Mathieu Sebilo

Background and aim: The natural abundance of nitrogen isotopes (δ ¹⁵ N) provides a powerful integrative tracer of nitrogen uptake and assimilation in plants, yet the internal partitioning of δ ¹⁵ N among plant tissues remains insufficiently understood. Methods: Here, we quantified isotopic discrimination during ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) uptake in maize ( Zea mays L.) grown under axenic hydroponic conditions at 0.2 and 2 mM N, representing low and high affinity transport systems, respectively. Results: Distinct and systematic isotopic offsets between roots and shoots revealed contrasting assimilation patterns for the two nitrogen forms. Under NH ₄ ⁺ nutrition, shoot and root δ ¹⁵ N were nearly identical, consistent with predominant assimilation in roots and limited translocation of reduced nitrogen. In contrast, NO ₃ ^– nutrition produced strong ¹⁵ N enrichment in shoots relative to roots, reflecting spatially partitioned NO ₃ ^– reduction and assimilation between organs. The δ ¹⁵ N of tissue NO ₃ ^– was inversely related with its concentration, supporting isotopic fractionation during nitrate reduction and dilution by vacuolar storage. These results demonstrate that intra‐plant δ ¹⁵ N partitioning primarily reflects the spatial localization of nitrogen assimilation. Conclusion: By establishing mechanistic baselines for intrinsic nitrogen isotope fractionation under controlled conditions, this study refines the physiological interpretation of plants δ ¹⁵ N and enhances its application to ecological and biogeochemical investigations.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools – nitrate and ammonium – while pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the principal dissolved inorganic intermediate linking the oxidative and reductive branches of the nitrogen cycle and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Its low and frequently undetectable concentration is the kinetic signature of this central position rather than evidence against it: rapid coupled turnover sustains high gross flux at near-zero standing concentration. Nitrogen retention, recycling and losses to the atmosphere are determined during nitrite turnover, where competing pathways partition fluxes according to kinetic and environmental constraints. Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled δ ¹⁵ N– δ ¹⁸ O measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of δ ¹⁵ N in resolving hidden turnover. However, its low concentration in natural environments can pose some challenges for analysis, requiring more sensitive approaches. Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N ₂ O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

Anthropogenic nitrogen inputs are increasing globally, while the conditions under which ecosystems transition from nitrogen retention to nitrogen loss remain poorly constrained. Environmental proxies commonly used to infer nitrogen loss, including redox status, carbon availability, and microbial gene inventories, provide limited power to predict when nitrate‐reduction pathways are expressed at the ecosystem scale. This limitation is particularly evident in coastal mangrove soils, where anaerobic and carbon‐rich conditions coexist with weak nitrate‐reduction activity. Here, we investigate nitrogen‐cycle organization along a spatially compressed mangrove–agricultural gradient on a Caribbean island by combining soil nitrogen pools, organic nitrogen isotopes (δ ¹⁵ N), inferred microbial metabolic functions putatively related to the microbial community (16S rDNA metabarcoding analysis), and controlled anoxic nitrate‐amended incubations. Across distances of less than 500 m, soils transitioned sharply from nitrogen‐rich, isotopically depleted mangrove sediments (total N up to 1.4%; δ ¹⁵ N≈2‰–3‰) to nitrogen‐poor, isotopically enriched agricultural soils (total N≈0.2%–0.6%; δ ¹⁵ N≈6‰–8‰). Despite widespread microbial functional potential associated with nitrate reduction throughout the transect, nitrate depletion under anoxic conditions remained slow in mangrove and back‐mangrove soils (0.56–1.15 nmol g ⁻¹ min ⁻¹ ) but was more than threefold faster in agricultural soils (2.32 nmol g ⁻¹ min ⁻¹ ). To integrate these long‐term isotopic and stoichiometric signals, we developed the Organic Nitrogen Index coupled to soil C/N ratios (ONI‐CN), an isotopic–stoichiometric framework combining δ ¹⁵ N and soil C/N ratios. Along the studied transects, ONI‐CN captured a sharp transition between two nitrogen‐cycle configurations, with mangrove and back‐mangrove soils clustering within a closed, recycling‐dominated state and agricultural soils in an open, loss‐dominated state, with no intermediate values. These observations support the interpretation that nitrate availability constrains the long‐term expression of nitrate‐loss pathways across the mangrove–agricultural gradient. The results are consistent with the presence of a functional nitrogen bank in mangrove soils, in which microbial nitrate reduction potential remains weakly expressed under chronically nitrate‐limited conditions. This framework highlights the sensitivity of nitrogen‐limited coastal ecosystems to increasing anthropogenic nitrate inputs and provides a conceptual basis for understanding threshold‐like reorganizations of nitrogen cycling under environmental changes.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools — nitrate and ammonium — and pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the only inorganic nitrogen species produced under both oxidative and reductive regimes and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Nitrogen retention, recycling and atmospheric loss are resolved at the stage of nitrite turnover, where competing pathways partition fluxes under kinetic and environmental constraints. Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled δ¹⁵N–δ¹⁸O measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of δ¹⁵N in resolving hidden turnover. Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N₂O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools — nitrate and ammonium — and pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the only inorganic nitrogen species produced under both oxidative and reductive regimes and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Nitrogen retention, recycling and atmospheric loss are resolved at the stage of nitrite turnover, where competing pathways partition fluxes under kinetic and environmental constraints. Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled δ¹⁵N–δ¹⁸O measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of δ¹⁵N in resolving hidden turnover. Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N₂O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools — nitrate and ammonium — and pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the only inorganic nitrogen species produced under both oxidative and reductive regimes and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Nitrogen retention, recycling and atmospheric loss are resolved at the stage of nitrite turnover, where competing pathways partition fluxes under kinetic and environmental constraints. Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled δ¹⁵N–δ¹⁸O measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of δ¹⁵N in resolving hidden turnover. Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N₂O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools — nitrate and ammonium — and pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the only inorganic nitrogen species produced under both oxidative and reductive regimes and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Nitrogen retention, recycling and atmospheric loss are resolved at the stage of nitrite turnover, where competing pathways partition fluxes under kinetic and environmental constraints. Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled δ¹⁵N–δ¹⁸O measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of δ¹⁵N in resolving hidden turnover. Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N₂O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

Selenium plays a crucial role in estuarine biogeochemistry, balancing essential nutrient functions with potential environmental toxicity. This study examines the seasonal distribution of dissolved Se species, including volatiles, in the Adour estuary in relation to anthropogenic influences. To characterize major Se inputs from upstream watersheds to downstream tributaries, water samples were collected at low tide during three different seasons in upstream freshwaters, industrial/urban effluents and downstream estuarine waters. A tidal-cycle sampling campaign was conducted under low discharge conditions to assess Se dynamics during downstream estuarine mixing. Total dissolved Se (TDSe) concentrations ranged from 71 (pristine river) to 656 ng L⁻¹ (industrial/urban-impacted tributaries). TDSe correlated strongly with nitrate (r = 0.84) in upstream waters, indicating significant agricultural and livestock contributions at the watershed scale. Selenate was the dominant species, followed by Se(-II+0) fraction and selenite. Volatile Se compound concentrations varied from 51 to 2757 pg L⁻¹. Seasonal changes suggest that Se speciation is mainly controlled by watershed inputs derived from land use (agricultural and livestock practices) rather than downstream estuarine inputs. This speciation study further indicates that Se reactivity/bio-availability in estuarine systems can be largely influenced by anthropogenic activities, although further characterization of the aqueous reduced Se fraction is still needed.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools — nitrate and ammonium — and pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the only inorganic nitrogen species produced under both oxidative and reductive regimes and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Nitrogen retention, recycling and atmospheric loss are resolved at the stage of nitrite turnover, where competing pathways partition fluxes under kinetic and environmental constraints. Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled δ¹⁵N–δ¹⁸O measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of δ¹⁵N in resolving hidden turnover. Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N₂O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

The increasing demand for reliable food authentication highlights the need for scalable and innovative tools to link geochemical fingerprints in food products with their geographic provenance. Food authentication is not only essential for preventing fraud but also offers a unique opportunity to relate agricultural products to their underlying geochemical signatures. Here we present a unified framework that combines stable isotopes (e.g. ⁸⁷Sr/⁸⁶Sr) and trace-element fingerprints measured in food products with explainable and cost-aware machine learning to support provenance verification.We first develop a cost-aware binary classification model for French sparkling wines, demonstrating how high-precision ⁸⁷Sr/⁸⁶Sr ratios can be partially substituted by low-cost elemental proxies (e.g. Rb) while maintaining strong discriminative power. To address scalability constraints, we extend this approach to a multiclass setting using cost-sensitive logistic regression to classify wines from multiple Portuguese and Chilean regions, explicitly handling class imbalance and feature redundancy. Finally, we introduce TeaPrint, an unsupervised multimodal clustering framework that jointly integrates isotopic, elemental and volatile organic compound data to uncover coherent regional geochemical patterns in international tea samples without requiring prior labels.Across these case studies, we show that food products carry integrated geochemical signatures that can be exploited for robust provenance authentication across heterogeneous datasets. By bridging forensic geochemistry and explainable machine learning, our approach offers a cost-efficient and scalable pathway towards robust provenance authentication and transparent food supply chains.