Nonlinear response of N2O and N2 emissions to increasing soil nitrate availability in a tropical sugarcane soil

Purpose: The reduction of the greenhouse gas nitrous oxide (N2O) to dinitrogen (N2) via denitrification and N2O source partitioning between nitrification and denitrification remain major uncertainties in sugarcane systems. We therefore investigated magnitude and product stoichiometry of denitrification and production pathways of N2O from a tropical sugarcane soil in response to increasing soil nitrate (NO3−) availability. Methods: Microcosms were established using a tropical sugarcane soil (Qld, Australia) and emissions of N2O and N2 were measured following fertilisation with 15NO3−–N equivalent to 25, 50 and 100 μg N g−1 soil, simulating soil NO3− contents previously observed in situ, and mimicking flood irrigation by wetting the soil close to saturation. Results: Cumulative N2O emissions increased exponentially with NO3− availability, while cumulative N2 emissions followed an exponential increase to maximum. Average daily N2 emissions exceeded 5 µg N2–N g soil−1 and accounted for > 99% of denitrification. The response of N2O suggests preferential NO3− reduction with increasing NO3− availability, increasing N2O even when NO3− levels had only a diminishing effect on the overall denitrification rate. The fraction of N2O emitted from denitrification increased with NO3− availability, and was a function of soil water, NO3− and heterotrophic soil respiration. Conclusions: Our findings show the exponential increase of N2O driven by excess NO3−, even though the complete reduction to N2 dominated denitrification. The low N2O/(N2O + N2) product ratio questions the use of N2O as proxy for overall denitrification rates, highlighting the need for in-situ N2 measurements to account for denitrification losses from sugarcane systems.