A re-evaluation of the agronomic effectiveness of the nitrification inhibitors DCD and DMPP and the urease inhibitor NBPT

Increasing evidence is emerging that enhanced efficiency nitrogen (N) fertilisers (EENFs) can lower nitrous oxide (N2O) emissions from soils, but five recently published meta-analyses reported marginal benefits to agronomic efficiency (biomass or grain yields) when assessed against conventional N fertilisers. Closer inspection of the experiments included in these meta-analyses reveals that the vast majority were designed to evaluate N2O emissions, and thus used only one N fertiliser rate, typically the recommended N fertiliser rate for the local crop production system. We suggest that EENFs are unlikely to increase yields beyond conventional N fertilisers when the control fertiliser treatment is applied at the recommended rate for achieving maximum N-limited yield. To demonstrate our perspective, we re-evaluated data from only those studies comparing yield responses to conventional N fertiliser with those of the nitrification inhibitors dicyandiamide (DCD) and 3,4-dimethylepyrazole phosphate (DMPP) and the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) that included a sub-optimal N rate as well as a control 'recommended' N rate. While only 11 published studies met these criteria, the available data suggested that EENF products achieve significantly higher yields over conventional N fertilisers at suboptimal N rates, with the greatest yield difference (11%, P < 0.05) generated at 50% of the recommended N rate. Due to the additional costs of EENF products per unit N applied, the question asked should not be 'can EENFs increase yields?' but rather 'to what extent can N application rate be reduced by applying EENFs without loss of yield, and is this economically viable?' To obtain such information, further studies across a range of crops and environments are needed to more accurately derive agronomic response curves for EENFs and simple calculator tools that factor in the cost of a given EENF at a given time can be used to determine economic viability. Finally, holistic assessment should also consider additional benefits of lower N application rates, such as a reduction in the rate of nitrate leaching-induced soil acidification which has associated longer term management costs.