Impact of N Rates and the Application of the Nitrification Inhibitor DMPP on N2O Emissions and the Agronomic Performance of Urea Applied to Subtropical Summer Sorghum Crops on Contrasting Soil Types in Australia.

Combining the nitrification inhibitor DMPP with traditional urea fertilizers (commercially sold as Entec^®), has been established as the most effective fertilizer product for reducing summer cropping N₂O emissions in the subtropical grains region of eastern Australia. Whilst a positive outcome for climate mitigation methodologies, a demonstrable increase in yield or a reduction in the optimum N rate across a range of soil types, micro-climates and years is required to encourage farmer uptake and ensure profitability. This comprehensive study examined the nitrogen use efficiency of Entec^® versus standard urea over two seasons (2013-14 and 2014-15) of sorghum (Sorghum bicolor L.) summer crops across four fertiliser rates on contrasting alkaline Vertisol and acidic Oxisol soil types. Agronomic data were collected over both years and a combination of manual and automated chambers were used to quantify N₂O losses in both cropping seasons.

Cumulative N₂O losses (1^st November to 25^th May 2015) ranged from 0.38 kg N ha^-1 from the Zero N treatment to almost 3 kg N ha^-1 in the 160 kg N/ha Urea treatment. Emissions from the Entec^® product on the Vertisol were identical to that from urea at the lowest N rate (80 kg N) but reduced N₂O by 42% and 63% in the 120 kg N and 160 kg N treatments, respectively. Cumulative seasonal emissions increased exponentially with increasing N rate in the urea treatments but only linearly with Entec^®, while emission factors increased linearly from 0.47 to 1.62% of added N with increasing urea application but remained constant for the Entec^® product.

Fertilizer rates to achieve 90% of site potential yield were consistently lower with Entec^® on the Oxisol, with savings on N rate balancing or exceeding the additional cost of the fertilizer product. There were no such effects on the Vertisol where Entec^® use would have not been financially viable, although emission intensities were reduced by more than half at the high N rate.