Biodegradable Polyesters and Low Molecular Weight Polyethylene in Soil: Interrelations of Material Properties, Soil Organic Matter Substances, and Microbial Community

Abstract

Conventional and also biodegradable polymer microplastics have started to be broadly present in the environment, if they end up in soil, they may influence both abiotic and biotic soil properties. In this study, the interactions of polyethylene wax together with three biodegradable polyesters PLA, PHB and PBAT with a soil matrix were investigated over a 1-year incubation period. Soil organic matter content was measured using UV-VIS, the microbial biomass amount was measured using qPCR, the mineralisation of polymers was measured using UGA 3000, the surface of polymers was observed with SEM, live/dead microorganisms were determined by fluorescent microscopy and microbial consortia diversity was analyzed using NGS. The amount of humic substances was generally higher in incubations with slowly degrading polyesters, but the effect was temporary. The microbial biomass grew during the incubations; the addition of PHB enhanced fungal biomass whereas PE wax enhanced bacterial biomass. Fungal microbial consortia diversity was altered in incubations with PHB and PBAT. Interestingly, these two polyesters were also covered in biofilm, probably fungal. No such trend was observed in a metagenomic analysis of bacteria, although, bacterial biofilm was probably formed on the PE520 surface. Different methods confirmed the effect of certain polymers on the soil environment.

Keywords: SEM; bacteria; biodegradation; fungi; polyesters; polyethylene; soil organic matter.

Figure 1

Mineralisation rate of tested samples.

Figure 2

Scanning electron micrographs and live/dead photos of PE sample. Magnification of initial SEM samples 1000×, incubated SEM samples 3000×, fluorescent microscopy samples 400×.

Figure 3

Scanning electron micrographs and live/dead photos of polyesters. Magnification of initial SEM samples 1000×, incubated SEM samples 3000×, fluorescent microscopy samples 400×.

Figure 4

Biomass of bacteria in incubations.

Figure 5

Biomass of fungi in incubations.

Figure 6

The percentual change in HA concentrations in the soil after 3, 6 and 12 months of incubation relative to HA content in blank incubation.

Figure 7

The percentual change in FA concentrations in the soil after 3, 6 and 12 months of incubation relative to FA content in blank incubation.

Figure 8

The ratio of spectral absorbances at 445 and 665 nm.

Figure 9

Heatmap of the bacterial community composition and time evolution at the class level.

Figure 10

Principal component analysis scatter plot based on 16S rDNA of bacteria.

Figure 11

Heatmap of the fungal community composition and time evolution at the family level.

Figure 12

Principal component analysis based on 18S rDNA.

Figure 13

The scheme of spectrophotometric method (NaOH method).