Soil under stress: The importance of soil life and how it is influenced by (micro)plastic pollution

Abstract

Soil organisms and specifically microorganisms are indispensable to life on Earth. They regulate essential ecosystem functions from carbon sequestration to primary production. These organisms often experience stress when the balance of the soil system is disrupted by agricultural practices and environmental disturbances. A new stressor is plastic, which can be found in soils, in and around soil-dwelling organisms, and close to plants. The presence of plastic can affect soil chemistry, plant growth and the survival of higher-order organisms. Microbial organisms respond sensitively to these changes in their surroundings and will thus be (in)directly affected by plastic. Eventually, this results in a different microbial activity, composition and reduced diversity. Plastic might even serve as a specific habitat for microorganisms, generally referred to as the plastisphere. In this review, we make predictions based on the observed effects of (micro)plastics and the potential impact on the plant-soil-microbiome system. We use prior knowledge of other disturbances (e.g. tillage and pesticides) which have been studied for many years in relation to the soil microbial community. Further research is needed to develop standardized methods to study smaller plastic particles (micro- and nanoplastics) as these play the most dominant role in terrestrial ecosystems.

Keywords: AMF, arbuscular mycorrhizal fungi; Agriculture; DOC, dissolved organic carbon; FDA, fluorescein diacetate hydrolase; HDPE, high density polyethylene; LDPE, low density polyethylene; MP, microplastic; Microplastics; NP, nanoplastic; PBAT, polybutylene adipate-co-terephthalate; PES, polyethersulfone; PET, polyethylene terephthalate; PGPF, plant growth promoting fungi; PGPR, plant growth promoting rhizobacteria; PLA, polyactic acid; PP, polypropylene; PS, polystyrene; PVC, polyvinylchloride; Plastisphere; Rhizosphere; SOC, soil organic carbon; SOM, soil organic matter; Soil functions; Soil microbiome.

Graphical abstract

Fig. 1

Illustration of the vital roles of organisms in soil. Numbers indicate the main soil processes to which microorganisms contribute. (1) carbon (C) sequestration; C enters the soil mainly via plants uptake and organic material. It is the second largest C sink, sequestering around 80% of the global terrestrial C, of which 58% is contained in the soil organic matter. Soil microorganisms contribute to this C cycle through respiration and decomposition from root deposits and plant litter (2) nutrient cycling; Macrofauna, such as earthworms, breaks down the larger organic material into smaller pieces, making it available for microorganisms which can either consume or degrade these smaller pieces. This breaks down the complex chemical compounds into more simple compounds that can again be taken up by plants. (3) soil structure; the soil structure consists of air and water-filled pore spaces created by organisms such as earthworms. Roots can reach these water-filled spaces, and these spaces are also inhibited by hydrophilic organisms. Fungi are able to bridge air-filled pore spaces with their hyphae. Microorganisms themselves contribute to the soil structure by converting organic material. The rhizosphere is magnified in black square and highlights the role of plant-growth promoting rhizobacteria and -fungi (PGPR and PGPF) for the growth, productivity and health of crops. These microorganisms can also suppress pathogens by their biological control activity. PGPR plant-growth promoting rhizobacteria; PGPF plant-growth promoting fungi; AM arbuscular mycorrhizal fungi; SOC soil organic carbon; C carbon; N nitrogen; P phosphorus; CO₂ carbon dioxide; CH₄ methane; N₂ dinitrogen; N₂O nitrous oxide; NO₃^– nitrate; NH₄ ammonium; SOM soil organic matter.

Fig. 2

Annual plastic production and overview of the number of research papers on Web of Knowledge (on 21/5/2021) regarding microplastic pollution in terrestrial, freshwater or marine environments. Annual plastic production is displayed by a black line starting from 1862. The search included the following keywords: terrestrial (soil + MP OR terrestrial + MP), freshwater (river + MP, freshwater + MP), marine (marine + MP, aquatic + MP, sea + MP). All reviews were excluded from the literature search.

Fig. 3

The known and potential effects of micro- and nanoplastics on the soil physicochemical and microbial characteristics. Proven effects of microplastics are indicated in black boxes: first it has been shown that the presence of MP changes the soil bulk density and is able to increase the dissolved organic carbon (DOC) in the soil. This is related to changes in the water availability (increase) and evaporation (increase). Effects on plant development have also been noted. These effects are still uncertain either (increase or a decrease) and are therefore indicated with a crossed-out tilde. MPs also affect the soil food web. Decreases in the survival and reproduction of earthworms and nematodes have been noted and also an active uptake of MPs by these organisms has been shown. The combination of these effects can explain at least in part the effect on the microbial community, with changes in the microbial activity, composition and a decrease in the microbial diversity. In addition, we added some hypothetical effects (in orange) plastics might cause. The degradation of plastic might result in an increase in the carbon:nitrogen (C:N) ratio. Also, differences in the rhizosphere microbiome are expected as the root architecture of the plant is different in plastic-polluted soil. The presence of plastic might also increase the abundance of plastic degrading organisms, either in the soil or residing on the plastic. Plastic can thus serve as a vector; however, it remains unclear if this will be more for pathogens or beneficial microorganisms as indicated by exclamation point and check mark, respectively. DOC dissolved organic carbon; MP microplastics; C carbon; N nitrogen; H₂O water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)