Nitrogen-fixation index set to help growers


SARDI researcher Elizabeth Farquharson says the development of an index of the nitrogen-fixation capacity of field peas will assist growers with their cultivar selections. Photo: Dr Rohan Kimber, SARDI

Research funded by the Grains Research and Development Corporation (GRDC) will lead to the development of an index of the nitrogen-fixation capacity of field peas to assist growers with their cultivar selections.

The South Australian Research and Development Institute (SARDI*) has been conducting field trials in SA and Victoria over the past four years to assess the nitrogen-fixation qualities of 15 different field pea cultivars and near commercial lines.

The data will be used to develop a “symbiotic index” for field pea – planned for release in 2017 – so growers can readily see which cultivars are likely to contribute the most fixed nitrogen (N) to their farming systems.

It will also provide pulse development programs with information on N-fixation potential in different environments and identify opportunities for symbiotic improvement.

Pulses such as field pea, along with pasture legumes, can provide an abundant, inexpensive and sustainable source of N for Australian cropping systems. Cereal yields are consistently greater following legumes due to these N inputs and other benefits, including disease and weed breaks and improvements to soil structure and biological function.

SARDI researcher Dr Elizabeth Farquharson says the work being undertaken through the GRDC’s southern region Optimising Nitrogen Fixation project is aiming to improve understanding of how cultivars differ in their N-fixation potential, clarifying where legumes respond to inoculation, identifying key agronomic factors affecting fixation and testing new inoculants.

While the work has so far been largely focused on field pea, Dr Farquharson said faba bean and lentil, along with vetch, were nodulated with the same species of rhizobia and therefore often faced similar N-fixation challenges in the field.

Development of a symbiotic index for other pulse crops in the future will depend on success of the index for field pea.

Speaking at GRDC Grains Research Updates in the southern cropping region, Dr Farquharson said the total amount of N fixed by a legume was determined by both its ability to fix N and by dry matter production.

“Since cultivars vary in their dry matter production, compatibility with rhizobia and tolerance of soil nitrate, it follows that their N-fixation potential also varies considerably,” she said.

Trials have demonstrated that while all 15 pea lines assessed form nodules and on average accumulate 128-150 kilograms of N/hectare, the amount of fixed N remaining in the pea stubble varied by up to 40 kg/ha.

“This is in part explained by variations in harvest index between lines, but also by differences in the percentage of N they derived from fixation,” Dr Farquharson said.

“Fixed N remaining in the stubble ranged on average from 30 kg/ha for some short-season and semi-leafless lines to 60 kg/ha for conventional lines. The forage pea, PBA Hayman, generally had a high amount of fixed N left in the stubble (70 kg/ha) due to its low harvest index.”

Dr Farquharson said that in poor seasons where dry matter production was reduced and harvest index was high (such as a dry spring), there was less difference between the pea lines, with as little as 10 kg/ha fixed N remaining in stubble.

“Ultimately, variety selection by growers will be based on a range of factors including rainfall, disease pressures and purpose,” she said.

“Nitrogen fixation may add another component to this mix or could be used to better guide future fertiliser inputs.”

* SARDI is a division of Primary Industries and Regions South Australia (PIRSA).

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