Probing the Nutrient Cycle in Forests using the Boron Isotopic Composition of Streams

Forests are essential for the regulation of Earth’s climate both locally and globally. Their ecosystems are dependent on a number of nutrients from mineral sources, which are derived from the underlying rocks and incorporated into clays and oxides in a process referred to as chemical weathering. Streams are ideal indicators of nutrient cycling at the catchment scale, and by extension an indicator of nutrient provision and stresses. As streams collect water from subsurface pathways including water from the root zone, their geochemical signals can be used to quantify nutrient uptake at the catchment scale.

Non-traditional stable isotopes offer opportunities to enhance our understanding of nutrient cycles in the Critical Zone. Biogeochemical processes cause measurable fractionation between metal isotopes, creating fingerprints for their pathway through the ecosystem. In this regard, the micronutrient boron presents an ideal tracer. At the catchment scale the B isotope signature is controlled by inorganic processes such as chemical weathering, atmospheric deposition and transport as dissolved species, but also by vegetational cycling. This can lead to significant deviation between the B-isotopic composition of Critical Zone compartments, in particular in streams, compared to its mineral sources. However, the understanding of the translation of B isotopic signals from the soil-plant system to streams needs to be further investigated in order to develop them as a catchment-scale proxy of nutrients.

Here we present B-isotopic data from the Quaraze instrumented catchment at the Mt. Lozère Critical Zone Observatory, Southern France, a long-term instrumented site covered by a mixed tree forest, which is experiencing water stress in summer. Stream samples were collected along the river profile from the outlet to the source over the course of four trips in April, June, August and October. Additionally, we collected groundwater from piezometers at 20m depth and solutions in the unsaturated zone from -2 to -10m depth. Generally, the stream displays strongly elevated δ11B compositions between 34.80‰ and 43.54‰, compared to the local groundwater (10.64‰ to 26.21‰) and soil solutions (11.75‰ to 38.41‰). Major changes in δ11B are observed from the stream source to the outlet, with the largest difference in August (43.52‰ at the source, 37.92‰ at the outlet). This behavior is exhibiting a strong seasonal dependency, since notably the source and outlet are identical within the analytical error in the month of October (40.55‰ and 40.39‰).

This dataset demonstrates that the B isotope signature is highly dynamic, both temporally and spatially, and sensitive to variations in source inputs. This in turn implies that nutrient provision is impacted by small changes in the ecosystem. Vegetational cycling might be playing a key role in explaining elevated stream δ11B compositions [Gaillardet and Lemarchand, 2018]. Using these data, a modelling framework will be applied to estimate the relative roles of recycled organic matter vs. chemical weathering as nutrient sources in this catchment.