From biological to lithological control of the B geochemical cycle in a forest watershed (Strengbach, Vosges)

There is a fast growing interest in understanding the coupling between mineralogical and biological processes responsible for the migration of elements through continental ecosystems. This issue has fundamental impacts at the soil/plant scale because it can explain the tight links between soil and plant development and at the watershed scale because it gives a direct access to the water quality. In the present study, we performed an extended investigation of the bio-geochemical cycle of boron, which is an element known to be suitable for investigating water/rock interactions and vegetation cycling. New B data are provided along the hydro-bio-geochemical continuum in a forest ecosystem (Strengbach basin, Vosges, France), from rainwaters down to the outlet of the basin including systematic analyses of throughfalls, soil solutions, springs and brooks scattered in the watershed. At the watershed scale, we evidence a relationship between the B isotopic composition of river waters and the weathering regime outlining a predominant control of the parent rock mineralogy on the B geochemical behavior. At the soil/plant scale, it appears that the B geochemical cycle is controlled by the vegetation cycling, which is characterized by an uncommon, easy to distinguish, B isotopic composition (δ¹¹B ranging from about +30‰ to +45‰). Each year the amount of B being involved in the vegetation cycle is about four times greater than that of B being exported out of the watershed. At 10 cm depth in soil, where the plant roots are expected to be the most active, we observe a marked seasonal oscillation of the B isotopic values, which is interpreted as resulting from the vegetation activity. A mass balance calculation based on the assumption that that ¹⁰B is preferentially accumulated in the biomass tends to indicate that the soil/plant system does not behave at steady state with respect to B.Because of the very distinct B isotopic signature of vegetation and minerals in soil, box modeling allows to quantify the part of the B fluxes involved in the vegetation cycling and the mineral reactions, respectively. This calculation reveals a clear correlation between the amount of B derived from soil weathering and the amount of B absorbed by plant roots. This result clearly supports the idea that a coupling exists between mineral weathering and plant activity, for the study of which B isotopes appear particularly suitable.