How rapidly do phospholipid fatty acids (PLFAs) degrade once an organism dies?

PLFAs are known to degrade rapidly and are therefore indicative of living microorganisms. However, the rate at which the fatty acids degrade depends on the environmental factors, particularly the temperature. 

A recent study has shown that the half-life t1/2 (the time after which exactly half of the original quantity of material remains) of various PLFAs is between 14 and 27 hours. Degradation at 15°C takes twice as long as at 20-25°C. The degradation rate was roughly the same in soils with a low and high microbial biomass.

Source: Zhang et al. (2019) - High turnover rate of free phospholipids in soil confirms the classic hypothesis of PLFA methodology.


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Related faqs

What is the 'PLFA method'?

The biological parameters of the Soil Life Monitor are analyzed using the PLFA method. PLFA stands for phospholipid fatty acids. These fatty acids occur in the cell membranes of living organisms. Different groups of organisms have unique compositions of these PLFAs. By measuring and quantifying the PLFAs, it is possible to obtain a fingerprint of the soil food web. For example, the cell membranes of fungi consist of different PLFAs than those found in bacteria. The PLFAs present are measured and quantified using gas chromatography-mass spectrometry (GC-MS).

What is the Soil Life Monitor used for?

Soil Life Monitor responds to the widespread demand for a better understanding of soil life and biological soil quality. As the number of permitted crop protection products declines and awareness of the importance of soil life grows, many sectors are paying more attention to soil life nowadays. 

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How long does a Soil Life Monitor analysis take?

The maximum duration of an SoilMonitor analysis is three weeks after arrival of the sample at the Eurofins Agro lab in Wageningen.

When is the best time to take sample for Soil LifeMonitor?

The best time to take a sample for Soil Life Monitor depends on the purpose for which they are intended. For plots being monitored over time, the best time to take the samples is at roughly the same time every year and in similar conditions. Soil life is generally more active in the growing season in warm, moist conditions. In winter, soil life activity slows down. In addition, very dry conditions can kill off or largely inactivate soil life.

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What is the sampling protocol for Soil Life Monitor?

The best way to take a sample depends on the substrate.
 

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Can the PLFA method also identify soil life at species level?

No, the PLFA method can only distinguish between groups of microorganisms such as actinomycetes and arbuscular mycorrhizae. The PLFA analysis provides a fingerprint of the soil food web. Plating methods and DNA techniques are more suitable for identifying specific species.

What are the target values based on?

The target values indicate how the sample scores compared with similar soils or matrices and are based on percentiles of samples taken in practice. The target values of outdoor and greenhouse soil samples are furthermore corrected based on their organic matter content. The target values of soils that are poor in organic matter are lower and lie closer together than soils that are rich in organic matter.

What are actinomycetes?

Actinomycetes form a special order of bacteria. They form thread-like filaments similar to hyphae, but they are not fungi: they have no cell nucleus and their cell walls contain no cellulose or chitin, unlike the cell walls of fungi.

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What are arbuscular mycorrhizae?

Arbuscular mycorrhizae (AM) are fungi which can establish a symbiotic association with around 80 percent of all plant species. 

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What types of mycorrhizae are measured with the Soil Life Monitor?

The PLFA analysis provides insights into the biomass of the active mycelium (network of fungal threads or hyphae) of arbuscular mycorrhizae in the soil. The fatty acid 16:1ω5 is used for this purpose. 

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What are protozoa?

Protozoa are single-celled microorganisms which contain a nucleus (eukaryotes). The most important function of protozoa is to make nutrients available to the crop by 'grazing' on micro-organisms (particularly bacteria). 

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What does the gram+/gram- ratio indicate?

Bacteria can be divided into two different groups: gram-negative (gram-) and gram-positive (gram+). 

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Why is the total number of fungi and bacteria in the soil not the same as the total microbial biomass?

Microbial biomass is a quantified total of a large number of fatty acids. Fungi and bacteria make up the largest proportion of this but do not contain the entire microbial biomass. The unit of the parameters measured is mg PLFA/kg soil. The biomass of fungi, bacteria and microbial biomass in mg C/kg soil is calculated using a conversion factor known from the literature.

What does the fungal-to-bacterial ratio indicate, and how is it calculated?

The fungal-to-bacterial ratio indicates the ratio between the total fungal biomass and the total bacterial biomass (expressed in g C/kg soil). 

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What is PLFA diversity and how is it calculated?

PLFA diversity is calculated using the Shannon-index. The Shannon-index is widely used in ecology to describe the diversity of species. The calculation takes account of both the number of species and their distribution.

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