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

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Rose, TJ, Wood, RH, Rose, MT & Van Zwieten, L 2018, 'A re-evaluation of the agronomic effectiveness of the nitrification inhibitors DCD and DMPP and the urease inhibitor NBPT', Agriculture, Ecosystems & Environment, vol. 252, pp. 69-73.

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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 nitrateleaching-induced soil acidification which has associated longer term management costs.

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