Nitrogen volatilised – factors affecting how much is lost

Fertiliser nitrogen may be lost from the
soil in several different ways, including:GS-icon

● ammonia volatilisation,
● nitrate leaching and
● nitrate denitrification.

Factors affecting these losses include the fertiliser compound, fertiliser form, type of application, timing of application, soil properties, rainfall amount and intensity, and temperature and wind after application.

Field trials on northwest NSW cracking clay soils (Vertosols) during 2011-2013 showed that surface application of urea led to ammonia volatilisation averaging 11% N loss when applied to fallow soils, and 5% N loss when applied to tillering wheat crops.

Amonia-capturing tube on a mast above a crop in one of Graeme Schwenke's trials (Photo supplied by Graeme Schwenke)

Amonia-capturing tube on a mast above a crop in one of Graeme Schwenke’s trials (Photo supplied by Graeme Schwenke)

 

 

 

 

 

 

 

 

 

 

Compared to urea, losses from ammonium sulfate were less, except when the soil contained >2% naturally occurring calcium carbonate (lime). Nitrogen losses from ammonium sulfate applied to fallow soils averaged >20% where soils contained >10% calcium carbonate. Calcium carbonate content did not affect losses from urea or other nitrogen fertilisers trialled.

A combined statistical analysis of all plots that had <2% calcium carbonate showed that ammonia volatilisation loss was principally affected by:
(a) the presence of a crop,
(b) fertiliser type, and
(c) the average windspeed at ground level.

Losses were greater in fallow paddocks than in-crop, and greater under windy conditions. A separate analysis of all urea plots in the study found that N loss was mainly influenced by:
(a) the presence of a crop,
(b) soil texture (sandier = greater loss), and
(c) soil moisture content at spreading (wetter soil = greater loss).

After the month of volatilisation measurements, most of the non-volatilised applied nitrogen was recovered in the topsoil or plant tissue. The exception was where paddocks had had intense rainfall which likely caused nitrate leaching and denitrification.
These results are from broadcasting in autumn for the fallow treatments and winter for the top-dressing treatments.

Volatilisation may be higher under warmer conditions (eg in summer).

Acknowledgements

Prepared by Luke Beange from an article written by Graeme Schwenke.

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