Soil carbon saturation and stabilization

1. Soil carbon saturation and stabilization • Richard Conant • Natural Resource Ecology Laboratory & • School of Global Environmental Sustainability • Colorado State University • Institute for Sustainable Resources • Queensland University of Technology

2. Soil C flows - a conceptual model Microbial death/turnover CO2 enzymes uptake • Available SOM • Unprotected, high molecular weight soil OM • Includes biochemically resistant OM Assimilable (low MW, dissolved OM) Mic. biomass Plant inputs microbial respiration depolymerization Adsorption/desorption Aggregate turnover leaching DOM loss • Protected SOM • Soil OM physically or chemically protected from decomposition • mineral-associated OM • occluded OM; • OM within aggregates

3. Temperature and decomposition - background • Reaction rates increase with temperature (Arrhenius, 1889) (Davidson and Janssens, Nature 2006)

4. Temperature and decomposition - background • Reaction rates increase with temperature • Q10 is greater at colder temperatures (Arrhenius, 1889) (Davidson and Janssens, Nature 2006)

5. Temperature and decomposition - background • Reaction rates increase with temperature • Q10 is greater at colder temperatures (Kirschbaum, SBB 1993)

6. Temperature and decomposition - background • Reaction rates increase with temperature • Q10 is greater at colder temperatures • Q10 increases with increasing Ea (Arrhenius, 1889) (Davidson and Janssens, Nature 2006)

7. Q10-q experiment: approach Labile Resistant t35 t25 tL-35 tL-25 tR-35 tR-25

8. Q10-q experiment: results

9. Q10-q experiment: results (G) (G) (G) (G) (F) (C)

10. Q10-q experiment: results

12. Temperature-decomposition - synthesis Incubation studies Field studies Cross-site studies Fierer 2005 Kahru 2010 Plante 2010 Feng and Simpson 2008 Leifeld 2005 Vancampenhout 2009 Hartley 2008 Conant 2008b Tsens= f(1/decomposability) Conant 2008a Rasmussen 2006 Biasi 2005 Curiel Yuste 2007 Vanhala 2007 Malcolm 2009 Haddix 2008 Waldrop 2004 Wetterstedt 2010 Tsens≠ f(decomposability) Dioumaeva 2002 Hakkenberg 2008 Luo 2001 Fang 2005 Plante 2006a Townsend 1997 Amelung 1997 Melillo 2002 Wagai 2008 Conen 2006 Cheng 2008 Fissore 2009b Tsens= f(decomposability) Fissore 2007 Trumbore 1996 Gillabel 2010 Amelung 1999 Fissore 2009a Liski 1999 Fissore 2009b turnover time turnover time turnover time months-yrs (5-15% of SOC) yrs-decades (40-50% of SOC) decades-centuries (~50% of SOC)

13. Soil C flows - a conceptual model Microbial death/turnover CO2 enzymes uptake • Available SOM • Unprotected, high molecular weight soil OM • Includes biochemically resistant OM Assimilable (low MW, dissolved OM) Mic. biomass Plant inputs microbial respiration depolymerization Adsorption/desorption Aggregate turnover leaching DOM loss • Protected SOM • Soil OM physically or chemically protected from decomposition • mineral-associated OM • occluded OM; • OM within aggregates

14. Soil C response to C input rates Semiarid Sites Mesic Sites • Linear relationships • R2 range from 0.81 to 0.98 • Similar for mesic and semiarid sites

15. Soil C response to C input rates -incl. high inputs

16. Soil C response to C input rates –Melfort, Sask.

17. Ho: soils have 4 saturating pools Carbon content Saturation level Non-protected Protection level Biochemically protected Protective capacity Microaggregate protected Silt + clay protected Carbon inputs

18. Saturation is different from steady-state (West and Six 2007 Clim. Change)

19. Testing our saturation hypothesis { input = respiration Saturation level Carbon content input  respiration Carbon inputs

20. Testing our saturation hypothesis (Stewart et al.2008 SBB)

21. Testing our saturation hypothesis - results (Stewart et al.2008 SBB)

22. Testing our saturation hypothesis - results (Stewart et al.2009 SBB)

23. Soil C flows - a conceptual model Microbial death/turnover CO2 enzymes uptake • Available SOM • Unprotected, high molecular weight soil OM • Includes biochemically resistant OM Assimilable (low MW, dissolved OM) Mic. biomass Plant inputs microbial respiration depolymerization Adsorption/desorption Aggregate turnover leaching DOM loss • Protected SOM • Soil OM physically or chemically protected from decomposition • mineral-associated OM • occluded OM; • OM within aggregates

24. C stabilization – biochemical protection Author Pool 14C Age Anderson and Paul (1984) Non-hydrolyzable 2820 Balesedant (1987) Non-hydrolyzable (clay size) 280 Campbell et al. (1967) Non-hydrolyzable humic acids 1400 Campbell et al. (1967) Non-hydrolyzable humin 1230 Jenkinson and Rayner (1977) Acid hydrolysis residues 2560 Jenkinson (1970) Acid hydrolysis residues 1995 Martel and Lasalle (1977) Non-hydrolyzable 1530 Martel and Paul (1970) Non-hydrolyzable 1100 Trumbore et al. (1989) Non-hydrolyzable 3530 Average = 1827

25. C stabilization – biochemical protection -expectation Cessation of C inputs

26. C stabilization – biochemical protection (Paul et al. 2006 SSSAmJ)

27. C stabilization – biochemical protection – acid hydrolysis • Easily measurable • Consistent • Measurement independent of soil mineral matrix • High signal to noise NO Lost slowly • NO Slow accrual • Old NO Related to litter quality ???? Correlated with SOC decomposability

28. C stabilization – biochemical protection – acid hydrolysis in mineral-associated fraction (Plante et al. 2007 SSSAmJ)

29. C stabilization – biochemical protection We may… “view the persistence of soil organic matter as codetermined by the interaction between substrates, microbial actors, and abiotic driving variables.” “…instead of being dependent on the ill-defined ‘recalcitrance’ of molecular structures, organic matter turnover would be seen as a function of microbial ecology and the resource availability within a given physical soil environment.” (Kleber et al. 2011 GCB)

30. Soil C flows - a conceptual model Microbial death/turnover CO2 enzymes uptake • Available SOM • Unprotected, high molecular weight soil OM • Includes biochemically resistant OM Assimilable (low MW, dissolved OM) Mic. biomass Plant inputs microbial respiration depolymerization Adsorption/desorption Aggregate turnover leaching DOM loss • Protected SOM • Soil OM physically or chemically protected from decomposition • mineral-associated OM • occluded OM; • OM within aggregates

33. Our conceptual model, circa 2001 CO2 CO2 Unprotected Soil C Unprotected soil C Litter quality Physically protected Soil C Chemically protected Soil C Aggregate formation/turnover Adsorption/desorption CO2 Microaggregate soil C Silt- and clay-associated soil C Condensation/complexation CO2 Biochemically protected Soil C Resistant soil C (Six et al. 2002)

34. Some soils enriched in mineral-associated fractions

35. Small management-induced changes in mineral-associated C fractions CT NT NG

36. Different CO2 efflux rates over incubation • despite same crop, similar soil textures, similar land use histories, etc

37. Testing our saturation hypothesis (Six et al. 2002)

38. Testing our saturation hypothesis - results (Stewart et al.2009 SBB)

39. Testing our saturation hypothesis - results (Stewart et al.2008 SSSAmJ)

40. Soil carbon saturation and stabilization • Part 2: stabilization

41. Soil carbon saturation and stabilization • Part 3: implications