Water–nitrogen interplay guides management

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A GRDC-funded project investigated the relationship between the yield-driving factors of nitrogen and water

Photo of urea spread

Fertilliser drives a trade-off between water use efficiency and nitrogren use efficiency. A broad 'rule of thumb' across soils, climates and practices is that fertiliser rates below 50kg nitrogen per hectare lead to more nitrogen being taken from the soil than put in.

PHOTO: Kellie Penfold

Nitrogen practices need to be continuously revised to make sure new wheat varieties can achieve their full yield and protein potential.

GRDC-supported research shows that selecting for yield and grain protein in winter-rainfall regions has significantly increased the capacity of new wheat varieties to absorb nitrogen from soil.

An example of this breeding advance is the fact that improved nitrogen uptake has helped to lift water use efficiency (WUE) from 20 kilograms of nitrogen uptake per hectare per millimetre of water use in 1980s varieties to 25kg/ha/mm in current varieties in southern Australia.

If a nitrogen deficiency is allowed to occur, then a gap will open between achieved yield and yield potential.

Preliminary estimates indicate 0.6kg/ha/mm of nitrogen uptake will close the yield gap, but grain protein influences this benchmark. For example, closing the gap may require 0.7kg nitrogen uptake per ha per mm for grain with 14 per cent protein, and 0.5kg nitrogen per ha per mm for grain with 11 per cent protein.

Matching nitrogen inputs to crop water availability and variety is a balancing act that directly influences productivity and profitability.

A scoping study led by the South Australian Research and Development Institute (SARDI), as part of GRDC’s More Profit from Crop Nutrition II, provided insights on the interplay between water and nitrogen from diverse perspectives.

These were captured by a team including regional agronomists and scientists from SARDI, CSIRO and the universities of Adelaide and Queensland. The aim was to identify gaps to guide future research, development and extension activities.

Water and nitrogen can have a complex relationship

  • The effects of water and nitrogen on crop yield can be cumulative. This means if nitrogen supply increases yield by 10 per cent, and water supply increases yield by 10 per cent, then the total combined yield gain is 20 per cent.
  • Or they can have a non-cumulative effect. This is when the combined effect of water and nitrogen can be higher (synergy) or lower than that expected from the sum of the individual effects.
  • Water and nitrogen can be involved in trade-offs. For example, increasing the rate of nitrogen improves water use efficiency but reduces nitrogen use efficiency.

Understanding these relationships between water and nitrogen is important to capture synergies and manage trade-offs.

The project looked at the role of physiological, agronomic, economic and breeding factors in the relationship between water and nitrogen.  

This agronomic component involved local information from experienced agronomists and data collected from 274 wheat crops grown in a range of soil, climates and management systems.

These trials showed that:
  • fertiliser rates of below 25kg nitrogen/ha
  • generated a median wheat yield of 2.7t/ha and median grain protein concentration of 10.6 per cent;
  • a rate of 150kg nitrogen/ha returned median yield of 5.3t/ha and median
  • grain protein concentration of
  • 13.1 per cent;
  • yield and biomass increase in response to fertiliser was two to three times higher under high-yielding conditions than under water stress;
  • variation in yield was mostly related to variation in biomass; and
  • median harvest index was relatively stable, ranging from 0.38 with less than 25kg nitrogen/ha to 0.41 at 150kg nitrogen/ha. (This contrasts with the perception that excess nitrogen leads to low harvest index.)

This project also looked at nutrient balance – comparing nitrogen export by grain with nitrogen fertiliser inputs.

A coarse “threshold” across soils, climates and practices indicated that fertiliser rates below 50kg nitrogen/ha would mean that nitrogen export exceeds nitrogen input so the crop is essentially ‘mining’ the soil of nitrogen.

This supports a long-term experiment in Victoria, where the amount of nitrogen removed was the equivalent of nitrogen fertiliser applied at rates of between 40 and 80kg N/ha.

From a physiological perspective, fertiliser drives a trade-off between water use efficiency and nitrogen use efficiency This means each additional unit of nitrogen will increase yield per unit of water use, and at the same time it will reduce yield per unit of fertiliser – irrespective of crop, soil, climate and management.

It is impossible to maximise both efficiencies at the same time, so any fertiliser rate decision must involve a trade-off that reflects individual environmental factors, economic considerations and attitudes to risk.

The project also looked at nitrogen management decisions under uncertain seasonal conditions.

In rain-fed cropping systems, yield and consequently profit depend not only on crop nutrition, but also on matching genotype and management to the growing environment and expected seasonal conditions.

More information:

Victor Sadras,
victor.sadras@sa.gov.au

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Understanding nutrient stratification to guide crop management

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Root response to fertiliser could guide nutrition

GRDC Project Code DAS00157

Region North, South