Potential benefits of enhanced-efficiency fertilisers in the Wet Tropics

Conventional fertiliser nitrogen (N) is prone to losses through leaching, runoff and denitrification on sugarcane farms in the Wet Tropics. One management option to increase fertiliser-N use efficiency is through the use of ?enhanced-efficiency fertilisers? such as nitrification inhibitor- or polymer-coated urea (NICU or PCU). We established a field trial in each of the Tully and Innisfail regions to assess the potential agronomic and environmental benefits of NICU (Entec?) and blends of PCU (Agromaster?) with normal urea at different ratios. The N-release dynamics from the PCU followed an exponential pattern, which matched the crop-N uptake dynamics fairly well. There were no marked differences in the PCU N-release dynamics between the two sites. Accordingly, lower soil mineral-N contents in the 0?20 cm depth occurred in the PCU-blend treatments than in the urea treatments in the first 1?2 months after application. Therefore, application of PCU could potentially mitigate the risk of N loss associated with high rainfall events during the early cropping season. Higher ammonium (NH4+) accumulation and/or lower nitrate (NO3?) accumulation in the 0?20 cm soil were recorded in the NICU treatments than in normal urea treatments in the first month after application. Substantial movement of fertiliser N into deep soil was evident at Tully, particularly in the urea-only treatments. Apart from the PCU treatments, most fertiliser N disappeared within 2.5?3 months after fertiliser application. At Innisfail, PCU-urea blends (25:75 and 50:50) and NICU significantly increased cane and sugar yields compared to conventional urea when applied at the recommended rate (110 kg N/ha, based on the SIX EASY STEPS guidelines). However, no significant yield benefits were achieved with application of PCU blends and NICU at 75% of the recommended N rate. The potential benefits of NICU and PCU, as demonstrated from soil mineral N dynamics, did not translate into higher sugar yield at Tully, perhaps due to serious crop lodging.