6.3 - Critical Zone 2 Flashcards
How do soils differ on topography?
- Steep terrain soils are shallow and have poorly differentiates profiles
- Soils at the bottom of a slope usually have better developed horizons
- Soils at depressions, are wetter and have more soil organic matter (SOM)
Some crops grow best on hillsides
What does the slope aspect affect?
The slope aspect affects temp, moisture, SOM and weathering
How does topography affect parent material?
Topography can determine the distribution of residual, colluvial, and alluvial parent material
Residual = upper slopes, well drained
Colluvium = covers the lower portion of the slope
Alluvium = tends to fill in the valley bottoms eg. Floodplain
What is soil formation a function of?
Climate
Organisms
Relief
Parent material
Time
How does climate affect weathering?
Climate = precipitation and temperature affects the nature and intensity of weathering over large geographic areas.
Latitudinal changes in relative precipitation and temperature affect depth of weathering
How does latitude affect weathering?
Latitudinal changes in relative precipitation and temperature affect depth of weathering
As well as the primary soil type which accumulate at depth
How does precipitation affect weathering?
H2O is essential for all major chemical weathering reactions.
The effectiveness of precipitation impacts the depth of penetration into the CZ.
What does water do that causes chemical weathering?
Water transports dissolved and suspended materials from upper to lower layers
Water carries away dissolved materials as water drains out to the groundwater or rivers.
Movement of water stimulates other weathering reactions (oxidation, cation exchange) and promotes differentiation of soil horizons
What happens when there is water deficiency?
It promotes the formation of salts (eg. Evaporites)
It promotes the accumulation of carbonates (calcrete)
There is limited chemical weathering which leads to very different chemical profiles.
What happens with H2O leaching of soils
To fully promote soil development, water must not only enter the profile and participate in weathering reactions, but also percolate through and translocate soluble products.
Where will more leaching occur?
Given changes in evaporation rate, the same amount of rain will cause more leaching/soil profile development in a cooler climate.
What does a high effective annual precipitation generally lead to?
Increasing clay and organic matter contents
Higher soil acidity (lower pH)
Lower ratio of Si/AI (an indicator of greater weathering of silicate minerals)
What happens to soil when temperatures increase?
- The rates of biochemical reactions double per increase in 10 degrees Celsius
- SOM (soil organic matter) content increases via plant growth and microbial decomposition
- High temps and moisture maximises rates of weathering, leaching and plant growth
There is a vegetation selection:
In warm and humid climates there are trees
In subhumid and semiarid regions there are grasses
In arid regions there are shrubs and brush
What climate is there greater rock weathering?
Under humid conditions there is greater rock weathering (deeper weathering)
Rock weathering in arid conditions is shallower
Forests vs grasslands
In forests, organic matter is primarily at or above the surface, while grassland soils have more soil organic matter.
In addition, conifer forests are more likely to be acidic, and therefore leached at depth, with more developed soil horizons
How is there nutrient cycling by trees?
- Conifers take up and store very small amounts of Ca, Mg and K
- Conifers retain leaves (needles) and debris is released by very acidic resinous
- Conifer debris accumulates as thick O horizon
- Conifer soils typically have a low pH and are highly leached
What is pyrite weathering?
Pyrite (FeS2) is a common mineral formed in hydrothermal systems with many associated valuable metals (eg. Ag, Cu, Au, REE)
When brought to the surface (by uplift or mining), pyrite under goes oxidative weathering.
- Lots of dissolved Fe2+ turns water bright red
- Lots of dissolved H+ turns water acidic
What can grow in acidic soils?
Pine trees can grow in acidic soils
- They also create acidic soils
How does soil biota influence soil formation?
- Macro and microorganisms in soils enhance:
- Organic matter accumulation
- Chemical weathering
- Profile mixing
- Nutrient cycling
- Aggregate stability - Vegetation cover reduces natural soil erosion rates
- Organic acids released fro, plants and decay of plant litter
- bring Fe and AI into solution by complexation
- Accelerates Fe AI transport
- Promotes accumulation of secondary minerals in B horizon
Time to produce materials in soils:
SOM accumulation in A horizon = 1-2 decades
Incipient B horizon = 4+ decades
Well-structured B horizon = 100’s years
Silicate clay minerals in B horizon = 1000’s years
Deeply weathered mature profile = 100,000’s years
Young and mature soil usually refers less to age, and more to degree of profile development
What causes changes to soils over time?
- Additions = OM- plant debris, dust, salts, pollution
- Losses = leaching and erosion
- Transformations = mineral weathering and OM decay
- Translocations = movement of material between horizons
How do chronosequences help identify time-dependencies?
Soil changes slowly with respect to a soil scientist
Locations with similar biota, climate, parent material and topography help isolate the effect of time since formation
What is stage 1 of development of a soil profile?
- No layering
- OM just beginning to accumulate
- Early colonisation by pioneer plants and microorganisms
- Weathering is getting going
- As OM accumulates, water and nutrients accumulate, allowing more/faster plant growth -> positive feedback
What is stage 2 of development of a soil profile?
- Activity of earthworms, termites, ants, microbes and plant decay -> several cm of OM-rich soil
- Soluble ions (eg. Ca2+, K+, Mg2+) move down, clays forming