A more predictable approach to sequestration of organic carbon in soil. Peter McGee
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?
FUNGUS %OC CONTROL 6.7 a 222 7.7b 347 7.6b 367 7.9bQuite clearly YES, but HOW?
Adding compost, and using no-till or green manure crops increases organic matter and necessarily increases organic carbon. WRONGRESPONSE: 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.
Stable, long-lived polyphenolic andpolyaromatic material physically protectedwithin micro-aggregates. OC is a fraction of OM. Well, lignin is a polyphenolic and lots enters the soil?
The main polyphenolic of plants (lignin) is the source of OC in soil. WRONGRESPONSE: 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.
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 notprovide food for microbes: REQUIRES OXYGEN
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?
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).
Arbuscular mycorrhizal (AM) fungiassociate with most agric plants.(AM) fungi constitute some 70% ofall microbial biomass in soil.Hyphae of AM fungi are 0.5 to 5mper g of soil.DO AM FUNGI CONTRIBUTE TOORGANIC CARBON IN SOIL?
We have been researching the role of fungi in the development of “top soil” using mine spoil, amended with composted councilMINE COMPOST PLANTS refuse. We useSPOIL ADDED with AM tubes as shown. then FUNGI PLANT 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.
1000Suction Pressure (cm / H2O) 0% Compost, with or without plants and AMF 100 10% 20% 30% 40% % Volume of water
1000Suction 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
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.
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.
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.
900 fungi isolated from Cultured melaniticroots: 13% were endophytic fungus onmelanitic. Work of T agar.Mukasa Mugerwa.
Steel mesh (43 Plant µm) Perspex side10 cm Hyphae FUNGAL (HYPHAL) PLANT (ROOT) COMPARTMENT COMPARTMENT 6.5 cm PVC body EXPERIMENTAL SETUP FOR TESTING FORMATION OF OC
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.
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.
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.
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.
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.
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.
I have presented the research of my studentsand collaborators. I especially wish to thankGreg Pattinson, Leonie Whiffen, CathalDaynes, Tom Mukasa-Mugerwa, Ning Zhang,Lucy Qi, Jenny Saleeba, Osu Lilje, JohnCrawford, Mike Cole and Bruce Sutton.Funding has come from the cotton industry,Waste Services NSW, Xstrata, and theEnvironmental Trust.