Yet, for the past decade or so, more and more attention, though still less than that given to aquatic ecosystems, has been paid to research into the changes caused by the presence of macro- and microplastics in the biotic and abiotic parts of the soil ecosystems. And with the implementation of the Bio-Plastics Europe project in the Horizon 2020 program, a research group from TUL has joined other scientists who work on this issue.
How bioplastics affect plants and earthworms
The research allowed the scientists to assess the impact of bioplastics on the biotic part of the soil ecosystem. The indicator organisms were land plants such as sorghum (Sorgum sacchcrataum), mustard (Sinapsis alba) and cress (Lepidium sativum), as well as earthworms Eisenia andrei. The tests were conducted against individual species of the aforementioned organisms, as were multispecies tests in a laboratory-scale model of the soil ecosystem with a working volume of 4.3 liters.
During the first run of the tests, the toxicity of five different bioplastics was evaluated, and in the second one, three. The examined bioplastics differed in chemical composition, properties, and function.
What did we learn about bioplastics from the ecotoxicity tests against soil organisms?
It was found that the presence of bioplastics in the soil had no effect on the germination of land plants even at relatively high concentrations of microplastic, reaching almost 12%. In contrast, some of the investigated bioplastics inhibited or stimulated the growth of plant roots and/or shoots. To put this into perspective, the concentration of plastics so far observed at different locations around the world various locations around the world has generally not exceeded 6.8%.
As for earthworms, the study has shown that their mortality does not increase when bioplastic microparticles are present, although, at the highest concentrations of certain types of bioplastics in the soil (about 12%), a reduction in the reproductive capacity of E. andrei is observed, as are elevated values of the oxidative stress markers.
The interesting thing is that earthworms which lived for a sufficiently long time (at least 28 days) in the model soil ecosystem tried to avoid coming into contact with the microplastic particles and to that end, most of them remained in the lower layers of the soil.
It is important to know that E. andrei are a species that lives close to the surface of the soil, and this was also true in the model soil ecosystems free of bioplastic. In contrast, when bioplastic microparticles were present, one could say that a fight or flight response was triggered in the earthworms and they attempted to take flight from the plastic-contaminated soil.
The bottom line is that although microplastics, the conventional ones as well as bioplastics, present in the soil at concentrations of less than 1% do not pose an immediate threat to plant and earthworm survival, their long-term impact may contribute to lowering their reproductive and migration abilities, and inhibition of the growth of some plants, which, in the long term, will lead to depletion of the soil biocenosis and degradation of the quality of the soil.
And what about the biodegradation of polymers?
The research on the biodegradation of bio-based polymers is being carried out by Professor Liliana Krzystek and dr inż. Radosław Ślęzak in collaboration with Professor Izabella Krucińska, Professor Michał Puchalski, and Professor Sławomir Sztajnowski of the Institute of Textile Materials Science and Polymer Composites.
It is concerned with the evaluation of the biodegradation of innovative polymeric plastics in soil under natural conditions, with particular emphasis placed on the role of microbial consortia.
The rate of degradation of biodegradable polymers in the natural environment depends both on the structure and properties of the material itself as well as on the environmental conditions where they are located. That said, the most important biological factor present in the natural environment that affects the rate of degradation is the activity of the microorganisms that the material contains, specifically bacteria and fungi.
The experiments performed on the samples of polymeric materials studied in the project (planted in the soil), their analysis with advanced measurement techniques - micro and macroscopic, combined with the analysis of the soil microflora and climatic conditions, allow the researchers to identify and characterize the individual phases as well as the degree and rate of the biodegradation process.
Only a slight reduction in the mass of the polymeric plastics studied over a period of 12 months in the soil was observed. Visually observable changes in the surface structure, changes in the chemical composition as well as in the mechanical properties also indicated a low degree of degradation in soil under natural climatic conditions prevailing in Poland. Some changes were observed in the profile of the microorganisms identified in the soil and on the surface of the investigated materials.