Australian study puts spotlight on nitrogen supply to wheat

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New research in Australia has highlighted the soil supply of nitrogen can be reduced in the presence of residue from the previous year’s wheat crop.

The results indicate that reducing the contact made between nitrogen fertiliser and crop residues on the soil surface could be a strategy that growers can use to optimise the availability of nitrogen to wheat plants.

These findings were among those made by University of Western Australia (UWA) student Nathan Craig as part of a PhD thesis supported by the Grains Research and Development Corporation (GRDC) through a Grains Industry Research Scholarship.

The aim of the study was to quantify the supply of nitrogen to wheat crops from the soil and crop residue and determine the effect on grain yield and grain nitrogen uptake – in a monoculture wheat rotation and a chickpea-canola-wheat rotation.

Research was conducted at the WA No-Tillage Farmers Association (WANTFA) long-term no-tillage trial site at Cunderdin in WA’s central grainbelt.

“Across the three seasons tested, there was an overall benefit of applying nitrogen fertiliser to monoculture wheat, which increased grain yield and grain nitrogen uptake in most years, and effectively closed the yield gap between wheat grown in monoculture and wheat grown in rotation with legume crops and with no nitrogen fertiliser added,” Mr Craig said.

“However, the application of nitrogen fertiliser actually reduced the mineralisation of nitrogen in the soil in the monoculture wheat system, whereas in the rotated wheat system there was less effect.”

“While retaining crop residue is known to improve nutrient recycling in no-tillage systems in the longer term, the potential for nitrogen to be immobilised in the short term during decomposition by microbial activity could provide significant competition for plant available nitrogen in the soil during the growing season.”

“More importantly, when soil mineral nitrogen levels were low at seeding, and the crop residue carbon-to-nitrogen ratio was high, there was a high risk of immobilisation of nitrogen in both of the wheat systems.”

Mr Craig suggested there was the potential to adapt crop management and fertiliser application techniques to optimise the soil supply of nitrogen to wheat crops seeded into wheat residue.

“An annual sampling program could be conducted at the start of each growing season to determine the carbon-to-nitrogen ratio of the crop residue mix left on the soil surface, and the amount of nitrogen in the soil to a depth of 10cm,” he said.
“From this, you could estimate the potential for immobilisation of nitrogen following fertiliser application.”

Mr Craig said reducing contact made between nitrogen fertiliser and crop residue and the soil surface – where mineralisation of nitrogen was concentrated – might increase the uptake of nitrogen from the soil in situations where there was a high risk of immobilisation.

“This could be achieved using existing technology where additional nitrogen fertiliser is applied at seeding and ‘banded’ below the seed at a depth greater than 5cm, rather than an in-crop broadcast application during the season,” he said.

“This approach would allow maximum rates of mineralisation of nitrogen in the soil surface layers while potentially increasing the efficiency of the uptake of nitrogen by the wheat plant.

“Another approach to minimise the immobilisation of applied nitrogen by decomposing crop residue is to develop fertiliser application technologies that can accurately deliver nitrogen fertiliser to the wheat plant.

“This could include more accurate foliar application of nitrogen fertiliser, or targeted application into the crop row at the base of the wheat plant.”

The picture shows Nathan Craig installing the tubes used to measure nitrogen mineralisation in wheat treatments at the WANTFA long-term no-tillage trial site. PHOTO: GRDC

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