Potential soil organic matter

benefits from mixed farming:evidence from long-term

experiments

David Powlson & “Johnny” JohnstonLawes Trust Senior Fellows

Department of Sustainable Soils & Grassland SystemsRothamsted Research, UK

Mixed livestock & arable farming

Pros

• Recycling nutrients from manure

• N inputs from biological N2 fixation in ley phase – decreased need for N fertilizer

• SOM content likely to be higher cfcontinuous arable

• Landscape, habitats

Cons

• Costs of fencing

• Costs & labour for

animal husbandry

• Economics

• Inability to specialise

(for small enterprises)

• Less total grain/arable

production during

rotation

Soil – Earth’s Living Skin

Soil

quality

Global carbon

cycle

• Food security

• Sustainability

Climate change:

• mitigation

OR

• worsening

SOM

Organic inputs and outputs

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

Eds. PJ Gregory & S Nortcliff, 2013)

Results in an equilibrium SOC content characteristic

of soil type, climate, cropping system

Extremes of SOC change in

agricultural soils

SOC changes following land use change,

Rothamsted

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

Started arable

Started grass

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

Movement

towards new

equilibrium

SOC content

Woburn Ley-arable Experiment (started 1938)

Continuous arable

with fallows

Continuous arable

3 year grass/clover ley

Woburn Ley/Arable Experimentsandy loam soil, 7% clay, 60 years

3 years “treatment” cropping followed by

2 years arable “test” crops

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

“Treatment”

cropping

3 year grass ley + N

%C increase ≈ 0.23%

SOC increases from leys cf

continuous arable cropping

• Sandy soil:

approx. 6.8 t ha-1 in 60 yrs (generous),

(but difference established in approx. 30 yrs

then no further increase)

≈ 0.2 t C ha-1 yr-1

Rothamsted Ley/Arable Experimentsilty clay loam soil, 23% clay, 36 years

3 years “treatment” cropping followed by

3 years arable “test” crops

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

Increase in %C

due to 3 yr leys

over 36 yrs

SOC increases from leys cf

continuous arable cropping

• Sandy soil:

approx. 6.8 t ha-1 in 30 yrs

= 0.2 t C ha-1 yr-1

• Silty clay loam soil:

approx. 7.2 t ha-1 in 36 yrs

= 0.2 t C ha-1 yr-1

Contribution to decreasing

UK GHG emissions• 4.6 Mha arable land

• Increased SOC from conversion to ley/arable =

0.2 t C ha-1 yr-1 (generous)

= 0.92 Mt C total UK

= 3.37 t CO2e total UK

• UK annual GHG emissions (DECC 2013,

provisional) = 569.9 Mt CO2e

• Annual UK GHG emissions savings through

soil C sequestration from converting all

arable land to ley/arable = 0.6%

A small change in SOC can have

a disproportionately large impact

on soil physical properties

(“soil quality”)

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, Glendining, Coleman, Whitmore (2011)

Agronomy Journal 103, 279 – 287

• Trend for soil organic C (SOC) content to

increase with straw incorporation – but

effects small.

• Increases only significant in 6 out of 25

experiments, mainly <10%.

• Increases in microbial biomass

proportionately much greater than for total

SOC.

Straw results (1/2)

Powlson, Glendining, Coleman, Whitmore (2011)

Agronomy Journal 103, 279 – 287

Straw results (2/2)• In some cases soil physical properties measured:

– aggregate stability, penetrometer resistance, water infiltration rate

– greatly improved with “straw returned” even where no measurable impact on total SOC.

– similar results for ‘conservation agriculture’ in Africa and South Asia

• Implications for seedling emergence, root growth, water infiltration, decreased soil erosion

…………………….

• Evidence from Rothamsted LTEs of positive impact of increased SOC on yield for short-duration crop (spring barley) but not long-duration crop (winter wheat).

Improved soil

structure

Greater root

exploration of soil

More efficient capture

of immobile nutrients,

e.g. P

SOC

%

Grain yield

t ha-1

Olsen P to

achieve

95% yieldField experiment (spring barley)

1.40 5.00 16

0.87 4.45 45

Pot experiment (grass)

1.40 23

0.87 25

Spring barley grown on low or high OM soil -

OM content differed from past manure applications

Concluding comments

Arable Mixed

arable/grass

• Small increase in SOC cf continuous arable

• Small climate change benefit from soil C sequestration– provided not counteracted by grass to arable conversions

• Some decrease in N fertilizer use if utilise legumes in ley phase

• Improved soil physical structure– agronomic and environmental benefits

• More diverse landscapes and habitats

BUT

• Decreased arable crop production during rotation

• Utilisation of pasture

• Large changes in agricultural structure required to make economically and practically feasible

Arable Mixed

arable/grass

Thanks for your attention

Extra slides

After: -

Continuous arable

3 year grass ley + N

3 year grass/clover ley

Winter wheat Spring barley

Johnston & Poulton (2005)

SOM influencing crop yield through N supply

Decline in SOC content in a sandy

soil (7% clay) over a 100 years (UK)

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

Continuous cereals, inorganic fertilizers

4 course rotation plus manure

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

at

85

% d

ry m

att

er

Cont wheat Unmanured

Cont wheat FYM

Cont wheat N3PK

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

1st wheat Best NPK

FallowingLiming

Herbicides

Fungicides

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

Cappelle D.

Flanders Apollo

HerewardBrimstone

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

PKPK

PK

Modern cultivars of spring-sown barley

(high yield potential):

grain yield is sensitive to SOC

concentration

only (5-6 month growing season)

FYM applied

2001 – 2006

only

Morrow Plots, IllinoisPoints: 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

Total SOC

Small changes from

straw return or removal

Microbial biomass

(and other “active fractions”

within total SOC)

Changes in response to straw return/removal

proportionately much greater

Soil physical properties

Larger impacts

- even when no measureable change in total SOC