Effects of development stage on organic matter transformation in Podzols

Olaf Brock,Karsten Kalbitz,Samira Absalah,Boris Jansen

doi:10.1016/j.geoderma.2020.114625

Abstract

Podzols are soils in which organic acid and mineral leaching results in the formation of a B horizon, which often contains a high amount of organic carbon. As Podzols cover large areas of the globe (±12% in European forests), carbon stocks of Podzols are important in the context of climate change mitigation. However, the dynamics of carbon storage in Podzols are not yet fully understood. In this study pyrolysis-GC/MS was used to investigate organic matter (OM) transformation from leaf litter to stabilized OM in the B horizon. It was analysed how OM transformation differed for different stages of Podzol development (leaf litter > incipient > active > buried) and for different vegetation types (deciduous, coniferous, mixed) in a temperate climate. The consequences for carbon storage in Podzols were evaluated. Identified pyrolysis products (105) were assigned to different chemical groups (lignin phenols, aliphatics, etc.) and relative abundances were calculated. The relative abundance of (lignin) phenols decreased from 42 to 58% in leaf litter and fermentation material to 34–49% in A and B horizons of incipient Podzols, to 14–47% in active Podzols, and to 10–18% in buried Podzols. At the same time, the relative abundance of aliphatics increased from 10 to 14% in leaf litter and fermentation material to 19–27% in the A and B horizons of incipient Podzols, to 13–56% in active Podzols, to 27–63% in buried Podzols. Vegetation type did not cause large changes in the OM composition. Incipient Podzols had a soil OM composition similar to the illuvial dissolved OM (DOM) input, as the buildup of stabilized OM has just started. Concluding, the soil OM composition changes for different Podzol development stages (incipient > active > buried), and stabilization of aliphatics increases when the relative input of illuvial DOM, and thus of lignin phenols, decreases.

Highlights

Podzols are extensively studied soils characterized by an illuvial horizon enriched in organic matter (OM) and aluminium or iron that have eluted from overlying horizons (Lundström et al, 2000; Sauer et al, 2007)
The OM composition is transformed from the original leaf litter derived dissolved OM (DOM) to the soil organic matter (SOM) in incipient Podzols, to the SOM in more well-developed active Podzols, and to the SOM that is stabilized in well-developed buried Podzols (Fig. 5)
The findings show that the molecular OM composition changes with increasing Podzol development, and these changes seem to be related to the amount of illuvial DOM input received from the surface

Summary

Introduction

Podzols are extensively studied soils characterized by an illuvial horizon enriched in organic matter (OM) and aluminium or iron (or both) that have eluted from overlying horizons (Lundström et al, 2000; Sauer et al, 2007). It is known that the formation of metal-organic complexes or precipitates plays an important role in the formation of the carbon rich and less permeable B horizons that are characteristic for Podzols (Jansen et al, 2005). The formation of these metal-organic complexes depends on environmental factors such as pH and metal/carbon (M/C) ratio (Jansen et al, 2002), on the availability of aluminium and/or iron cations, and on the OM composition (Scheel et al, 2007)

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