physical movement of soil occurs virtually everywhere root penetration shrinking/swelling of clay
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/1.jpg)
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
![Page 2: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/2.jpg)
Soils on Hillslopes
•K = a constant for a site that captures parent material effects, biological processes, abiotic processes
Flux = K • (gradient)
![Page 3: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/3.jpg)
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:
![Page 4: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/4.jpg)
A. Slope Dependent Transport: Surface Transport (p. 267)
“I t was shown in the third chapter that on Lieth Hill Common, dry earth weighing atleast 7.453 lbs. was brought up by worms to the surface on a square yard in the course ofa year. If a square yard be drawn on a hill-side with two of its sides horizontal, then it isclear that only 1/36 part of the earth brought up on that square yard would be near enoughto its lower side to cross it, supposing the displacement of the earth to be through oneinch. But it appears that only 1/3 of the earth brought up can be considered to flowdownwards; hence 1/3 of 1/36 or 1/108 of 7.453 lbs. wil l cross the lower side of oursquare yard in a year. Now 1/108 of 7.453 lbs is 1.1 oz. Therefore, 1.1 oz of dry earthwil l annually cross each linear yard running horizontally along a slope having the aboveinclination (9o 26’); or very nearly 7 lbs wil l annually cross a horizontal li ne, 100 yards inlength, on a hill- side having this inclination.”
B. Soil Creep (p. 270).
“If it could be shown that worms generally excavate their burrows at right angle to aninclined surface, and this would be their shortest course for bringing up earth frombeneath, then as the old burrows collapsed from the weight of the superincumbent soil,the collapsing would inevitably cause the whole bed of vegetable mould to sink or slideslowly down the inclined surface. But to ascertain the direction of many burrows wasfound too diffic ult and troublesome. A straight piece of wire was, however, pushed intotwenty-fi ve burrows on several sloping fields, and in eight cases the burrows were nearlyat right angles to the slope; whilst in the remaining cases they were indifferently directedat various angles, either upwards or downwards with respect to the slope
![Page 5: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/5.jpg)
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:
![Page 6: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/6.jpg)
![Page 7: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/7.jpg)
Describing soil movement downslope
Soil flux (mass/distance•time)=•K• (slope)
Where slope = dz/dx
K=distance2/time
K is affected by:
-bedrock type
-Climate (?)
-Biological type and activity
![Page 8: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/8.jpg)
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)
![Page 9: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/9.jpg)
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
![Page 10: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/10.jpg)
Nunnock River , Australia (bio-zone by ants, termites, etc.)
![Page 11: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/11.jpg)
Nunnock River Australia
![Page 12: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/12.jpg)
Nunnock River Austrailia
![Page 13: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/13.jpg)
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
![Page 14: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/14.jpg)
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.
![Page 15: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/15.jpg)
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 102 to 104 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
![Page 16: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/16.jpg)
• 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)
![Page 17: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/17.jpg)
Tennesse Valley Hillslope
•Hollow
•Erosional “noses”
![Page 18: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/18.jpg)
Tennessee Valley Erosional Segment Soil
•A1
•biomantle
•A2
•AC
•Cr1
•Cr2
![Page 19: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/19.jpg)
Tennessee Valley Hollow (Depositional Soil)
•A1
•A2
•A3
•A4
•AC1
•AC2
![Page 20: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/20.jpg)
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):………..
![Page 21: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/21.jpg)
Black Diamond
Soil on Summit
•A
•AC
•Cr
![Page 22: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/22.jpg)
Black Diamond Shoulder
•A
•AC
•Cr
cracks
![Page 23: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/23.jpg)
Soil thickness vs. curvature
![Page 24: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/24.jpg)
Range in Soil Residence Times on Hillslopes
=soil thickness/prod
•Approximate range is 102 to 104 for Tenn. Valley
![Page 25: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/25.jpg)
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)
![Page 26: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/26.jpg)
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
![Page 27: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/27.jpg)
Effect of hillslope processes on soil C and N cycles
CO2
CO2
Soil C cycle on flat land
![Page 28: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/28.jpg)
Effect of hillslope processes on soil C and N cycles
CO2
CO2
Soil C cycle on sloping land
How important is erosion on soil C cycle locally and globally?
erosion
![Page 29: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/29.jpg)
Is erosion (and burial of eroded sediments) a part (or the) residual terrestrial sink?
![Page 30: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/30.jpg)
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)
![Page 31: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/31.jpg)
![Page 32: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/32.jpg)
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
![Page 33: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/33.jpg)
What happens to eroded C?
(Tenn Valley)
![Page 34: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/34.jpg)
![Page 35: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/35.jpg)
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
![Page 36: Physical movement of soil occurs virtually everywhere Root penetration Shrinking/swelling of clay](https://reader035.vdocuments.net/reader035/viewer/2022062723/56813f35550346895da9dcce/html5/thumbnails/36.jpg)