Natural nitrogen a fuel buffer

Photo of Frances Hoyle

By Brendon Cant

[Photo by Brendon Cant: Frances Hoyle: "In many farming systems, the amount of nitrogen cycling through soils annually is more than enough to satisfy crop nitrogen demand."]

As oil prices hit record highs, graingrowers have more reasons to be concerned than simply the cost of powering their machinery - it takes the energy from about one litre of oil to produce one kilogram of urea. Frances Hoyle, of the University of Western Australia, says one way of fighting the energy "crisis" could be to increase the mass of micro-organisms in soil, resulting in greater soil nitrogen supply, thereby reducing dependence on nitrogenous fertilisers. The GRDC is supporting her research into stubble retention practices and their impact on soil biology.

"Stubble retention positively increases micro-organism mass in soil, and in WA those soil micro-organisms are highly efficient deliverers of biologically derived nitrogen," Ms Hoyle says. "Managing soils for optimal biological fertility can therefore positively influence potential yield."

The GRDC project found nitrogen supply in dryland agricultural systems was derived mainly from microbial decomposition of soil organic matter and crop residues, making stubble retention an important factor.

"This is a response to water-limited yield potentials and low inorganic fertiliser inputs, for example. It is therefore important to identify and understand the primary influences on microbial mass and activity in this environment and the potential limitations to gross nitrogen transformation rates."

In WA"s typically low-input agricultural systems (less than 50kg nitrogen/ ha is applied for wheat production), up to 80 per cent of soil nitrogen supply in wheat crops is from microbial transformation of organic matter, compared with 50 per cent in temperate soils. "In many farming systems, the amount of nitrogen cycling through soils annually is more than enough to satisfy crop nitrogen demand, even where no fertiliser is applied."

Differences in plant residues and management will influence organic matter quality and decomposition rate and therefore the timing and amount of nutrient released. Ms Hoyle says there is also potential to improve the synchrony of plant growth and nitrogen uptake in WA farming systems, thereby reducing potential nitrogen losses.

For more information: Frances Hoyle, UWA, 08 6488 7345