Particulate plastics persist both in terrestrial (i.e. soil) and aquatic (i.e. marine and freshwater) ecosystems. Primary particulate plastics are manufactured and are a direct result of anthropogenic use of plastic-based materials. They are used in a wide variety of applications, such as cosmetics (e.g. microbeads), clothing and industrial processes. Secondary particulate plastics are plastic fragments resulting from the breakdown of larger plastic pieces. Both can enter natural environments.
Particulate plastic inputs to terrestrial and aquatic ecosystems include a range of particle sizes (including microplastics and nanoplastics) and types of polymer. Although medium and large plastic fragments are generally sieved out during the composting process, a significant portion of small plastics makes it through the sieve. Because compost is subsequently milled, most plastics end up as secondary microplastics or nanoplastics.
Biowastes – including biosolids (treated sewage sludge) and composts – are excellent sources of nutrients and organic matter for agricultural and degraded soils. Although biowastes offer agronomic benefits, they also contain a number of contaminants, including heavy metals, pharmaceuticals, per- and poly-fluoroalkyl substances and particulate plastics. Particulate plastics end up in soils when biowastes are applied to land (https://ab.co/2HeQHuY). It has been estimated that 107 000–730 000 tonnes of particulate plastics are added to European and North American farmlands each year. The weathering of plastic film mulch used over agricultural fields is also a source of particulate plastic input in soils.
The United Nations Environment Programme identified that large quantities of particulate plastics found within the marine environment globally have originated from land-based sources (UNEP. Marine plastic debris and microplastics: global lessons and research to inspire action and guide policy change, 2016. Jambeck J.R. et al. Science 2015, vol. 347, pp. 768–77). Sediment transfer during soil erosion is one process that allows the transport of particulate plastics from terrestrial to aquatic ecosystems. Despite this link to land-based sources, most scientific research on particulate plastics has focused on their effects in aquatic environments.
Impacts of particulate plastics in soils
Particulate plastics input to soils can have both beneficial and detrimental impacts on soil characteristics and organisms. For example, polyacrylamide is used to promote flocculation (particle clumping) and soil aggregation, thereby mitigating soil erosion. Particulate plastics in soils can serve as a hidden source of carbon sequestration, thereby contributing to climate change mitigation. However, since particulate plastics are introduced through human activities, it may not be considered a ‘direct action’ approach to mitigate climate change.
Particulate plastics affect environmental contaminant interactions in soils by acting as vectors for contaminant transport and by altering contaminant bioavailability. Plastic polymer resins are mixed with various additives to improve their utilisation performance. These additives include inorganic carbon- and silica-based filler materials to reinforce final products, plasticisers to render the plastic polymers pliable, thermal and ultraviolet stabilisers, and flame retardants. Some additive chemicals such as tributyl tin in polyvinyl chloride, phthalates and various bisphenol analogues in polycarbonates are leached when plastics undergo photochemical weathering, and have adverse effects on soil microbial diversity and function.
Particulate plastics interact with a wide range of organic and inorganic pollutants in the environment. Adsorption and desorption processes of pollutants in particulate plastics in the soil environment are influenced by factors such as weathering and surface area, interaction with natural organic matter (NOM), and microbial activity (i.e. biofilm formation). The association of NOM with particulate plastics in terrestrial environments plays a crucial role in promoting particulate plastics as a vector for contaminant transport. The high surface area of particulate plastics, which can be increased through weathering, not only aids adsorption of contaminants, but also supports chemical transport of the pollutants through leaching.
Measuring particulate plastic input to soils
Although particulate plastics are recognised as emerging contaminants in soils, their impact in the soil environment remains largely unclear, particularly on microbial functions and contaminant mobility. An international research group, including our Global Centre for Environmental Remediation at the University of Newcastle, quantified the amount of particulate plastics in a range of biowastes, including biosolids, composts and manures (doi.org/10.1016/j.chemosphere.2018.01.166). We also examined the impact of particulate plastics on microbial activity and contaminant mobility in soils.
In the study, uncontaminated and copper-contaminated soil samples were treated with particulate plastics. Copper is used agriculturally as a fungicide and is toxic to soil microbial communities. These soil samples were subsequently analysed for bioavailable copper concentration, soil respiration, microbial biomass carbon and dehydrogenase activity. Bioavailable copper concentration is an index for toxicity to soil microorganisms. Soil respiration, microbial biomass carbon and dehydrogenase enzyme activity provide an index of microbial function.
As our results showed, biosolids contain much higher amounts of particulate plastics. We estimated the amount of particulate input to soils through biowaste application as a rate of 10 t/ha as a source of organic matter input to soils. Although the particulate plastic input to soils through biowaste application ranged from only 0.15 to 1.05% as a mass percentage of biowaste input, the number of particulate plastics added to soils through these biowaste applications was 5 000 000–12 000 000/ha.
The incubation study indicated that the bioavailability of copper in contaminated soils decreased with the addition of particulate plastics. This may be attributed to the adsorption of copper by the NOM associated with the particulate plastics. The results also showed that there was a significant difference in soil respiration between copper-contaminated and uncontaminated soil samples (see graph). Particulate plastics addition resulted in a significant increase in soil respiration in both uncontaminated and contaminated soils. Particulate plastics retain contaminants such as copper, thus reducing their toxicity to soil microorganisms. The improved soil aeration (porosity) caused by particulate plastics addition could be another reason for the increase in the microbial activity. Similarly, uncontaminated and copper-contaminated soils treated with particulate plastics appeared to increase dehydrogenase activity and microbial biomass carbon, indicating increased microbial function.
Particulate plastics in terrestrial ecosystems can have both beneficial and detrimental effects on soil health. Our findings indicate that adding particulate plastics helps modulate contaminant (e.g. copper) toxicity on soil microbial activity. However, it is important to examine the long-term effects of particulate plastics on soil biology and function, which is a major focus of our current research on particulate plastics.