A more predictable approach to sequestration

of organic carbon in soil

Peter McGeeSchool of Biological SciencesUniversity of Sydney

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?

Background

FUNGUS %OCCONTROL 6.7 a

222 7.7b

347 7.6b

367 7.9b

Can Organic Carbon be increased in soil? Our result.

Quite clearly YES, but HOW?

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.

Misconceptions I

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?

So What is Organic Carbon?

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.

Misconceptions II

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

Decomposition: Two Processes(simplified)

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?

Microbes critical for the Carbon Cycle

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).

Where is OC held?

Reminder: Arbuscular Mycorrhizas transport P to plants AND

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?

Development of Soil Structure?

We have been researching the role of fungi in the development of “top soil” using mine spoil, amended with composted council refuse. We use tubes as shown.

MINE SPOIL

COMPOST ADDED then PLANT

PLANTS with AM FUNGI

Data from C Daynes

* * **

* * **

* * **

Aggregation by AM fungi after 12 m

* Indicates stat significance compared with 100% mine spoil.

Consists of both aggregates and the pore space distribution.

Pore space distribution can be measured by water-holding capacity.

Soil Structure

Water-holding capacity of Mine Spoil

10% 20% 30% 40%100

1000

% Volume of water

Su

cti

on

Pre

ss

ure

(c

m /

H2

O)

0% Compost, with or without plants and AMF

10% 20% 30% 40%100

1000

% Volume of water

Su

cti

on

Pre

ss

ure

(c

m /

H2

O)

Plants with AM Fungi in composted spoil

Plants alone in compos-ted (6%) spoil

No plants, 6% composted spoil

Water holding capacity of compost amended, planted, and plants with AM fungi in spoil

Data from C Daynes

OC Content of amended Mine Spoil after 6m.

AM fungi increase AGGREGATION (enmesh, create pores) but not OC.

Where does the OC come from?

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.

Readily cultured fungi

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.

Which Endophytes?

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.

Fungal Polyphenolic?

All plants yielded endophytes from their roots.

900 fungi isolated from roots: 13% were melanitic. Work of T Mukasa Mugerwa.

Cultured melanitic endophytic fungus on agar.

Steel mesh (43 µm) Perspex side

PVC body

PLANT (ROOT)COMPARTMENT

FUNGAL (HYPHAL)COMPARTMENT

6.5 cm

10 cm

Plant

Hyphae

EXPERIMENTAL SETUP FOR TESTING FORMATION OF OC

MEF ISOLATE

MWD %OC

CONTROL 990a 6.7 a

222 1030b 7.7b

367 1010a 7.9b (18%)No melanin 358

1003a 6.7a

10 out of 24 Melanised Endophytic Fungi increased OC in an already aggregated soil within 3m.

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.

How does OC increase?

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.

How does OC increase (2)?

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.

Current Model (1)

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.

Current Model (part 2)

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.

Implications for Planted Habitats

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.

Implications (2)

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.

Acknowledgements