Window opened on root behaviour
The relentless pursuit of knowledge that can lift crop performance has turned to root behaviour using fascinating technology that allows researchers to see what is happening within the soil
At first glance, the crops growing at the Charles Sturt University (CSU) rhizolysimeter site near Wagga Wagga, New South Wales, look like any other. However, on closer inspection, the exposed circular tops of 24 buried steel ‘silos’ provide a clue that beneath the surface, something more interesting than the norm is happening.
Inside each of the 24 silos, a core of soil is intersected by several tiny transparent tubes, each fitted with sensors and cameras recording in intimate detail the behaviour of roots and water as crops grow – information that would usually be concealed from view.
It is a unique perspective, and one made possible thanks to the CSU rhizolysimeter – the largest underground laboratory of its kind in the southern hemisphere.
In fact, with the aid of time-lapse photography speeding up the process, it is possible to watch a root heading wards the direction of a particular nutrient, or being rebutted by a hostile soil.
The purpose of the technology is not to merely produce fascinating slideshows. “While that might make for some spectacular photographs, it’s the data you’re getting that is more important,” says CSU’s associate professor of soil science Dr Philip Eberbach, who is using the facility to gain an insight into the dynamics of roots, water and soil.
Through the network of transparent three-millimetre tubes known as mini-rhizolysimeters, researchers are observing the roots of a variety of grain and cover crops growing in real time in both natural and controlled-environment conditions.
This allows the effects of rainfall, crop rotations and various inputs to be monitored and evaluated. It is this new and important agronomic information that is generating the most interest.
In some of its latest findings, the rhizolysimeter shows just how valuable a lucerne crop can be to a following wheat or canola crop – speeding up root growth rate and depth respectively and allowing the plants greater access to water and nutrients.
In the case of wheat, it grew a root system faster – achieving a particular soil depth 10 days earlier than a wheat plant growing in a soil that did not have a history of lucerne. Canola roots tended to grow deeper than they would otherwise.
This, explains Dr Eberbach, is because lucerne’s four-metre root systems create tunnels – or stable biopores – for the roots of subsequent crops to exploit, enabling them to grow deeper and faster than they would otherwise.
“There is a legacy from lucerne,” he says. “Rather than having to shift soil particles out of the way, the new roots find a plane of weakness and follow it down.”
Furthermore, this benefit was evident in the second crop after lucerne as well. “So it was more than a transient effect,” Dr Eberbach says. “You wouldn’t say ‘permanent’ but it lasted a reasonable amount of time.”
The findings are part of an eight-year program conducted by Dr Eberbach with colleagues Dr Jeff Hoffman and Dr Sergio Moroni, part-funded by the GRDC and prompted initially by Dr Eberbach’s idea to test the effectiveness of lucerne in mitigating processes important in contributing to landscape salinisation.
A major agricultural environmental issue, it was thought salinisation could be combated by perennial crops such as lucerne or phalaris through their ability to capture surface water and prevent leakage into groundwater.
But, contrary to expectations, the rhizolysimeter research found that after its removal, the lucerne phase could encourage elevated rates of recharge – especially in a wet year – because once it was removed, the deep biopores left by its root system enhanced the rate at which water in the surface soils could drain to the deeper soil profile. “You might sow lucerne to try to de-water deep subsoils, but the effect could be negated in subsequent wet years,” Dr Eberbach says.
This water may also potentially move beyond the capture zone of subsequent annual crops.
In a dry year, though, the biopores could aid crops because if water from a single autumn storm event penetrated further into the soil profile, it could be captured by annual roots the following spring to see them through a drought period. “So a history of lucerne in a dry year is probably beneficial to subsequent crops but hydrologically less favourable in wet years,” Dr Eberbach says.
Cover crop knowledge
Dr Eberbach believes this knowledge would help growers determine the best cover and crop mix for their conditions. “In a highly variable climate we can’t write absolute truths,” he says. “You can only say it will do this in one circumstance and it might do this in another.”
While lucerne provides a good summer feed there are also other tangential benefits, Dr Eberbach says. “It can capture water but there is also a legacy for future crops and the environment through its deeper roots system.”
The rhizolysimeter is the ideal facility to study such dynamics because not only can root behaviour be determined, but core soil samples can also be removed intact, enabling very accurate measurement of soil water content.
The eight-year research period also allowed the simulation of realistic farming practice (growing lucerne for a full four years, followed by crop rotations and fallow) and the effects of lucerne’s removal on soil water and subsequent crops to be tested.
Dr Eberbach says his passion lies in replicating real farm scenarios to understand how plants function and use water in a working landscape. He says the rhizolysimeter has enhanced his ability to do this.
“Being able to measure how the system is responding to particular plants and how those plants are using water is fascinating,” he says. “You can look at how an agricultural system actually functions and there’s no doubt that what we have learned has been almost directly attributable to that facility.
“A new extension of the existing facility effectively quadrupling the number of soil cores from 24 to 96 will only enhance this capability,” he says. “It will allow rotational experiments to run simultaneously, so you can have canola trials or wheat trials every year in alternating cores.”
The advantage of rotational experiments is that the legacy of one crop on another can be measured and knowledge built up over time. “The idea in agriculture of one crop having a legacy on another is really important and it’s something that single-year experiments just don’t bring through,” he says.
Dr Eberbach says there is still much to learn about chemical and biological processes in the rhizosphere – the zone of soil surrounding plant roots. “We know a lot about what happens above the ground but our knowledge about what happens beneath the soil surface is still largely in its infancy,” he says. “There are big challenges and big opportunities.”
More information:Dr Philip Eberbach,
0428 422 785,
GRDC Project Code UCS29, UCS00003
Region South, North