The project expanded on PICCC's earlier study of nitrous oxide emissions in the high rainfall cropping zone in south west Victoria, aiming to improve nitrogen use efficiency through a better understanding of carbon and nitrogen cycling, and their contribution to grain production and nitrous oxide emissions. The work also evaluated the potential of inhibitor-coated fertiliser for reducing emissions without compromising income from grain production.
Soils in the high rainfall cropping region of south west Victoria are characterised by heavy clay subsoils and high organic carbon contents (>3%). These characteristics, in association with high rainfall (>600 mm), can result in prolonged periods of saturated anaerobic conditions which are conducive to very high levels of nitrous oxide production. Losses of up to 35 kg N2O-N / ha / year have been measured in the area. Furthermore nitrogen fertiliser applied in excess of crop nitrogen demand to these soils, has been found to produce unnecessary emissions without any production gain.
The research team undertook a series of field experiments comparing the effect of different nitrogen fertiliser management strategies (including inhibitors) on crop production and nitrous oxide emissions. This knowledge was used to determine the optimal balance between nitrogen supply and management strategies needed to maintain profitable cropping and minimise nitrous oxide emissions and other losses.
Management treatments assessed were those designed to alter carbon and nitrogen dynamics following conversion of leguminous pastures to cropping. The experiments were conducted across the high rainfall zone of western Victoria, in soils varying in background carbon and nitrogen.
The results suggest that matching the supply of nitrogen with crop demand to avoid large surpluses is the most profitable approach to reducing nitrous oxide losses, and requires knowledge of the supply of nitrogen from the soil.
A series of studies compared continuous pasture with early termination, late termination and a spring crop. The data indicated that pasture terminated shortly before the winter crop was sown can reduce the supply of labile nitrogen and assoiated subsequent nitrous oxide losses. Late termination reduced nitrous oxide emissions by more than 80% compared with early termination, over a 12-month period.
Early termination of long-term pasture prior to cropping is the district practice, on the basis that a longer fallow helps control weeds prior to cropping. However the researchers found that weed pressures and crop yield were the same irrespective of the timing of pasture termination.
The research team also found that nitrogen fertiliser coated with the nitrification inhibitor DMPP (3, 4-dimethylpyrazole phosphate) can reduce or delay nitrous oxide losses. However no significant improvement in crop yield was found, and given inhibitor-coated fertilisers cost significantly more than conventional urea, the result suggests their use is not currently cost-effective.
A limited study to determine whether grain yield response to nitrogen application could be predicted based on background soil carbon or nitrogen suggested that there was no simple relationship.
The research has provided farmers in the high rainfall zone with conclusive evidence that maximising nitrogen fertiliser effi ciency, by matching timing and rate with crop demand, is the most economically feasible option for reducing nitrous oxide emissions.
The recommendation to delay the timing of pasture termination prior to planting winter crops to minimise emissions is also signifi cant for the high rainfall zone cropping region, where early termination is district standard practice, for the weed control benefits.
|Nitrous oxide in cropping systems fact sheet||Fact sheet profiling the project 'Improving nitrous oxide abatement in high rainfall cropping systems'.|
|Nitrous oxide publications||A bibliographic survey of research publications produced by PICCC's nitrous oxide projects.|