Dynamics of rare earth elements and associated major and trace elements during Douglas-fir (Pseudotsuga menziesii) and European beech (Fagus sylvatica L.) litter degradation
Authors: Montemagno Alessandro , Hissler Christophe , Bense Victor , Teuling Adriaan J. , Ziebel Johanna , Pfister Laurent 
Published: July 2022

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.