The soil plastisphere

Abstract

Understanding the effects of plastic pollution in terrestrial ecosystems is a priority in environmental research. A central aspect of this suite of pollutants is that it entails particles, in addition to chemical compounds, and this makes plastic quite different from the vast majority of chemical environmental pollutants. Particles can be habitats for microbial communities, and plastics can be a source of chemical compounds that are released into the surrounding environment. In the aquatic literature, the term 'plastisphere' has been coined to refer to the microbial community colonizing plastic debris; here, we use a definition that also includes the immediate soil environment of these particles to align the definition with other concepts in soil microbiology. First, we highlight major differences in the plastisphere between aquatic and soil ecosystems, then we review what is currently known about the soil plastisphere, including the members of the microbial community that are enriched, and the possible mechanisms underpinning this selection. Then, we focus on outlining future prospects for research on the soil plastisphere.

Fig. 1

The definition of the soil plastisphere (A) and its place in the context of other soil compartments (B). The soil plastisphere consists of a specific environment shaped by the plastic material and the microbial community inhabiting this environment. The microbial community of the plastisphere consists of the attached biofilm (or microbes colonizing the plastic surface), but also of the microbes in the soil under the influence of the plastic particle (A). (B) The soil plastisphere in the context of other important soil features and hotspots^,, such as the rhizosphere (soil under the influence of roots), the aggregatusphere (soil aggregates as major building blocks of soil structure), the drilosphere (soil influenced by earthworms) and the detritusphere (soil under the influence of dead organic material, detritus). We here re-define the soil plastisphere as the soil under the direct influence of plastic particles, which includes the biotic community but also the changed physicochemical soil environment itself. The soil plastisphere is envisioned as being embedded within the hierarchy of the other soil compartments.

Fig. 2

Overview of the general difference between plastic in aquatic (A) and terrestrial (B) ecosystems in terms of microplastic behavior, with a focus on comparing movement of the particles. These two contrasting environments differ in sources of microplastic, in properties of the environment, and especially the mobility of the plastic particles. For example, plastic in aquatic systems adds a surface to the water body, whereas soil is already a particle-rich environment. In soil, particles are substantially less mobile, which means they experience a much more heterogenous set of localized micro-environments (see also Fig. 1). In soils, microplastic can become incorporated into soil aggregates, which makes these particles even less mobile. By contrast, in aquatic environments, plastic debris can be moved by currents until it settles to the sediment at the sea floor.

Fig. 3. The soil plastisphere microbial community differs from that in other soil compartments.

(A) The soil plastisphere is an environmental filter, leading to lower microbial richness and diversity compared to the source community in the bulk soil; and (B) functional composition of microbial communities in the soil plastisphere shifts from that in other soil compartments. Soil plastisphere community composition is influenced by (C) a range of environmental and soil factors and (D) plastic characteristics.

Fig. 4. Future developments and outlook.

There are several key questions that need to be addressed to drive progress in our understanding of the soil plastisphere. (A) Overall effects and roles of the soil plastisphere in ecosystem processes and Earth system feedbacks, (B) the spatial extent of the plastisphere in the soil environment, (C) existence of a core microbiome in the soil plastisphere, (D) successional changes of the plastisphere with fate of the plastics in soil environment, (E) interactions between soil plastisphere and other factors of global change.