1. The C sequestration efficiency of soils
Pete Smith
Professor of Soils & Global Change
School of Biological Sciences,
University of Aberdeen,
Scotland, UK
CarboEurope-IP Annual Meeting, Poznan, Poland, October 2007

2. Outline Soils in the global C cycle
Mechanisms for soil C sequestration
Global potential for soil C sequestration
Response of soil C sinks to future climate change
Conclusions

3. Soils in the global C cycle

5. Mechanisms for soil C sequestration

6. How does soil C sequestration work?
e.g. residue management,
organic amendments,
increased plant C input…
Increase C inputs…
e.g. restore &
rewet farmed
organic soils
...or reduce C losses
Organic
carbon
source
CO2
Some C is stabilised
in the soil
Add to soil
Soil C cycle
Soil
C in soil

7. How does soil C sequestration work?
– reduced disturbance
No-till
Tillage C
Tillage breaks
open aggregates C C
Organic material (C)
more exposed to
microbial attack and
weathering
Key: C
= C inside
aggregate
= weathering
= microbe

8. Mechanisms for soil C sequestration in agriculture
Smith et al. (2007a)

9. Global potential for soil C sequestration

10. Method
Database of over 200 experiments to derive per-area / per-animal mitigation efficiencies for >60 agricultural mitigation options, for four climate zones
Mean estimates and low and high 95% CI values derived from mixed effects modelling
Applied to appropriate agricultural (crop, grass, livestock) areas / numbers in each climate zone in each region
Smith et al. (2007a)

11. Ogle et al. (2003)

12. Soil C sequestration rates for cool dry climate
Data from: Smith et al. (2007a)

13. Soil C sequestration rates in different climates
Cool
Warm
Dry
Moist
Data from: Smith et al. (2007a)

14. Soil C emission reduction rates for organic soil restoration
Data from: Smith et al. (2007a)

15. Organic soil restoration vs. mineral soil sequestration
Data from: Smith et al. (2007a)

16. IPCC AR4 Agricultural GHG Mitigation
FAO AEZ Database (e.g. showing regions)

17. Global mitigation potential in agriculture
Smith et al. (2007a)

18. Regional breakdown of mitigation potential
Smith et al. (2007a)

19. High and low estimates of the mitigation potential in each region
Smith et al. (2007a)

20. Effect of C price on implementation
Smith et al. (2007a)

21. Global economic mitigation potential for
different sectors at different carbon prices
IPCC WGIII (2007)

22. Response of soil C sinks to future climate change

23. Uncertainty in cropland & grassland SOC stock changes due to climate, NPP & technology change
(HadCM3-A2)
Including potential improvements due to technology results in, with the minimum likely effect, a small increase in both arable and grassland systems, and with the maximum likely effect an average increase of over 5 t/ha.
Minimum
Climate Only
Climate & NPP
Climate & NPP & Tech
Maximum
J.U. Smith et al. (2005)

24. Vulnerability of the Carbon Cycle in the 21st century
Uncertainty of the
Biospheric-Carbon-Climate
Feedback
Uncertainty on
Anthropogenic Carbon
Emissions
Up to
250 ppm
Up to
200 ppm
IPCC SRES 2000; Friedlingstein et al. 2006
Slide courtesy of Pep Canadell, GCP

25. Conclusions
Soil C sequestration is a globally significant climate mitigation measure
Soil C sequestration is cost competitive with climate mitigation measures in other sectors
Soil C sequestration is not permanent and is of limited duration (due to sink saturation)
Response of soil C sinks to future climate change remains uncertain

26. Thank you for your attention