Nitrification (DMPP) and urease (NBPT) inhibitors had no effect on pasture yield, nitrous oxide emissions, or nitrate leaching under irrigation in a hot-dry climate

Modern dairy farming in Australia relies on substantial inputs of fertiliser nitrogen (N) to underpin economic production. However, N lost from dairy systems represents an opportunity cost and can pose several environmental risks. N-cycle inhibitors can be co-applied with N fertilisers to slow the conversion of urea to ammonium to reduce losses via volatilisation, and slow the conversion of ammonium to nitrate to minimise leaching of nitrate and gaseous losses via nitrification and denitrification. In a field campaign in a high input ryegrass-kikuyu pasture system we compared the soil N pools, losses and pasture production between (a) urea coated with the nitrification inhibitor 3,4-dimethyl pyrazole phosphate (b) urea coated with the urease inhibitor N-(n-butyl) thiophosphoric triamide and (c) standard urea. There was no treatment effect (P > 0.05) on soil mineral N, pasture yield, nitrous oxide flux or leaching of nitrate compared to standard urea. We hypothesise that at our site, because gaseous losses were highly episodic (rainfall was erratic and displayed no seasonal rainfall nor soil wetting pattern) that there was a lack of coincidence of N application and conditions conducive to gaseous losses, thus the effectiveness of the inhibitor products was minimal and did not result in an increase in pasture yield. There remains a paucity of knowledge on N-cycle inhibitors in relation to their effective use in field system to increase N use efficiency. Further research is required to define under what field conditions inhibitor products are effective in order to be able to provide accurate advice to managers of N in production systems.