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

[摘要] Given the diverse physico-chemical properties of elements, wehypothesize that their incoherent distribution across the leaf tissues,combined with the distinct resistance to degradation that each tissueexhibits, leads to different turnover rates among elements. Moreover, litterlayers of varying ages produce diverse chemical signatures in solutionduring 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 thesolid fractions and in the respective leachates of fresh leaves anddifferent 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 thein situ REE composition of the soil solutions to clarify the impact that the litterdegradation processes may have on soil solution chemical composition. Both tree species showed similar biogeochemical processes dominating theelement dynamics during litter degradation. REE, Al, Fe and Pbwere preferentially retained in the solid litter material, in comparison withthe other cations, and their concentrations increased over time duringthe degradation. Accordingly, different litter fractions produced differentyields of elements and REE patterns in the leachates, indicating that thetree species and the age of the litter play a role in the chemical releaseduring degradation. In particular, the evolution of the REE patterns,relative to the age of the litter layers, allowed us to deliver newfindings on REE fractionation and mobilization during litter decay. Specifically, the degradation of the litter was characterized by a decrease inthe Y / Ho ratio and an increase in the La N / Yb N ratio. Therelationship between these ratios provided information on the litterspecies-specific resistance to degradation, with Douglas-fir litter material showing a lower resistance. During the litter degradation of the two tree species, two maindifferences 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 degradationleads 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 inCe enrichment of soil solutions, which appears to be associated with thedissolution and/or direct transport of Ce-enriched MnO 2 accumulated on the surface of the old litter due to the metabolicfunctioning of these microorganisms. Similar MREE and heavy REE (HREE) enrichments werealso found in the leachates and the soil solutions, probably due to thehigher affinity of these elements for the organic acids, which represent theprimary products of organic matter degradation.

[学科分类] 大气科学