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

Soils on Hillslopes

•K = a constant for a site that captures parent material effects, biological processes, abiotic processes

Flux = K • (gradient)

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:

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

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:

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

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)

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

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

Nunnock River Australia

Nunnock River Austrailia

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

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.

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

• 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)

Tennesse Valley Hillslope

•Hollow

•Erosional “noses”

Tennessee Valley Erosional Segment Soil

•A1

•biomantle

•A2

•AC

•Cr1

•Cr2

Tennessee Valley Hollow (Depositional Soil)

•A1

•A2

•A3

•A4

•AC1

•AC2

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):………..

Black Diamond

Soil on Summit

•A

•AC

•Cr

Black Diamond Shoulder

•A

•AC

•Cr

cracks

Soil thickness vs. curvature

Range in Soil Residence Times on Hillslopes

=soil thickness/prod

•Approximate range is 102 to 104 for Tenn. Valley

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)

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

Effect of hillslope processes on soil C and N cycles

CO2

CO2

Soil C cycle on flat land

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

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

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)

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

What happens to eroded C?

(Tenn Valley)

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