Carbon in the Soil–Plant–Atmosphere System

All terrestrial C originates from atmospheric CO\(_2\)

The reaction:

6 CO\(_2\) + 6 H\(_2\)O + light → C\(_6\)H\(_{12}\)O\(_6\) + 6 O\(_2\)

Requires:

• Light energy
• Open stomata (for CO\(_2\))
• Water supply (for transpiration)

This is where carbon enters ecosystems

Gross primary productivity (GPP):

Total C fixed by photosynthesis.

Plant respiration (R\(_a\)):

Plants burn 30–50% of GPP for energy (growth, maintenance).

Net primary productivity (NPP) = GPP − R\(_a\)

• Carbon available for biomass growth and soil inputs.
• Units: g C m\(^{-2}\) yr\(^{-1}\)

Aboveground:

• Leaves, stems and branches
• Reproductive structures

Belowground:

• Coarse and fine roots
• Root exudates
• Mycorrhizal fungi

Allocation reflects resource limitation

Leaves: Turnover: 1–3 years (evergreens or deciduous)

• Eventually fall as litter

Stems/wood: Long-lived: decades to centuries

• Eventually fall as coarse woody debris

Reproductive structures: Flowers, fruits, seeds

• Relatively small C pool

All eventually become surface litter

Fine roots: Rapid turnover: 1–2 years

• Often 30–50% of NPP
• Decompose in place (in soil)

Root exudates: Continuous “leakage” from living roots

• Sugars, amino acids, organic acids
• 10–40% of photosynthate
• Drives rhizosphere microbes (Week 5)

Mycorrhizal fungi:

Plants allocate 10–20% NPP to symbionts

Both WA systems invest heavily belowground (c.f. fertile temperate forests)

• Nutrient-poor soils drive high root and mycorrhizal investment
• Sclerophyllous litter slows aboveground decomposition in both systems

Decomposition ➡ microbial and faunal breakdown of OM

\[R_{eco} = R_a + R_h\]

• R\(_a\) = autotrophic respiration (plants and roots)
• R\(_h\) = heterotrophic respiration (decomposers) — the primary C output from soil

What controls decomposition rate?

• Substrate quality
• Temperature
• Moisture
• Oxygen availability

Labile carbon (fast decomposition):

• Sugars, starches, simple proteins
• Decompose in days to weeks

Recalcitrant carbon (slow decomposition):

• Lignin, cellulose, waxes
• Decompose in months to years (decades)

C:N ratio:

• C:N >30: Slow (microbes N-limited)
• C:N <20: Fast (microbes have enough N)

Temperature sensitivity:

• Decomposition rate approx. doubles every 10\(^o\)C
• Q\(_{10}\) ≈ 2

Implications:

• Tropical soils (hot): very fast decomposition
• Boreal soils (cold): very slow decomposition
• Mediterranean (e.g. WA): seasonally variable

Ma et al. (2025)

Moisture:

• Optimal: Near field capacity
• Too dry: Microbial activity limited
• Too wet: Anaerobic conditions (slow decomposition)

Oxygen:

• Aerobic respiration: Fast, efficient
• Anaerobic respiration: Slow, incomplete

Three possible outcomes:

1. Inputs > Outputs → Accumulation
2. Inputs < Outputs → Loss
3. Inputs ≈ Outputs → Steady state

Water use efficiency (WUE)

• Open stomata → more photosynthesis, more water loss
• Closed stomata → less water loss, little photosynthesis
• Drought: WUE can rise, but total C gain usually falls

WA context

• Mediterranean climate → summer stomatal closure
• Banksia and Jarrah: high WUE under summer drought
• Hot, dry summers: very low carbon uptake
• Carbon gain concentrated in cooler, wetter months

Example calculation:

• Soil C stock: 10 kg C m\(^{-2}\) (top 1 m)
• Annual output (R\(_h\)): 400 g C m\(^{-2}\) year\(^{-1}\)
• Mean residence time: \(\tau\) = 10,000 / 400 = 25 years

Reality is more complex:

• Labile pool: \(\tau\) ~ months to years
• Stable pool: \(\tau\) ~ decades-centuries

High productivity, low soil C

• NPP: 1500 g C m\(^{-2}\) year\(^{-1}\) (very high)
• Decomposition: Very fast (warm + wet)
• Litter residence time: ~1 year
• Soil C stock: 5–10 kg C m\(^{-2}\) (moderate)

Most carbon in living biomass (trees), not soil

Low productivity, very high soil C

• NPP: 300 g C m\(^{-2}\) year\(^{-1}\) (low)
• Decomposition: Very slow (cold + waterlogged)
• Litter residence time: decades to centuries
• Soil C stock: 30–60 kg C m\(^{-2}\) (extremely high)

Carbon accumulates because outputs << inputs

Low inputs, slow outputs, moderate stocks

• NPP: 400 g C m\(^{-2}\) year\(^{-1}\) (low)
• Decomposition: Slow (dry summer + recalcitrant litter)
• Soil C stock: 3–5 kg C m\(^{-2}\) (moderate)
• Steady state: slow inputs balanced by slow outputs

Fire periodically resets the balance, but also creates stable charcoal

Moderate inputs, moderate outputs

• NPP: 800 g C m\(^{-2}\) year\(^{-1}\) (moderate)
• Decomposition: Moderate (Mediterranean climate)
• Soil C stock: 5–8 kg C m\(^{-2}\) (higher than SCP)
• Deeper A horizon (lateritic profile)

Fire + charcoal contribute to stable C pool

Jarrah forest → agricultural land

Three mechanisms drive C loss:

1. NPP inputs stop
2. Decomposition increases (tillage, warmer soil)
3. Aggregate breakdown exposes protected C

Some systems never fully recover

1. Forest removal — transpiration stops
2. Recharge increases — water table rises
3. Saline groundwater reaches surface
4. Salinity inhibits revegetation
5. C inputs cannot recover — persistent C loss

Positive reinforcement:

1. High soil C ➡ good structure, water retention, nutrients
2. Supports productive vegetation ➡ high NPP
3. High C inputs ➡ maintains/increases soil C
4. Cycle reinforces itself

Example: Temperate deciduous forests on loess soils

Negative reinforcement:

1. Low soil C ➡ poor structure, low water retention, low nutrients
2. Supports only low-productivity vegetation ➡ low NPP
3. Low C inputs ➡ soil C cannot rebuild

Example: Cleared SCP sands

Fire management:

• Shapes fire-adapted vegetation
• Maintains ecosystem structure
• Manages fuel loads

Carbon impacts:

• Combustion: C → CO₂
• Charcoal: Stable C formation
• Regrowth: Vegetation recovery

Low frequency (>10 years)

• Fuel accumulates
• High-intensity fires
• Large C loss per event
• Long recovery

High frequency (<5 years)

• Limited fuel
• Low-intensity fires
• Small C loss per event
• Frequent cumulative loss

Cultural burning (Indigenous practice)

• Frequent, low-intensity
• Mosaic patterns
• Moderate total C loss
• Prevents catastrophic fires
• Maintains biodiversity

Discussion (5 min):

1. How do different fire frequencies and intensities affect carbon balance?

2. Why might cultural burning lead to less carbon (C) loss than large wildfires?

3. How does fire change vegetation and soil in ways that affect soil C?

4. What lessons about managing Country come from Indigenous fire practice?