Nitrogen in terrestrial ecosystems

Learning Goals – Nitrogen

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

Explain the nitrogen cycle and key soil N processes (fixation, mineralisation, nitrification, denitrification)
Describe how C:N stoichiometry controls microbial immobilisation vs mineralisation and affects N availability
Compare N cycling across SCP sands, Jarrah laterites, and pasture systems, linking to carbon storage from Hour 1
Discuss how fire regime influences N losses and retention in WA landscapes
Connect soil N dynamics to SDGs and sustainable land management

Nitrogen in Landscapes and Soils

From carbon to nitrogen
N as a key control on productivity and C cycling
Soil N processes: fixation, mineralisation, nitrification, denitrification
Focus: SCP–Scarp systems and C:N

Why nitrogen matters for ecosystems

Important

N is critical for the survival of all living organisms. It is most important in regulating productivity and species diversity in terrestrial and aquatic ecosystems.

Nitrogen (N): essential for:
- Proteins and enzymes
- DNA/RNA
- Chlorophyll (photosynthesis)

Often limits terrestrial productivity
In Australia: P often most limiting, but N can also limit, especially post‑fire

The nitrogen cycle

Major pools:

Atmosphere: N₂ (not directly usable by most plants)
Soil organic N: in SOM and microbes
Soil inorganic N: NH\(_4^+\), NO\(_3^-\)
Plant biomass: proteins, nucleic acids

Major fluxes: fixation, mineralisation (ammonification), nitrification, assimilation, denitrification, leaching

Mineralisation releases NH₄⁺ Microbes decompose C → CO₂. C and N cycle together.

Key soil N processes

Fixation:
N₂ → NH₄⁺ (by bacteria — Rhizobium in legume nodules, free‑living soil bacteria)
Mineralisation:
organic N → NH₄⁺ (released during C decomposition)
Nitrification:
NH₄⁺ → NO₃⁻ (aerobic bacteria; NO₃⁻, mobile and leachable, unlike NH₄⁺)
Denitrification:
NO₃⁻ → N₂, N₂O (in wet/anaerobic zones; N₂O is a potent greenhouse gas)

Plant uptake, resorption, and N conservation

Uptake:
- Plants take up: NH\(_4^+\), NO\(_3^-\), sometimes small organic N
Resorption: recover N from leaves before they fall
- Removes N → leaf C:N rises → litter arrives N‑poor
Australian sclerophylls:
- High resorption efficiency
- Conserves N in biomass
- Results in high C:N litter reaching the soil

Joly (2021)

Nitrogen losses from ecosystems

Carbon-to-nitrogen ratio (C:N) - A simple diagnostic tool

\[ \large \text{C:N} = \frac{\text{carbon}}{\text{nitrogen}} \]

High C:N

e.g. 60:1 C-rich, N-poor
Slow decomposition
Microbes immobilise N

Low C:N

e.g. 15:1 more balanced
Fast decomposition
Microbes release N (mineralisation)

Microbial biomass C:N

Typically around 8–10:1
the reference point

N in WA landscapes – old, nutrient‑poor soils

Swan Coastal Plain (SCP)

Deep, leached sands
Banksia woodland, low biomass
High litter C:N, tight N cycling

Darling Scarp – Jarrah forest

Lateritic profiles, low available N
Moderate biomass, high litter C:N
Deep roots, evergreen canopy

Many WA systems: low N availability and strong N conservation

Banksia woodland – N characteristics

SCP Bassendean sands:
- Low total N and SOM
- High litter C:N, slow mineralisation
N sources:
- Atmospheric deposition (small)
- Biological N fixation (Acacia, legumes)
N cycling:
- Tight, with strong immobilisation in decomposing litter

Jarrah forest – N cycling on laterites

Jarrah (Eucalyptus marginata) on laterites:
- Moderate biomass, low N (and P)
- Deep roots and evergreen canopy ⬇ N leaching
N cycling:
- Low external inputs
- Low stream N losses
- Efficient retention and internal recycling

Pasture systems – N inputs and losses

Cleared Banksia/Jarrah land → pasture:
- Fertiliser N additions
- Short‑lived, shallow‑rooted plants
Consequences:
- Higher NO₃⁻ in soil water
- Greater risk of N leaching and gaseous loss
- Lower C:N in litter and soil

Litter and soil C:N along the SCP–Scarp

Typical patterns:

Banksia:
litter C:N ~60–80, soil C:N ~20–30
Jarrah:
litter C:N ~40–60, soil C:N ~15–25
Pasture:
litter C:N ~20–30, soil C:N ~10–15

Microbial C:N and the immobilisation–mineralisation balance

Microbial biomass C:N ≈ 8–10:1

When litter C:N is higher than microbial C:N:

Microbes are N‑limited
Net immobilisation (microbes take soil N)

When litter C:N is close to microbial C:N:

Enough N for growth
Net mineralisation (release of N)

Worked example – litter C:N and N availability

Microbial requirement: C:N = 10:1

1: Banksia litter, C:N = 60:1

For 60 units C decomposed:
- Microbes need 6 units N
- Litter supplies 1 unit N
- Extra 5 units N must come from soil → immobilisation

2: Pasture litter, C:N = 25:1

For 25 units C decomposed:
- Microbes need 2.5 N
- Litter supplies 1 unit N
- 1.5 units short → immobilisation, but less severe

3: Pasture litter, C:N = 15:1

For 15 units C decomposed:
- Microbes need 1.5 N
- Litter supplies 1 unit N
- Close to balance → potential mineralisation
- Risk of leaching

Demo C:N – mineralisation vs immobilisation

Click here to access the interactive model

or copy into your browser: https://ravr19.github.io/lsse_teaching/cn_miner_app.html

In the app, see how :

changes in litter C:N and microbial growth fraction affect N mineralisation vs immobilisation

Test for different ecosystems:

Banksia sands vs. Jarrah laterites vs. pasture
Answer the three questions.

Indigenous fire and nitrogen

Hot wildfire: high temperatures oxidise organic N to NOₓ and NH₃ → major N loss to atmosphere
Indigenous cultural burning:
- Lower‑intensity, patchy burns
- Cooler flames → less N volatilisation, more char
- Supports N‑fixers (Acacia) and tight nutrient cycling

- Creates local nutrient “hot spots” without landscape‑scale loss

Linking N back to carbon storage

N availability is a key control on carbon storage in soils and vegetation

Low N availability
→ limits NPP and litter inputs (Recall \(I\) in \(\Delta C = I - kC\))

High litter C:N
→ slow decomposition, N immobilisation → affects which C pools build up (POC vs MAOC)

N losses via fire or leaching
→ reduce long‑term N capital → constrain future C storage

Key takeaways

N cycles through fixation, mineralisation, nitrification, uptake, and losses (leaching, denitrification, fire) in WA’s old, nutrient‑poor landscapes.
High litter and soil C:N in Banksia and Jarrah systems → strong microbial N limitation and tight N conservation, while pasture has lower C:N and higher N loss risk.
Microbial C:N vs litter C:N controls net mineralisation vs immobilisation, shaping N availability and linking N cycling to C decomposition and storage.
Indigenous cultural burning moderates N losses in fire → maintain resilient C–N cycling.

Tomorrow: We add phosphorus and then integrate C–N–P.