David Powlson's presentation from the Sustainable Food Trust's meeting: What role for grazing livestock in a world of climate change and diet-related disease?

1. Potential soil organic matter
benefits from mixed farming:
evidence from long-term
experiments
David Powlson & “Johnny” Johnston
Lawes Trust Senior Fellows
Department of Sustainable Soils & Grassland Systems
Rothamsted Research, UK

2. 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 cf
continuous 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

3. Soil – Earth’s Living Skin

4. Soil
quality
Global carbon
cycle
• Food security
• Sustainability
Climate change:
• mitigation
OR
• worsening
SOM

5. 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

6. Extremes of SOC change in
agricultural soils

7. SOC changes following land use change,
Rothamsted
40
30
20
10
0
1960
90
1940
70
50
80
100
60
20001980
Year
-1
Started arable
Started grass
Johnston et al (2009) Advances in Agronomy 101, 1-57
Movement
towards new
equilibrium
SOC content

8. Woburn Ley-arable Experiment (started 1938)

9. Continuous arable
with fallows
Continuous arable
3 year grass/clover ley
Woburn Ley/Arable Experiment
sandy 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%

10. 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

11. Rothamsted Ley/Arable Experiment
silty 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

12. 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

13. 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%

14. A small change in SOC can have
a disproportionately large impact
on soil physical properties
(“soil quality”)

15. 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

16. • 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

17. 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).

18. Improved soil
structure
Greater root
exploration of soil
More efficient capture
of immobile nutrients,
e.g. P

19. SOC
%
Grain yield
t ha-1
Olsen P to
achieve
95% yield
Field 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

20. Concluding comments
Arable Mixed
arable/grass

21. • 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

22. Thanks for your attention

23. Extra slides

24. 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

25. 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

27. 0
1
2
3
4
5
6
7
8
9
10
1840 1860 1880 1900 1920 1940 1960 1980 2000 2020
Grain,t/haat85%drymatter
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
HerewardBrimstone
1st Wheat
Cont. wheat
Modern cultivars
FYM
Broadbalk wheat yields, varieties and major chang
NPK
Grain yield of winter-sown wheat:
not very sensitive to SOC concentration,
despite improved soil structure in FYM treatment
(10 month growing season)

28. 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
only (5-6 month growing season)
FYM applied
2001 – 2006
only

29. Morrow Plots, Illinois
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/hato15cm)
Bluegrass
Border
Continuous
Corn
Corn-oats
-clover
C lost to
atmosphere
Gollany et al (2011) Agronomy Journal 103, 234-246

30. 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