The temperature sensitivity of decomposition of soil organic matter Ivan Janssens & Eric Davidson

1. The temperature sensitivity of decomposition of soil organic matter Ivan Janssens & Eric Davidson

2. Acknowledgments Earlier works by: Liski; Ågren; Giardina; Ryan; Trumbore; Knorr; Fang; Reichstein; Cotrufo; several others Special thanks to Pete Smith

3. Everybody’s tired. Let’s make a deal …

4. What we think we know: Mechanism = Arrhenius kinetics Mechanism = Arrhenius kinetics What we do not know: Mechanism for Q10  Q10 of labile vs recalcitrant SOC What we know from observations T : Decomp rate  Q10  Drought: Decomp rate  Q10  Decomposition rate labile SOC> recalcitrant SOC

5. incomplete What we think we know: Mechanism = Arrhenius kinetics Mechanism = Arrhenius kinetics What we do not know: Mechanism for Q10  Q10 of labile vs recalcitrant SOC What we know: T : Decomp rate  Q10  Drought: Decomp rate  Q10  Decomposition rate labile SOC> recalcitrant SOC Objectives Explain drought effects on rate and Q10 mechanistically Provide new insight on Q10 and recalcitrance

6. The temperature sensitivity of decomposition and chemical laws 1884: van ‘t Hoff : exponential temperature response = Q10 function = incorrect but good approximation Still OK for interpolation, but not a chemical law, so not OK for extrapolation

7. 1887: Arrhenius: Post-doc with van ‘t Hoff The temperature sensitivity of chemical reactions is given by:

8. Q10 goes down with temperature • Q10 is higher for recalcitrant compounds with higher Ea (quality determines Q10) Activation energy (kJ mol-1)

9. Ågren & Bosatta, 1999; Fang et al., 2005; Knorr et al., 2005; Reichstein et al., 2005 Activation energy (kJ mol-1)

10. However, … Decomposition = enzymatic process that follows Michaelis-Menten kinetics (1913) Davidson et al., Global Change Biology, in press

11. Characteristics : At high [S], Km = insignificant and Q10 of R = Q10 of Vmax (Arrhenius kinetics) At low [S] : Km becomes important Also Km increases with Temp At low [S]: Q10 of R << Q10 of Vmax Davidson et al., Global Change Biology, in press

12. Q10 always goes down with temperature Davidson et al., Global Change Biology, in press

13. Under low [S], decomposition rates decrease Davidson et al., Global Change Biology, in press

14. Thus: in agreement with observations, Michaelis-Menten explains T effects on rates and on Q10 of decomposition ! So What ?? M-M-kinetics also prescribe reduced Q10 as soil carbon stores decline

15. Under low [S], Q10 goes down Davidson et al., Global Change Biology, in press

17. Global Relevance Michaelis-Menten kinetics suggest that as climate warming depletes available substrates in a SOC pool Q10 of decomposition of that pool will decrease

18. M-M : substrate depletion will lower Q10

19. + if labile pools deplete faster than recalcitrant pools • relative enrichment of recalcitrant SOC • apparent Q10 of total SOC decomposition may increase (Knorr et al. 2005)

20. Integrated response could go either way

21. Part IIHow to avoid confounding effects in soil carbon models-intrinsic versus apparent Q10 • Intrinsic Q10 = Temperature sensitivity when everything is constant except temperature • Apparent Q10 = Temperature sensitivity when other factors covary with temperature

22. Both rates and Q10 are affected by covarying substrate availability  APPARENT Q10

23. Apparent Q10 = What we measure ?? Is it what is needed ?? • Apparent Q10 = the one that matters : a simple molecule may be locked inside an aggregate and thus be very recalcitrant with a Q10 = 1, although the intrisic Q10 is much higher. • However, … observed apparent Q10cannot be extrapolated to future climate because constraints/primers of decomposition may respond differently to climate change

24. APPARENT ST

25. Towards a new conceptual approach to model decomposition: We suggest to incorporate a M-M function 1 : Substrate quality determines Ea of Vmax 2 : Need to separately predict future changes in [S] (not in [SOC] !!!)

26. Substrate quality Ea Climatechange Apparent Q10 Intrinsic Q10 [S] [SOC] constraints Sorption/desorption to mineral surfaces Future decomposition COV ([S], T) Q10 of adsorption/release

27. Substrate quality Ea Climatechange Apparent Q10 Intrinsic Q10 [S] [SOC] constraints Sorption/desorption to mineral surfaces Aggregate formation/breakdown Permafrost Future decomposition

28. Why does Q10 decline with drought ? Can this be explained mechanistically ? • Decomposition is extracellular process • Enzymes have to diffuse to [S] • Soluble [S] have to diffuse to cell • Diffusivity in soils depends on thickness of the water film • Drought = lowering [S] availability

29. Reason for declining Q10 with drought: = decline in [S] Km becomes relatively more important Michaelis-Menten kinetics prescribe lower rates and lower Q10

30. Substrate quality Ea Climatechange Apparent Q10 Intrinsic Q10 [S] [SOC] constraints Sorption/desorption to mineral surfaces Aggregate formation/breakdown Drought Permafrost melting Future decomposition

32. Global relevance #2 Climate change may reduce [SOC] But still enhance [S] and thus decomposition rates and their Q10 Need to separately model [S] if we want correct extrapolations

33. M-M : substrate depletion will lower Q10

34. M-M : increase in [S] would increase Q10

35. Conclusion: Future temperature sensitivity of decomposition of soil organic matter We don’t know yet, but cooperation will get us there

36. Thank you Competivity enhances productivity at the expense of creativity and quality

37. CONCLUSIONS • M-M kinetics should be the base of SOC models, because decomposition is an enzymatic process • M-M kinetics can explain all drought-related responses via a reduction in [S] due to reduced diffusion rates • Fundamental chemical principles suggest that any intrinsic and apparent Q10 of decomposition is possible

38. CONCLUSIONS (2) • Climate sensitivity of constraints to decomposition are at least of equal importance as the Q10 determined by the chemical nature of the substrate • Recalcitrant substrates with high “apparent” Ea may intrinsically be very labile (glucose in aggregate) • All of this is theory, knowledge is lacking, testing and implementation in joint research project ??

39. Part IIDrought effects on decomposition • Rates decline = OK in current models • Q10 declines = not OK in current models

41. Typical models fail to reproduce this effect of drought on Q10

42. Typical model and model that accounts for drought effect on Q10 differ strongly outside of current climate = Important Not under current climate because models always perform well under conditions at which they have been calibrated But not necessarily outside of these conditions 1.0 -4 -3 -2 -1 0 0.8 1 2 3 4 5 0.6 6 0.4 RWC 0.2 0.0 5 10 15 20 25 30 35 40 Temp

43. Labile recalc Total Pool size 18 2 20 Q10 2 4 2.2 Pool size 8.2 1.8 10 Q10 1.5 3.8 1.9 Pool size 3.2 1.8 5 Q10 1.3 3.8 2.2