Interaction Between Soil and Fertiliser Nitrogen (N) Drives Plant N Uptake and Nitrous Oxide (N2O) Emissions in Tropical Sugarcane Systems

Abstract Aims: High nitrogen (N) fertiliser inputs in intensive sugarcane systems drive productivity but also significant emissions of nitrous oxide (N2O), a potent greenhouse gas. The effects of N fertiliser inputs on native soil N availability for plant N uptake as well as N2O emissions remain unknown, hindering efficient N management. This study investigated the contribution of fertiliser and soil N and their interaction to plant N uptake and N2O emissions in two intensively managed tropical sugarcane systems with different farming practices. Methods: High temporal resolution N2O measurements were combined with 15N recoveries across four N fertiliser rates, (100, 150, 200 and 250 kg N ha-1) in soil, plant and N2O emissions. Results: Cumulative N2O emissions ranged from 0.3 to 4.1 kg N ha-1, corresponding to emission factors ranging from 0.7 to 2.4%. Native soil N accounted for > 60% of cumulative N2O emissions and total plant N uptake. Fertiliser N addition increased N2O emissions from native soil N compared to the unfertilised control. These findings highlight the interaction between fertiliser and soil N, with the latter being the dominant source for N uptake and loss. Overall fertiliser 15N loss responded exponentially to N rates with 50% of applied N fertiliser permanently lost even at the recommended N rate. Conclusions: The response of soil-derived N2O emissions to N rates largely determined the magnitude of total N2O emissions, demonstrating the importance of site and soil specific interaction between soil and fertiliser N for N2O emissions.