Fatty acid 16:1ω5 as a proxy for arbuscular mycorrhizal fungal biomass: current challenges and ways forward

Abstract

Fatty acid biomarkers have emerged as a useful tool to quantify biomass of various microbial groups. Here we focus on the frequent use of the fatty acid 16:1ω5 as a biomarker for arbuscular mycorrhizal (AM) fungi in soils. We highlight some issues with current applications of this method and use several examples from the literature to show that the phospholipid fatty acid (PLFA) 16:1ω5 can occur in high concentrations in soils where actively growing AM fungi are absent. Unless the study includes a control where the contribution of other microbes can be estimated, we advocate for the use of the neutral lipid fatty acid (NLFA) 16:1ω5. This biomarker has higher specificity, is more responsive to shifts in AM fungal biomass, and quantification can be conducted along with PLFA analysis without doubling analytical efforts. We conclude by contrasting various methods used to measure AM fungal biomass in soil and highlight future research needs to optimize fatty acid analyses.

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Biology and Fertility of Soils
https://doi.org/10.1007/s00374-022-01670-9
POSITION ANDOPINION PAPERS
Fatty acid 16:1ω5 asaproxy forarbuscular mycorrhizal fungal
biomass: current challenges andways forward
YlvaLekberg1,2· ErlandBååth3· ÅsaFrostegård4· EdithHammer3· KatarinaHedlund3· JanJansa5·
ChristinaKaiser6· PhilipW.Ramsey1· TomášŘezanka5· JohannesRousk3· HåkanWallander3· MonikaWelc7·
PålAxelOlsson3
Received: 14 February 2022 / Revised: 14 September 2022 / Accepted: 27 September 2022
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022
Abstract
Fatty acid biomarkers have emerged as a useful tool to quantify biomass of various microbial groups. Here we focus on the
frequent use of the fatty acid 16:1ω5 as a biomarker for arbuscular mycorrhizal (AM) fungi in soils. We highlight some issues
with current applications of this method and use several examples from the literature to show that the phospholipid fatty
acid (PLFA) 16:1ω5 can occur in high concentrations in soils where actively growing AM fungi are absent. Unless the study
includes a control where the contribution of other microbes can be estimated, we advocate for the use of the neutral lipid fatty
acid (NLFA) 16:1ω5. This biomarker has higher specificity, is more responsive to shifts in AM fungal biomass, and quanti-
fication can be conducted along with PLFA analysis without doubling analytical efforts. We conclude by contrasting various
methods used to measure AM fungal biomass in soil and highlight future research needs to optimize fatty acid analyses.
Keywords PLFA 16:1ω5· NLFA 16:1ω5· Arbuscular mycorrhizal fungi· Biomass· Gram-negative bacteria
Background
Soil microbes are integral components of soil health, but
how we measure and interpret microbial abundance and
function is less clear (Fierer etal. 2021). Arbuscular mycor-
rhizal (AM) fungi are obligate root symbionts with most land
plants and obtain carbon (C) in exchange for phosphorus
and other putative services, such as increased drought tol-
erance and pathogen protection (Smith and Read 2008).
Many methods are used to quantify AM fungal biomass in
soils, including hyphal length and spore counts, DNA-based
approaches, and fatty acid analysis, each with its pros and
cons. For the fatty acid analysis, phospholipids are gener-
ally separated from neutral lipids to yield phospholipid fatty
acids (PLFA), which are components of cell membranes,
and neutral lipid fatty acids (NLFA) that are associated with
storage (Fig.1). The number of C atoms, number and posi-
tions of double bonds, and cyclic rings in fatty acid residues
follow the phylogeny of organisms, and their analysis can
therefore be used to estimate the biomass of various micro-
bial groups (Tunlid and White 1990). More than 350 papers
have been published using the fatty acid 16:1ω5 to quantify
AM fungal biomass since this method was first developed in
the early 1990s (Olsson and Lekberg 2022). Our concern is
that the method is not always used accurately, which likely
affects assessments of AM fungal biomass.
A recent review published in this journal describes cur-
rent challenges and future use of phospholipid fatty acid
(PLFA) analysis as an indicator for soil microbial biomass
and the composition of microbial communities (Joergensen
2022). The review is timely and highlights important issues
* Ylva Lekberg
ylekberg@mpgranch.com
1 MPG Ranch, Missoula, MT, USA
2 Department ofEcosystem andConservation Sciences,
W.A. Franke College ofForestry andConservation,
University ofMontana, Missoula, MT, USA
3 Department ofBiology, Lund University, Lund, Sweden
4 Faculty ofChemistry, Biotechnology andFood Science,
Norwegian University ofLife Science, Aas, Norway
5 Institute ofMicrobiology oftheCzech Academy ofSciences,
Prague, CzechRepublic
6 Department ofMicrobiology andEcosystem Science,
Division ofTerrestrial Ecosystem Research, University
ofVienna, Vienna, Austria
7 The Soil andPlant Laboratory, Swedish University
ofAgricultural Science, Uppsala, Sweden

Biology and Fertility of Soils
1 3
associated with the method, but we take issue with one argu-
ment put forward, which is that PLFA 16:1ω5 should be used
to quantify the biomass of AM fungi. This has been advised
against previously because PLFA 16:1ω5 is also found in
Gram negative (G −) or diderm bacteria, which makes it
a less suitable marker for AM fungi (Nichols etal 1986;
Fig. 1 Extraction and quan-
tification pathways using gas
chromatography (GC) of neutral
lipids and phospholipids, as
well as organisms known to
contain 16:1ω5 phospholipid
and neutral lipid fatty acids,
respectively

Biology and Fertility of Soils
1 3
Frostegård etal. 2011). The recent review (Joergensen 2022)
argues that this is not problematic based on two assump-
tions: (1) PLFA 16:1ω5 is found only in minute amounts in
G − bacteria and (2) this group of bacteria contribute at most
10% to the total microbial biomass. As such, any contribu-
tion by bacteria to the PLFA 16:1ω5 ought to be minimal.
Here we question those two assumptions and use exam-
ples from the literature to illustrate that PLFA 16:1ω5 may
be a poor biomarker for actively growing AM fungi. This is
important because more than half of all the papers published
to date have used PLFA 16:1ω5 to quantify AM fungal bio-
mass, predominately in soils and lacking proper controls
where the non-AM fungal contribution can be estimated
(Olsson and Lekberg 2022). We then contrast the perfor-
mance of PLFA 16:1ω5 with NLFA 16:1ω5, compare fatty
acid analysis with other methods to quantify AM fungal
biomass, and conclude by outlining current challenges and
future approaches. Our objective with this Position paper is
to provide a sound basis for choosing methods to quantify
the biomass of this important group of symbionts as accu-
rately as possible.
PLFA 16:1ω5 insources other thanactively
growing AM fungi
While several G − bacteria contain no detectable or only
trace amounts of PLFA 16:1ω5 (Zelles 1997), this fatty acid
can dominate in other isolates, representing up to 50% of all
PLFA or 30% of the total fatty acids in their cells (Walker
1969; Livermore etal. 1969; Joung etal. 2015; Islam etal.
2020; Chen etal. 2020). Indeed, PLFA 16:1ω5 is consid-
ered a biomarker for some G − bacteria (Findlay etal. 1990;
Findlay and Dobbs 1993). If the biomass of G − bacteria
is small relative to AM fungi, this may not be a problem,
but what support do we have for this assertion? One way to
address this question is to look at environments where AM
fungi are expected to be absent or in very low abundance,
and here results vary. For example, PLFA 16:1ω5 was in
low abundance or apparently absent in lake sediments and
compost (Findlay and Dobbs 1993; Cooper etal. 2002), but
it was the fourth most abundant fatty acid based on mol %
in wood fiber collected from other lake sediments (Regnell
etal. 1996). Also, the PLFA 16:1ω5 responded more than
any other fatty acid to lime additions in coniferous forests
and spruce plantations that contained very few AM hosts
(Frostegård etal. 1993; Cruz-Paredes etal. 2017). Another
way to address this is to consider results from microcosm
experiments where soils are incubated without plants. Here,
some experiments have found that PLFA 16:1ω5 was among
the markers responding most to the addition of leaf litter
(e.g., Benito-Carnero etal. 2021), or to treatments that
increase soil pH (Cruz-Paredes etal. 2017). Because AM
fungi are obligate biotrophs they may survive for some time
without hosts, but their biomass will not increase without
carbon supplied by a living host plants (with some nota-
ble exceptions in highly sophisticated invitro systems; e.g.,
Tanaka etal. 2022). Thus, AM fungi cannot be the source of
the increasing PLFA 16:1ω5 concentrations in these types of
experiments. Likewise, despite assertions that plants do not
produce PLFA 16:1ω5 (e.g., Brands etal. 2020), some spe-
cies within the family Proteaceae are metabolically capable
of synthesizing this fatty acid (Vickery 1971). A public data-
base tool (https:// plant fadb. org/ tree; Ohlrogge etal. 2018)
based on the Seed Oil Fatty Acid (SOFA) database (Mat-
thäus 2012) reports 93 occurrences of 11-Z-hexadecenoic
acid (i.e., 16:1ω5) in 60 out of 8000 plant species, albeit in
seeds or leaves, not roots. Combined, these studies illustrate
that PLFA 16:1ω5 can be quite abundant and show treat-
ment effects that cannot be explained by shifts in AM fungal
biomass.
What is the support for AM fungal biomass greatly
exceeding G − bacterial biomass in AM-dominated systems?
Clearly such data are very difficult to obtain and will be
rough estimates at best. In a Danish linseed field, AM fungal
biomass was estimated to be ~ 0.3–4 times that of total
bacterial biomass (Olsson etal. 1999), and in arable soils
in Germany, the value ranged from 1.3 to 1.6 (Faust etal.
2017) when applying a conversion factor of 322nmol mg−1
dry biomass for bacteria and 19nmol mg−1 for AM fungal
biomass (Olsson etal. 1999). These conversion factors
incidentally also illustrate that bacteria, with a higher surface
area per volume, contain an order of magnitude more PLFAs
per g biomass than AM fungi (Joergensen and Wichern
2008). Stable isotope probing (SIP) combined with fatty acid
analysis offers another way to assess whether AM biomass
exceeds G − bacterial biomass by tracing the relative
proportion of 13C in PLFA 16:1ω5 originating from living
plants (indicative of AM fungi) and dead organic matter
(indicative of bacteria, assuming that AM fungi are fully
biotrophic and acquire all their carbon from plants under
most situations). The study by Elfstrand etal. (2008) showed
more 13C in PLFA 16:1ω5 originated from green manure
than from actively growing leek plants in an agricultural
setting, and that G − bacteria dominated irrespective of
C-source. Likewise, Ven etal. (2020) used host plant and
soil with different δ13C signatures (C4 plant grown in soil
collected underneath C3 vegetation) and found that the
δ13C signal in the PLFA 16:1ω5 extracted from sand-filled
mesh bags was 30–50% higher and more similar to plant
root than the δ13C signal extracted from the soil, which was
more similar to soil organic matter. This probably signifies a
much greater AM fungal contribution to PLFA 16:1ω5 in the
mesh bags (where the only source of C for bacteria would be
AM fungal exudates or microbial necromass), and a much
greater bacterial contribution to PLFA 16:1ω5 in soils.

Biology and Fertility of Soils
1 3
Another indication that this was the case was a significant
correlation between PLFA 16:1ω5 and AM hyphal density
in the mesh bags, but not in the soil. Thus, the assumption
that AM fungal biomass dominates in AM systems, which
justifies the use of PLFA 16:1ω5 as an indicator specific for
this group with negligible interferences from other microbial
groups, is not supported under all situations. As such, PLFA
16:1ω5 should only be used when the contribution from
sources other than AM fungi can be estimated.
Comparisons betweenPLFA andNLFA
16:1ω5
Given the issues raised above, we argue that NLFA 16:1ω5
is a more specific biochemical signature compound to use
in AM studies because other dominant fungi (ascomycetes
and basidiomycetes) do not produce it in significant amounts
(Müller etal. 1994) and the common storage material for
prokaryotes is polyhydroxyalkanoic acids, which do not con-
tain 16:1ω5 or other long-chain fatty acids (Wältermann and
Steinbüchel 2005). Some actinomycetes can store triacylg-
lycerols or wax esters (Wältermann and Steinbüchel 2005)
but there is no evidence that NLFA 16:1ω5 is produced by
these organisms. Thus, there is no reason to believe that the
16:1ω5 fatty acid detected in the neutral fraction of lipid
extracts has prokaryote origin. Despite this, NLFA analyses
are less common than PLFA analyses for quantifying AM
fungi (Olsson and Lekberg 2022), possibly because of the
doubling of sample numbers postfractionation (Fig.1) and/
or lack of awareness among researchers regarding issues
associated with PLFA.
Some studies that have analyzed both 16:1ω5 PLFA and
NLFA show similar responses to treatments (Faust etal.
2017), but others do not. In a field experiment where a non-
AM host (amaranth) replaced an AM-host plant (wheat),
the NLFA 16:1ω5 in soils decreased to almost zero whereas
the PLFA 16:1ω5 increased (Ngosong etal. 2012). Given
that amaranth does not supply C to AM fungi, this increase
in PLFA 16:1ω5 cannot reflect greater AM fungal biomass.
More likely, the increase in PLFA 16:1ω5 in Ngosong etal.’s
study was due to G − bacteria. This is supported by Elfstrand
etal. (2008) where most of the 13C in PLFA 16:1ω5 came
from green manure, whereas most of the 13C in NLFA
16:1ω5 came from plants. Moreover, soil NLFA, but not
PLFA 16:1ω5, correlated well with AM colonization of bait
plants in old field successions (Hedlund 2002), and with
inoculum potential in bulk soil collected around plants that
varied in host quality (Vestberg etal. 2012).
To quantitatively compare PLFA and NLFA 16:1ω5 in
soils where actively growing AM fungi were present or not,
we selected all publications from a recent literature review
(Olsson and Lekberg, 2022) where both fatty acids were
reported. We also included values extracted from roots to
assess if patterns found in soils were similar in roots. We
found seven papers in total. In each publication, a micro-
bial inoculum lacking AM fungi was added to all treat-
ments, and AM fungal inoculations resulted in substantial
AM colonization of host plants whereas control plants were
non-mycorrhizal (TableS1). This is not a perfect compari-
son if other microbial groups respond to the absence of AM
fungi, which can occur sometimes (Gryndler etal. 2018) but
not always (Olsson etal. 1996). However, it allows for some
rough comparisons of the two fatty acids when viable AM
fungi are present or not. This analysis showed that PLFA
16:1ω5 did not differ substantially between soil without AM
fungi (0.80nmolg soil−1 ± 0.23) and soil with AM fungi
(1.3nmolg soil−1 ± 0.74, means ± SD), and that actively
growing AM fungi contributed on average 29% to the PLFA
16:1ω5 (Fig.2). That is, roughly 70% of this fatty acid origi-
nated from sources other than actively growing AM fungal
biomass, such as G − bacteria and dead AM fungi that had
not yet decomposed. In contrast, AM fungi accounted for on
average 92% of all NLFA 16:1ω5 in soil (Fig.2; TableS1).
Taken together, we argue that NLFA 16:1ω5 is not only more
specific but also more responsive to changes in AM fungal
biomass than PLFA. This is illustrated by the fact that the
difference between mycorrhizal and non-mycorrhizal soils
was 1.6-fold for PLFA 16:1ω5 and 22-fold for NLFA 16:1ω5.
The contribution by AM fungi to PLFA 16:1ω5 in roots,
however, was greater and more similar to the contribu-
tion to NLFA 16:1ω5 (94% vs. 98%, respectively; Fig.2;
TableS1). This greater contribution by AM fungi to PLFA
Fig. 2 Contribution by actively growing arbuscular mycorrhizal
(AM) fungi to 16:1ω5 phospholipid fatty acids (PLFA) and neutral
lipid fatty acids (NLFA) extracted from soil (n = 5 papers) or roots (6
papers) in studies that quantified both fatty acids were conducted in
sterilized soil inoculated with AM fungi as well as a microbial wash.
While variable among studies, it highlights that the contribution to
PLFA [PLFAAM − PLFANM)/PLFAAM] is much smaller than NLFA in
soil, but not in roots. Data used to generate this figure is in TableS1,
mean ± SD

Biology and Fertility of Soils
1 3
16:1ω5 in roots relative to soil is presumably because the
biomass of AM fungi can constitute several percent of
root biovolume (Toth etal. 1991), i.e., substantially more
than in soil, whereas the abundance of bacteria is orders
of magnitude lower in roots than in the rhizosphere (Bul-
garelli etal. 2013; Liu etal. 2017). Based on this, PLFA
16:1ω5 could probably be used to quantify AM fungal
abundance in roots, but not soil, unless accompanied by a
proper control where AM fungi are absent.
There are a few situations where care should also be
taken with NLFA 16:1ω5. Like PLFA, it has been found
in small amounts in plants (TableS1), although AM roots
contained on average 50 times more NLFA 16:1ω5 than
non-mycorrhizal roots (TableS1). This suggests that most
NLFA 16:1ω5 found in roots likely comes from AM fungi
rather than plants (or plant endophytes). The exception
may be when dried plant matter is used to stimulate hyphal
colonization into ingrowth bags (Hammer etal. 2011),
especially insituations where hyphal abundance is low.
In such cases, including mesh bags impenetrable by AM
fungal hyphae as controls is a good idea. Also, NLFA
16:1ω5 measures AM fungal biomass associated with
storage structures (spores and vesicles primarily) where
persistence in soil is little known (discussed more below).
High background values due to slow turn-over or non-
specificity do not seem to be a problem, however, because
we observed rapid responses in NLFA when C allocation
to AM fungi changed (see previous references) and high
ratios in both soil and roots relative to non-mycorrhizal
controls (TableS1). Finally, NLFA should never be used to
assess AM fungal biomass when the ratio NLFA/PLFA for
16:1ω5 is < 1 as it indicates that NLFA values primarily
derive from sources other than AM fungi (Olsson 1999).
A common misunderstanding from Olsson (1999) is that
PLFA (not NLFA) 16:1ω5 primarily measures AM fungal
biomass when ratios > 1, which may not be the case.
When estimates of bacterial background for 16:1ω5
PLFA can be obtained (e.g., by including mesh bags and
non-mycorrhizal treatments), using this fatty acid to quantify
AM fungi allows for direct comparisons with other microbial
groups also quantified by PLFA profiling. There are also
advantages of using both PLFA and NLFA to quantify AM
fungi as they can provide insights into (i) AM fungal energy
status (Lekberg etal. 2013), (ii) relative allocation to growth
vs. storage (van Aarle and Olsson 2003), and (iii) shifts in
AM fungi and G − bacteria across environmental gradients,
including changes in host abundance, soil organic matter,
and pH (Högberg etal. 2006; Welc etal. 2012). If estimates
of AM fungal biomass in soils are of interest and bacterial
contributions cannot be estimated, however, we argue that
NLFA 16:1ω5, not PLFA 16:1ω5, should be used because
of its higher specificity.
AM fungal biomass measurements —
comparisons withother methods andway
forward
It is important to remember that all methods are estimates
at best and can be biased to various extents. For exam-
ple, most measurements of AM fungal abundance in roots
have been based on microscopic assessment of presence/
absence in root intercepts (McGonigle etal. 1990). This
method does not differentiate between viable and non-
viable AM fungi and does not consider the proportion of
each intercept filled (Trouvelot etal. 1986), which may
result in poor correlations with AM fungal biomass. Like-
wise, molecular approaches targeting DNA or RNA, such
as qPCR, may overestimate the abundance of particular
taxa that sporulate readily or contain high concentration
of DNA per unit weight of biomass and/or high copy
number of marker genes per genome (Thonar etal. 2012).
Internal transcribed spacer region (ITS) sequences have
also been used to assess relative abundance of different
fungal clades including AM fungi, but amplification of
Glomeromycotan fungi can sometimes be low relative to
other fungal taxa, especially in soil (Lekberg etal. 2018;
Fu etal. 2022). Such methods are also sensitive to the
choice of primers and amplification (Suzuki etal. 2020).
A study involving consecutive harvests and characteriza-
tions of both intraradical and extraradical AM fungal bio-
mass using both PLFA and NLFA analyses, microscopy
and molecular approaches would allow for assessments
of congruence. We are not aware of many such studies
to date, although some exists (e.g., Vestberg etal. 2012;
Voříšková etal. 2017), which highlights an important
knowledge gap.
One method that holds great promise is SIP where 13C
incorporation into specific fatty acids or RNA is measured.
This technique has been used to trace recently assimilated
C from plants to AM fungi and other soil biota groups,
including individual microbial taxa (Olsson and Johnson
2005; Drigo etal. 2010; Kiers etal. 2011; Kaiser etal.
2015; Forczek etal. 2022). The SIP method can be cou-
pled with radioactive isotopes and molecular approaches
targeting functional genes such as nutrient/carbon trans-
porters (Watts-Williams etal. 2015; Sawers etal. 2017)
or measurements of enzyme activity (e.g., phosphatase;
Van Aarle etal. 2001) to calculate cost–benefit ratios
in the AM symbiosis, or nanoscale secondary-ion mass
spectrometry (NanoSIMS) to visualize C flow through AM
fungal structures in roots (Kaiser etal. 2015).
Interpretation of fatty acid analyses could be improved
by increased attention to a few areas. Opinions differ
regarding persistence after cell death, with some argu-
ing that fatty acid analysis are not suitable for short-term

Biology and Fertility of Soils
1 3
experiment (Joergensen 2022), while others claim rapid
degradation upon cell death (Tunlid and White 1992; Her-
zberger etal. 2014). This may be particularly relevant for
AM fungi because, depending on environmental condi-
tions, they can survive on stored carbon for an extended
period of time (Lekberg and Koide 2008). How this may
influence NLFA and PLFA 16:1ω5 values and corre-
spond to AM fungal viability is little known but could be
assessed by sampling stored soil over time for parallel fatty
acid analyses and infectivity assays. Future experiments
should also quantify the possible contribution of extracel-
lular lipids stabilized on soil minerals and/or microbial
necromass in soil to both NLFA and PLFA fractions after
cell death (Zelles 1999; Joergensen 2022). One approach
would be to isotopically (13C) label AM fungal mycelium
and then track microbial decomposition by measuring 13C
incorporation into metabolites (fatty acids, proteins, and
nucleic acid) within specific microbial groups.
Other areas that would benefit from more informa-
tion are these analyses’ degree of specificity and context
dependency. For example, we currently do not know if
fungi that can co-occur with AM fungi, such as arbus-
cule-forming fine root endophytes from the Mycoromy-
cotina clade (Hoysted etal. 2019) produce the fatty acid
16:1ω5. Also, the presence and absence of certain endo-
bacteria seem to influence fatty acid content (Salvioli
etal. 2010), as can the particular type of chloroform used
to elute NLFA (Drijber and Jeske 2019). Shifts in AM
fungal community composition may also influence meas-
urements because different taxa can vary considerably in
their fatty acid concentration (Graham etal. 1995) and
some AM fungal genera (e.g., Gigaspora, Paraglomus,
and Ambiospora) may produce little or no 16:1ω5. These
genera may instead produce more 20:1ω9 (Graham etal.
1995; Bentivenga and Morton 1996), which has prompted
some researchers to include this fatty acid in order not to
underestimate AM fungal biomass (Voříšková etal. 2017).
Combining community characterization with fatty acid
analyses may be a good idea wherever possible to estimate
the relative abundance of AM fungal families.
Still another approach would be to identify AM fungi-
specific compounds with a greater complexity than fatty
acids, such as phosphatidylcholines (PC) with acyl combina-
tions in specific positions, such as 20:5–20:5-PC, 20:5–20:4-
PC, or 20:5–20:3-PC (Wewer etal. 2014). Although greater
specificity towards the target organisms could be achieved
due to their unlikely presence in plants and bacteria, con-
cerns remain about variation in quantity of such compounds
per unit biomass among the different AM fungal species/
families. Furthermore, much more sophisticated and expen-
sive biochemical analyses would be required to identify and
quantify such compounds in biological samples as compared
to fatty acid methyl ester analyses, limiting the throughput
and cost-effectiveness of such an approach.
Conclusions
To achieve greater consistency and the ability to compare
results across studies, it is imperative that soil ecologists
discuss pros and cons of various methods. Here we outline
what we know about 16:1ω5 PLFA and NLFA and under
which conditions each of them can be used to assess AM
fungal biomass. Both PLFA and NLFA 16:1ω5 are reli-
able biomarkers for AM fungal biomass in roots. However,
NLFA 16:1ω5 is preferred in soils as it is more specific
and more responsive than PLFA 16:1ω5, especially if non-
mycorrhizal controls are difficult or impossible to include
in studies. More information about congruence between
fatty acid analyses and other methods used to estimate via-
ble AM fungal biomass, the persistence of lipids and fatty
acids in soil, and the degree of specificity would further
improve applicability and interpretation of these methods.
Supplementary information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s00374- 022- 01670-9.
Declarations
Conflict of interest The authors declare that no competing interests.
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