Decreasing nitrous oxide emissions in high rainfall legume / wheat cropping systems

Nitrous oxide chamber

Decreasing nitrous oxide emissions in high rainfall legume / wheat cropping systems

Background

Cropping is expanding in the southern Australian high rainfall zone, with an increasing proportion of land being converted from pasture to cereals. This change in land use can have significant ramifications for greenhouse gas emissions: soils under grass and clover pasture typically have high levels of fixed nitrogen which breaks down and is lost as N₂O when the soil is prepared for cropping.

To reduce these emissions, it is important to provide farmers with management options that minimise the loss of N₂O from the soil without reducing productivity.

Project outline

The research team are evaluating the effect of different management options on N₂O emissions from soil:

  • direct drilling of wheat is being compared with conventional tillage, to determine the effect of soil disturbance on emissions
  • application of a nitrification inhibitor, dicyandiamide (DCD), is being tested. DCD shuts down some of the microbes in the soil nitrogen cycle, disrupting the production of N₂O.

In the first experiment, a legume-rich pasture was sprayed out and planted (April 2010) with winter wheat by either direct drilling or sowing into cultivated soil. Half the area was also sprayed with DCD. Automatic gas collection chambers sited across the experimental plots monitored the N₂O gas losses from February 2010 to February 2011.

Results to date

Cumulative N₂O emissions across all treatments in the first year was 35 kg nitrogen per hectare - a relatively high level of emissions even by international standards. The high level of N₂O was most likely the result of a combination of factors:

  1. The high rainfall in the region encouraged significant nitrogen fixation under the previous clover and grass pasture, resulting in high levels of organically fixed nitrogen in the soil.
  2. Soil nitrogen was further increased by spraying out the grass and encouraging clover growth in the clean-up phase in the year before cropping.
  3. Soil at the site was relatively acid, which can increase N₂O emissions.
  4. Most N₂O is formed when a soil is saturated with water. The soil at the site has a perched water table, meaning that heavy clay in the subsoil slows down the drainage. This causes the soil to stay wet after rain for longer than a soil that drains freely.

Next steps

The research team are now repeating the experiment at an adjacent site. The results of the project will be communicated to government, policy makers and scientists through scientific and conference papers, and to the farming community through field days, workshops and a series of best management practice recommendations.

The project will result in options for practice change in the farming community that reduce emissions and provide information to inform models such as the National Carbon Accounting System for industry and government decision makers.

This project was part of the national Nitrous Oxide Research Program, funded by the Australian Government Department of Agriculture, Fisheries and Forestry under its Australia’s Farming Future Climate Change Research Program.

Related resources

Titlesort descending Excerpt
Nitrous oxide publications A bibliographic survey of research publications produced by PICCC's nitrous oxide projects.

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