No evidence for higher agronomic N use efficiency or lower nitrous oxide emissions from enhanced efficiency fertilisers in aerobic subtropical rice
Rose, TJ, Quin, P, Morris, SG, Kearney, LJ, Kimber, S, Rose, MT & Van Zwieten, L 2018, 'No evidence for higher agronomic N use efficiency or lower nitrous oxide emissions from enhanced efficiency fertilisers in aerobic subtropical rice', Field Crops Research, vol. 225, pp. 47-54.
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Enhanced efficiency nitrogen (N) fertilisers (EENFs) contain chemical urease or nitrification inhibitors, or physical barriers (e.g. polymer coating) to minimise the rapid build-up of nitrate (NO3−) in soils. They have the potential to improve N use efficiency and lower N2O emissions from soils. However, evidence of the efficacy of EENFs in warm, wet subtropical conditions is lacking. We therefore developed N response curves (0, 30, 60, 90, 120 and 150 kg N ha-1) for urea, polymer coated urea (PCU), 3,4-dimethylpyrazole phosphate (DMPP)-urea (Entec®), N-(n-butyl) thiophosphoric triamide (NBPT)-urea (Green urea®) and a carbon-coated urea (Black urea®) in subtropical, aerobic rice (Oryza sativa L) crops in two fields with contrasting soils (Gleysol and Histosol), and quantified N2O emissions from nil-N and 90 kg N ha-1 treatments for all EENFs and urea. In the Gleysol, cumulative in-crop N2O emissions were relatively high (approximately 2 kg N2O-N ha-1 season-1 with 90 kg N ha-1 applied) with no significant mitigation from any EENFs compared to urea. Grain yield data fitted with an exponential model indicated that 95% of the estimated maximum grain yield (6.8 t ha-1 at 14% moisture) was achieved with 81 kg N ha-1 for the urea treatment. The yield response curves for all tested EENF products did not differ significantly from the urea-N yield response curve. In the Histosol, cumulative in-crop N2O emissions were negligible (around 0.05 kg N2O-N ha-1 season-1), with no significant difference (p < 0.05) between N fertiliser or nil-N treatments, and 95% of the estimated maximum grain yield (5.63 t ha-1 at 14% moisture) was achieved with only 11 kg N ha-1 for urea. There was no evidence that EENFs could achieve the maximum yield at a lower applied N rate. We hypothesised that the low soil pH of 4.9 (1:5 CaCl2 extract) may have inhibited nitrification in the Histosol, leading to low N2O emissions and a limited response to N fertiliser. Ultimately, the results of this study found no evidence that EENF products could improve agronomic N use efficiency or lower N2O emissions in the two aerobic rice crops studied.