Dirt(y) plastics

                              

Breaking down the microbiology world one bite at a time


Dirt(y) plastics

We have all heard about the tons and tons of plastic that end up in the ocean. But did you know that 2-23 times more plastics annually end up in the soil than in oceans? At least, according to calculations in this study. Plastics can enter soil through sewage sludges, fertilisation of organic manures, disposal of wastewater effluent and/or plastic mulching, and atmospheric deposition.

Plastics are on their way to becoming microplastics. Source:
https://www.wtert.net/news/433/Microplastic-Pollution-Caused-by-Laundry.html

Soil is made from three different primary components, sand, silt, and clay, together with organic matter and other particles. When these components clump together, we call them ‘soil aggregates’. These aggregates form different shapes, and this combination allows for the formation of pores that can contain air or water. Plastics, however, increase soil aggregation and disrupt the formation of ‘healthy soil’. In other words, adding plastics will change the soil porosity, which, in turn, has an influence on the growth of plant roots but also on nutrient dispersal and/or availability and pH.

The plastics not only change soil properties, but also the microbial communities within. Unfortunately, our knowledge of this is still limited because the effect of each type of plastic changes depending on the soil and on the kind of plastic polymer. For instance, an earlier study found that plastics could reduce the pH of acidic soil, but could increase the pH of alkaline soil.

In their study, Shi et al. looked at the effect of three kinds of microplastics in four types of soils. Since carbon processing and storage is a crucial function of a well-functioning soil ecosystem, the researchers focused on CO2 emission and dissolved organic matter characteristics. More interestingly, they also looked at the response of bacterial communities to plastic-induced stress.

The four soils used in this study were collected from different Chinese provinces: 

Cinnamon soil (loam) contains organic matter and is suitable for growing most plants.

Yellow cinnamon soil (loam) is a widespread soil type in China. However, its poor soil structure and lower fertility limit crop productivity.

Black soil (silty loam) is part of the topsoil; the granules are smaller than sand. If you rub it between your fingers it feels loamy. With a high density of organic matter, black soil is very suitable for growing crops. 

Red soil (silty clay) is widely distributed in tropical and subtropical regions in China and is potentially the most important soil for food production in the world.

Soil types by clay, silt and sand composition as used by the United States Department of Agriculture. Source:
https://en.wikipedia.org/wiki/File:SoilTexture_USDA.svg

The three plastics, (PA, PE and PET) were all added as a powder in an environmentally relevant concentration of 0.5%. However, some soils have been reported to contain up to 6.5% plastic!

Polyamide-6 (PA) is the most significant construction material used in many industries, such as the automotive industry, aircraft industry, electronic and electrotechnical industry, clothing industry and medicine. 

Polyethylene (PE) is the most common plastic in use today, and is mainly found in packaging such as plastic bags and plastic films.  

Polyethylene terephthalate (PET) is used to make fibers for clothing and plastic food containers. 

The researchers found that there was a small but insignificant change in the alpha-diversity (e.g. species diversity within each soil type) in the soil where PA was added. Similarly, when looking at beta-diversity (e.g. species diversity between soil types), there was no clear difference between the control samples (no plastics added) and different plastic treatments.

Alpha-diversity vs. beta-diversity. Source:
https://en.m.wikipedia.org/wiki/File:Biodiversity.png

However, the effect of the various plastics differed per soil type: three plastic types increased the percentage of Proteobacteria in the cinnamon soil while decreasing the number of Proteobacteria in the red soil. So, these results indicate that the type of soil has the most influence on microbial changes, not the kind of plastic.

Next, the researchers looked at the co-occurrence of bacteria via so-called ‘interaction networks’. In short, these networks show if bacteria are commonly found together or if microorganisms have a positive or negative relationship with each other. They found that, compared to the control, the networks of plastic groups had fewer nodes (e.g. species that can be connected), fewer edges (e.g. the species that were connected to fewer other species), and fewer keystone species (e.g. species that form a connection to many other species and act as a ‘hub’). The researchers also noticed that the percentage of negative interactions in the network decreased.

Microplastics influence microbial networks. The amount of species (dots) and their connections (lines) decreases when plastics are added to the soil (PA, PET and PE), compared to the control (CK). Source: the original article

Although the species diversity might not change when plastics are in the soil, the results described above show that microplastics can influence bacterial co-occurrence, and, therefore, lower the stability of the network. Unstable ecosystems are susceptible to small changes, leading to a collapsed community that cannot perform its original function. Microbes play an important role in nutrient cycling in soil, breaking down crop residues, disease suppression, and simulating plant growth. When these ecosystems are disrupted, crop yields could decrease or the soil might even become inhabitable for plants.


Link to the original post: J. Shi, Y. Sun, X. Wang, J. Wang. 2022. Microplastics reduce soil microbial network complexity and ecological deterministic selection. Society for Applied Microbiology. 2157-2169.

Featured image: https://www.flickr.com/photos/chesbayprogram/16999295662