The origin of calcium in forest ecosystems: use of natural strontium isotopes

Calcium is essential for tree mineral nutrition and plays a major role in the neutralization of acid inputs in forest ecosystems. The decline in bioavailable Ca reserve in the soil, linked to acid deposition produced by industrial activity, was highlighted in a lot of forest sites. This requires a precise knowledge of the supplying sources of Ca. Measurements of natural strontium isotopes allow to determine accurately the origin of Ca in the vegetation. By means of this method, we highlight the extreme dependence of some forest ecosystems to the Ca input by atmospheric precipitation. The use of this isotopic tracer also allows characterizing the Ca exchanges between the ecosystem components. Finally, the coupling of this method with dendrochemistry adds an important piece to the knowledge of the soil acidification story. The change of the Sr isotopic ratio in the tree-rings of several Belgian sites and the comparison with similar trends in other regions suggest that the soil acidification in industrial regions started in the beginning of the XXth century, i.e. earlier than it is generally admitted.