Nonlinear response of N2O and N2 emissions to increasing soil nitrate availability in a tropical sugarcane soil
Robert Kirkby, Johannes Friedl, Naoya Takeda, Daniele De Rosa, David Rowlings, Peter Grace
Abstract
Abstract Purpose The reduction of the greenhouse gas nitrous oxide (N 2 O) to dinitrogen (N 2 ) via denitrification and N 2 O 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 N 2 O from a tropical sugarcane soil in response to increasing soil nitrate (NO 3 − ) availability. Methods Microcosms were established using a tropical sugarcane soil (Qld, Australia) and emissions of N 2 O and N 2 were measured following fertilisation with 15 NO 3 − –N equivalent to 25, 50 and 100 μg N g −1 soil, simulating soil NO 3 − contents previously observed in situ, and mimicking flood irrigation by wetting the soil close to saturation. Results Cumulative N 2 O emissions increased exponentially with NO 3 − availability, while cumulative N 2 emissions followed an exponential increase to maximum. Average daily N 2 emissions exceeded 5 µg N 2 –N g soil −1 and accounted for > 99% of denitrification. The response of N 2 O suggests preferential NO 3 − reduction with increasing NO 3 − availability, increasing N 2 O even when NO 3 − levels had only a diminishing effect on the overall denitrification rate. The fraction of N 2 O emitted from denitrification increased with NO 3 − availability, and was a function of soil water, NO 3 − and heterotrophic soil respiration. Conclusions Our findings show the exponential increase of N 2 O driven by excess NO 3 − , even though the complete reduction to N 2 dominated denitrification. The low N 2 O/(N 2 O + N 2 ) product ratio questions the use of N 2 O as proxy for overall denitrification rates, highlighting the need for in-situ N 2 measurements to account for denitrification losses from sugarcane systems.