D.

Flanders Apollo

Hereward Brimstone

1st Wheat

Cont. wheat

Modern cultivars

FYM

Broadbalk wheat yields, varieties and major changes

NPK

Grain yield of winter-sown wheat: not very sensitive to SOC concentration, despite improved soil structure in FYM treatment (10 month growing season)

Example of SOM influencing crop yield –

through soil structure or water availability?

Hoosfield spring barley - changing yield trends with changes in variety

1976-79

Julia

1988-91

Triumph

1996-99

Cooper

2004-2007

Optic

Johnston et al (2009) Advances in Agronomy 101, 1-57

FYM

FYM FYM

FYM

PK

PK PK

PK

Modern cultivars of spring-sown barley (high yield potential): grain yield is sensitive to SOC concentration (5-6 month growing season)

FYM applied 2001 – 2006 only

Soil C and climate change

(Burial)

107

Environment regulation: Global C cycle

62

0.1

105107

60

60

120

0.4

Global carbon: stocks and flows

Vegetation (560)

Soil (1500)

1000 million tonnes per year

(Pools) 1000 million tonnes

flows

Ocean (38000)

Atmosphere (720)

(Burial)

107

Environment regulation: Global C cycle

62

0.1

105107

60

60

120

0.4

Global carbon: stocks and flows

Vegetation (560)

Soil (1500)

1000 million tonnes per year

(Pools) 1000 million tonnes

flows

Ocean (38000)

Atmosphere (720)

C sequestration

Either: Greater movement from atmosphere to land (increased plant growth) Or Decreased movement from land to atmosphere (slower SOM decomposition)

Where the opposite to sequestration is happening

Deforestation in Brazil down 23% - only 2040 km2 in last 12 months!

Oil palm, SE Asia, on high-C soils - for biofuel - to decrease GHG emissions!

Brazil - Cerrado native vegetation

Was Cerrado - now soybeans

- mainly for export to Europe and China

C loss from:

1. Clearing natural vegetation

2. Soil organic C

Arable Forest

Poulton et al (2003) Global Change Biology 9, 942 - 955

Broadbalk and Geescroft Wildeness sites, Rothamsted.

Formerly arable fields.

Reverted to (semi) natural vegetation from 1881.

pH 4.4 in 1999 pH 7.4 in 1999

roots

soil

trees

litter

Poulton et al (2003) Global Change Biology 9, 942 - 955

C sequestered

in trees + soil

Re-vegetation: loess plateau, NW China

New vegetation

Inter-crops, cover crops

• Less bare soil

• Genuine C sequestration (additional

transfer of C from atmosphere to soil)

• In addition to numerous other benefits

• Provided water is not limiting total biomass

production

Legume inter-cropping

Adding manure or straw to soil

Increases SOC content.

BUT - is it climate change mitigation through soil C sequestration?

• Depends on alternative fate of manure or straw.

• Manure – mostly applied to soil anyway – so usually no additional C retention in soil.

• Straw - if alternative is burning in the field, then soil C retention from straw application is genuine climate change mitigation.

Does no-till sequester C ?

Nature Climate Change (2014) 4, 678-683

Impact of 26 years reduced tillage on soil C (Brazil)

0 5 15 20 25 0

5

10

15

20

30

40

Soil

depth

(cm)

1.0 2.0

Carbon content (mg/g soil)

0

Whole soil Free light fraction

Machado et al (2003)

Soil Use and Management

19, 250-256

+ 50 % +100 %

10

- - - - - Dashed lines = Conventional tillage; Solid lines = no-tillage

Impact of zero tillage on SOC: meta analysis

– 23 studies, 47 site comparisons, 237 data points

Angers & Eriksen-Hamel (2008)

Soil Science Society of America Journal 72, 1370-1374

(SOC in no till – SOC in ‘full inversion tillage’) / SOC in ‘full inversion tillage

Reduced tillage

“No-till agriculture can deliver significant

benefits for farmers and sustainability in

many (though not all) situations: reduced

GHG emissions are a small but important

additional benefit, not the key policy driver

for its adoption.”

Powlson et al (2014) Nature Climate Change 4, 678-683

Concluding comments • SOM content influences virtually all soil properties

• Source of nutrients for crops.

• Source of energy for microbes.

• Chemical structure is complex and difficult to define – derived from microbial metabolites.

• Arable agriculture normally leads to a decline in SOM cf natural vegetation.

• Various practices can slow the decline – but quite difficult to achieve increases.

• Small changes in SOM content can have disproportionately large impacts on soil properties.

• A major pool of C in biosphere.

Thanks for your attention