C, N, P Coupling in Terrestrial Ecosystems

Landscape soils and surface environments - Week 6 Workshop 2b

R.A. Viscarra Rossel & Lewis Walden

2026-03-24

Recap of week 6 so far


Tuesday Workshop 1 – Carbon:

C pools (POC, MAOC, PyC), fluxes, \(\Delta C = I - kC\)

Tuesday Workshop 2 – Nitrogen:

N cycle, C:N stoichiometry, N immobilisation in native systems

Today Workshop 1 – Phosphorus:

P cycle (no atmospheric pool), P sorption, C:P stoichiometry, cluster roots vs mycorrhizae

Learning goals


By the end of this session, you will be able to:

  • Explain why co-limitation by N and P is common in WA’s old soils

  • Use C:N:P stoichiometry to predict coupled nutrient cycling

  • Analyse how land-use change affects C, N, and P simultaneously

  • Describe how cultural burning maintains coupled C–N–P cycling

  • Apply the nutrient budget framework to management scenarios

A short video on C, N, P to get us going (2.5 min)

Why integrate C, N, and P?

  • Plants and microbes need C, N, P in fixed proportions

  • Co-limitation: shortage of N or P constrains growth, even if the other is adequate

  • C storage depends on N and P supply

  • Land-use change affects all three simultaneously

The same budget logic applies (for C, N, and P):

\[\Delta \text{nutrient} = \text{inputs} - \text{outputs}\]

Nutrient budgets: same logic applies to C, N, and P

\[ \large\Delta \text{nutrient} = \text{inputs} - \text{outputs} \]

C N P
Inputs NPP (litter, roots) Fixation, fertiliser, deposition Weathering (slow!), fertiliser
Outputs Respiration, fire, harvest Leaching, denitrification, fire Erosion, runoff, harvest

Key differences:

  • C and N can be replenished (photosynthesis, fixation)
  • P cannot—weathering is geological timescale; losses are permanent

C:N:P stoichiometry — Typical ratios (mass basis, approximate)


  • Plant biomass:

C:N:P ≈ 500 : 10 : 1

  • Soil microbes:

C:N:P ≈ 60 : 7 : 1

  • Soil organic matter:

C:N:P ≈ 100–200 : 10 : 1

Same microbial stoichiometry, just written with C, N and P together

For microbes, 60:7:1 gives:

  • C:N ≈ 60/7 ≈ 9 (we used ~10 on Tuesday)
  • C:P = 60/1 = 60 (same as in workshop 2a)

From individual elements to coupled C–N–P


Banksia / Jarrah (high C:N and high C:P)

  • Both N and P immobilised during decomposition
  • Very tight nutrient cycling
  • Low availability constrains NPP

Fertilised pasture (lower C:N and lower C:P)

  • Potential mineralisation of both
  • Higher nutrient availability
  • Higher loss risk

Nutrient limitation patterns in WA systems

Banksia (SCP sands)

  • Extremely P-limited
  • Also N-limited
  • → Co-limited

Jarrah (laterites)

  • P-limited (strong sorption)
  • N-limited to co-limited
  • Tight cycling of both

Pasture (fertilised)

  • N and P inputs via fertiliser
  • Often still P-limited (rapid re-sorption)
  • Risk of N leaching, P runoff

Co-limitation example: Adding N alone to P-poor pasture → weak response.
Adding P alone when N is low → weak response. Both needed for sustained productivity.

Land-use change: effects on C, N, P

Native woodland/forest → pasture/agriculture:

Carbon

  • Loss of biomass C
  • SOC decline (especially POC)

Nitrogen

  • Fertiliser inputs lower C:N
  • Higher leaching and gaseous losses

Phosphorus

  • Fertiliser inputs
  • P runoff and erosion
  • Sorption limits effectiveness


Net result: altered C:N:P stoichiometry, changed cycling and loss pathways

Activity (handout 15 min) — C:N:P and land-use change (parts i., ii.)

Two transitions:

  1. Banksia woodland → pasture on SCP sands
  2. Jarrah forest → fertilised restoration on laterites

For each: fill in C:N:P values (litter and soil, native vs managed).

Then answer:

  1. Largest increase in N loss risk? Why?
  2. Largest increase in P loss risk? Why?
  3. Effects on long-term soil C storage?

Discussion (5 min) — C–N–P and soil carbon


  • Which transition will improve short-term productivity more? Why?

  • Which transition most reduces long-term C storage? Why?

  • Which transition has the highest risk of nutrient losses? Why?

  • How do N and P losses feed back on SOC pools?

  • Management options that improve productivity without large nutrient losses?

Indigenous cultural burning — C, N, P together


From earlier sessions:

  • C: creates char (PyC), less C loss than intense wildfire
  • N: reduces N volatilisation, supports N-fixers
  • P: not volatilised; ash redistributes P locally

Integration:

cultural burning maintains coupled C–N–P cycling at landscape scale

Nutrient “hot spots” and landscape mosaics


Burned patches:

  • Ash → ↑ available N and P
  • Higher pH, enhanced microbes
  • Supports regeneration

Unburned patches:

  • Refugia and OM sources
  • Tight C–N–P cycling
  • No net nutrient loss

Activity (handout 15 min) — Cultural burning and C–N–P (part iii.)

Compare three patch types:

1. Recently burned

(1–2 yr post-fire)

2. Unburned

(5+ yr since fire)

3. Landscape average

(mosaic)

Fill in: Soil C, Available N, Available P (low/med/high), implication for regeneration

Think:

  1. How does the mosaic balance local fertility with landscape conservation?
  2. How does cultural burning compare to fire exclusion or intense wildfire?

C–N–P in your stakeholder assessment


How to use in your brief:

  • Diagnose current C–N–P status of the landscape
  • Describe stoichiometry patterns
  • Assess nutrient budgets qualitativelyN, P
  • Reference cultural burning where relevant
  • Connect to SDGs

For example, think about:

  • C–N–P cycling under native vegetation
  • Feedback loops and Co-limitation by N, P
  • P sorption limits on laterites
  • Cultural burning and nutrient budgets
  • Management options that maintain tight cycling

Key takeaways

  • Co-limitation by N and P is common in WA’s old soils, so managing one element alone is usually ineffective.

  • High litter C:N and C:P in native systems → tight coupled cycling; conversion to pasture/restoration changes stoichiometry and opens N and P loss pathways.

  • Cultural burning creates nutrient “hot spots” within a mosaic, while largely conserving C, N, and P at the landscape scale.

  • Nutrient budgets (inputs − outputs) provide a simple framework to think about long‑term gains and losses of C, N, and P.

  • Using C–N–P stoichiometry and budgets helps design resilient, place‑based management for SCP–Scarp landscapes and supports your stakeholder briefs.

Week 6 complete. C–N–P is the foundation for Weeks 7+.

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