The NUTRIBOR project addresses a critical gap in our understanding of tree mineral nutrition under global change by leveraging non-traditional boron stable isotopes (δ¹¹B) as tracers of geo- versus bio-nutrient sources. Forest ecosystems depend on a complex interplay between organic recycling and mineral weathering to supply essential nutrients beyond carbon, nitrogen, and phosphorus. By combining controlled ecotron experiments, field manipulations in Brazilian eucalyptus plantations, and observations across diverse critical zone observatories (CZOs), NUTRIBOR seeks to quantify how environmental stressors—drought, soil depletion, and climate change—alter the balance of nutrient pathways and leave discernible isotopic fingerprints in soil, plant, and watershed compartments.
Project Overview and Objectives
- Rationale: Forests sequester a quarter of anthropogenic carbon emissions annually yet face increasing nutrient limitations that undermine productivity and climate-feedbacks. Traditional models inadequately capture multi-element nutrient stress, especially for mineral-derived micronutrients.
- Innovation: Boron isotopes offer a unique proxy because biological processes enrich ¹¹B, whereas weathering releases ¹⁰B-enriched pools. By tracing δ¹¹B across compartments, we can diagnose shifts between recycling (“bio”) and weathering (“geo”) nutrient sources.
- Approach: Four work packages integrate data acquisition and modeling:
- WP1: Analytical and experimental design, establishing robust δ¹¹B protocols and ecotron and field setups.
- WP2: Boron nutrition at the water-soil-plant scale (highlighted below).
- WP3: Development of isotope-enabled ecophysiological and reactive-transport models to simulate nutrient budgets and B isotope fractionation from root to river.
- WP4: Upscaling δ¹¹B diagnostics to catchment scales, validating model predictions with multi-site river monitoring.
Spotlight on WP2: Boron Nutrition at the Water–Soil–Plant Scale
WP2 tests the central hypothesis that the δ¹¹B signature of tree and soil compartments reflects an ecosystem’s nutritional status and stress. It comprises three interlinked tasks:
Task 2.1: Controlled Fractionation Experiments
In the ecotron (France) and Brazilian plantations, a factorial design manipulates water availability (ambient vs. −40% rainfall) and boron supply (deficient vs. fertilized). Periodic sampling of soil solution, fine roots, sap, wood, and leaves quantifies δ¹¹B fluxes and establishes mass balances. These controlled perturbations elucidate how drought and B limitation drive preferential accumulation of ¹¹B in plant tissues.
Task 2.2: Coupling B Isotopes with Macronutrient Cycles
Concentrations and fluxes of key macro- and micronutrients (K, P, Ca, Mg) are measured alongside δ¹¹B to determine whether boron isotopic shifts serve as proxies for broader nutrient stress. This comparative analysis informs parameterization of the CASTANEA ecophysiological model and identifies correlations between δ¹¹B and nutrient uptake efficiency under varying fertilization regimes.
Task 2.3: Transposability to Natural Ecosystems
To assess the generality of experimental findings, WP2 extends sampling to non-manipulated forests within the Strengbach, Mont Lozère, and Guadeloupe CZOs. Here, intra- and inter-species δ¹¹B variability in roots, trunks, branches, leaves, litter, and throughfall is mapped. Coupled with existing meteorological and soil metrics, this “B isoscape” validates whether patterns observed in eucalyptus can predict nutrient dynamics across diverse forest types.
Anticipated Deliverables and Impact
- Theoretical Isoscapes linking δ¹¹B distributions to bio- versus geo-nutrient contributions at the plant-soil interface.
- Quantitative Relationships between δ¹¹B and macronutrient cycling, enabling novel diagnostic tools for ecosystem nutrient status.
- Simplified Sampling Protocols for rapid assessment of forest nutritional stress using boron isotopes.
- Cross-ecosystem Validation demonstrating δ¹¹B’s utility beyond model systems.
By illuminating the mechanistic links between nutrient stress and isotopic signatures, WP2 lays the foundation for scalable, isotope-based diagnostics of forest health. These insights will refine process-based models, inform sustainable management of forest plantations, and enhance predictive capabilities of Earth System Models under nutrient limitation scenarios.