Boron isotopes in soils: investigation of horizon reactivity and vegetation cycling

Investigating the soil/plant coupling requires the development of specific approaches being unambiguously sensitive to mineral- and biology-derived reactions. Boron presents chemical properties that, a priori, well meet the conditions for tracing bio-geochemical reactions. In particular, it is present in moderate to high concentrations in minerals; it is very sensitive to water/rock interactions during which it is partitioned between solid and liquid phases and undergoes a great isotopic fractionation and, finally, it is an essential nutrient for plants. Here, we present an extensive study on B isotopes in two distinct soil/tree systems from the well-characterized Strengbach basin (http://ohge.u-strasbg.fr/indexuk.html). Both bulk soil samples and granulometric fractions were analyzed. Soil solutions (down to 60 cm depth) were monitored every 6 weeks over two years (2005-2006). Tree samples (spruce needles and beech leaves) punctually sampled during this period. A Mass budget based on B concentration and hydrology model clearly first indicates that trees largely control the distribution of B in soil uppermost layers by yearly mobilizing 4 times more B than it is drained by soil solutions below 60 cm depth. B isotopes in soil solution depth profile highlight the presence of a highly reactive layer a 10 cm depth, which is interpreted as resulting from seasonal chemical oscillations caused by the biology and hydrology cycles. Isotopic budget indicates that this layer is not at steady state and accumulates B over years. The increasing contribution with depth of the weathering-derived B flux is clearly observable by a shift of the δ11B values towards low values. At the soil scale, mass and isotopic budgets help distinguishing both the B fluxes related to the mineral weathering reactions and the vegetation cycling and even show a strong correlation between them. Detailed analyses of granulometric fractions permit the determination of the B-carrier phases in these two soils and help understanding the long-term B geochemical cycle in forest soils. This approach evidences the transfer of B from the coarse (primary) fractions at greater depth to the finest (secondary) fractions in uppermost layers. Investigation of B isotopes in the bulk soil samples shows opposite weathering regime with respect to the soil type. The brown acidic (dystrochrept) soil developed on an hydrothermally altered granite and under spruce trees shows a dominant dissolving behavior with little precipitation of secondary phases whereas the ochreous podzolic (Haplorthod) soil developed on the less hydrothermally altered granite and under beech trees shows an increasing contribution on the B isotopic signature of secondary minerals with decreasing depth. These observations bring conceptual basics on the use of B isotopes in investigating bio-geochemical reactions and further help to quantify the mass budget of the dissolution/precipitation reactions in forest soils as well as to identify with more details the soil layers being presently the most reactive.