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Soil microbes – the soil fertility engine room

Producers across the southern tablelands recognise phosphorus (P) is a key driver of pasture growth. Soil microorganisms play a fundamental role in the cycling of soil P as well as all other soil nutrients. These organisms are the ‘engine room’ in plant/soil systems ensuring that nutrients are being continually made available to plants for growth. Soil microbes are responsible for breaking down organic matter (decaying plant material) and converting the nutrients held in this material into forms that are available to plants. They also secrete substances that solubilise slow release mineral forms of nutrients such as P in rock phosphate. In addition, within themselves they hold nutrients and as microbes live and die they release nutrients into the soil solution in a plant available form. Soil microorganisms are critical for the turnover of organic matter as well as maintaining soil physical, structural and associated hydrology aspects of the soil.

There is little evidence however that soil biology can solely provide all nutrient requirements for plants in highly productive commercial grazing situations where there is requirement for high pasture growth, and ultimately P removed from agricultural systems must be replaced. Australian soils being highly weathered are also intrinsically low in P. So by enriching the cycle with P-based fertilizer to grow more pasture, we effectively also build the organic matter component, which is the food source that allows soil microbial function and, with that, maintenance of soil fertility.

Often producers ask what effect do mineral, manure, compost or microbial formulation type fertilizers have on microbes in the soil and do some of our conventional P-based fertilizers harm the soil microbial function? There are numerous anecdotal claims made in the agricultural arena concerning the effect of various fertiliser types on soil microbial communities, with both positive and negative claims being reported, but with little or no scientific evidence to support them.

Research into these areas is still very recent, however it is clear that microbial communities that are present in our soils are highly complex and diverse in structure and are highly adaptive and resilient to change. These communities are represented by a wide range of microbial types, including fungal, bacterial and archaeal groups. Collectively, and in association with other soil biota, these microbial communities play an important role in soil health and particularly in relation to cycling of nutrients such as phosphorus. Current research indicates that there is a high level of functional sharing of processes that occur in the soil, indicating that even with marked shifts in microbial community structure, eg, a move from fungal dominance to bacterial dominance, important soil functions such as nutrient cycling are maintained

The Native Pasture and Alternative Fertiliser Project with trial sites at Binalong and Bookham in southern NSW is currently investigating the effect of various alternative and conventional fertilisers on soil microbial diversity and their functions performed in relation to P cycling. The trial is being coordinated by Fiona Leech from the South East Local Land Services. Soil was collected in the sixth year (2014) of the study providing an excellent base for the investigation. The data being collected for this project is one of very few field research studies in Australia.

As part of the study, DNA has been extracted directly from soil in all replicates for each fertiliser product treatment and then processed using new molecular laboratory techniques for high-throughput DNA analysis to identify the range and diversity of microbial communities present. The communities have been divided up into fungal, bacterial and archaeal groups. Using the DNA sequencing technique allows all of the microbes present in the soil to be identified to the species level, including the discovery of many new organisms. Detailed analyses of the microbial DNA datasets is currently being undertaken and key messages from the work will be presented at a future fertilizer and pasture productivity seminar to be held in Bookham in 2017. The event will be advertised in the South East Circular when a date is decided – please watch this space.

This project has been a collaborative initiative between Binalong Landcare, NSW DPI and Local Land Services.

The microbial investigative part of the project is supported through funding from the Australian Government’s National Landcare Programme and is being conducted in collaboration with CSIRO Agriculture in Canberra.