Root Zone Hydrology and the Rhizosphere

• Soil PAW and \(\Psi\) set the potential water supply

Today:

• Which parts of the soil actually supply that water?
• How do roots and soils co-control uptake?
• What happens when we change the vegetation?

• Soil volume that actively supplies water to plants
• Defined by function (active uptake), not just structure (where roots are present)
• Determined by root presence and hydraulic properties

Components

• Taproot vs lateral roots
• Coarse roots (support, transport); fine roots (uptake)
• Root hairs increase surface area; rhizosheaths (soil bound to roots)

Why it matters

• Controls where water and nutrients are accessed
• Sets the geometry of the effective root zone

• Most species: highest root length density near surface (~30 cm), declining exponentially with depth
• Some trees: significant fine roots in deeper horizons

Implications for water supply

• Shallow-dominated systems: rely on PAW in top 0.5 m (e.g., annual crops, shallow-rooted shrubs)
• Deep-distributed: accesses more buffered deeper storage

What limits root penetration?

• Physical barriers: compaction, hardpans, waterlogging
• Chemical barriers: aluminium toxicity, salinity
• Set the maximum depth of the effective root zone

Recall

• PAW = water held between FC and PWP
• Texture controls how much PAW there is

Now add roots

• Root distribution determines which depths contribute to uptake

• Root uptake lowers \(\Psi\) at root surface
• Radial gradient: bulk soil → rhizosphere → root

Consequence

• Water moves thru unsaturated flow to roots
• As soil dries, conductivity drops faster than gradient increases → uptake slows

• Zone of drier soil around active roots (mm to cm scale)
• Scale: mm–cm around fine roots

Controlled by

• Transpiration demand (uptake rate)
• Soil hydraulic conductivity
• Root density: soil dries faster when DZ overlap

Under drought, the rhizosphere dominates resistance to water flow—even if bulk soil still holds water.

• Soil directly influenced by living roots (mm–cm)
• Modified by exudates (carbon-rich compounds), mucilage (gel-like secretions), root growth

Water-related effects

• Altered pore geometry and aggregation
• Mucilage maintains hydraulic contact between root and soil at moderate dryness

• \(\Psi_m\) threshold below which plants cannot extract water fast enough to meet transpiration demand
• Near this, uptake declines sharply

Near critical \(\Psi_m\)

• Most of the potential drop occurs within 1–2 mm of root. The rhizosphere becomes the bottleneck for water supply

- Hydraulic properties dominate resistance

Traits affecting water uptake

• Fine root density — maximises surface area in water-rich horizons
• Rhizosheaths and mucilage — maintain hydraulic contact as soil dries
• Deep rooting — accesses buffered stores below seasonal drying front
• Plastic root growth — roots grow toward available moisture

Soil–climate context

• Deep, leached quartz sands (very low clay)
• Low PAW (~30–50 mm/m); rapid drainage
• Mediterranean climate: wet winter, 4–5 month summer drought

Hydrological implication

• Effective root zone shallow, transiently wet

• Dense fine roots in upper 0.5 m capture transient winter–spring moisture
• Some Banksia sp. send roots to shallow groundwater (phreatophytic), but most rely on shallow storage
• Strong rhizosphere specialisation (cluster roots, rhizosheaths)

Soil–climate context

• Sandy A over ferruginous B, pallid zone (retains moisture) at depth
• Higher integrated PAW over 2 m depth (100–150 mm)
• Similar climate, different storage profile

Hydrological implication

Strategy: deep miner

• Shallow laterals capture nutrients and winter rain in the A horizon
• Deep taproots penetrate laterite (via fractures) into the moist pallid zone (1–3 m depth)
• Pallid zone retains moisture through summer, which supports evergreen canopy and high ET
• Dimorphic root system: shallow roots for nutrients, deep roots for water

Clearing Jarrah → pasture (Scarp)

• Shallow roots replace deep taproots
• ET drops, pallid zone no longer accessed
• ↑ deep drainage, ↑ watertable, salinity risk

Clearing Banksia → pasture (SCP)

• Smaller change in root-zone depth
• Increased leaching of nutrients on deep sands
• Salinity risk lower ’cause deep watertable (>5 m)

Two Scarp catchments (same rainfall, geology):

• A: Intact Jarrah forest
• B: Cleared 40 years ago, pasture

Compare the two catchments: (7 min)

1. Effective root-zone depth in summer?
2. Soil moisture at 1–2 m depth in summer?
3. Which has higher recharge? Why?
4. Salinity and baseflow implications?

Share one key difference + one management implication (3 min)