B and δ11B biogeochemical cycle in a beech forest developed on a calcareous soil: Pools, fluxes, and forcing parameters

Abstract

Rock weathering and biological cycling hold the development and sustainability of continental ecosystems, yet the interdependence of macro- and micro-nutrients biogeochemical cycles and their implications for ecosystem functioning remains unclear, despite being of particular importance in the context of global changes. This study focuses on the stocks, fluxes and processes constituting the biogeochemical cycle of boron. Vegetation, soils and solutions were monitored for a full year in a temperate beech forest developed on calcareous soil. Despite an overwhelmingly large B pool in soils, this study points to limited influence of weathering emphasizing the importance of vegetation cycling on this site. The biological imprint on the B cycle is marked by (1) a strong ¹¹B enrichment of solutions compared to the mineral source and (2) systematic correlations observed between B and other strongly recycled elements in all water samples. B isotopes are fractionated within the beech stand with higher values in leaves (23.5‰) and lower in fine roots (-11.7‰), suggesting that the light ¹⁰B isotope is preferentially assimilated during plant growth. B isotopic data are consistent with a Rayleigh-like behaviour during xylem transfer leading to an ¹¹B enrichment in the higher parts of the trees, putting internal B transfer as the main driver of the large range of isotopic compositions between plant tissues. B apparent isotopic fractionations are observed in the annually produced biomass and total beech stand, albeit with different values: α_{xylem-biomass} = 0.980 ± 0.009 and 0.990 ± 0.002, respectively, suggesting ¹¹B transfer from old to new tissue. The developed model also points to an isotopic fractionation factor during B uptake much higher than previously evaluated (0.979 < α_uptake < 0.994). Overall, this study demonstrates that B isotopes appear as a promising tracer of soil-plant interactions with particular emphasis on tree adaptation to B bioavailability in soil.

Keywords: Biogeochemical cycle; Boron isotopes; Forest ecosystem; Plant uptake; Vegetation cycling.