The Soil-Water-Plant-Atmosphere Continuum

Weeks 1–3 (ULO1): Static patterns

• Where soils occur across landscapes
• Why they vary (formation processes, CLORPT)
• Properties determined by formation history

Weeks 4–6 (ULO2): Dynamic processes

• Soils and landscapes as living, functioning systems
• How soil, water, and vegetation interact
• Fluxes, cycles, and feedbacks

Continuous pathway for water movement driven by energy gradients

An integrated hydraulic network:

• links soil, plants and atmosphere into a functioning system.

The mechanism that couples plant transpiration to soil water availability:

• water and carbon fluxes are jointly controlled along the continuum.

Brady & Weil (2008)

Water moves continuously through connected components:

1. Soil ➡ water stored in pores
2. Roots ➡ uptake from soil into root xylem
3. Xylem ➡ transport through stem to leaves
4. Leaves ➡ evaporation from mesophyll cells
5. Atmosphere ➡ water vapor diffuses away

• Force → water potential gradient (\(\Psi\))

• Water potential (\(\Psi\)) = free energy of water

• Units: MPa (megapascals) or bars

• Direction of flow: High \(\Psi\) ➡ Low \(\Psi\)

Attraction of water to soil particles

• Capillary forces (menisci in pores)
• Adsorptive forces (water films on particle surfaces)

Key properties:

• Always negative (water held by attraction)
• Becomes more negative as soil dries
• Dominant component in unsaturated soils

Effect of elevation on water potential

• Increases +0.01 MPa per meter of elevation
• Drives vertical water movement

Examples:

• Drainage: water moves downward (gravity)
• Capillary rise: water moves up if \(\Psi\) forces > gravity

• Solutes reduce water \(\Psi\) (make it more negative)
• Plant must overcome osmotic barrier to extract water

When important:

• Saline soils (coastal areas, evaporite deposits)
• Dryland salinity (WA wheat belt, cleared Scarp)

Plant and soil water potential:

In plants:

\(\Psi = \Psi_{\text{pressure}} + \Psi_{\text{solute}}\)

In soils:

\(\Psi_{\text{soil}} = \Psi_{\text{matric}} + \Psi_{\text{gravitational}} + \Psi_{\text{osmotic}}\)

• Not all soil water is accessible to plants

Plant-available water (PAW) = water held between two thresholds

• Controls how long plants can transpire after rain stops – directly determines drought resilience and growing-season length.

• Set by texture, structure, and organic matter – sands have low PAW; clays hold more but tighter; improving structure/OM increases PAW.

• Typically 24–48 hours after saturation (e.g., heavy rain)
• Water potential: \(\Psi_m\) ≈ −0.01 to −0.03 MPa (−10 to −33 kPa)

Characteristics:

• Macropores drained (good aeration)
• Meso and micropores still filled (water available)

Soil “full” but not waterlogged

• Water potential: \(\Psi_m\) ≈ −1.5 MPa (−1500 kPa)
• Roots cannot generate sufficient \(\Psi\) gradient

Characteristics:

• Soil contains some water (not dry)
• Water held too tightly by soil matrix

Plants wilt irreversibly

The difference in volumetric water content (mm per m depth) between FC and PWP

Mechanism:

1. Root absorbs water ➡ lowers \(\Psi\) in rhizosphere
2. Creates radial gradient: bulk soil ➡ root surface
3. Water moves along gradient (unsaturated flow)

Depletion zone:

• Forms cylinder of drier soil around active roots
• Typically extends 1–5 mm from fine roots
• Depends on uptake rate, conductivity, root density

Bassendean sands:

• Soil: Quartz sands (<3% clay)
• PAW: 40–60 mm in top 1 m
• Rainfall: 700–900 mm, 5–6 month summer drought
• Water table: 5–20 m depth (inaccessible to roots)

Water dynamics:

• Winter: rain infiltrates rapidly, drains quickly
• Brief window of high \(\Psi\) (days to weeks)
• Summer: soil \(\Psi\) declines to PWP fast

Plant adaptations:

• Shallow, dense roots, 80% of biomass in top 50cm
• Proteoid roots: cluster roots (Week 5)
• Sclerophylly: thick, waxy leaves reduce transpiration
• Low stomatal conductance: conservative water use

• Soil: Sandy A horizon over clayey B (laterite)
• PAW: 150–200 mm in top 2 m
• Pallid zone: Moist kaolinitic layer at 5–15 m depth
• Rainfall: 800–1200 mm, Mediterranean climate

Water dynamics:

• Winter: A horizon saturates, B horizon may have perched water
• Summer: A horizon dries but pallid zone retains moisture

Plant adaptations:

• Dimorphic root system:
  • Shallow lateral roots (0–50 cm): seasonal rainfall
  • Deep tap roots (5–15 m): pallid zone moisture
• Higher transpiration: 100–300 L tree⁻¹ day⁻¹ (summer)
• Evergreen canopy, no summer dormancy

High conductance (wide xylem):

• ✔️ High water flow and transpiration
• ❌ High cavitation risk (vulnerable to drought)

Low conductance (narrow xylem):

• ✔️ Safer: lower cavitation risk
• ❌ Limited water flow and photosynthesis

Wet (\(\Psi\) > −0.5 MPa)

• High transpiration and photosynthesis

Moderate (−0.5 to −1.5 MPa)

• Reduced transpiration and photosynthesis

Severe (\(\Psi\) < −1.5 MPa)

• Zero growth, survival mode

Scenario: Two sites SCP both 800 mm rain

• Site A: Banksia woodland (never cleared)
• Site B: Cleared 50 years ago, now revegetated (10-year-old seedlings)

Soil differences:

• Site A: 2–3% organic matter, intact structure
• Site B: <1% organic matter, compacted surface

Questions (7 min, pairs, then share 3 min):

1. How does lower OM at Site B affect PAW?
2. How does compaction affect infiltration & roots?
3. Predict soil \(\Psi\) dynamics through summer (A vs B)
4. Why might seedlings at B experience more stress?
5. Management to improve Site B?