Week 6 – Phosphorus in terrestrial ecosystems

1 – Carbon:

• C pools and fluxes; \(\Delta C = I - kC\)
• SOC pools (DOC, POC, MAOC, PyC) with different turnover times
• Banksia sands vs Jarrah laterites: different stabilisation capacity

2 – Nitrogen:

• N cycle: fixation, mineralisation, nitrification, losses
• High litter C:N → strong N immobilisation and tight cycling
• Microbial C:N vs litter C:N controls N availability

• Phosphorus (P): essential for

  • ATP, DNA/RNA, cell membranes
  • Key for energy transfer, growth, reproduction

• No atmospheric P pool — entirely from rock weathering

• Ancient, weathered Australian soils often chronically P-limited

• Rock P (primary minerals, e.g. apatite)
• Soil P (sorbed to minerals, unavailable)
• Available P in soil solution (tiny pool)
• Organic P (in biomass and SOM)
• Plant P (in tissues)
• Sedimentary P (in lakes, oceans)
• Atmospheric P (dust, volcanic ash - negligible)

• Weathering → soil P (very slow)
• Mineralisation ↔︎ immobilisation (biological, like N and C, but P as limiting)
• Sorption ↔︎ desorption (chemical)
• Plant uptake and tight recycling

• Sedimentation and geological burial
• Erosion and runoff losses
• Fertiliser inputs (in managed systems)
• Atmospheric deposition (dust, volcanic ash - minor)

• Fe/Al oxide surfaces carry positive charges at typical WA soil pH

• Dissolved PO₄³⁻ binds tightly to these surfaces

• Initially adsorbed (reversible) → progressively occluded (irreversible)

• Result: total P can be moderate but available P is tiny (< 5 mg/kg)

Syers and Cornforth (1983)

• Deep Bassendean sands: very low total and available P

• Cluster (proteoid) roots:

  • Dense mats of fine rootlets
  • Release organic acids that dissolve P from mineral surfaces
  • Release phosphatases that cleave P from organic molecules

• High P resorption from leaves (>80%)

Lambers et al. (2014)

• Laterites: moderate total P, but strongly sorbed

• Jarrah forms ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) associations

• Mycorrhizal strategy (ECM + AM):

  • Hyphae extend beyond the root depletion zone
  • Access P in fine pores roots cannot enter
  • Some ECM fungi also produce organic acids

• Deep roots access P from less-weathered horizons

• Cleared native land → pasture:
  • Fertiliser P (superphosphate)
  • Shallow-rooted, short-lived plants
  • Higher available P in topsoil
• Consequences:
  • P leaches on sands, runs off on slopes
  • Elevated P disrupts native species adapted to low P

Soil C:P is an ecological indicator of P availability and microbial activity.

Mineral topsoils C:P 10s to a 100s → varies strongly by ecosystem and depth.

• Higher C:P → relatively lower P availability (stronger P limitation)
• Lower C:P → relatively higher P availability (P less limiting; other limits possible)

Typical patterns in the SCP sand–laterite–pasture gradient (approximate):

For each system (Banksia, Jarrah, pasture):

Part (i) (5 min)

– Fill in the Notes column

• What drives the litter and soil C:P patterns?

• P conservation strategies, fertiliser inputs

Part (ii) (10 min)

– Predict immobilisation vs mineralisation

• Compare litter C:P to microbial C:P ≈ 60:1

• Classify: microbial P limitation (low/med/high)

• Classify: P loss risk if fertiliser added (low/med/high)

• Answer the two short questions

• P is not volatilised by fire (unlike N and C)

• Ash redistributes P locally → temporary increasing available P

• Cultural burning:

  • Low-intensity, patchy burns
  • P redistributed via ash without erosion
  • Creates local P “hot spots” for regeneration