Aims High nitrogen (N) fertiliser inputs in intensive sugarcane systems drive productivity but also significant emissions of nitrous oxide (N2O), a potent greenhouse gas. Fertiliser and soil N availability for both plant N uptake and N2O emissions across different N rates 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. 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, highlighting the interaction between fertiliser and soil N, which determined the overall magnitude but also the response of total N2O emissions to N rates dependent on the site conditions. 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 interaction between fertiliser and soil N and its contribution to N uptake and N2O emissions demonstrate the importance of integrating soil fertility management with N fertiliser rate recommendations for sugarcane systems to maintain crop productivity and reduce environmental impacts.

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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.

Denitrification is a key process in the global nitrogen (N) cycle, causing both nitrous oxide (N2O) and dinitrogen (N2) emissions. However, estimates of seasonal denitrification losses (N2O+N2) are scarce, reflecting methodological difficulties in measuring soil-borne N2 emissions against the high atmospheric N2 background and challenges regarding their spatio-temporal upscaling. This study investigated N2O+N2 losses in response to N fertiliser rates (0, 100, 150, 200 and 250 kg N ha-1) on two intensively managed tropical sugarcane farms in Australia, by combining automated N2O monitoring, in-situ N2 and N2O measurements using the 15N gas flux method and fertiliser 15N recoveries at harvest. Dynamic changes in the N2O/(N2O+N2) ratio (< 0.01 to 0.768) were explained by fitting generalised additive mixed models (GAMMs) with soil factors to upscale high temporal-resolution N2O data to daily N2 emissions over the season. Cumulative N2O+N2 losses ranged from 12 to 87 kg N ha-1, increasing non-linearly with increasing N fertiliser rates. Emissions of N2O+N2 accounted for 31–78% of fertiliser 15N losses and were dominated by environmentally benign N2 emissions. The contribution of denitrification to N fertiliser loss decreased with increasing N rates, suggesting increasing significance of other N loss pathways including leaching and runoff at higher N rates. This study delivers a blueprint approach to extrapolate denitrification measurements at both temporal and spatial scales, which can be applied in fertilised agroecosystems. Robust estimates of denitrification losses determined using this method will help to improve cropping system modelling approaches, advancing our understanding of the N cycle across scales.

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.