The breakdown of soil aggregates and the extraction of
particulate organic matter (POM) by ultrasonication and density
fractionation is a method widely used in soil organic matter (SOM) analyses.
It has recently also been used for the extraction of microplastic from soil
samples. However, the investigation of POM physiochemical properties and
ecological functions might be biased if particles are comminuted during the
treatment. In this work, different types of POM, which are representative
of different terrestrial ecosystems and anthropogenic influences, were
tested for their structural stability in the face of ultrasonication in the range
of 0 to 500 J mL
The mechanical disintegration of soil aggregates by use of ultrasonication following the method of Edwards and Bremner (1967a) and subsequent density fractionation of particulate organic matter is widely used in the assessment of soil organic matter (SOM) stability. This includes characteristics such as aggregate composition and stability (Edwards and Bremner, 1967b), the constitution of SOM pools (Golchin et al., 1994), the stabilization of SOM in forest ecosystems (Graf-Rosenfellner et al., 2016) and the occlusive strength of particulate organic matter (POM) (Büks and Kaupenjohann, 2016). Ultrasonication is also applied to assess quantities and qualities of anthropogenic contaminants such as microplastics (Zhang and Liu, 2018; Zhang et al., 2018).
In studies on soil carbon pools, ultrasound is applied to a soil slurry to
break down soil aggregates. The disaggregation allows density fractionation
of the free and occluded light fractions (fLF and oLF), which largely
consist of material with densities below the fractionation medium, from the
heavy fraction (HF), which has higher densities. These operational fractions
largely correspond to the free particulate organic matter (fPOM), the
occluded particulate organic matter (oPOM) and the mineral-associated
organic matter (MOM). This organic matter is assigned to the labile,
intermediate and stable carbon pool, respectively, and has turnover times
of
Furthermore, the extracted POM fractions may contain not only natural
but also anthropogenic components such as microplastic. Recent studies have
reported soil microplastic concentrations between 1 mg kg
The common method of ultrasonication is carried out with a piezoelectric converter, which uses electric energy to generate axial vibration of a sonotrode, which is dipped into a flask containing a fluid and a submerged soil sample. The oscillating sonotrode emits acoustic pulses within the fluid. In front of the shock waves the medium is compressed, and the increased pressure causes an increased gas solubility. Behind the wave the medium relaxes and the pressure drops below the normal level, leading to an explosive outgassing (Ince et al., 2001). This so-called cavitation effect produces lots of exploding microbubbles between particles and within cavities of the soil matrix, generating very local pressure peaks of 200 to 500 atm accompanied by temperatures of 4200 to 5000 K (Ince et al., 2001). It provokes the detachment of physiochemical bondings between soil primary particles and soil aggregates and, thus, causes disaggregation. Depending on the type and settings of the device, the vibration frequency can vary up to 10 000 kHz, but low frequencies around 20 to 100 kHz are recommended for soil aggregate dispersion to avoid chemical alteration of OM, and the use of 40 kHz is very common (Kaiser and Berhe, 2014; Graf-Rosenfellner et al., 2018).
As an artifact of the method, ultrasonication is known to provide mechanical
and thermal stress strong enough to comminute mineral particles at energy
levels
The aim of this work was to test how susceptible different types of POM are to
comminution by ultrasonic treatment under standardized conditions. We
embedded three types of POM (farm oPOM, forest oPOM and pyrochar, applied as an
analog for soil black carbon and biochar amendments) and also six
differently weathered microplastics (fresh and weathered low-density
polyethylene (LD-PE), polyethylene terephthalate (PET) as well as
polybutylene adipate terephthalate (PBAT), a common biodegradable material)
into a fine sand matrix. Then, we treated these mixtures with 0, 10, 50, 100
and 500 J mL
In advance of the treatment, the nine materials showed different mechanical
stabilities. Unlike all six types of plastic particles, the occluded POM
and the pyrochar were easy to grind between two fingers and therefore
assumed to be prone to ultrasonication. An examination of the recent
literature on microplastic extraction from soils showed that the stability
of microplastic in the face of ultrasound has not been studied yet, neither with
weathered nor juvenile material. Experiments with polymer-based adsorber
resins indicated fractures on microbead surfaces after treatment with
100 J s
The farm and forest oPOM were extracted from air-dried soil aggregates of
630 to 2000
In order to test their stability against ultrasonication, the nine POM types
(farm and forest oPOM and pyrochar as well as fresh and weathered LD-PE, PET
and PBAT) were each exposed in triplicates to different mechanical stress
levels (0, 10, 50, 100 and 500 J mL
We chose acid-washed and calcinated fine sand to simulate the soil mineral
matrix. This texture can be easily suspended by ultrasonication (coarse sand
cannot), has a low tendency to coat POM or coagulate (like clay does) and
shows a fast sedimentation when the sample is centrifuged. Fine sand,
moreover, represents soils that originated from Weichselian sanders or
eolian sand deposition. In this methodical paper, our aim, however, was not
to simulate a set of soil textures, but to have a proof of concept to find
out whether natural or artificial POM is damaged by ultrasonication. Then,
quantities of 1 %
These artificial soils (each 20 g) were stored in 100 mL of 1.6 g cm
After lyophilization, the recovery rate
All samples continued to be used for particle sizing. After pre-trials have
shown that mainly the hydrophobic particles (microplastics and pyrochar)
coagulated in distilled water, aggregation was avoided by suspension in
0.1 %
A second set of triplicates of pyrochar and farm soil oPOM were treated
similarly at 0 and 500 J mL
All microplastic samples (LD-PE, PET and PBAT) show a constantly high
recovery rate of about
Recovery rates of natural POM and microplastics from after
ultrasonic treatment with 0, 10, 50, 100 and 500 J mL
None of the plastics shows a significant reduction of particle size due to
ultrasonic treatment within the 10 % and 50 % quantile. In contrast,
at
Particle size distribution of natural POM
Particle size distribution (10 % and 50 % quantile) and
comminution factor of natural POM and microplastics after ultrasonic
treatment with 0, 10, 50, 100 and 500 J mL
NA – not available.
The treatment of pyrochar triplicates with 500 J mL
Mass balance that indicates the fate of OM fractions during the
ultrasonication/density fractionation treatment. Bold numbers indicate
differences with
Our experiments indicate that soil-derived oPOM and pyrochar embedded into a
fine sand matrix are prone to comminution by ultrasonic treatment at energy
levels of
The concurrent decrease of particle size and recovery rate of soil-derived
POM and pyrochar and its absence after ultrasonic treatment of
microplastics might indicate a causal relationship of these measures. The
underlying process, however, has not been studied before. We assume a
mechanism that prevents POM from density fractionation. This effect appeared
in our experiment from energies around 50 J mL
As a consequence of the reduction of the recovery rate, farmland, forest and
pyrochar POM remain within a sandy matrix the stronger they are treated by
ultrasound. If these findings are applied to ultrasonication/density
fractionation of natural soils, not only an increasing number of particle
size artifacts can be expected, but also the extraction of occluded POM is
increasingly hindered at a certain energy level. After each extraction step,
parts of the released oPOM remain within the sedimenting fraction, a
carry-over artifact. This leads to an underestimation of the extracted oPOM
fractions and an overestimation of the mineral-associated organic matter
fraction (MOM), a natural part of the soil organic matter (SOM), which is
adsorbed on mineral surfaces of the heavy fraction and mainly assumed to be
molecular. According to our data, a reduction of recovery rates would appear
at 10 J mL
An overestimation would have an impact, for example, on the assessment of
operationally defined carbon pools within landscapes: POM is assigned to
carbon pools with turnover times orders of magnitude shorter then MOM, which
endures hundreds of years. Misquantification of these pools, such as
counting POM to the MOM as implied by this work, would have an influence on,
for example, the estimation of SOM decomposition and CO
Microplastic particles, whether they are weathered following DIN
ENISO4892-2/3 or pristine, are not prone to disruption by ultrasonic
treatment, and their recovery rates are stable in a wide range of energy
levels. We therefore assume that there will be significantly less
carry-over of particles due to comminution when extracting microplastics
from soils with ultrasonication/density fractionation. In consequence, the
extractive performance is higher and subsequent particle size measurements
give more valid information about the original particle size spectrum
compared to the measurement of farmland, forest and pyrochar POM. This is a
positive sign for research on soil microplastic; however, it does not mean
that microplastic will be fully extracted from soils with this method. Soil
microplastics appear within a wide range of sizes between some nanometers
and its upper limit of 5 mm by definition. Their smallest part, fibers and
microfragments produced by physical, chemical and biological erosion within
the soil, might also be affected by chemical alteration due to both
weathering and ultrasonication causing enhanced retention in the sedimenting
fraction. Although we have introduced billions of metric tons of microplastics into
ecosystems since the 1950s (Thompson et al., 2009; Geyer et al., 2017),
there are still problems in producing microplastic fragments
Unlike weathered and fresh PE, PET and PBAT microplastic, soil-derived POM like occluded POM from farm and forest soils and pyrochar concurrently shows comminution and a reduced recovery rate after ultrasonication and subsequent extraction from a sandy matrix. Applied to natural soils, parts of the farmland, forest and pyrochar POM remain within the sedimenting fraction and can be misinterpreted as more strongly bound oPOM or MOM. An overestimation as shown in this study might lead to fundamentally different interpretations of physical protection of SOM, functional carbon pools and the expected mineralization rates in consequence of, for example, land-use change. On the contrary, the extraction of microplastics neither causes additional retention of particles nor alienates the particle size spectrum due to ultrasonic-driven comminution. We conclude that density fractionation in combination with ultrasonication is an appropriate tool for analyzing occlusion of microplastics within soil aggregates and studying the size distribution of particulate microplastics.
All of the data are published within this paper and in the Supplement.
The supplement related to this article is available online at:
FB developed the experimental concept, extracted all samples and prepared the manuscript. GK performed the particle size analysis. AZ supported the development of the experimental concept. MK and FL supervised the whole study.
The authors declare that they have no conflict of interest.
Many thanks to Zoltán Mátra, who kindly helped us to conduct the QICPIC analysis.
This open-access publication was funded by Technische Universität Berlin.
This paper was edited by Yakov Kuzyakov and reviewed by two anonymous referees.