1. Soil Processes on Hillslopes Based on work by Arjun Heimsath (Dartmouth), Bill Dietrich (UCB), and Kyungsoo Yoo (UCB) Physical movement of soil occurs virtually everywhere –Root penetration –Shrinking/swelling of clay –Earthworms –gophers Hillslopes are special environments in that a driving gradient (gravity) exists to cause a NET movement downslope

2. Soils on Hillslopes K = a constant for a site that captures parent material effects, biological processes, abiotic processes Flux = K (gradient)

3. Importance of Biological Mixing/Movement Processes Charles Darwin: earthworms –10,500 kg soil ha -1 yr -1 –  = ~700 years for upper 50 cm –  is consistent with archaeological observations of Roman ruins:

5. Other Bioturbation Examples Earthworm invasion into Canada mixed upper 10 cm of soil in 3 years Upper 75 cm of soil in San Joaquin Valley mixed by ground squirrels in 360 years Formation of “Mima mounds” of Great Valley:

7. Describing soil movement downslope Soil flux (mass/distancetime)= K (slope) Where slope = dz/dx K=distance 2 /time K is affected by: -bedrock type -Climate (?) -Biological type and activity

8. Soil profiles on hillslopes are affected by net soil movement: input-outputs… Soil mass = erosion in + soil production - erosion out Slope in Slope out Soil production from rock Difference in slope (in vs. out) is curvature (derivative of slope)

9. Soil thickness on hillslopes: Key Implications: 1. Soil thickness is proportional to land curvature on hillslopes 2. Soil production rate is modulated by soil thickness Soil thickness ~ curvature (x other variables) Soil production ~ soil thickness ~ curvature

10. Nunnock River, Australia (bio-zone by ants, termites, etc.)

11. Nunnock River Australia

12. Nunnock River Austrailia

13. The linkage between erosion and deposition on hillslopes Sites with negative curvature (increasing slope) are erosional Sites with positive curvature are depositional (hollows) -experience continous deposition -Experience periodic evacuation due to landslides

14. Soil properties on Bay Area hillslopes Tennesse Valley (Marin County) Sandstone pm Gopher bioturbation Black Diamond State Park (Contra Costa County) Shale pm Few gophers, shrink swell impt.

15. Summary of Objectives of Hillslope Soil Discussion OBJECTIVES OF THIS COMPARISON 1. Typical soil thickness on hillslopes Convex areas: soils < 1 m Concave areas: soils variable but much thicker 2. Typical soil residence time on hillslopes Varies with k (rate of downslope movement) Varies with rate of soil production from rocks Range is 10 2 to 10 4 years for soils on convex areas 3. Soil profiles on hillslopes and hollows Convex soils have no or weak B horizons (commonly Bw) Soils on nearby flat areas (no curvature) can have Bt Soils in concave areas have over-thickened A horizons due to accumulation of sediment (burial of A horizons) and eroded OM 4. Effect of erosion/deposition on soil organic matter Globally significant

16. production rates and transport (K) not necessary related K and prod not necessarily related to precipitation Bedrock important for production (shale>sandstone>granite) K related to process (shrink/swell>wombats/ants/termites>gophers>earth worms)

17. Tennesse Valley Hillslope Hollow Erosional “noses”

18. Tennessee Valley Erosional Segment Soil A1 biomantle A2 AC Cr1 Cr2

19. Tennessee Valley Hollow (Depositional Soil) A1 A2 A3 A4 AC1 AC2

20. Conceptual View of Tennessee Valley Soils (and all others) Rate of downslope movement may not be constant with depth Rate depends on biological/physical mixing processess Extensive mixing by gophers at Tenn. Valley suggest rates are somewhat constant with depth (soils lack a Bw horizon) (compare to Australia w/ lower production rates and bio-mixing near surface):………..

21. Black Diamond Soil on Summit A AC Cr

22. Black Diamond Shoulder A AC Cr cracks

23. Soil thickness vs. curvature

24. Range in Soil Residence Times on Hillslopes  =soil thickness/prod Approximate range is 10 2 to 10 4 for Tenn. Valley

25. Time that (some) soil material has weathered on downslope path There is an order of magnitude difference in transport rates between sites Shrink-swell relatively more effective than gophers Velocity=(K)(soil thickness)(slope)

26. Summary of Soil Physical Processes and Properties on Hillslopes Soil production varies with bedrock, etc. Soil ‘diffusivity’ varies with transport mechanism –Varies with soil depth Soil thickness/morphology reflect rapid movement –Soils approx. 50 cm or so thick –May lack B horizons entirely Soil residence time 10 2 to 10 5 years Transport rate (and time on downslope travel) varies around same time range

27. Effect of hillslope processes on soil C and N cycles CO 2 Soil C cycle on flat land

28. Effect of hillslope processes on soil C and N cycles CO 2 Soil C cycle on sloping land How important is erosion on soil C cycle locally and globally? erosion

29. Is erosion (and burial of eroded sediments) a part (or the) residual terrestrial sink?

30. Erosion in soil C model: C(t) = I - (k d +k e )C C ss = I/ (k d +k e ) Where k d = decomposition constant and k e = erosion constant At Tennessee Valley: Inputs (grass production)= ~ 100g C m -2 yr -1 I = (kd + ke)C Erosive C losses= ~ 5 to 15 g C m -2 yr -1 (~5 to 15% of total)

32. Global Scale Effects: Ball Park Estimates of Natural Rates Global uplands draining to oceans = 90 x 10 12 m 2 Soil C loss = ~ 5 g C m -2 yr -1 Total C flux= ~ 0.5 Gt yr -1

33. What happens to eroded C? (Tenn Valley)

35. Humans and Accelerated Erosion (R. Stallard, 1998) Cultivation enhances natural rates of erosion by an order of magnitude Accelerated erosion generally considered detrimental –Loss of A horizon (N, P, etc) –Eutrophication of lakes and rivers Accelerated erosion may have positive impact on C cycle –Erosion of C in soil compensated by accelerated inputs via farming Part of reason soil C declines after farming starts Eventually inputs compensate for losses –Much of eroded soil never leaves immediate area Floodplains Basins Lakes/dams