Italian rhizobia boost for production on acid soils

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Key points

  • Research is seeking new acid-tolerant rhizobia that will allow pulse crops to be grown more productively on acid soils
  • Elite rhizobia have been tested on pulse crops in Western Australia, South Australia, Victoria and New South Wales to assess nitrogen fixation, nodulation and grain yield
  • Researchers are looking for rhizobia that will persist and colonise the soil when the host is not present

Half of Australia’s cropping soils are too acid to reliably grow pulses such as field peas and faba beans, but GRDC-supported researchers are testing new rhizobia to improve pulse production on these hostile and yield-robbing soils

Photo of Dr Ron Yates

Dr Ron Yates is looking for resilient rhizobia that will colonise and persist in the soil when the host plant is not present.

PHOTO: Brad Collis

Researchers across Australia have spent the past four years looking for elite acid-tolerant rhizobia to improve the nitrogen fixation and yields of pulse crops grown on low-pH soils.

Rhizobia are soil bacteria that colonise the roots of pulse plants and form nodules that biologically convert nitrogen from the atmosphere into a plant-available form. But not all rhizobia are equal and some are better than others at fixing nitrogen in low-pH soils.

The idea for research on acid-tolerant rhizobia came in 2009 when Department of Agriculture and Food, Western Australia researcher Dr Ron Yates was setting up a small demonstration trial into acid soils for the 15th Australian Nitrogen Fixation Conference at Margaret River in Western Australia. The Murdoch University-based researcher planted field peas treated with Group E rhizobia (the commercial inoculant for field peas and vetch) and compared them with field peas inoculated with Group F rhizobia (the commercial inoculant for faba beans and lentils).

Interestingly, he found field pea plants inoculated with Group F grew far better because the inoculant is known to be slightly more acid-tolerant than Group E.

When the results were analysed, Dr Yates found the field peas inoculated with the Group F rhizobia performed better than those inoculated with Group E rhizobia in terms of nitrogen fixation, nodulation and yield.

“After seeing the results, I put in a successful research proposal to the GRDC,” he says. “A lot of peas are grown in alkaline-to-neutral soils, but I wondered if having new strains of acid-tolerant rhizobia would allow more growers to plant field peas across a larger area.”

Dr Yates checked Murdoch University’s gene bank for rhizobia isolated from field peas, but was surprised to find only 16 strains were stored, and nearly all were from alkaline-to-neutral soils.
International search

To search for new strains of acid-tolerant rhizobia, Dr Yates sourced strains isolated from field pea plants in acidic soils in Australia and the Mediterranean basin.

Luckily, the South Australian Research and Development Institute’s Dr Elizabeth Farquharson had recently collected strains from southern Australia that he added to his own collection from WA. Also, Dr Yates travelled to southern Italy to target granitic acid soils, similar to Australian soils.

Two-and-a-half months before leaving Australia for Italy, he sent packets of surface-sterilised Kaspa field pea seeds to his research collaborators in Sardinia (CNR-ISPAAM, Consiglio Nazionale delle Ricerche, Sassari) and Reggio Calabria (Mediterranean University of Reggio Calabria), who had agreed to plant the seeds into acid soils from a range of locations in southern Italy.

His collaborating colleagues planted the Australian KaspaA field pea seeds in local acid soils and, by the time Dr Yates arrived in Italy, the acid-tolerant rhizobia resident in the soils had produced nodules on the field pea roots.

Dr Yates dug up the field peas, collected the nodules and brought them back to Australia, where the strains of acid-tolerant rhizobia remained in quarantine until finally they were isolated from the desiccated nodules.

By 2012, Dr Yates had collected more than 100 new strains of rhizobia to be tested in Australian agriculture. His next step was to run multiple experiments over several years on acidic and other challenging soils to whittle down the number to just one or two.

By late October last year, he had reduced the number of acid-tolerant rhizobia strains to four. He hopes to soon be able to recommend two potential replacements to the current commercial Group E and Group F inoculants.

This year he plans to run further experiments and report his findings to the Australian Nitrogen Fixation Committee, which will decide whether further tests are needed or if one strain can be endorsed and delivered to inoculant manufacturers.

Persistent rhizobia

Part of Dr Yates’s research has also involved looking for resilient rhizobia that will colonise and persist in the soil when the host plant is not present.

“With increased reliance on fungicide seed treatments and a trend toward dry sowing, I want growers to have the option to establish the new commercial inoculant with their canola, wheat or barley in the year preceding the field pea crop,” he says.

“Applying the rhizobia the year before field peas are planted means growers can treat their seed with fungicide or insecticide without killing the rhizobia, which would happen if they were co-inoculated on the seed.”

To isolate the most resilient rhizobia, Dr Yates initially used glasshouse trials. From the first strains collected, 22 strains showed potential to persist from one season to the next.

His next step involved testing those 22 strains over two years in paddock trials in acid soils near Brookton, WA, where field peas are not currently grown.

In the first year, he inoculated field peas with the 22 new rhizobia strains together with the current commercial strains of Group E and F. He planted them into acid soils and rated their performance by dry matter production at peak biomass.

In the second year, surface-sterilised field peas (not inoculated) were planted across the original sowing rows to see which rhizobia had survived in the dry soil over summer.

Dr Yates checked the field peas for nodulation where the two sowing rows crossed. If nodules had formed on the plants where the rows crossed, the rhizobia were considered to have persisted in the soil. If nodules formed on plants further along the row, the rhizobia were considered to have colonised the soil.

“That’s how I identified my top six acid-tolerant rhizobia with the resilience to persist over summer and colonise the soil,” he says. “And that’s really important if we’re going to put the rhizobia out with crops the year before when the field pea host is not present.”

Research over several years has shown the elite rhizobia strains consistently outperform the commercial Group E and F inoculants.

“Last year at a low-pH soil trial site in Katanning, WA, the new strains produced more nodules per plant,” Dr Yates says.

“While there is a lot of data still to be analysed, including 15N measurements (to calculate how much nitrogen is contributed to the plant by atmospheric nitrogen) and yield assessments, the initial results are promising.”

Northern region trials

In central and southern New South Wales, Dr Belinda Hackney of Central West Local Land Services spent 2016 evaluating some of Dr Yates’s acid-tolerant field pea rhizobia strains in on-farm trials at Greenethorpe and Wagga Wagga. To do this, she planted field peas into soils with pH levels of 4.3 to 4.8 and assessed the strains for nodulation, nitrogen fixation and grain yield.

“In central and southern NSW, most field peas are grown on mildly acidic to strongly acidic soils,” she says. “As a result, growth and nitrogen fixation is significantly less than potential.”

Development of more acid-tolerant field pea rhizobia will make a significant difference to field pea growth, grain yield and, importantly, the quantity of nitrogen fixed, Dr Hackney explains.

She says that because of the intolerance of the current rhizobia strain to low-pH soils, growers in NSW are unable to capture the full potential of field peas in their cropping systems, even if their other agronomic management is perfect. She says the elite strains scored more highly in terms of nodulation than the current Group E and F inoculants.

Further, she says, there was an increase in the size of the nodules in the elite strains and better distribution of the nodules through the field pea roots.

“Nodulation in the current commercial strains was more sporadic,” she says.

“When nodulation did occur, it was only at the plant crown, where soil pH is generally higher, and nodules are smaller than for the elite strains.”

No silver bullet

Although the results are encouraging, Dr Hackney and Dr Yates caution growers not to rely completely on new rhizobia strains to increase crop production on acid soils.

“At pH levels of 4.3 to 4.8, cereal and pulse production is likely to be compromised and therefore liming to increase soil pH to between 5 and 5.5 is desirable,” Dr Hackney says. “The elite rhizobia would also be expected to perform better than the current commercial strain at a pH of 5 to 5.5.”

Dr Yates agrees and is analysing the performance of the elite rhizobia strains from recent trials on neutral-to-alkaline soils to assess how well they performed against the current commercial strains of rhizobia in terms of nitrogen fixation, nodulation and grain yield.

More information:

Dr Ron Yates,
08 9368 3665,
ronald.yates@agric.wa.gov.au

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New rhizobia tested in the south

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GRDC Project Code DAW00221

Region North, Overseas, South, West