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.