Reply on AC2

doi:10.5194/bg-2021-268-rc3

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 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.

Highlights

Nutrient cycling is key to forest ecosystem sustainability and productivity, especially in sites characterized by low fertility or degraded soils
Our results clearly show that Rare Earth Elements (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
When normalized to the REE concentrations of the local atmospheric deposition, the REE patterns of bulk fresh leaves and litter material show some similarities between the two tree species

Summary

Introduction

Nutrient cycling is key to forest ecosystem sustainability and productivity, especially in sites characterized by low fertility or degraded soils. There are three types of nutrient cycles, which relate to geochemical, biochemical or biogeochemical processes (Morris, 2004). The biochemical cycle refers to processes involved in the transport and retention of nutrients inside the trees (such as the withdrawal of specific nutrients from leaves before senescence). The biogeochemical cycle encompasses the processes that occur outside of the trees and lead to the degradation of the organic waste material (such as exudates of leaves and stems, dead leaves and branches or even a whole tree) into its primary components and to the release of nutrients in a form that is reusable by trees (Morris, 2004). Organic matter degradation, which represents part of the biogeochemical cycle, is a major contributor to the nutrient stock available to trees in forest ecosystems

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