Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay

1. Soil Processes on HillslopesBased 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 Flux = K • (gradient) K = a constant for a site that captures parent material effects, biological processes, abiotic processes

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

4. 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:

6. Describing soil movement downslope Soil flux (mass/distance•time)=•K• (slope) Where slope = dx/dz K=distance2/time K is affected by: bedrock type Climate (?) Biological type and activity

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

8. Soil thickness on hillslopes: Key Implications: Soil thickness is proportional to land curvature on hillslopes Soil production rate is modulated by soil thickness

9. Nunnock River Australia

10. Nunnock River Austrailia

11. 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

12. 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. OBJECTIVES OF THIS COMPARISON Typical soil thickness on hillslopes Typical soil residence time on hillslopes Soil profiles on hillslopes and hollows Effect of erosion/deposition on soil organic matter

13. Summary of Objectives of Hillslope Soil Discussion OBJECTIVES OF THIS COMPARISON Typical soil thickness on hillslopes Convex areas: soils < 1 m Concave areas: soils variable but much thicker Typical soil residence time on hillslopes Varies with k (rate of downslope movement) Varies with rate of soil production from rocks Range is 102 to 104 years for soils on convex areas 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 Effect of erosion/deposition on soil organic matter Globally significant

14. 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)

15. Tennesse Valley Hillslope Hollow Erosional “noses”

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

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

18. 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):………..

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

20. Black Diamond Soil on Summit A AC Cr

21. Black Diamond Shoulder A AC Cr cracks

22. Black Diamond

23. Soil thickness vs. curvature

24. Tennessee Valley Curvature • soil thickness declines with increasing curvature • Soil residence time increases with soil thickness • Age/weathering of soil particles related to slope position and distance they have traveled (ie. Material becomes more weathered with distance from nose ridge)

25. Range in Soil Residence Times on Hillslopes =soil thickness/prod Approximate range is 102 to 104 for Tenn. Valley

26. Comparison to other watersheds : Black Diamond differs due to higher production rates from soft rock

27. 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)

28. 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 102 to 105 years Transport rate (and time on downslope travel) varies around same time range

29. Effect of hillslope processes on soil C and N cycles Soil C cycle on flat land CO2 CO2

30. Effect of hillslope processes on soil C and N cycles Soil C cycle on sloping land How important is erosion on soil C cycle locally and globally? CO2 CO2 erosion

31. Soil Carbon in Global Perspective • Two main anthropogenic C inputs are form fossil fuel and soil/plants • Main C sinks are atmosphere, oceans and (???) ecosystems

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

33. Erosion in soil C model: C(t) = I - (kd+ke)C Css= I/ (kd+ke) Where kd = decomposition constant and ke= 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)

35. Global Scale Effects: Ball Park Estimates of Natural Rates Global uplands draining to oceans = 90 x 1012 m2 Soil C loss = ~ 5 g C m-2yr-1 Total C flux= ~ 0.5 Gt yr-1

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

38. 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

41. Summary of erosion and C cycle Natural erosion is an uncharacterized C flux that has never been incorporated into global C budget Maybe on order of 0.5 Gt (very crude estimate) Accelerated erosion is large enough to account for much or most of the “missing” anthropogenic CO2 Stallard puts range of 0.6 to 1.5 Gt year-1 If erosion of OM is important, it should have measureable effects on N cycle for example:

42. Effect of hillslope processes on soil C and N cycles Atm deposition and N fixation 15N/14N = 0 o/oo As erosive losses vs. microbial losses increase, N isotope composition should approach that of atmospheric inputs…. Erosion 15N/14N = 0 o/oo relative to soil N Nitrate, N gases: 15N/14N = ~ - 15 to 30 o/oo relative to soil N

43. Soil N at Tennesse Valley Conforms to this hypothesis As slope increases, erosion rates increase and soil N isotope values approach plausible range of atm inputs Upper limit of inputs