Nitrous oxide greenhouse gas emissions from dryland cropping soil

• Soil type: Sand over clay (duplex), deep yellow sand
• Average annual rainfall: 291-374 mm

How much N₂O is emitted from N-fertilised crops?

Annual losses from typical N-fertilised cropping soils in Western Australia have ranged from 0.02 to 0.13 kg N₂O-N ha^-1 yr^-1. The proportion of N fertiliser emitted as N₂O after a year, and correcting for the ‘background’ emission (no N fertiliser applied), has ranged from 0.01 to 0.06%.

When is the greatest risk of N2O emissions?

The greatest N2O emissions from Western Australian cropping soils has occurred following harvest and in response to summer-autumn rainfall when the soil is fallow. This is because the soil was warm and the rainfall enhances soil organic matter mineralisation, increasing the soil mineral N pool at a time when there was no plant growth. As a result, post-harvest N2O emissions have contributed 55 to 85% of annual N2O emissions.

Does growing a grain legume increase N₂O emissions?

Growing lupins did not enhance N₂O emissions during the growing season in comparison to the soil that was not cropped (bare) or cropped to wheat, with annual emissions ranging from 0.002 to 0.05 kg N₂O-N ha^-1 yr^-1. Incorporating grain legumes in cropping rotations did not increase cumulative N₂O emissions from the lupin-wheat cropping rotation in comparison to the wheat-wheat cropping rotation; total cumulative N₂O losses were approximately 0.1 kg N₂O-N ha^-1 two years for both rotations when averaged across a liming treatment.

Will increasing soil organic carbon increase N₂O emissions?

Directly increasing soil organic matter by incorporating crop residues at Buntine in the northern grainbelt increased N₂O emissions during a 2.5 year study. However, emissions were still low by international standards (<0.12% of the N fertiliser applied).

What strategies can decrease N₂O emissions?

Raising the soil pH was an approach proposed for mitigating N₂O emissions that occur following summer-autumn when the soil is fallow.  At Wongan Hills applying lime decreased N₂O emissions from a two-year cropping rotation (wheat-wheat) with a history of annual N fertiliser applications, but had no effect on a two-year cropping rotation that included a grain legume (lupin-wheat).