Better information is needed to quantify and mitigate the impact of denitrification and volatilisation in Australian cropping systems.

Key points

• Not enough is known about denitrification and volatilisation, the two main pathways for nitrogen losses in cropping systems
• National field trials aim to improve the understanding of these processes to help growers and advisers reduce these losses on-farm

High prices for nitrogen fertiliser and recent wetter-than-average conditions have focused attention on the spectre of denitrification, which can cause substantial losses of soil and fertiliser nitrogen.

Denitrification and volatilisation are the two main gaseous loss pathways for nitrogen from cropping systems, but there are still many gaps in our understanding of the mechanisms and importance of these losses across diverse Australian grain growing regions.

Quantifying these losses is a significant component of the national GRDC nitrogen investment led by the University of Queensland. The results will be used to improve the ability of the existing Agricultural Production Systems sIMulator (APSIM) model’s soil nitrogen module to estimate total denitrification and will introduce a volatilisation component to the model for the first time.

Nitrogen losses are highly variable and depend on many factors including the soil type, cropping system, location and seasonal conditions. This research will study these losses over three seasons at field trials in a range of cropping systems and soil types across Australia.

Losses of ammonia (NH₃), dinitrogen gas (N₂) and nitrous oxide (N₂O) will be measured using stable isotope-labelled nitrogen fertiliser recovery experiments with the aim of identifying opportunities to reduce these losses on-farm.

Volatilisation

Total volatilisation losses from surface-applied urea and manures are typically less than 20 per cent of applied nitrogen for urea and less than 30 per cent for manures, but can range from zero to 65 per cent. When these nitrogen sources are incorporated into the soil, losses are generally negligible.

Ammonia (NH₃) gas can be volatilised from soil, plants or plant residues, fertiliser and animal manures. In the air it causes environmental pollution that can travel long distances, causing soil acidification and nitrogen inputs that reduce water quality and biodiversity.

Alkaline soil conditions increase the likelihood of volatilisation and the addition of nitrogen fertiliser as urea to soils tends to increase alkalinity in the short term, depending on soil pH and the ability of the soil to buffer these changes. Soils rich in clay minerals and organic matter are better able to buffer changes in pH and reduce volatilisation than are sandy soils.

Denitrification

Denitrification is a significant nitrogen loss pathway in some agricultural soils, with the microbially mediated process producing harmless dinitrogen (N₂) gas as well as the potent greenhouse gas nitrous oxide (N₂O).

While the loss of dinitrogen  simply represents a waste of a valuable crop input, nitrous oxide emissions have received considerable attention as a potent greenhouse gas that has 273 times greater global warming potential than carbon dioxide (CO₂). The typically smaller losses of nitrous oxide can therefore have a big impact on industry emissions profiles. The processes and soil characteristics that determine the ratio of dinitrogen to nitrous oxide in gaseous nitrogen emissions are currently not well understood.

Denitrification is typically more common in poorly drained and high-clay soils. Seasonal gaseous nitrogen losses in eastern Australian grains systems may exceed 50 per cent of the applied nitrogen fertiliser, averaging 27 per cent on the high clay vertosols in north-eastern Australia.

The potential for large losses of nitrogen due to denitrification can be reduced by practices that reduce the concentrations of nitrate in the soil, such as split fertiliser applications and adjusting fertiliser application rates to account for existing soil mineral nitrogen stores in the soil profile.

More information: Dr Graeme Schwenke, 0418 636 421, graeme.schwenke@dpi.nsw.gov.au; Professor Peter Grace, 07 3138 9283, pr.grace@qut.edu.au, Podcast: Predicting nitrogen cycling and losses in Australian cropping systems