Anthropogenic nitrogen inputs cause major negative environmental impacts, including emissions of the important greenhouse gas N₂O. Despite their importance, shifts in terrestrial N loss pathways driven by global change are highly uncertain. Here we present a coupled soil-atmosphere isotope model (IsoTONE) to quantify terrestrial N losses and N₂O emission factors from 1850-2020. We find that N inputs from atmospheric deposition caused 51% of anthropogenic N₂O emissions from soils in 2020. The mean effective global emission factor for N₂O was 4.3 ± 0.3% in 2020 (weighted by N inputs), much higher than the surface area-weighted mean (1.1 ± 0.1%). Climate change and spatial redistribution of fertilisation N inputs have driven an increase in global emission factor over the past century, which accounts for 18% of the anthropogenic soil flux in 2020. Predicted increases in fertilisation in emerging economies will accelerate N₂O-driven climate warming in coming decades, unless targeted mitigation measures are introduced.

Intensifying agricultural production and coastal urbanization are increasing nitrogen (N) loads to estuaries, potentially increasing emissions of the greenhouse gas nitrous oxide (N₂O). Here we present a first assessment of how changes in land use intensity affect estuarine N₂O fluxes. We measured N₂O concentrations over marine‐freshwater transects in the wet and dry seasons in eight subtropical estuaries selected for differences in land use intensity. Daily estuary N loads ranged from 0.5 ± 0.4 kg N km⁻² d⁻¹ (minimally impacted) to 51 ± 30 kg N km⁻² d⁻¹ (highly impacted), corresponding to higher concentrations of all inorganic N species (nitrate, ammonium, and N₂O) in the highly impacted estuaries. Net N₂O fluxes from the eight estuaries ranged from −20 μg N₂O‐N m⁻² d⁻¹ (sink) to +300 μg N₂O‐N m⁻² d⁻¹ (source). However, neither N concentrations nor N loads explained the variations in N₂O fluxes. Instead, seasonal differences in freshwater flushing times increased either N₂O uptake (minimally impacted systems) or N₂O efflux (moderately impacted systems) relative to N load. The lack of relationship between freshwater flushing times (kinetics) and N₂O fluxes from the highly impacted estuaries, combined with evidence for both low carbon quality and phosphorous limitation in those systems, suggests that N₂O emissions from highly impacted estuaries were controlled by stoichiometry rather than kinetics. This study shows that estuaries can shift from net sinks to sources of N₂O as land use intensity increases but that the magnitude of this switch cannot be predicted based on N loads alone. image

Estuaries play a key role in moderating the flow of nitrogen (N) to marine ecosystems. However, the magnitude of this N removal can vary dramatically both within and between estuaries due to the benthic habitats present. Here, we compare denitrification, coupled nitrification–denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) across a mosaic of benthic habitats in the subtropical Noosa River Estuary, Australia. Using ¹⁵N tracer techniques and passive pore‐water samplers, we show that coupled nitrification–denitrification was the dominant pathway for N₂ production across all habitats, with higher rates in vegetated habitats (10–70 μmol N m⁻² h⁻¹) compared to bare sediments (0.9–2 μmol N m⁻² h⁻¹). Unusual pore‐water profiles in the macroalgal sediments suggest the presence of sulfur‐driven anoxic nitrification of NH₄⁺ to NO₃⁻ and N₂. A benthic N budget showed that combined denitrification and coupled nitrification–denitrification accounted approximately 96% of the N₂ production, while DNRA accounted for 9% of total NO₃⁻ reduction pathways in the Noosa River Estuary. The macroalgae habitat contributed 76% of total N removal via N₂ production and 65% of N retention via DNRA, despite accounting for only 25% of the total surface area. We show a strong relationship between seagrass and macroalgae area and N₂ production (r² = 0.8; p < 0.01), and as such, the capacity to mitigate reactive N loads in estuaries may decrease with the large‐scale loss of seagrass and other vegetated habitats.