Dynamics of rare earth elements and associated major and trace elements during Douglas-fir (<i>Pseudotsuga menziesii</i>) and European beech (<i>Fagus sylvatica</i> L.) litter degradation

Abstract

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 different turnover rates among elements. Moreover, litter layers of varying 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 litter layers of two forested soils developed under Pseudotsuga menziesii and Fagus sylvatica L. trees. The results from the leaching experiment were also compared with the in situ REE composition of the soil solutions to clarify the impact that the litter degradation processes may have on soil solution chemical composition. Both tree species showed similar biogeochemical processes dominating the element dynamics during litter degradation. REE, Al, Fe and Pb were preferentially retained in the solid litter material, in comparison with the other cations, and their concentrations increased over time during the 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 degradation. In particular, the evolution of the REE patterns, relative to the age of the litter layers, allowed us to deliver new findings on REE fractionation and mobilization during litter decay. Specifically, the degradation of the litter was characterized by a decrease in the Y/Ho ratio and an increase in the LaN/YbN ratio. The relationship between these ratios provided information on the litter species-specific resistance to degradation, with Douglas-fir litter material showing a lower resistance. During the litter degradation of the two tree species, two main differences were highlighted with the help of the REE: (i) in Pseudotsuga menziesii the behaviour of Eu appeared to be linked to Ca during leaf senescence and (ii) species-specific release of organic acids during litter degradation leads to a more pronounced middle REE (MREE) enrichment in the Fagus sylvatica leachates. Finally, we showed the primary control effect that white fungi may have in Ce enrichment of soil solutions, which appears to be associated with the dissolution and/or direct transport of Ce-enriched MnO2 accumulated on the surface of the old litter due to the metabolic functioning of these microorganisms. Similar MREE and heavy REE (HREE) enrichments were also found in the leachates and the soil solutions, probably due to the higher affinity of these elements for the organic acids, which represent the primary products of organic matter degradation.