1. Organic carbon is critically important for soil health but is often depleted, especially in agricultural soils. 2. Certain fungi, especially arbuscular mycorrhizal and melanin-producing endophytic fungi, can help restore organic carbon levels by developing soil structure and depositing stable polyphenolic compounds like melanin within soil aggregates. 3. Inoculating soils with select fungi may be a more effective strategy for long-term sequestration of organic carbon compared to methods like adding compost or leaving soils fallow.

1. Peter McGee
School of Biological Sciences
University of Sydney

2.  Most topsoil in Australia is shallow and ancient.
Organic Carbon (OC) has been lost esp from A
horizon of cropping soil. Global problem.
 OC is the most critically important factor for
sustainable use of soil. Can OC be restored to soil?

3. FUNGUS %OC
CONTROL 6.7 a
222 7.7b
347 7.6b
367 7.9b
Quite clearly YES, but HOW?

4. Adding compost, and using no-till or green
manure crops increases organic matter and
necessarily increases organic carbon.
WRONG
RESPONSE: Review by Goaverts et al 2007
showed occasional increase, occasional
decrease but mostly no change in OC.
ie. conservation agriculture has unpredictable
impact on OC in soil.

5. Stable, long-lived polyphenolic and
polyaromatic material physically protected
within micro-aggregates.
OC is a fraction of OM. Well, lignin is a
polyphenolic and lots enters the soil?

6. The main polyphenolic of plants (lignin) is the
source of OC in soil.
WRONG
RESPONSE: Lignin is NOT the main constituent
of OC. Lignin is polyaromatic but it is left in
oxic zone. Lignin is oxidised rapidly
(oxidative enzymes or directly). Labelling
experiments show lignin is never found in
stabilised OC fraction of the soil.

7.  Hydrolysis - process results in food and
minerals for microbes. Takes place in
Aerobic or anaerobic
 Oxidation – action of the highly reactive
oxygen on recalcitrant compounds eg
polyphenolics. Microbes (mostly fungi)
release oxidative enzymes. Carbon dioxide is
released immediately. Oxidation does not
provide food for microbes: REQUIRES OXYGEN

8.  Carbon cycle: Microbes decompose, transform
and deposit organic materials, eg some fungi
deposit melanin, a complex polyphenolic
compound, in their walls.
 Fungi develop soil structure (aggregates and
pore spaces) where OC is protected (from
oxygen)
Can we use these features to store OC?

9.  Empirical research strongly supports storage
of OC in protected aggregates.
 Aggregates the “structure” of soil structure.
 Hierarchical model of soil structure (Tisdall &
Oades 1982) – specifically mentions organic
materials, plant roots and hyphae (AM fungi).

10. Arbuscular mycorrhizal (AM) fungi
associate with most agric plants.
(AM) fungi constitute some 70% of
all microbial biomass in soil.
Hyphae of AM fungi are 0.5 to 5m
per g of soil.
DO AM FUNGI CONTRIBUTE TO
ORGANIC CARBON IN SOIL?

11. We have been
researching the
role of fungi in the
development of
“top soil” using
mine spoil,
amended with
composted council
MINE COMPOST PLANTS refuse. We use
SPOIL ADDED with AM tubes as shown.
then FUNGI
PLANT
Data from C Daynes

12. Aggregation by AM fungi after 12 m
* * **
* * **
* * *
*
* Indicates stat significance
compared with 100% mine spoil.

13.  Consists of both aggregates and the pore
space distribution.
 Pore space distribution can be measured by
water-holding capacity.

14. 1000
Suction Pressure (cm / H2O)
0% Compost,
with or without
plants and AMF
100
10% 20% 30% 40%
% Volume of water

15. 1000
Suction Pressure (cm / H2O)
Plants alone in
composted (6%) spoil
No plants, 6% Plants with AM Fungi
composted in composted spoil
spoil
100
10% 20% 30% 40%
% Volume of water
Data from C Daynes

16. OC Content of amended Mine
Spoil after 6m.
AM fungi increase AGGREGATION
(enmesh, create pores) but not OC.
Where does the OC come from?

17.  Tisdall & Oades: OC in micro-aggregates
consists of hyphal fragments.
 Few saprotrophic (free-living) fungi survive
for long. Saprotrophic fungi require source of
energy. Plants provide energy to endophytes.
 We next isolated endophytic fungi from roots.

18.  Humus is what remains of organic matter after
degradation (fungi) in aggregates (Tisdall).
Black due to the presence of transformed
polyphenolics and various polyaromatic
compounds.
 Therefore we tested endophytic fungi that
express polyphenolic or polyaromatic
compounds in walls.

19.  Little known about polyphenolics of fungi.
 Melanin is polyphenolic, melanin is a common
compounds found in fungi and other
organisms, 60% hyphae in soil are melanised.
 Fungal melanin can degrade to humus.
Therefore, we isolated and tested melanitic root
endophytes.

20. 900 fungi isolated from
Cultured melanitic
roots: 13% were
endophytic fungus on
melanitic. Work of T
agar.
Mukasa Mugerwa.

21. Steel mesh (43
Plant µm) Perspex side
10 cm Hyphae
FUNGAL (HYPHAL)
PLANT (ROOT) COMPARTMENT
COMPARTMENT
6.5 cm
PVC body
EXPERIMENTAL SETUP FOR
TESTING FORMATION OF OC

22. MEF ISOLATE MWD %OC
CONTROL 990a 6.7 a
222 1030b 7.7b
367 1010a 7.9b (18%)
No melanin 1003a 6.7a
358

23.  One crucial experiment showed that
endophytic fungi translocate N but not C
through the mycelium.
 As a consequence, as energy runs out the
fungus withdraws and dies. If in an
aggregate, the melanised wall remains behind.

24.  Normal anaerobic hydrolysis continues in the
aggregate.
 If an MEF colonises an anaerobic
aggregate, melanin remains because it will
not be oxidised.
 Thus repeated colonisation of aggregates by
MEF results in the ongoing deposition of
stable OC (melanin) in aggregates.

25.  Plant materials are hydrolysed and oxidised
within 7-10 m in warm, moist aerobic soil.
 AM fungi form aggregates. Some organic
matter essential for aggregation.
 MEF deposit OC in aggregates.

26.  Protection from oxygen due to soil particles
esp clay embedded on surface. Pores in
aggregates clogged by hyphae (walls).
 Pores continue to form from anaerobic
hydrolytic activity of microbes esp fungi.
 Melanin increases because it is left behind.
 Protection from oxygen ensures stability of
polyphenolic deposits in aggregates.
 Aggregate breakdown results in oxidation.

27.  Cultivation increases movement of oxygen
into soil esp to surface of
aggregates, increasing rate of oxidation of
polyphenolics. Short term gain (minerals for
plants) for a long term loss of soil carbon.
 Restoration of soil requires action of AM
fungi for structure. AM fungi present in many
cultivated soils. AM fungi require presence of
at least 3% organic matter such as crop
residues to aggregate the soil.

28.  Restoration of soil carbon requires the
inoculation of soil with specific fungi selected
to deposit polyphenolic (melanin).
 Simply leaving the soil alone (for how
long?), adding compost or green manure are
slow and ultimately unpredictable approaches
to sequestration of OC in soil.

29. I have presented the research of my students
and collaborators. I especially wish to thank
Greg Pattinson, Leonie Whiffen, Cathal
Daynes, Tom Mukasa-Mugerwa, Ning Zhang,
Lucy Qi, Jenny Saleeba, Osu Lilje, John
Crawford, Mike Cole and Bruce Sutton.
Funding has come from the cotton industry,
Waste Services NSW, Xstrata, and the
Environmental Trust.