Key points

• Drought is a 'double whammy' - with a lack of water also impacting on crops' nitrogen uptake
• Field trials in South Australia showed that varieties adapted to drought also had improved nitrogen uptake
• With water becoming more limited, research on the connections between nitrogen and water will become increasingly important.

Research has shown that drought produces a 'double whammy' for crop yields, with a lack of water also impacting on nitrogen uptake.

In work published last year, South Australian Research and Development Institute (SARDI) senior research agronomist, Dr Mariano Cossani, and principal crop ecophysiologist, Professor Victor Sadras, tested the theory that wheat yield increases when the crop is not restricted by water and nitrogen - in an equal manner.

The theory is called co-limitation. Professor Sadras explains that - in this example - it means that there needs to be sufficient water to make the most of the nitrogen available. Similarly, there also needs to be enough nitrogen to make use of the water available.

"First and most obvious, water deficit restricts growth," Professor Sadras says.

"Secondly, nitrogen uptake is impaired in dry soil - we call this a drought-induced nitrogen deficit."

Multiple limitations

Spindly, stunted wheat plants affected by nitrogen deficiency. Photo: NSW Department of Primary Industries

Professor Sadras says the notion that single factors sequentially limit growth and yield is inadequate, which is where the theory of co-limitation helps. It offers researchers an alternative method to investigate simultaneous limitations of multiple resources.

After highlighting empirically that yield correlates with water/nitrogen co-limitation for a given intensity of stress, Dr Cossani and Professor Sadras wanted to further this theory in field experiments.

"We have strong evidence that a good deal of adaptation to drought in improved varieties is related to improved nitrogen uptake per unit of root length," Professor Sadras says.

"We also know that sorghum is more tolerant than maize to drought because sorghum is better able to cope with drought-induced nitrogen deficits."

To test the theory in wheat, field experiments were undertaken with cultivars released during the past 60 years to understand whether selecting varieties for improved yield and drought adaptation had also increased the water/nitrogen co-limitation.

New varieties are more demanding in nitrogen to achieve yield potential and protein.

The varieties were grown under four or five environmental conditions, resulting from the combination of sites, seasons and supply of nitrogen and water - at Hart, Roseworthy and Minnipa in SA. Soils varied from:

• a clay loam over light to medium clay at Hart;
• a sandy loam over light clay at Roseworthy; and
• red, sandy clay loam at Minnipa.

Weeds, pests and diseases were controlled using grower practices.

The results showed that improving wheat varieties over the years has also played an important part in improving nitrogen use efficiency (NUE), with nitrogen uptake per millimetre of water used by the crop increasing with each year of cultivar release.

The results are also likely to be relevant across northern and western Australia.

Professor Sadras says that selection for grain yield, agronomic adaptation and grain quality has reduced nitrogen stress, increased seasonal uptake of nitrogen per unit of water used, and the degree of water/nitrogen co-limitation in Australian wheat.

"This reinforces the critical role of nitrogen for the adaptation of wheat to low-rainfall environments and the scope for further improvement based on traits that integrate resources," he says.

Future

Professor Sadras says the importance of this information is many-fold, especially in drought adaptation work.

"There are pre-breeding investments for drought adaptation and NUE, but these are often 'silos' with little collaboration," he says.

"There is a lot to be gained if we bring nitrogen and water deficits together.

"The water use efficiency (WUE) of Australian wheat increased from 20 kilograms per hectare per millimetre in the 1970s to 25kg/ha/mm with current varieties, and this is not because of typical drought adaptation traits, but nitrogen efficiency traits.

"Another reason this is important is for the National Variety Trials. Our initial and limited evidence suggests trials are under-fertilised and this may introduce a bias in the ranking of varieties."

Professor Sadras says there are also two concerning upcoming trends:

• firstly - water is becoming more limited because of changes in rainfall patterns and total seasonal rainfall, and this will impact on yields; and
• secondly - there is the issue of soil mining.

"Mining of soil nitrogen reserves and declining soil organic matter results from long-term under-fertilisation or poor use of legumes in rotations," he says.

"The export of nitrogen in grain has been higher than the inputs, so the balance is paid by soil organic matter.

"But there is a limit to this decline. Added to this, new varieties are more demanding in nitrogen to achieve yield potential and protein."

NOTE: This work was funded by the bilateral agreement between the South Australian Research and Development Institute and GRDC.

More information: Dr Mariano Cossani, mariano.cossani@sa.gov.au; Professor Victor Sadras, victor.sadras@sa.gov.au