DMPP-urea restricts nitrification in the first month without improving agronomic N use efficiency

Enhanced efficiency N fertilisers (EENF) including DMPP-urea (Entec®) can lower nitrous oxide emissions compared to urea, but evidence for higher agronomic efficacy—the ability to attain optimum grain yields at lower N application rates than standard urea fertiliser—is lacking. Three multi-N fertiliser rate trials were established in the Australian wet subtropics to investigate whether Entec® could attain maximum rice and maize yields with lower N rates than urea. Crop biomass, N uptake and grain yields were quantified, and soil mineral N concentrations in the 0–10 cm layer were tracked for 30 d after N fertiliser application in the nil-N and the 90 kg N ha −1 (Entec® and urea) plots in two trials. There was no evidence at any site that optimum biomass yields, crop N uptake, or grain yields could be achieved at a lower dose rate of N as Entec® than urea. Entec® inhibited nitrification for up to a month where N fertiliser was broadcast and incorporated, and the lack of impact on agronomic efficiency was attributed to climatic conditions in the first month after N fertiliser application not being conducive to large N losses (i.e., leaching from rainfall, or denitrification). Where N fertilisers were banded, it is likely that high concentrations of ammonia within the urea bands were detrimental to soil microbes, resulting in suppression urea hydrolysis and inhibition of nitrification. In these circumstances, nitrification inhibitors such as DMPP are effectively redundant during the month following N fertiliser application. Further studies are therefore required to examine the role of soil properties on the efficacy of EENFs, combined with modelling studies that examine the frequency of high intensity rainfall events that may cause NO 3 − leaching or denitrification in the weeks after N fertiliser application.