Exponential response of nitrous oxide (N2O) emissions to increasing nitrogen fertiliser rates in a tropical sugarcane cropping system

High rainfall, irrigation and large fertiliser nitrogen (N) inputs create conditions highly conducive for nitrous oxide (N₂O) emissions in sugarcane systems. A positive non-linear response of N₂O emissions to increasing N fertiliser rates has been reported from a range of cropping systems, where N inputs exceed crop requirements. However, the response of N₂O emissions to N fertiliser rates in sugarcane remains largely unclear. This study monitored N₂O emissions in a tropical sugarcane system in Australia to examine the response of N₂O emissions to increasing N fertiliser application rates. Soil-borne N₂O emissions were measured at high temporal resolution with an automated monitoring system across four N fertiliser treatments (0, 150, 200 and 250 kg N ha⁻¹). Furrow irrigation and rainfall events triggered large N₂O emissions with peak N₂O emissions > 100 g N ha^-1 d⁻¹. Nitrous oxide emissions peaked when high soil nitrate (NO₃-) content (> 40 kg N ha⁻¹) coincided with water-filled pore space (WFPS) near saturation. Emissions of N₂O in the first three months after N fertilisation accounted for 74–96 % of total N₂O emitted over the year. Cumulative N₂O emissions increased exponentially with N fertiliser rates, ranging from 0.29 to 3.06 kg N ha⁻¹. Emission factors (EF) ranged from 0.69 to 1.11 %, exceeding the IPCC default of 1 % at 250 kg N ha⁻¹ of N rate. The amount of N₂O emitted per kg sugar yield doubled above the recommended N rate of 200 kg N ha⁻¹. The exponential response of N₂O to fertiliser N suggests that N₂O emissions from intensive sugarcane systems with high fertiliser N inputs are considerably larger than currently assumed and that a non-linear model may be more appropriate for estimating N₂O emissions from sugarcane soils. The steep increase in N₂O intensity demonstrates environmental inefficiency at high fertiliser N rates, emphasising the importance of avoiding excessive N fertiliser application in tropical sugarcane systems from both agronomic and environmental perspectives.