Mg and Ca root uptake and vertical transfer in soils assessed by an in situ ecosystem-scale multi-isotopic ( super(26)Mg & super(44)Ca) tracing experiment in a beech stand (Breuil-Chenue, France)

Background and aims: The sustainability of forest ecosystems may be at stake especially in forests on base-poor soils due to reduced nutrient deposition and intensified silvicultural practices. Understanding nutrient availability and cycling is therefore essential to manage forest soil fertility. This study aims to assess in a beech plot Mg and Ca vertical transfer in soil and root uptake using an isotopic tracing experiment. Methods: A simulated rainfall containing a small amount (960 gMg.ha super(-1); 530 g Ca.ha super(-1)) of highly enriched super(26)Mg and super(44)Ca was sprayed on the forest floor of a 35-yr-old beech plot. The isotopic composition of fine roots and of the soil exchangeable Mg and Ca pool was monitored during 1 year. A pool and flux model (IsoMod) was developed to predict the labeling of the soil and vertical transfer of tracers. Results: Tracers ( super(44)Ca and super(26)Mg) were immediately retained in the thin litter layer. During the following year, Mg and to a lesser extent Ca were progressively released. After 1 year, the exchangeable Mg and Ca pools of the upper mineral layer (0-5 cm) were strongly labeled (~660ppt, representing ~55 % of the tracer input and ~370ppt, ~41 % of the tracer input respectively). A significant proportion (~8 % super(26)Mg, ~2 % super(44)Ca) of tracer was leached through the soil, below 10 cm. This amount was much larger than what was predicted using a simple mixing model. The Ca and Mg isotopic composition of fine roots at all depths was close or lower than that of exchangeable Ca and Mg respectively. Conclusions: An in situ ecosystem-scale super(26)Mg and super(44)Ca isotopic tracing experiment was successfully carried out. Tracers were at first strongly retained in the litter layer, then progressively transferred to soil horizons below. Nutrient cycling of Mg and Ca were proven to be very different. Mg had a higher mobility in the soil than Ca, and nutrient uptake sources were proven to be different.