A project undertaken at Southern Cross University, and supervised by Dr Luke Jeffrey.
Nitrogen is the most abundant element in our atmosphere and is critical to plant growth and health. However, plants cannot utilise atmospheric nitrogen gas and instead rely upon specialist microbes to convert nitrogen gas into biologically available forms, such as ammonia or nitrate. This process is called nitrogen fixation and is one of the most fundamental processes supporting life on earth. Nitrogen-fixation sustaining our forests is conventionally understood to occur predominantly below ground within the soil microbiome. The importance of nitrogen-fixation performed by tree-dwelling microbial communities in bark, is currently unknown and unquantified.
When recently investigating the methane-cycling microbial communities within the common Paperbark tree (Melaleuca quinquenervia), we discovered that about 30% of the bark-dwelling microbial community appear capable of nitrogen-fixation, in stark contrast to only 1% reported in temperate Australian soil communities. If microbial nitrogen fixation occurs within the bark surface of all tree species, our current understanding of the nitrogen cycle, and the role of trees within ecosystem nutrient budgets, may require a re-think.
This project aims to determine whether bark-dwelling nitrogen-fixing microbes (diazotrophs) are present and active within a range of endemic Australian tree species. We will sample the tree-associated microbiomes within strategic locations such as the wet tropics, subtropics and cool temperate regions, spanning a latitudinal and habitat gradient. Through a combination of next-generation genetic sequencing, lab and field-based experiments, we will:
1) Characterise the microbial communities within several tree / bark types (i.e. who they are)
2) Determine their metabolic functions (i.e. what they do)
3) Quantify bark-derived N-fixation rates (i.e. how much nitrogen they fix)
This data will for the first time quantify this potentially overlooked yet important biogeochemical process and represents a new research area within the forest nitrogen cycle. This project will provide unique insights into the diversity and metabolic capabilities of Australian tree-dwelling microbial communities.
By identifying this knowledge gap, the data may one day be used in climate model projections surrounding forest growth and carbon sequestration capacity. This research may also have implications for global reforestation efforts, plantation timber and/or agricultural applications, by paving the way for alternative approaches to fertilisation whereby bark-dwelling N-fixation may be enhanced or cultured, to achieve required nitrogen inputs in more economically and ecologically favourable ways.