Study of Ca and Mg cycles in five beech forests in the northern France and modifications induced by liming: combined use of elementary and isotopic approaches δ^26/24Mg, δ^44/40Ca and ⁸⁷Sr/⁸⁶Sr)

In France, many forest ecosystems grow on acidic and nutrient poor soils. In numerous cases, the chemical fertility of forest soils is slowly degrading due to increasing external pressures (e.g. decreasing atmospheric inputs, increasing biomass exportations) and is a growing concern in the international forest community. Forest management often must resort to restoration/remediation practices to ensure the sustainability of the ecosystem. Forest liming with a carbonate product is a known practice to reduce soil acidity, globally improve the ecosystem functioning and compensate nutrient losses caused by biomass harvest and exportation. However, our understanding of the effects of liming on magnesium and calcium biogeochemical processes and cycling in forest ecosystems is still incomplete. This study focuses on the fate of magnesium and calcium originating from the dissolution of liming products in the soils and trees of five beech forests located in Northern France over a period up to 45 years. In order to do so, 'conventional' total-element approaches were combined with a multi-isotopic approach (^26/24Mg, ^44/40Ca et ^87/86Sr). At all five sites, the data measured in control plots throughout the study period suggest that the exchangeable pools of nutrient cations in the soil has decreased over the last several decades and that this trend, together with the low chemical fertility level of the soil, was reflected in foliar nutrition (Ca, Mg, K, P). Compared to the control plots, soil exchangeable pools of Mg and Ca in the 0-40 cm layer in the limed plots increased during the first decade after liming but differences were no longer observable in soil pools after 20 to 30 years. However, the effect of liming on tree nutrition was still observed after 40 years, most probably because the biological cycling of these elements was enhanced by the lime input. The ecosystem response to the lime input varied between sites depending on the nature of the liming product (CaCO₃ or Ca-MgCO₃) and the initial chemical fertility level of the soil, the poorest sites showing the strongest responses. The natural variations of Mg, Ca and Sr isotopic ratios combined with conventional approaches highlight the importance of the BIO component of the biogeochemical cycling of Mg and Ca in the studied low chemical fertility beech forests. These isotopic tools also enabled to show the long-lasting incorporation of the Mg and Ca released from the liming product in the biological cycle and highlight the changes in the biogeochemical functioning of these forest ecosystems after the liming operation. (author)