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
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
Radiocarbon provides valuable constraints on carbon turnover in soils
Here we examine the
Despite their rapid adoption by biogeochemists and paleoclimatologists as molecular tracers and proxies of environmental conditions, there are numerous aspects regarding their production, turnover and fate that remain enigmatic. While isoGDGTs in soils are most likely produced by ammonia-oxidizing Crenarchaeota and heterotrophic methanogens
Previous estimations of the turnover time of GDGTs in soils have been based on stable isotopes and incubation experiments
Prior
In the present study, we used molecular-level, natural-abundance
Soils were sampled by taking soil cores at two forested sites in Switzerland, namely one near Lausanne (46.5838
The sub-Alpine soil from the site at Beatenberg is a podzol, which has a thick organic layer followed by a 10 cm A horizon and carbonate-free sandstone as parent material. The MAT at the site is 4.6
At each site, soil cores were taken from 16 locations on a regular grid on a 1600 m
Evaluation of the isolation method involved an assessment of the purity of separated fractions (i.e., potential interference from compounds other than the desired compounds) and determination of the amount and isotopic composition of external contamination introduced in course of the preparation sequence. For the latter, composites of different topsoil samples (0–5 cm) from a
Despite the relative ease of detection of GDGTs using modern HPLC mass spectrometry (MS) techniques, one challenge in the radiocarbon analysis of GDGTs in soil and sediment samples is their low abundance, with ambient concentrations of brGDGTs and isoGDGTs that are typically in the range of 10 to 1000 and 1 to 100 ng gdw
Polar fractions were separated on an Agilent 1260 HPLC system coupled to an Agilent 1260 fraction collector. Separation was achieved on two Waters Acquity UHPLC HEB hydrophilic liquid interaction chromatography (HILIC) columns (1.7
High-performance liquid chromatography (HPLC) mass chromatograms (
Even at the compound scale, SOM cycles on a continuum of timescales
Repetitive preparation of samples with 10 injections each reveals a recovery efficiency of 0.85
The extraneous contamination added in the preparatory process is assumed to be of constant mass
The blank assessment (Fig.
Blank assessment associated with GDGT isolation.
The recommended sample size to reach a precision of
In the Lausanne soil, concentrations of GDGTs are generally highest in the topsoil, where the isoprenoid and branched GDGTs comprise 0.6 and 2
Branched GDGT (brGDGT) abundances and parameter ratios in Lausanne and Beatenberg soil profiles. In both soils (
Changes in the relative abundance of the individual brGDGTs (Fig.
The GDGT fractions prepared for AMS measurement contained between 30 and 80
Radiocarbon content expressed as
In the Lausanne profile,
Similar patterns exist in the Beatenberg profile. Radiocarbon signatures of both iso- and brGDGTs decrease with
The
Turnover times of the compounds are calculated based on a two-pool model that requires the following three parameters to be fitted: the turnover time of the fast-cycling pool, the turnover time of the passive pool and the proportion of the fast-cycling pool. As only one radiocarbon measurement per compound and depth interval is available, two of the parameters need to be estimated, while one can be fitted accordingly. We use the proportion of the low-density particulate organic matter consisting of some decomposed residues, the so-called light fraction of the samples
Turnover times of isoprenoid and branched GDGTs. Turnover times are based on the single-pool turnover time of the light fraction (fPOM) or the short-chain fatty acids (SCFA – in parentheses) as approximation of the fast pool. Note: Loc – location.
Compared to prior methods used to achieve individual isoGDGT separation by HPLC
The GDGT-specific
Our study reveals low
The
Relationship between
We next consider the long-term stabilization microbially derived carbon as the source of
Soil properties related to the stability of soil organic matter. Effective cation exchange capacity (CEC), Fe
The
Overall,
The relative abundance of different brGDGTs, as expressed in the MBT
In addition to the insights into soil carbon turnover, the observed
We modified and validated a normal-phase HPLC method to isolate isoprenoid and branched GDGTs at the compound class level for radiocarbon analysis. Although further refinements in the method would be desirable, this new approach yields reliable GDGT
Application of the method to depth profiles for two well-studied sub-Alpine soil profiles in Switzerland reveals a marked decrease in
Our findings also provide motivation for further work to validate our interpretations and assess the broader significance of the current limited suite of observations. For example, comparison of the proportions and isotopic signatures of intact polar lipid GDGTs relative to the core lipids measured here could shed light on the significance active GDGT-producing communities residing at a specific soil depth versus remnants of past microbial activity (necromass). The potential sorptive stabilization of GDGTs could be verified by measuring GDGTs and their
Molecular structures of GDGTs analyzed in this study. The equations to determine the MBT
The data set and script for the turnover model used in this study are available at
HG, ML and TE conceptualized the study. HG and DM designed the method. HG isolated the compounds and wrote the turnover model. NH performed the radiocarbon measurements. TSvdV provided the data. HG, FH, ML and TE interpreted the data. HG prepared the paper with contributions from all coauthors.
The authors declare that they have no conflict of interest.
We thank Urs Overhoff and Markus Neuroth from RWE Power AG, for providing the lignite sample, and Marco Griepentrog and Cindy de Jonge, for the helpful discussions. We thank the Laboratory for Ion Beam Physics (ETH) for supporting us with the accelerator mass spectrometry measurements. Finally, this paper benefited from the thoughtful comments of two anonymous reviewers.
This research has been supported by the Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (grant no. 184865).
This paper was edited by Yakov Kuzyakov and reviewed by two anonymous referees.