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:

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 = dx/dz

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€

h =1

α(−ln(

ρ sρ p

netdiffusiveflux

soilproductionrate@0)

whereα = constan trelevanttolocation

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.

OBJECTIVES OF THIS COMPARISON

1. Typical soil thickness on hillslopes

2. Typical soil residence time on hillslopes

3. Soil profiles on hillslopes and hollows

4. Effect of erosion/deposition on soil organic matter

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):………..

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

Black Diamond

Soil on Summit

•A

•AC

•Cr

Black Diamond Shoulder

•A

•AC

•Cr

cracks

Black Diamond

Soil thickness vs. curvature

Tennessee Valley Curvature

• soil thickness declines with increasing curvature

•Soil residence time increases with soil thickness

•Age/weathering of soil Age/weathering of soil particles related to slope particles related to slope position and distance they position and distance they have traveled (ie. Material have traveled (ie. Material becomes more weathered becomes more weathered with distance from nose with distance from nose ridge)ridge)

Range in Soil Residence Times on Hillslopes

=soil thickness/prod

•Approximate range is 102 to 104 for Tenn. Valley

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

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

€

SoilThickness∝ (−ln(1

productionoK(curvature))

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

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

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

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:

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….

Erosion15N/14N = 0 o/oo relative to soil N

Nitrate, N gases:

15N/14N = ~ - 15 to 30 o/oo relative to soil N

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