The C sequestration efficiency of soils

1. The C sequestration efficiency of soils Pete Smith Professor of Soils & Global Change School of Biological Sciences, University of Aberdeen, Scotland, UK E-mail: pete.smith@abdn.ac.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. Increase C inputs… ...or reduce C losses e.g. restore & rewet farmed organic soils e.g. residue management, organic amendments, increased plant C input… CO2 Add to soil Some C is stabilised in the soil Soil C cycle How does soil C sequestration work? Organic carbon source Soil C in soil

7. C C Tillage breaks open aggregates C C C C C Organic material (C) more exposed to microbial attack and weathering How does soil C sequestration work? – reduced disturbance No-till Tillage C 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) Minimum Climate Only Climate & NPP Climate & NPP & Tech Maximum J.U. Smith et al. (2005)

24. Uncertainty on Anthropogenic Carbon Emissions Up to 250 ppm Vulnerability of the Carbon Cycle in the 21st century Uncertainty of the Biospheric-Carbon-Climate Feedback 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