Millennial-age glycerol dialkyl glycerol tetraethers (GDGTs) in forested mineral soils: &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C-based evidence for stabilization of microbial necromass

Daniel Montluçon,Maarten Lupker,Tessa Sophia Van Der Voort,Hannah Gies,Negar Haghipour,Timothy Ian Eglinton,Frank Hagedorn

doi:10.5194/bg-18-189-2021

Abstract

Abstract. Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate–carbon cycle model predictions. Emerging concepts regarding the formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues; however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for the isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic membrane lipids of archaea and bacteria, respectively – for subsequent natural abundance radiocarbon analysis. The method is applied to depth profiles from two Swiss pre-Alpine forested soils. We find that the Δ14C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low Δ14C values of these membrane lipids reflect heterotrophic acquisition of 14C-depleted carbon. We therefore attribute the 14C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.

Highlights

Soil organic matter (SOM) represents the largest reservoir of carbon in terrestrial ecosystems, exchanging large quantities of carbon with the atmosphere and supplying aquatic systems with organic and inorganic C (Parry et al, 2007; Battin et al, 2009; Bradford et al, 2016)
Analysis of isolated fractions on a quadrupole mass spectrometer operated in scan mode (Agilent 6130) for all masses between m/z 500 and 1500 reveals that more than 95 % of compounds in either fraction are comprised of masses assigned to glycerol dialkyl glycerol tetraethers (GDGTs) (Fig. 1)
As GDGTs are microbial membrane lipids, these findings reveal the presence of 14C-depleted, millennial-age microbial residues as a component of organic matter in deeper soils

Summary

Introduction

Soil organic matter (SOM) represents the largest reservoir of carbon in terrestrial ecosystems, exchanging large quantities of carbon with the atmosphere and supplying aquatic systems with organic and inorganic C (Parry et al, 2007; Battin et al, 2009; Bradford et al, 2016). Emerging concepts of SOM suggest that only a small fraction of annual C inputs from plants persists in the soils, and that microbial products and residues stabilized by the interaction with reactive minerals comprise the majority of the soil C pool (Schmidt et al, 2011; Cotrufo et al, 2015; Lehmann and Kleber, 2015; Kallenbach et al, 2016; Kästner and Miltner, 2018). Evidence of the entombment of microbial necromass is presently limited and largely circumstantial, being primarily based on the finding of increasing contributions of microbial biomarkers compared to plant-derived compounds with increasing soil depth (Amelung et al, 2008; Miltner et al, 2012; Kallenbach et al, 2016; Liang et al, 2019; Ma et al, 2018)

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Discussion

Conclusion