Why Ecologists Need Soil Physics, and Vice Versa

1. Why Ecologists Need Soil Physics, and Vice Versa Dennis Baldocchi Ecosystem Sciences Division Dept of Environmental Science, Policy and Management University of California, Berkeley Campbell Lecture Washington State University Feb 16, 2010

2. The Big Picture • Soil Physics Drives Many of the Biological Processes in the Soil that are of interest to Ecologists and Relevant to Life in the Soil • Soil Temperature, Moisture, Trace Gas Diffusion • Ecologists Have Many interesting Questions that relate to Mass and Energy Transfer and Require Collaboration with Soil Physicists • Greenhouse Gas (H2O, CO2, N2O, CH4,) Production through Soil Evaporation and Respiration, Organic Matter Decomposition, DeNitrification and Methanogenesis

3. Outline • Flux Measurement Methods/Models • Better Experimental Design • Eddy Covariance, an alternative to Chambers • Soil CO2 probes & Fickian Diffusion • Improved Incubation Measurement Protocols • How Temperature modulates Soil Respiration • How Photosynthesis modulates Soil Respiration • How Moisture and Rain Regulates Soil Respiration & Evaporation • Lessons on Soil Evaporation Modeling using Understory Eddy Covariance Measurements

4. Soil Respiration and Evaporation: Chambers Perturb Solar Energy Input & Wind and Turbulence, Humidity and Temperature Fields

5. Scalar Fluxes Diminish with Time, using Static Chambers, due to C build-up and its negative Feedback on F Ability to measure dC/dt well is a function of chamber size and F

6. Understory Eddy Flux Measurement System: An Alternative Means of Measuring Soil CO2 and Energy Fluxes: LE and H

7. Reasonable Energy Balance Closure can be Achieved Jack pine Baldocchi et al. 2000 AgForMet

8. Design Experiment and Interpret Data by Modeling CO2 in Soil Model derived from Campbell’s ‘Soil Physics with Basic’

9. Continuous Soil Respiration with Soil CO2 Sensors and Eddy Covariance

10. Theory/Equations Moldrup et al. 1999 Fcp: fraction silt and sand; b: constant; f:porosity; e: air-filled pore space

11. Validation with Chambers Tang et al, 2005, Global Change Biology

12. Validation with Understory Eddy Covariance System Baldocchi et al, 2006, JGR Biogeosciences

13. ‘Dennis, you should perform incubation studies to interpret you field data’ Paraphrase, Eldor Paul

14. Continuous Flow Incubation System Intact soil core of known volume and density to assess water potential

15. Re-Designing Incubation Studies • Use Closed path IRGA • Data log CO2 continuously with precise time stamp to better compute flux from dC/dt at time ‘zero’. • Avoid/ exclude P and C perturbation when closing lid • Use soil samples with constrained volume • If you know bulk density and gravimetric water content, you can compute soil water potential from water release curve • Expose treatment to Temperature range at each time treatment, a la Fang and Moncreif. • Reduces artifact of incubating soils at different temperatures and thereby burning off different amounts of the soil pool • Remember F = [C]/t • Because T will be transient sense temperature at several places in the soil core.

16. Temperature and Soil Respiration

17. Soil Respiration vs Soil Temperature, measured at one depth, yields Complicated functional responses, Hysteresis and Scatter Irvine and Law, GCB 2002 Janssens and Pilegaard, 2003 GCB

18. Soil Temperature Amplitude and Phase Angle Varies with Depth It is critical to measure Soil Temperature at Multiple Depths and with Logarithmic Spacing

19. Measure Soil Temperature at the Location of the Source Otherwise Artificial Hysteresis or Poor Correlations may be Observed

20. Soil Respiration Acclimates with Temperature over Time: Cold Incubated Soils (10C) doubled their rates of respiration compared to warm soil incubations (30 C) after 140 days Curiel-Yuste, Ma, Baldocchi, 2010 Biogeochemistry

21. Savanna: Ideal Model to Separate Contributions from Autotrophs (Roots) and Soil Heterotrophs (Microbes) Under a Tree: ~Ra+Rh Open Grassland: ~Rh (summer)

22. But Sometimes Hysteresis between Soil Respiration and Temperature is Real:The Role of Photosynthesis and Phloem Transport Tonzi Open areas Soil Respiration Tang, Baldocchi, Xu, GCB, 2005

23. Soil CO2 is Greater Under Trees than Senescent Grassland Baldocchi et al, 2006, JGR Biogeosciences

24. Impact of Rain Pulse and Metabolism on Ecosystem respiration: Fast and Slow Responses Baldocchi et al, 2006, JGR Biogeosciences

25. Lags and Leads in Ps, Soil Temperature and Soil Respiration Tang et al, Global Change Biology 2005.

26. Continuous Measurements Enable Use of Inverse Fourier Transforms to Quantify Lag Times Tang et al, Global Change Biology 2005.

27. Photosynthetic Priming Effect? Phoelem Transport Model shows that Sucrose Fronts travel at ~ 2 m/hour; Blue Oak trees are 13 m tall…~6 hour lag Thompson and Holbrook 2003 J Theor Biol

28. Other Evidence that Soil Respiration Scales with GPP Understory Eddy Flux Auto Chambers Irvine et al 2005 Biogeochemistry Misson et al. AgForMet. 2007

29. Impact of Rain Pulses on Soil Respiration:

30. Sustained and Elevated Respiration after Fall Rain from Senescent Grassland

31. Rains Pulsed Do Not have Equal Impacts Xu, Baldocchi Agri For Meteorol , 2004

32. Why Do Rain pulses Produce more C from Open Grassland than Understory? Xu et al. 2003 GBC

33. What Happens to the Grass?; It is too Dry to Promote Microbial Decomposition June October

34. PhotoDegradation Can Be a Important Pathway for Carbon Loss in Semi-Arid Rangelands (~20-30 gC m-2 season-1) Rutledge et al 2010 Global Change Biology

35. Type I pulse is the first pulse occurring right after summer drought; • Type II pulse is the subsequent pulse. Data of S. Ma and D. Baldocchi, unpublished

36. Forming a Bridge between Soil Physics and Ecology: Refining Sampling and Analytical Measurements Protocols

37. Use Appropriate and Root-Weighted Soil Moisture, Not Arithmetic Average

38. Use of Root-Weighted Soil Moisture Enables a ‘Universal’ relationship between normalized Evaporation and Soil Moisture to be Observed Soil Moisture, arithmetic average Soil Moisture, root-weighted Chen et al, WRR 2009

39. Evaporation and Soil Moisture Deficits Baldocchi et al, 2004 AgForMet

40. Combining Root-Weighted Soil Moisture and Water Retention Produces a Functional Relation between lE and Water Potential Water Retention Curve Provides a Good Transfer Function with Pre-Dawn Water Potential Baldocchi et al. 2004 AgForMet

41. Understory Latent Heat Exchange Can be a Large Fraction of Total Evaporation: Baldocchi et al. 2004 AgForMet

42. Overstorey Latent Heat Exchange Partitioning: Homogeneous vs Patchy Pine Forests Baldocchi et al. 2000 AgForMet

43. LE is a Non-Linear Function of Available Energy Baldocchi et al. 2000 AgForMet

44. Why Does Understory LE Max out at about 20-30 W m-2 in closed canopies? Consider Evaporation into the Canopy Volume and feedbacks with vapor pressure deficit, D

45. Periodic and Coherent Eddies Sweep through the Canopy Frequently, and Prevent Equilibrium Conditions from Being Reached t, 10 Hz Timescale for Equilibrium Evaporation (~1000s) >> Turbulence Timescales (~200s)

46. Modeling Soil Evaporation

47. Below Canopy Energy Fluxes enable Us to Test Model Calculations of Soil Energy Exchange Baldocchi et al. 2000 AgForMet

48. Lessons Learned: 1. Convective/Buoyant Transport Has a Major Impact on Understory Aerodynamic Resistances Daamen and Simmons Model (1996)

49. Ignoring Impact of Thermal Stratification Produces Errors in H AND Rn, LE, & G Baldocchi et al. 2000 AgForMet

50. Sandy Soils Contain More Organic Content than May be Visible