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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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Nitrification inhibitors (NIs) have been shown to reduce emissions of the greenhouse gas nitrous oxide (n 2 O) from agricultural soils. However, their N 2 O reduction efficacy varies widely across different agroecosystems, and underlying mechanisms remain poorly understood. To investigate effects of the NI 3,4-dimethylpyrazole-phosphate (DMPP) on N-turnover from a pasture and a horticultural soil, we combined the quantification of N 2 and N 2 O emissions with 15 N tracing analysis and the quantification of the N 2 O-reductase gene (nosZ) in a soil microcosm study. Nitrogen fertilization suppressed nosZ abundance in both soils, showing that high nitrate availability and the preferential reduction of nitrate over N 2 O is responsible for large pulses of N 2 O after the fertilization of agricultural soils. DMPP attenuated this effect only in the horticultural soil, reducing nitrification while increasing nosZ abundance. DMPP reduced N 2 O emissions from the horticultural soil by >50% but did not affect overall n 2 + N 2 O losses, demonstrating the shift in the N 2 o:n 2 ratio towards N 2 as a key mechanism of N 2 O mitigation by NIs. Under non-limiting NO 3 − availability, the efficacy of NIs to mitigate N 2 O emissions therefore depends on their ability to reduce the suppression of the N 2 O reductase by high NO 3 − concentrations in the soil, enabling complete denitrification to N 2 .Agricultural soils have become the main source of anthropogenic nitrous oxide (N 2 O), a powerful greenhouse gas and the single most important substance depleting stratospheric ozone 1 . Delaying the conversion of ammonium (NH 4 + ) to nitrate (NO 3 − ), nitrification inhibitors (NIs) have been suggested as a means to reduce N 2 O emissions from agricultural soils. NIs demonstrated their efficacy across different cropping soils 2 , but results vary widely, and in particular in pasture soils the use of NIs had no or little effect on N 2 O emissions 3-5 . Despite a growing body of research on NIs, mechanisms and factors determining their efficacy to reduce N 2 O emission remain poorly understood 6 . The challenges to understand these mechanisms derive from the fact that N 2 O is formed via several different pathways in the soil matrix 7 , tightly coupled to different processes of N supply and consumption 8 . Critically, N 2 O can be further reduced to N 2 via the microbial-mediated process of denitrification, and the sole quantification of N 2 O as affected by NIs provides therefore only a limited insight into mechanisms of N 2 O mitigation using NIs.Microbial metabolic pathways can contribute via a wealth of different processes to N 2 O production and consumption, i.e. the reduction to N 2 in soils. Apart from abiotic processes, N 2 O formation can be categorized into www.nature.com/scientificreports www.nature.com/scientificreports/ nitrification-mediated pathways, denitrification and biotic formation of hybrid N 2 O 9 . Denitrification is generally assumed to be the main process contributing to overall N 2 O production from agricultu...

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