Amplifying nitrogen effects using sulfur

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Photoof Professor Wujun Ma
Professor Wujun Ma and his team have gained new insights into the influence of sulfur on the effectiveness of soil nitrogen. PHOTO: Evan Collis

Getting the most out of nitrogen fertilisers on wheat crops can depend a lot on sulfur availability in soils, which can decline after a canola rotation

The value of nitrogen fertiliser for improving wheat yield and grain quality is well understood. However, more recently researchers at Murdoch University have characterised the vital role played by sulfur in mediating and potentiating nitrogen’s yield and grain-quality enhancing effects.

Professor Wujun Ma, who supervised the work, says that under field trials in Western Australia’s wheatbelt region sulfur was found to beneficially impact nitrogen uptake by a crop, thereby improving the crop’s nitrogen use efficiency (NUE) and grain yield. Strikingly, in sulfur-deficient conditions, adding more nitrogen to a crop does not result in higher NUE, but this ‘levelling off’ of NUE can be reversed by providing nitrogen in combination with sulfur, due to the stimulation of nitrogen uptake.

In addition, laboratory studies on grain storage proteins revealed additional beneficial effects. Sulfur was found to play a special role providing the biochemical glue to a grain storage protein that endows glutenin with its elasticity. In turn, this sulfur-based agglutination affects grain performance during food processing, thereby influencing grain grade and market price.

These grain protein benefits are mediated by a type of chemical bond (called a disulfide bond) that affects protein structure, aggregation and polymerisation.

“Wheat is not able to reach its full yield potential and make efficient use of nitrogen for grain protein synthesis without adequate sulfur availability,” Professor Ma says. “Furthermore, it is sulfur metabolism that provides grain with the chemistry that makes protein aggregation possible and which, in wheat, determines the viscoelasticity of the dough matrix.”

Professor Ma warns that not all soils in Australia’s wheat-growing regions are necessarily sulfur-deficient. However, he has identified it is a factor in western growing regions, where his field trial sites were located and more than 20 wheat cultivars tested. There is a possibility that sulfur deficiency is less of an issue in eastern growing regions.

“Sulfur deficiency is an emerging issue,” Professor Ma says. “In part, a possible cause is the purity of modern nitrogen fertilisers, since in previous years the formulations did contain plant-available sulfur. The main reason, however, is canola cultivation, as this crop tends to strip soils of sulfur. A few years of cultivating canola will tend to lead to a deficiency.”

An optimal sulfur-to-nitrogen ratio to maximise yield and grain quality benefits at the trial sites was identified. However, Professor Ma says that additional studies are needed across Australia’s wheat-growing regions before more general agronomic recommendations can be made with confidence for different soil types. An economic cost-benefit analysis is also needed to determine the return on investment in sulfur inputs.

In the meantime, several morphological traits, including peduncle length and straw diameter, were found to be useful as markers to help optimise sulfur fertilisation strategy and NUE and grain yield gains.

In addition, the study defined the molecular pathways – genetic and metabolic – that mediate the sulfur effect. Included is the discovery that sulfur is one of those environmental factors that can alter gene expression epigenetically – meaning that it induces chemical modification to DNA rather than relying on encoded DNA sequence. The epigenetic signal is active during the plant’s lifetime, but is erased before the DNA is passed to the next generation.

These findings were the result of GRDC investment in crop nutrition research undertaken over three years by PhD student Zitong Yu, who was supervised by Professor Ma, Chair of Grain Chemistry and Quality at Murdoch University and director of the Australia–China Joint Centre for Wheat Improvement. The project also benefited from a collaboration with the WA Department of Primary Industries and Regional Development.

The work was recently published in Nature Scientific Reports (www.nature.com/articles/s41598-018-20935-8).

More information

Professor Wujun Ma
w.ma@murdoch.edu.au