Biology · Fungal Biology & Symbioses
Decomposition, nutrient cycling and fungal networks
A patch of leaf litter becomes smaller while fungal tissue appears nearby. The lost litter is not necessarily all fungal tissue. Some carbon can remain in undecomposed material, some can enter new biomass, and some can leave as CO2. A useful explanation follows each destination across a stated interval.
Phylogeny, Classification & the Tree of Life introduces clades and the difference between similarity and ancestry. Energy, Enzymes & Metabolism supplies enzyme action, fermentation and separate material and energy accounts. Meiosis & Patterns of Inheritance supplies chromosome sets and DNA-copy accounting for the optional nuclear-cycle calculation. Qualitative explanations come before those calculations; the model's units and counting rules are introduced where they are used. Cells, Membranes & Organisation is a useful return path for membrane boundaries, and Microbial Eukaryotes & Algal Diversity introduces photosynthetic partners.
Back to the Biology mapBy the end, you can
Trace conserved carbon and nitrogen through decomposition, and distinguish network geometry, transport flows and evidence of ecological effects.
Breaking material changes its accessibility
Decomposition alters dead organic material through biological and chemical processes. Fungi contribute external enzyme activities and other mechanisms, while other organisms participate too. Cellulose, hemicellulose and lignin are different plant-wall components. Their breakdown does not use one universal enzyme or occur at an identical rate in every fungal species.
Wood-decay fungi illustrate this diversity. Familiar white-rot and brown-rot descriptions summarise different decay patterns, but comparative genomes and decay assays reveal combinations that do not fit a strict two-box mechanism. A label is a starting description; testing an activity and examining the resulting material provide more specific evidence.
A sequence associated with an enzyme can suggest capacity. It does not alone show expression, secretion, active chemistry in the substrate or a field decomposition rate. Those are successive empirical questions. Similarly, material that is visibly softened does not identify which polymer has changed or how much carbon has been respired.
Carbon and nitrogen have separate accounts
The constructed litter model begins with 100 carbon-account units. In one assigned interval, 60 are processed: 25 end in fungal material and 35 in CO2. Forty remain in unprocessed litter. The ending sum is 40 + 25 + 35 = 100. These values illustrate conservation without estimating a real conversion efficiency.
The nitrogen inventory begins with 10 separate nitrogen-account units. Six are processed: four end in fungal material and two in a released ammonium pool. Four remain in litter. The sum is 4 + 4 + 2 = 10. The carbon and nitrogen numbers have different units and cannot be added to form 110 interchangeable food units.
The released ammonium is a nitrogen-containing form outside the fungal biomass account. Whether a neighbouring plant acquires it is a further event, requiring uptake and competition with other sinks. Released material is not automatically plant nutrition. Likewise, nitrogen retained in fungal tissue can re-enter other pools later; the model ends before that turnover.
Separate destinations for carbon and nitrogen
Read the diagram in words
Carbon begins at 100; 40 remain in litter, 25 end in fungal material and 35 in CO2. The processed amount is 60. Nitrogen begins at 10; 4 remain in litter, 4 end in fungal material and 2 in ammonium. The processed amount is 6. Each element balances independently.
| Model identity | Meaning | Drawing x | Drawing y |
|---|---|---|---|
| c-litter | Initial litter: 100 | 0 | 0 |
| c-remaining | Remaining litter: 40 | 4 | -2 |
| c-fungal | New fungal material: 25 | 4 | 0 |
| c-co2 | CO2: 35 | 4 | 2 |
| From | To | Relationship | Element-account amount |
|---|---|---|---|
| Initial litter: 100 | Remaining litter: 40 | Unprocessed retention | 40 |
| Initial litter: 100 | New fungal material: 25 | Retention in new fungal material | 25 |
| Initial litter: 100 | CO2: 35 | Respired carbon | 35 |
| Assigned quantity | Value |
|---|---|
| Initial inventory | 100 |
| Amount processed | 60 |
| Assigned quantity | Value |
|---|---|
| Remaining litter: 40 | 40 |
| New fungal material: 25 | 25 |
| CO2: 35 | 35 |
| Model identity | Meaning | Drawing x | Drawing y |
|---|---|---|---|
| n-litter | Initial litter: 10 | 0 | 0 |
| n-remaining | Remaining litter: 4 | 4 | -2 |
| n-fungal | New fungal material: 4 | 4 | 0 |
| n-ammonium | Released ammonium: 2 | 4 | 2 |
| From | To | Relationship | Element-account amount |
|---|---|---|---|
| Initial litter: 10 | Remaining litter: 4 | Unprocessed retention | 4 |
| Initial litter: 10 | New fungal material: 4 | Retention in new fungal material | 4 |
| Initial litter: 10 | Released ammonium: 2 | Released nitrogen | 2 |
| Assigned quantity | Value |
|---|---|
| Initial inventory | 10 |
| Amount processed | 6 |
| Assigned quantity | Value |
|---|---|
| Remaining litter: 4 | 4 |
| New fungal material: 4 | 4 |
| Released ammonium: 2 | 2 |
Initial inventory
Amount processed during the interval
Ending pools
Different elements: separate units and accounts
One assigned interval
Downstream uptake not represented
Original closed element accounts over one interval; no measured rates, complete chemistry, downstream plant uptake, transport exports or energy recycling are implied.
A material account needs a boundary and an interval
Sixty processed carbon units is a flow during the interval. Twenty-five carbon units ending in new fungal material is an assigned stock change; 40 remaining in litter is an ending stock. Describing the time window keeps these entries comparable. Counting the 60 again alongside all three destinations would double-count the processed material.
Element conservation does not imply that usable energy returns unchanged. Fungal metabolism transfers and transforms energy, with dissipation as heat. A carbon atom can be incorporated into new organic compounds after returning to CO2, but that does not restore an earlier molecule's usable energy without a new energy input. A nutrient cycle and an energy flow require different accounts.
For a real observation, missing destinations matter. Dissolved carbon exported by water, other microbial biomass, gases and fragments carried away could all change the chosen inventory. A failed balance can reveal an incomplete boundary or measurement rather than a violation of conservation. The deliberately closed teaching account omits those routes.
A connection is a possible route, not a result
A mycelium can connect patches and support internal movement of materials. The transport model uses five positions within one assigned fungal network. Initially, 12 carbon units occupy the base. Eight pass to the junction; five continue to tip A and three continue through the intermediate position to tip B. Four remain at the base. The final inventory is 4 + 5 + 3 = 12.
Adding all arrows gives 8 + 5 + 3 + 3 = 19 passage units because some carbon crosses more than one segment. Nineteen is not the initial inventory and is not newly created carbon. The example distinguishes cumulative segment traffic from distinct material in the system. Its arrow directions are assigned flows, not ancestry or a universal rule that every hypha transports only outward.
When mycorrhizal hyphae link different plant roots, additional questions arise. Did a tracer move through fungal tissue, through soil solution, or by another route? Did the recipient obtain usable material, and did that change its growth or survival? A connection and tracer arrival do not settle every step. Network research includes evidence and debate about particular effects; neither a claim of universal benefit nor a denial of all transfer follows from geometry alone.
One inventory crossing several segments
Read the diagram in words
The initial 12 carbon units at the base supply 8 to the junction. Five continue to tip A; three continue through position B to tip B. Ending stocks are 4 at the base, 5 at tip A and 3 at tip B, totalling 12. Segment passages total 19 because some units pass through multiple segments.
| Model identity | Meaning | Drawing x | Drawing y |
|---|---|---|---|
| base | Base | 0 | 0 |
| junction | Junction | 4 | 0 |
| tip-a | Tip A | 8 | 0 |
| mid-b | Intermediate position B | 6 | 3 |
| tip-b | Tip B | 8 | 3 |
| From | To | Centreline length (drawing units) |
|---|---|---|
| Base | Junction | 4 |
| Junction | Tip A | 4 |
| Junction | Intermediate position B | 3.605551275463989 |
| Intermediate position B | Tip B | 2 |
| From | To | Relationship | Element-account amount |
|---|---|---|---|
| Base | Junction | 8 carbon units pass | 8 |
| Junction | Tip A | 5 carbon units pass | 5 |
| Junction | Intermediate position B | 3 carbon units pass | 3 |
| Intermediate position B | Tip B | 3 carbon units pass | 3 |
| Local component | Initial stock | Ending stock |
|---|---|---|
| Base | 12 | 4 |
| Junction | 0 | 0 |
| Tip A | 0 | 5 |
| Intermediate position B | 0 | 0 |
| Tip B | 0 | 3 |
| Assigned quantity | Value |
|---|---|
| Initial inventory | 12 |
| Cumulative segment passages | 19 |
Hyphal connections
Assigned transfer over one interval
Ending carbon stock
Carbon-account units
Some units cross more than one segment
Internal fungal transport; plant benefits unspecified
Constructed internal transport within one fungus, without plant-to-plant transfer, an actual flux measurement, directional growth claim or intentional sharing.
Worked example
Compare loss from litter with gain in fungus
Carbon removed from the litter account is 100 − 40 = 60. New fungal carbon is 25. Therefore the processed amount exceeds fungal gain by 35, which the model assigns to CO2. The ratio 25/60 is about 0.417 of processed carbon; 25/100 is 0.25 of the initial carbon inventory. These are different denominators.
For nitrogen, fungal retention is 4/6, or about 0.667 of processed nitrogen, while released ammonium is 2/6, or about 0.333. Neither fraction is a carbon efficiency. The different allocations show why element accounts cannot be inferred from one another simply because both involve the same litter and fungus.
Check your understanding
How much of the initial carbon inventory is in CO2 at the end of the decomposition model?
Show reasoning
35 out of 100 carbon units, or 35%. It is 35/60, about 58.3%, of the processed carbon. The denominator must be named; neither value describes a measured natural rate.
Why do the transport arrows total 19 when only 12 carbon units exist?
Show reasoning
Eight cross the first segment, and subsets cross later segments. Those passages count some units again. Ending stocks sum to 12: four at the base, five at tip A and three at tip B. Cumulative segment traffic is not a count of distinct carbon.
A tracer supplied to one plant later appears in another plant near a fungal connection. Which conclusions still need evidence?
Show reasoning
The particular transport route, uptake into the recipient's usable material, net transfer relative to other directions, and effects on growth or survival need relevant controls and measurements. Tracer arrival alone does not establish all those steps or intentional sharing.
What to carry forward
Balance each element across stated pools and intervals, and distinguish a network's connections from its traffic and ecological consequences.
References
Resources supporting this lesson’s explanations and examples.
- Extensive sampling of basidiomycete genomes demonstrates inadequacy of the white-rot/brown-rot paradigm for wood decay fungi (PNAS 111(27):9923–9928 (2014); doi:10.1073/pnas.1400592111)Robert Riley and colleaguesPrimary research paper · PDF
- Fungal evolution: diversity, taxonomy and phylogeny of the Fungi (Biological Reviews 94(6):2101–2137 (2019); doi:10.1111/brv.12550)Miguel A. Naranjo-Ortiz and Toni GabaldónSpecialist review
- Positive citation bias and overinterpreted results lead to misinformation on common mycorrhizal networks in forests (Nature Ecology & Evolution 7:501–511 (2023); doi:10.1038/s41559-023-01986-1; author correction 15 March 2023)Justine Karst, Melanie D. Jones and Jason D. HoeksemaResearch abstract
- Opinion: Response to questions about common mycorrhizal networks (Published 21 January 2025; Frontiers in Forests and Global Change 7:1512518; doi:10.3389/ffgc.2024.1512518)Suzanne W. Simard, Teresa (Sm'hayetsk) L. Ryan and David A. PerryResearcher opinion and response