Dynamics of Rare Earths and associated major and trace elements during Douglas-fir (Pseudotsuga menziesii) and European beech (Fagus sylvatica L.) litter degradation

Given the diverse physico-chemical properties of elements, we hypothesize that their incoherent distribution across the leaf tissues, combined with the distinct resistance to degradation that each tissue exhibits, leads to distinct turnover rates between elements. Moreover, litter layers of different ages produce diverse chemical signatures in solution during the wet degradation. To verify our hypothesis, Na, K, Mg, Mn, Ca, Pb, Al and Fe were analysed together with the Rare Earth Elements (REE) in the solid fractions and in the respective leachates of fresh leaves and different humus layers of two forested soils developed under Pseudotsuga menziesii and Fagus sylvatica L. trees. The results from the leaching experiment were also compared to the in situ REE composition of the soil solutions to clarify the impact that the litter degradation processes may have on soil solution chemical compositions. Our results clearly show that REE, Al, Fe and Pb were preferentially retained in the solid litter material, in comparison to the other cations, and that their concentrations increased over time during the litter degradation. Accordingly, different litter fractions produced different yields of elements and REE patterns in the leachates, indicating that the tree species and the age of the litter play a role in the chemical release during the degradation. In particular, the evolution of the REE patterns according to the age of the litter layers allowed us to deliver new findings on REE fractionation and mobilization during litter degradation. In particular, the LaN/YbN ratio highlights differences in litter degradation intensity between both tree species, which was not shown with major cations. We finally showed the primary control effect that litter degradation can have on the REE composition of the soil solution, which presents a positive Ce anomaly associated with the dissolution and/or transportation of Ce-enriched MnO2 particles accumulated onto the surface of the old litter due to white fungi activity. Similar MREE and HREE enrichments were also found in the leachates and the soil solution, probably due to their higher affinity to the organic acids, which represent the primary products from the organic matter degradation.