NLFA vs. PLFA: What Do These Soil Biology Tests Measure?
NLFA and PLFA are lipid-biomarker tests, but they measure different lipid pools. Learn what each result means, how AMF-associated 16:1ω5 should be interpreted, why fungal:bacterial ratios are not universal health scores, and what to ask a laboratory before comparing results.
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By Soil Health Exchange Team||16 min read|49 reads
Gas chromatography in a USDA ARS mycorrhiza study. NLFA and PLFA workflows also separate and quantify fatty-acid derivatives by gas chromatography, although their extraction and analytical targets differ from the isotope analysis pictured. — Photo: Peggy Greb, USDA Agricultural Research Service (public domain)
**PLFA is mainly a membrane-lipid test; NLFA is mainly a storage-lipid test.** PLFA is useful for estimating total living microbial biomass and detecting broad shifts in community composition. For AMF in mixed soil, NLFA 16:1ω5 is usually the more defensible lipid proxy. Neither result is a direct count of organisms or a measurement of microbial activity, diversity, plant benefit, or soil health [1][2][8][9].
The acronyms look almost identical, which is why reports are often misread. PLFA means phospholipid fatty acid. NLFA means neutral lipid fatty acid. The important difference is not the instrument at the end of the analysis; it is the lipid fraction separated from the sample before the fatty acids are measured.
NLFA and PLFA at a glance
Feature
PLFA
NLFA
Full name
Phospholipid fatty acid
Neutral lipid fatty acid
Main biological pool
Cell-membrane phospholipids
Neutral storage lipids, especially triacylglycerols in fungi
Common soil use
Total microbial biomass proxy and broad community fingerprint
Storage-lipid or reserve-carbon proxy; targeted use for AMF
AMF-associated marker
PLFA 16:1ω5, with a disputed bacterial background in mixed soil
NLFA 16:1ω5, generally more sensitive and specific for AMF-associated biomass
Does it identify species?
No
No
Does it measure activity or crop benefit?
No
No
Both are operational laboratory measurements: the result depends on the extraction, fractionation, marker rules, unit, and sample condition [1][2][3][5].
Start with the lipid, not the acronym
A fatty acid is a chemical building block that can occur in several kinds of lipids. Phospholipids form much of the membrane around microbial cells. Neutral lipids are uncharged compounds used prominently for energy and carbon storage. AMF store large quantities of lipids in spores, vesicles, and mycelium. The same fatty acid can therefore occur in both a membrane phospholipid and a neutral storage lipid; the fraction tells you which pool the laboratory recovered [2][5][6].
16:1ω5 and C16:1cis11 name the same fatty acid
Both notations describe a 16-carbon fatty acid with one cis double bond. Counting from the methyl, or omega, end puts the double bond at omega-5; counting from the carboxyl end puts it at carbon 11. A report may also write 16:1ω5c or 16:1ω5cis. The notation does not tell you whether it came from the PLFA or NLFA fraction—that must be stated separately.
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The fraction changes the biological meaning. Teal outlines represent membrane phospholipids contributed by many living organisms; amber droplets represent neutral storage lipids concentrated here in AMF hyphae and spores. In a common workflow, total lipids from one soil extract are separated into neutral-, glycolipid-, and phospholipid fractions; the figure follows only the neutral- and phospholipid paths. Fatty acids are converted to methyl esters (FAMEs) and analyzed by gas chromatography. The color key is conceptual, not a stain, and neither fraction by itself identifies AMF, species, or biological activity [1][2][3][5][6]. — Photo: Soil Health Exchange scientific illustration (AI-assisted; scientifically reviewed)
How the laboratory produces the number
Collect and preserve the soil. The sample is kept under the laboratory's specified temperature and holding time so the living community does not change before extraction.
Extract the lipids. An organic-solvent mixture removes a broad set of lipids from a known mass of soil.
Separate lipid classes. Solid-phase chromatography separates neutral lipids from glycolipids and polar phospholipids. This fractionation is what distinguishes NLFA from PLFA.
Convert fatty acids to methyl esters. A chemical reaction produces fatty-acid methyl esters that are suitable for gas chromatography.
Separate and quantify the peaks. Gas chromatography identifies compounds by retention behavior and measures them relative to standards.
Apply biomarker rules. The laboratory sums selected peaks into categories such as total PLFA, Gram-positive bacterial markers, Gram-negative bacterial markers, saprotrophic fungal markers, or AMF-associated 16:1ω5 [1][2][3].
The method produces a chemical fingerprint, not a microscope image, culture, or DNA identification. Several organisms can share a fatty acid, and one organism contains many fatty acids. Group assignments are therefore proxies defined by a biomarker list, not a taxonomic census [1][2].
What PLFA can tell you
Total PLFA concentration is widely used as a proxy for microbial biomass because phospholipids are central membrane components and generally do not accumulate as long-lived soil residues. A controlled paddy-soil incubation found half-lives of 14–27 hours for added phospholipids at 25°C, with an average near 20 hours; turnover was nearly twice as slow at 15°C [4]. This supports the membrane-biomass interpretation, but the exact values are not universal decay constants for every soil.
Figure
Rapid phospholipid turnover in one controlled soil study
Shortest measured half-life
14 hours
Average total phospholipid half-life
20 hours
Longest measured half-life
27 hours
Minimum, average, and maximum half-lives reported for added bacterial and fungal phospholipids in one paddy-soil incubation at 25°C. The study supports using intact phospholipids as a living-biomass proxy; it does not establish one half-life for all soils [4].
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Individual or summed PLFA markers can also reveal whether two samples have different lipid profiles. That makes PLFA useful for monitoring a management change, land-use contrast, contamination gradient, or treatment experiment. It is rapid and sensitive to community shifts, but it cannot resolve individual species and should not be converted into a conventional species-diversity index [1][2].
How to read common PLFA outputs
Report line
Defensible meaning
What it does not mean
Total PLFA
Sum of quantified phospholipid fatty acids under that protocol; a broad living-microbial-biomass proxy
Total cells, activity, diversity, or universal soil health
Bacterial PLFA
Sum of fatty acids the laboratory assigns to bacterial groups
Every bacterium or a species count
Fungal PLFA
One or more fungal-associated membrane markers, often including 18:2ω6,9
All fungi; plant tissue can contribute some markers
AMF PLFA 16:1ω5
A membrane-associated fatty acid that may include AMF and non-AMF background
A stand-alone AMF census in mixed soil
Fungal:bacterial ratio
Ratio of the laboratory's selected fungal marker sum to its selected bacterial marker sum
A literal biomass ratio or a universal optimum
Stress ratios
Changes in selected fatty-acid ratios that may be consistent with physiological or community shifts
A diagnosis of the cause of stress
Marker names and sums vary among laboratories. Ask for the exact fatty acids included in every category [1][2].
PLFA is a living-biomass proxy, not a viability assay
Rapid phospholipid turnover supports association with cells alive near sampling, but PLFA does not test whether an organism can grow, infect a root, reproduce, or perform a desired function. Use a culture, host bioassay, or another purpose-built viability test when viability is the question [4].
What NLFA can tell you
NLFA analysis measures fatty acids in neutral lipids. In fungi, that fraction commonly represents stored energy and carbon reserves rather than cell membranes. For AMF, the fatty acid 16:1ω5 can be abundant in neutral lipids of spores, vesicles, and mycelium. Controlled work linked spore number most strongly with NLFA 16:1ω5, while hyphal length related more closely to the phospholipid fraction in that experimental system [5][6].
A later comparison grew maize in three soils and compared root colonization and spore density with three lipid methods. NLFA 16:1ω5 had the highest correlation with spore counts. PLFA 16:1ω5 was not significantly correlated with spore density and did not reproducibly correlate with root colonization [7]. That study supports NLFA as an AMF-associated storage-biomass proxy, not as a substitute for every AMF measurement.
A higher NLFA number is not automatically better
Higher NLFA 16:1ω5 means more of that AMF-associated storage lipid per stated sample unit. It does not by itself show more arbuscules, greater infectivity, faster nutrient transfer, higher crop yield, or a beneficial fungal community. Storage allocation changes with AMF taxon, life stage, sporulation, host carbon supply, soil phosphorus, and season [5][6][10].
The hard question: can PLFA 16:1ω5 measure AMF?
In a sterile or otherwise well-controlled AMF experiment, PLFA 16:1ω5 can track membrane-rich AMF mycelium. The difficulty is assigning the same peak in mixed field soil. Olsson and Lekberg's 2022 review found that more than half of published studies had used PLFA 16:1ω5 to quantify AMF biomass without controls that could estimate the non-AMF contribution. They argued that bacterial background cannot be separated reliably and recommended NLFA 16:1ω5 for more accurate and sensitive AMF biomass assessment [8].
There is a published scientific disagreement. Joergensen argued that there is no experimental evidence for marked amounts of PLFA 16:1ω5 in Gram-negative bacteria and supported its use as an AMF indicator [2]. A multi-author response concluded that specificity remains uncertain and recommended NLFA unless a control estimates contributions from other microbes [9]. The careful interpretation is therefore not that PLFA 16:1ω5 is useless, nor that it is universally AMF-specific: its strength depends on the sample, comparison, and controls.
When the 16:1ω5 marker is more or less convincing
Situation
Interpretation strength
Reason
Controlled AMF culture with a matched non-mycorrhizal control
Stronger for PLFA and NLFA
Background 16:1ω5 can be estimated
Field soil with NLFA 16:1ω5 and standardized comparison
Useful AMF-associated biomass proxy
Neutral fraction is generally more responsive and has less bacterial interference
Field soil with PLFA 16:1ω5 but no control
Weak as a stand-alone AMF estimate
AMF and non-AMF contributions cannot be separated confidently
Generic fungal PLFA or fungal:bacterial ratio
Not an AMF measurement
Pools broad groups and depends on the laboratory's marker definitions
Specificity is a study-design property as much as a chemical property [2][8][9].
What about the NLFA:PLFA ratio?
The ratio of NLFA 16:1ω5 to PLFA 16:1ω5 has been used in controlled AMF studies as an index of carbon allocation to storage relative to membranes, sometimes described as fungal nutritional status. In the 1997 phosphorus experiment, the ratio changed with phosphorus supply [6]. But both numerator and denominator inherit the marker and extraction limitations, and the PLFA denominator may include non-AMF background. There is no universal ratio that defines a healthy field, an active fungus, or an optimal crop–AMF partnership.
Evidence snapshot: the methods do not move together
What selected studies actually found
Evidence
Data or result
Lesson
Controlled AMF phosphorus experiment [6]
Spore number related most closely to NLFA 16:1ω5; hyphal length to PLFA 16:1ω5
The lipid fraction changes the biological pool represented
Maize grown in three soils [7]
NLFA had the highest correlation with spore counts; PLFA did not significantly correlate with spores and was inconsistent for root colonization
One AMF endpoint cannot be inferred reliably from another
2022 critical review [8]
More than half of reviewed studies used PLFA 16:1ω5 for AMF biomass without proper controls
Published use does not guarantee valid specificity
Three-year cover-crop field study [10]
Propagule counts and NLFA C16:1cis11 were the most consistently responsive AMF measures; PLFA response was marginal
Treatment sensitivity and crop-performance relevance are separate questions
Seventeen prairie soils under alternative handling [11]
Air-drying, room-temperature storage, and oven-drying all caused significant PLFA losses and shifted biomarker ratios
Sample handling can change both totals and apparent community structure
These results are method-specific observations, not calibration values for every soil.
Units matter as much as the biomarker
Absolute lipid results are commonly reported as nanomoles of a fatty acid per gram of dry soil, but laboratories may use other mass, volume, or moisture bases. Relative abundance may be reported as mole percent of the detected lipid pool. These are not interchangeable: mole percent can rise because one marker increased or because other markers decreased. A value per gram of field-moist soil cannot be compared directly with a value per gram of oven-dry soil without a moisture correction.
nmol g−1 dry soil: an absolute concentration under the stated extraction and calibration.
mol%: the marker's share of the quantified lipid pool; compositional, not absolute.
Total PLFA: a sum whose included peaks and detection limits may differ by laboratory.
Group biomass: a biomarker sum, sometimes converted with a factor; ask whether the conversion is directly validated for that sample type.
Ratio: a quotient of two marker sums; uncertainty and interference in both parts affect the result.
Sampling and storage can dominate the result
Lipids respond to roots, crop stage, moisture, temperature, depth, residue placement, and recent management. They can also change after collection. In the 17-soil handling study, all tested alternatives to fresh, promptly lyophilized material—including air-drying, field-moist room-temperature storage, and oven-drying—lost PLFA markers, and microbial groups were affected disproportionately [11]. That means a shifted fungal:bacterial ratio can be a storage artifact rather than a field response.
Ask the laboratory before sampling. Use its container, cooling or freezing instructions, maximum holding time, and shipping method.
Fix the soil depth and position. A root-zone sample and a bulk-soil sample answer different questions.
Composite consistently. Use the same number and pattern of cores, but preserve independent field replicates when statistical inference is needed.
Record soil moisture and crop stage. Sample comparisons made after different weather or at different growth stages can confound treatment with time.
Do not compare fresh and dried protocols. Keep preservation and dry-mass correction identical.
Use the same laboratory and method over time. Cross-laboratory differences in extraction, peak identification, marker sums, and reporting can be larger than the management effect.
Which test should you choose?
Match the test to the question
Question
Best starting method
Useful confirmation
Did broad living microbial biomass change?
Total PLFA with standardized sampling
Microbial biomass C and a process measurement such as respiration
Did the broad lipid profile change?
PLFA profile analyzed multivariately
Targeted DNA or process data chosen in advance
Did AMF-associated soil biomass change?
NLFA 16:1ω5 with consistent controls and units
Root colonization microscopy plus infective-propagule or hyphal measurement
Are crop roots colonized by AMF?
Cleared and stained root microscopy
AMF-targeted qPCR when taxonomic scope is validated
Is an AMF product viable?
Host-plant bioassay in controlled substrate
Root microscopy and a viable positive control
Did AMF improve nutrient delivery or yield?
Controlled field or mesh-compartment experiment with plant endpoints
Plant tissue nutrients, yield, and an isotope tracer when causal pathway matters
Choose the least complex method that directly measures the target. Add a second, independent endpoint when the claim matters [10][12].
More sophisticated is not always more relevant
A well-replicated PLFA comparison can answer a broad community question better than poorly sampled sequencing. Root microscopy can answer colonization more directly than NLFA. A host bioassay can answer infectivity more directly than DNA. Start with the noun you need to measure.
Questions to ask a commercial laboratory
Are you measuring PLFA, NLFA, or both, and are the fractions separated before analysis?
Which exact fatty acids are included in total bacteria, fungi, AMF, Gram-positive, and Gram-negative categories?
Is 16:1ω5 reported separately in the neutral and phospholipid fractions?
What unit and moisture basis are used: nmol per gram dry soil, another absolute unit, or mole percent?
What are the extraction recovery, internal standard, detection limit, and quantification limit?
How should samples be collected, cooled, frozen, lyophilized, shipped, and held—and for how long?
Are current values directly comparable with the laboratory's historical database, or has the extraction or marker algorithm changed?
What validation supports any categories labeled 'AMF,' 'active,' 'stress,' 'diversity,' or 'soil-health score'?
Can you provide the raw marker table and method version, not only colored gauges or percentile ranks?
What agronomic calibration supports a sufficiency threshold for this crop, soil, region, depth, and season?
Frequently asked questions
Is NLFA better than PLFA?
Only for certain questions. NLFA 16:1ω5 is generally the stronger lipid proxy for AMF-associated biomass in mixed soil. PLFA is better for total living microbial biomass and broad lipid-community profiles. Neither is universally superior because they measure different pools.
Does PLFA measure living microbes?
It is a useful proxy for living microbial biomass because intact phospholipids are membrane components and turn over rapidly after release. It is not a direct live/dead test and does not show culturability, infectivity, growth rate, or function [2][4].
Does NLFA measure AMF spores?
NLFA 16:1ω5 often correlates strongly with AMF spores because spores contain storage lipids, but the signal can also come from AMF vesicles and mycelium. It is an AMF-associated lipid concentration, not a microscopic spore count [5][6][7].
Is a high fungal:bacterial ratio healthier?
Not universally. The ratio depends on the selected markers, extraction, vegetation, crop stage, soil type, and management. Different ecosystems support different communities. Interpret the ratio as a protocol-specific comparison, not a universal target or a literal count of fungi divided by bacteria [1][2].
Can NLFA or PLFA identify a microbial inoculant strain?
No. Many organisms share fatty acids. Demonstrating establishment of an introduced strain requires a validated strain-specific molecular assay or another strain-resolving method, sampled through time. Product viability requires a growth or host-infectivity test.
Can I compare my result with a value from another laboratory?
Only after confirming the same lipid fraction, extraction and calibration, marker list, unit, dry-mass basis, sampling depth, preservation, and season. A shared acronym does not guarantee a comparable measurement.
How to interpret an NLFA or PLFA report
Read the fraction first. PLFA 16:1ω5 and NLFA 16:1ω5 are not interchangeable.
Read the unit and denominator. Absolute concentration and relative abundance answer different questions.
Request the marker definitions. A group label is a laboratory rule, not a universally fixed organism list.
Check sample handling. Drying, warming, and holding time can alter totals and ratios.
Demand a meaningful comparator. A matched treatment, repeated time point, or well-described reference is more useful than an unexplained percentile.
Separate biomass, composition, activity, and function. A lipid change proves only a lipid change under the method used.
Avoid universal thresholds. USDA-NRCS describes soil biological values as generally directional—higher is better—and notes they may not be well defined regionally; it offers no calibrated critical limits for lipid biomarkers [12].
Link the result to an endpoint. For farm decisions, pair the lipid measurement with crop nutrition, yield or quality, soil fertility, and economics.
The bottom line
**PLFA asks mainly about membrane lipids and the broad living microbial community. NLFA asks mainly about storage lipids; NLFA 16:1ω5 is the preferred lipid proxy for AMF in mixed soil when a proper control is unavailable.** Neither is a species test, activity test, colonization score, viability assay, or proof of crop benefit. The fraction, marker, unit, controls, handling, and comparison define what the number can mean.
NLFA vs. PLFA Knowledge Check
Each attempt draws 8 questions from a 12-question pool and shuffles both the questions and answer choices. Score 85% or higher to earn a Certificate of Completion.
1.What is the main biological distinction between PLFA and NLFA analysis?
2.A report lists the marker as both 16:1ω5 and C16:1cis11. What does that tell you?
3.Why are lipid biomarker group assignments described as proxies rather than a taxonomic census?
4.What is the most defensible interpretation of total PLFA?
5.What does a higher NLFA 16:1ω5 concentration most directly mean?
6.Why is PLFA 16:1ω5 difficult to interpret as an AMF biomass marker in mixed field soil?
7.What is a defensible use of the NLFA 16:1ω5 to PLFA 16:1ω5 ratio?
8.Why can 5 mol% of a marker not be interpreted as the same thing as 5 nmol per gram of dry soil?
Source material for every claim in this article, plus a citation-ready record for reference managers and scholarly indexes.
Cite this
Reference this work
SHE-ART-2026-0043
Soil Health Exchange Team (2026). NLFA vs. PLFA: What Do These Soil Biology Tests Measure?. Soil Health Exchange. SHE-ART-2026-0043. https://soilhealthexchange.com/cite/SHE-ART-2026-0043
Soil Health Exchange Team (2026). NLFA vs. PLFA: What Do These Soil Biology Tests Measure?. Soil Health Exchange. https://soilhealthexchange.com/blog/nlfa-vs-plfa
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MLA
Soil Health Exchange Team. "NLFA vs. PLFA: What Do These Soil Biology Tests Measure?." Soil Health Exchange, 2026-08-04, https://soilhealthexchange.com/blog/nlfa-vs-plfa.
Chicago
Soil Health Exchange Team. "NLFA vs. PLFA: What Do These Soil Biology Tests Measure?." Soil Health Exchange. Published 2026-08-04. https://soilhealthexchange.com/blog/nlfa-vs-plfa.
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