Expanding on PICCC's earlier enhanced efficiency fertiliser study, this project examined the impact of nitrogen fertilisers amended with urease and nitrification inhibitors on indirect nitrous oxide emissions via ammonia volatilisation and nitrate leaching.
Up to 48% of applied urea is lost through ammonia volatilisation, and studies conducted in Australia have shown that urease inhibitors may reduce this loss enough to make their use economically viable. In systems such as dairy farms, where urea is the most common source of fertiliser nitrogen, there is great potential for reduction of ammonia volatilisation through the use of urease inhibitors. In vegetable systems, high ammonia and nitrate leaching loss is predicted to occur as a result of high manure inputs. In these systems, the use of nitrification inhibitors to address leaching and nitrous oxide losses may increase ammonia loss but there is no information of the extent to which this might occur.
Researchers aimed to provide verification data to increase the confidence in modelling ammonia volatilisation and nitrate leaching in conventional and inhibitor fertiliser and manure treatments.
Researchers merasured changes in ammonia volatilisation and nitrate production resulting from the application of:
- a urease inhibitor – used to reduce ammonia emissions
- a nitrification inhibitor (3,4-dimethylpyrazole phosphate [DMPP]) – used to reduce nitrous oxide emissions and nitrate leaching losses, but can increase ammonia volatilisation
- manures, which are increasingly being applied in vegetable production and are potentially a large source of ammonia volatilisation.
Data on the potential mitigation of, or increase in ammonia volatilisation through the use of inhibitors, and nitrogen mass balance are essential for establishing methodologies to reduce indirect nitrous oxide emissions in intensive agricultural systems.
The urease inhibitor-amended fertiliser (Green UreaNV®) reduced ammonia loss from dairy pastures in both temperate (by 45– 56%) and sub-tropical (by 40%) regions, while the nitrifi cation inhibitor-amended fertiliser (urea with ENTEC®) reduced nitrous oxide emissions in all of the field trials.
The ENTEC® fertiliser, however, had a variable impact on ammonia volatilisation - for example, ammonia loss from manure increased by up to 20% in one vegetable trial but was not affected in the other. This highlights the importance of considering all potential loss pathways when developing strategies to reduce nitrogen loss.
The inhibitor products only increased plant productivity in the sub-tropical dairy pasture (22–36% increase), most likely due to the low background mineral nitrogen levels of this system. The high background levels and inputs in the vegetable systems, in comparison, masked any potential savings in nitrogen from using the ENTEC® fertiliser.
This finding suggests that simply reducing nitrogen inputs into these systems will have significant environmental, economic and social outcomes, with many intensive vegetable production systems using fertiliser well in excess of crop requirements.
Creating strategies to reduce ammonia loss from urea and manures in these intensive industries will lead to real benefits for Australian agriculture.
The research provided evidence for industry that urease and nitrification inhibitors can be effective at reducing ammonia and nitrous oxide loss, respectively. However, the variable productivity benefits resulting from inhibitor use highlight remaining questions about the economic feasibility of applying them to commercial dairy and vegetable systems.
The key recommendation for industry for reducing indirect emissions and improving sustainability is to reduce nitrogen inputs to better match plant demand.