The plastisphere in soil
Questions arising from a clarified version of the definition of the soil plastisphere
The plastisphere was a term coined in the aquatic realm, and referred to the microbial community colonizing plastic debris. The same term has then also been applied to the soil, where it also took on a new meaning: the soil around plastic particles, including its microbial community. So when people talk about the community on the surface of plastic pieces in soil, they write about the plastisphere microbial community.
As we recently wrote in a paper in Nature Reviews Microbiology, this way of viewing the plastisphere makes a lot more sense, rather than just referring to the microbial community, because then this terminology also intersects with other well-known “spheres” in the soil, like the rhizosphere or the drilosphere. Before this paper, there was actually quite a lot of confusion about the use of this term, exemplified by the three reviewers of the paper, who gave excellent insights into the manuscript, and all had different takes on this term.
But more importantly, taking this concept of the plastisphere seriously also has other consequences. The most important of which is that it moves the soil under the immediate influence of plastic pieces into focus. What are its characteristics, how does it differ from soil away from plastic surfaces? And most importantly perhaps: how far into the soil does the plastisphere effect really extend? As it turns out we do not know the answer to these questions. We know quite a lot about how plastic, especially microplastic, in soil affects soil properties as a whole, and we now have also quite a lot of information what the microbial community looks like at the surface of plastic particles. But we don’t know how the soil immediately around plastic surfaces differs from the rest of the soil.
How could this be studied? This is not very easy. Studying the attached biofilm of plastic pieces is comparatively simple, and also measuring whole-soil effects is straightforward. One problem with the plastisphere soil is that there is so little soil material around any individual piece; this makes it difficult to measure things. What could be done is to artificially increase the soil material available for study by creating an interface: one could have a microcosm system where a fine mesh separates an area of microplastic from a bulk soil compartment (just like what has been done for the rhizosphere) and then sample soil at increasing distances from such a mesh. This could give some first answers about the extent of the plastisphere effect: how far away from this mesh interface can we measure any changes in soil characteristics.
Why is this interesting? It could tell us when a saturation point of effects could be reached: such a saturation point could occur perhaps when the individual plastispheres start overlapping. And it might give insights into a minimum amount of plastic in soil to initiate effects that are measurable at the whole-soil level.
What do you think? What else could we ask with this version of the plastisphere definition?
This is ground breaking research. I have often wondered what happens to plastics in soil.