The plastics industry is booming. Especially during the Covid19 pandemic there has been an increasing demand for acrylic glass shields, face masks, gloves, medical disposables, plastic wrappings for online shopping and takeaway food containers. The calls for the reduction of single-use plastic items have all but faded in the face of efforts to reduce the spread of the virus. But wherever plastic is used, waste is being generated – and unfortunately not always disposed of correctly.
In a business-as-usual scenario, the global amount of mismanaged plastic waste is projected to nearly triple from around 80 million metric tons per year in 2015 to an estimated 210 million metric tons per year in 20601. Unfortunately, the stable characteristics and longevity of plastics causes them to accumulate in the environment. There they may fragment into continuously smaller pieces, but do not decompose completely, at least not within a reasonable time frame. This effect has been under scientific scrutiny ever since the term “microplastics” was minted in a tipping-point publication in 20042. Initial research was mainly focused on microplastic pollution and its effects in the marine environment, later followed by studies concerning freshwater bodies that were deemed to be significant pathways of microplastics into the oceans. But only recently the focus has shifted from aquatic environments to soils, especially arable soils.
“the global amount of mismanaged plastic waste is projected to be an estimated 210 million metric tons per year in 2060”
Sources of plastic in soils
Soils receive plastics from multiple sources such as from incorrect waste disposal, from agricultural practices, and from atmospheric deposition of very fine particles. Therefore, it is almost certain that plastics can be found in almost any topsoil – at variable amounts. In arable soils plastic concentrations differ enormously and can reach up to several percent of soil weight, depending on the land use history and especially, the ploughing-in of used mulch foils. Sources of plastics in agriculture are manifold, for example by using plastic contaminated irrigation water, fertilising with sewage sludge, mulching with plastic foils, as is typical for strawberry and asparagus production, or the addition of compost which is produced with plastic-contaminated household biowaste.
Potential consequences of the microplastic contamination in soils include changes in soil properties and the soil microbiome which may affect relevant soil functions, such as water, carbon, and most likely also nutrient storage. This of course affects plant performance, which is highly relevant for global food security, and may reduce the efficiency of soils to mitigate climate change.
In an attempt to better understand formation, migration and effects of microplastics in the environment as a whole, the German Research Foundation has funded the Collaborative Research Centre 1357 Microplastics at the University of Bayreuth. Next to other microplastic research questions an interdisciplinary team of 34 research groups are developing methods to assess the transport of microplastics between soil, air and water. With a focus on microplastic pollution in terrestrial environments, the CRC 1357 is also studying the fundamental physical, chemical and biological processes to which microplastics are subjected once they enter the environment.
Organic Fertiliser as Source for Microplastics in agricultural soils
Plastic is a man-made material, so whatever contamination is found in the environment originates with us. While the intentional use of primary microplastic, for example in cosmetics or detergents, is on the decline, due to new regulations, large quantities of plastic enter the environment through mismanaged waste. A particular concern in this regard is the contamination of household biowaste. This waste fraction is valuable and can be recycled in the form of organic fertiliser (compost). Many countries already mandate the collection and recycling of household biowaste. The recycling takes place in municipal biowaste treatment plants, where the biowaste is typically mixed with cuttings from greeneries to add bulk and then composted. Since biowaste is very rich in energy, modern plants often use a two-stage process, where the biowaste is first fermented to produce biogas and only the residues are then composted. This improves the economic balance of the waste treatment, which reduces the waste removal costs for the community, while providing a renewable source of energy in the form of biogas. The amount of methane, a much worse climate gas than CO2, potentially released during simple composting is also reduced.
“liquid fertilisers contain several thousand microplastics particles of a size between 10 and 500 micrometres per litre”
Unfortunately, household biowaste is nearly always contaminated by plastic, mostly bags, presumably because a significant fraction of the population prefers to collect its biowaste in such bags, if they agree to collect at all. These bags and other plastic contaminants, such as coffee capsules or fast-food containers, have to be removed from the incoming biowaste by elaborate cleaning procedures. In spite of all efforts, however, some plastic will still enter the biowaste treatment process. This is taken into consideration by the current regulations concerning quality compost. For instance, in Germany there is a limit of 0.1 wt% for residual plastic. Since any analysis is limited by what can be measured, only fragments larger than 1 millimetre are considered in this quality control measure. Particles larger than 1 millimetre can still be seen with the naked eye. In consequence, they can be picked out and identified by spectroscopy. For smaller particles an analysis is much more difficult. Thus, microplastics smaller than 1 millimetre are not yet included in the current quality tests.
Investigating compost contamination
A recent research project financed by the Ministry of the Environment, Climate Protection and the Energy Sector in Baden-Württemberg, Germany, investigated the contamination of quality composts by microplastics. None of the investigated composts exceeded the legal limit. Composts are regularly added on agricultural soils. Therefore, microplastics in composts add to the amount of microplastics in agricultural soils used for food and feed production with all the possible consequences for soil quality and vitality. More importantly, liquid fertilisers contained no plastic fragments larger than 1 millimetre, but several thousand smaller microplastics particles of a size between 10 and 500 micrometres per litre.
Biodegradable plastics are proposed as possible alternatives to commodity plastics, in particular for packaging and bags. By definition, a biodegradable material is supposed to be fully decomposed by microbial activity into carbon dioxide, water and biomass. Under oxygen-limited conditions, the production of the greenhouse gas methane is also possible. In supermarkets several synthetic bags have appeared that are certified as biodegradable and intended for biowaste collection. Most communities prohibit these bags like any other plastic bag since they cannot be easily distinguished from, e.g. polyethylene bags, and have to be removed anyhow. In addition, the “compostability” of such bags has been questioned. In principle, conditions during industrial composting should be perfect for biodegradation, for instance the elevated temperatures, the active supply of oxygen, and the active microbial decomposer communities. Still, several biowaste treatment plant operators complain that they find residues of biodegradable bags in their finished composts. In fact, the above-mentioned study found fragments smaller than 1 millimetre of biodegradable plastics alongside commodity plastics such as polyethylene in most compost samples. Moreover, studies of the fate of biodegradable plastics in the environment have shown that other features of a material besides its chemical nature influence biodegradation, most importantly the crystallinity. In consequence, unless these aspects are much better understood, not even biodegradable plastics can be fully recommended for use in environmentally sensitive areas.
Detection and Quantification of Microplastics in Soils
As a prerequisite for any kind of risk assessment of contaminants in the environment, it is necessary to assess the extent of the actual environmental pollution, which can then be put into context with eco-toxicological studies. So far, independent research groups have confirmed that soils around the globe contain microplastics to various degrees. However, these reports are seldomly comparable, simply because different monitoring methods are being used, and efforts to standardise and harmonise sampling, sample preparation and analysis methods have not yet succeeded. Scientists are in agreement, that the only secure way of identifying microplastics as such is via chemical characterisation – purely optical identification has been proven to be quite unreliable3.
“independent research groups have confirmed that soils around the globe contain microplastics to various degrees”
Concerning reliable methods for microplastic identification, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy have been established for particle-based analysis, while pyrolysis gas chromatography mass spectrometry (Pyr-GC/MS) and thermal extraction-desorption gas chromatography mass spectrometry (TED-GC/MS) have been established for mass-based analysis. Both approaches (mass-based and particle-based) have their justification, but each also has drawbacks that need to be considered. The here named mass-based methods allow a relatively fast mass-quantitative identification of microplastics in complex environmental samples.However, as only 0,5-20 milligrams of a sample can be analysed, the original sample has to be very well homogenised and several aliquots measured in order to achieve representative results. The particle-based spectroscopy methods (FTIR and Raman) on the other hand also allow the collection of ecologically relevant data, such as information on particle numbers, shape and size distribution. To apply spectroscopy methods the microplastic particles need to be isolated from the soil samples which contain a complex mixture of mineral and organic matter. Therefore, an extensive sample purification process is indispensable4.
A recently published method by Julia Möller and colleagues allows the purification of up to 250 g of soil sample of over 99.9% by weight5. First particles larger than 500 micrometres are sorted out manually and analysed with ATR-FTIR spectroscopy. The particles smaller than 500 micrometres undergo a sequence of purification steps before they can be analysed by µ-FTIR spectroscopy. By following a two week long purification protocol mineral matter, plant residues and soil organic matter are removed from the sample. This admittedly long purification period is one of the main drawbacks, but the time efficiency can be increased by purifying multiple samples simultaneously in a parallel setup. The excellent purification efficiency then allows the sample to be scanned under an FTIR microscope to generate a chemical image (FTIR Imaging).
“the long-term enrichment of microplastics in arable soils has the potential to compromise soil fertility and to endanger crop yields and food security”
The microplastics are identified by an overlay of an optical and a chemical image. The microplastics in the chemical image are identified by an automated analysis software tool. Here, the particles that were chemically identified as a known plastic type are highlighted in different colours and microplastic particles down to a size of 10 micrometres can be reliably identified. This way the number, exact size, shape, plastic type and even colour of every single microplastic particle on the sample carrier can be identified. While FTIR Imaging is a well-established method, microplastics smaller than 10 micrometres and the even smaller nanoplastics still cannot be routinely identified and quantified.
The effects of microplastics on soil quality and the growth of agricultural crops
Soil functions and soil fertility heavily depend on its relative contents of natural components such as clay mineral, humus (organic matter), and also soil microbes. Microbes release nutrients from the weathering of rocks and minerals and the recycling of organic matter, clay and humus adsorb water and carry surface charges which enable the storage of nutrients for plants. Here, the pollution of soil with microplastics firstly dilutes the contents of such beneficial constituents – to some extent comparable to the addition of glass spheres or inert quartz sand. Depending on the polymer type and also particle size and shape, this may change the water and nutrient holding capacity of soils. Furthermore, microplastics have been shown to be incorporated into soil aggregates6,7, thus interfering with one of the most important mechanisms for the long-term stabilization of organic carbon in soil. For plastics of some millimetres in size, a significant disturbance of soil structure was reported, soils were more loose and tended to dry out more quickly6,7. Thus the accumulation of microplastics in soil, next to degrading soil fertility, must also be expected to affect carbon storage and greenhouse gas emissions8. Furthermore, the accumulation of microplastics in soil has been reported to alter soil microbial communities9. Soil microbes are key for both, plant nutrition and any potential biodegradation of the introduced xenobiotic polymers. Especially in the rhizosphere of crop plants, the impacts of microplastics on the intricate interplay between the plant host and its associated, plant-beneficial microbiota are far from well understood. However, a certain enrichment of opportunistic pathogens has been reported for “plastisphere” microbiomes of marine systems10.
If similar effects prevail also in soil, the accumulation of microplastics in the rhizosphere of crop plants, especially plants intended for direct human consumption (vegetables, berries), could thus even come with a yet-to-be recognised risk for food security. In summary, the long-term enrichment of microplastics in arable soils has the potential to compromise soil fertility and to endanger crop yields and food security. However, respective research is still in an early stage and robust predictions are still not possible to date.
This short overview summarises the sources of microplastics for agricultural soils, explained the challenges in analysing and quantifying microplastics in soil samples and discusses the potential impacts on agricultural soils and plants. Lots of interdisciplinary research is still needed to understand the fundamental processes and effects. In field samples microplastics exist in different sizes and shapes.
Microplastics found in the environment are not only made of one sort of polymer type, they are complex material mixtures made of different polymers, additives and colours. Thus, scientists are interested to a better and robust detection of microplastics and to understand the impact of size, shape and material on diverse ecosystems. Since microplastics in agricultural soils are closely linked to plastic pollution the whole society needs to tackle this problem.
The recently published report by the UN Environment Programme11 showcases that the impacts of plastics on marginalised populations are severe, and exist at all stages of the production cycle, from extracting raw materials and manufacturing, through to consumption and disposal. Several targeted strategies are needed – there is not only one source and one action that needs to be taken.