DR TENDO MUKASA MUGERWA, MELANISED ENDOPHYTIC FUNGI ENHANCE CARBON IN SOIL
Soil function is greatly dependent on the amount, distribution and quality of organic matter (particularly organic carbon) in soil. Current methods of soil management that aim to increase organic carbon in soil have unpredictable outcomes primarily due to misunderstandings regarding the effective long-term storage of organic carbon. Organic carbon will only be maintained in soil for significant periods of time if it is protected from oxidation. Oxidation is a process that occurs in the presence of oxygen and one that readily results in the breakdown of organic carbon. Organic carbon will therefore only be protected if it is deposited within soil aggregates, the centres of which are anaerobic.
Currently, a number of soil management practices widely utilised in efforts to enhance soil organic carbon involve the addition of plant-derived compounds (such as lignin) to soil. Though recalcitrant, such compounds remain in the aerobic zone in soil (outside of soil aggregates) and can be readily oxidised. A number of studies have demonstrated that this process can occur rapidly. Plant-derived compounds remain outside of aggregates due to their size. So what other recalcitrant material is readily available and is small enough to be deposited within aggregates? Melanin. Melanin is a complex polyaromatic (recalcitrant) material that is deposited in the hyphal walls of some fungi. Fungal hyphae, unlike plant material, are small enough to penetrate soil aggregates. Therefore, if fungi with melanised hyphae are able to penetrate soil aggregates they can potentially deposit recalcitrant material (melanin) within these aggregates. This material would be protected from oxidation within soil aggregates and the accumulation of such material would contribute to increased levels of soil organic carbon, long-term.
Under the supervision of Associate Professor Peter McGee, I tested this hypothesis as part of my PhD at The University of Sydney (completed 2012). In summary, I was able to identify 20 melanised endophytic fungi (MEF) that increased levels of organic carbon in an agricultural soil over 14 weeks. An increase in soil carbon of up to 17% was recorded within experimental trials using subterranean clover. Increases (up to 32%) in organic carbon were also recorded in a subsequent 1 month field trial conducted on canola (Guy Webb, Gaia2112).
I chose to utilise endophytic fungi in the study as endophytic fungi form mutual associations with plants. Endophytic fungi therefore have a direct supply of energy from plants. As such, the longevity of endophtyic fungi in soil is enhanced in comparison to saprotrophic (freeliving) fungi. Endophytic, however, can also be cultured independently of any plant. Cultures can therefore be independently maintained in the absence of a plant, unlike mycorrhizal fungi, for example. This capability was utilised when carrying out pot-trials in order to determine whether MEF enhance soil carbon. During this process, the isolates were simply grown on sterilised wheat seed in order to bulk up an inoculum and the wheat seed added to soil. This method of inoculating soil was simple and cost-effective, yet produced significant results. The inoculum could be prepared within the space of 2 to 3 weeks (to allow the fungi to grow on the wheat seed) and was done so in a laboratory with basic equipment. With input from experts within the field of inoculum development, I believe that this process can easily be shortened and made more efficient. An inoculum grown on what seed will most likely not be the method by which such fungi may be applied to soil on large scale plots, however, again, this method demonstrated the simplicity by which such an inoculum can be developed.
In conclusion, the research I conducted in previous years has indicated the potential that melanised endophytic fungi have to enhance carbon in soil. In laboratory studies, these fungi significantly enhanced levels of carbon in an agricultural soil within 14 weeks. The results of this study will soon be published in an international peer-reviewed soil science journal. Initial field trials in which select melanised endophytic fungi were applied to an agricultural soil in which Canola was grown further confirmed this potential. I believe that the role of fungi in carbon sequestration may be greatly underestimated. The application of melanised endophytic fungi to soil may be a simple yet efficient method by which organic carbon can be enhanced in soils.