The second CLIMECS experiment

A key objective of the PAPILLONS project is to understand the fate and effects of microplastics from agricultural plastics. We need to investigate where plastic residues end up and how they might interact with animals and plants. One of the ways we will research this is through mesocosm experiments at the CLIMECS facility in Vrije Universiteit Amsterdam. During the last week of January, PAPILLONS scientists from Leiden University and the Norwegian Institute for Water research travelled to Vrije Universiteit Amsterdam to assist with the end of the second CLIMECS experiment of the project. Over 2,000 samples were collected or measurements taken during the week. 

The second CLIMECS experiment 

The second experiment run in the CLIMECS facility focused on fate: where microplastics in soil end up. More specifically, we investigated the influence of different watering regimes and the presence or absence of earthworms on the vertical transport of microplastic through soil columns. 

At the start of the experiment, 30 cm of compacted soil was added to the bottom of each column and 10 cm of less compact soil was added on top – to represent a plough layer. The plough layer had already been mixed with different concentrations of microplastics composed of two important polymers used in agricultural plastics: linear low density polyethylene (LLDPE) and a biodegradable starch-polymer blend. These microplastics are small fragments of real mulching films used in European agriculture, representing the residues that these products may leave behind in soils. 

Different treatments were applied to distinguish between the effects of the presence or absence of two species of earthworm (Lumbricus terrestris and Dendrobaena hortensis) and two different watering regimes, a “high” and a “low”. To simulate a more complete ecosystem, three Collembola species (Heteromurus nitidus, Ceratophysella denticulata and Protaphorura fimata) were also introduced into all columns and lettuce (Lactuca sativa) was sown. Once the soil columns were established, the experiment was run for 12 weeks.

Image 1: Close up of a column at the end of the 12 week experiment showing the lettuce that was grown.

Image 2: Removing the soil columns for sampling. 

Sampling and analysis 

During the experiment, samples of the plants were collected. At the end of the experiment, the final plants were removed for analysis and each column was carefully sliced into 6 layers. We took photos of each layer and measured the penetrability of the soil. From each layer, samples were taken for the analysis of microplastics, soil physicochemical properties (bulk density, pH, water holding capacity and organic content), plastic additive chemicals and microbial activity. The earthworms that were added into some of the treatments were retrieved, counted and collected for analysis of stress markers and microplastic content. Any water that had passed through the columns and leached out of the bottom was collected for analysis of microplastic content. In total, 2,272 different samples were collected or measurements taken. These samples will now travel across Europe to different laboratories where the analysis will commence.

Many different sample types were collected from the columns as they were broken down into different layers.

What will our results tell us? 

Once all the samples have been analysed, we will have a good picture of the vertical transport of microplastics from mulching film residues in soils. By comparing the microplastic content in different layers, we will see whether microplastics mostly stay in the upper part of the column or if they are actually transported to deeper layers and, in the case of the latter, what particle types (e.g. size or polymer) are most likely to be transported. By comparing the different treatments – the presence or absence of earthworms and the different watering regimes – we will be able to unpick which factors encourage more movement of particles down the columns. Our analysis of plastic additive content in the soils will tell us whether these chemicals are released from microplastics under typical environmental conditions and how they may also undergo vertical transport. Analysing the microplastic content of the earthworms will show whether these animals ingest microplastics. 

Understanding where microplastic residues from mulching films end up in soils is important. There is already evidence that the presence of microplastics can affect our soils. We need to know how microplastics that enter the surface layers of soils can be transported to different parts of the ecosystem or be taken up by organisms. This experiment will reveal whether surface contamination can spread to deeper soil layers, where different organisms are present and could be affected. Analysis of microplastic in leachate samples will also show whether residues could enter groundwater supplies. By analysing different soil physicochemical properties and microbial activity, we will see whether microplastics are changing the structure or functioning of the soil at different layers and how this could actually encourage transport. 

Whilst this experiment focused mainly on assessing fate, we also took samples of the plants and animals to measure indicators of stress. This will begin to shed a light on the potential effects of microplastics – something we will continue to examine in much more detail in the rest of the PAPILLONS project, as well as the third CLIMECS experiment planned for later this year. Stay tuned to learn more about these experiments and our results! 

PAPILLONS team with just one of the crates full of samples collected by the end of the week. 

Scientists involved included:

• Anne Wilts, Intern at Leiden University
• Laura Zantis, PhD candidate at Leiden University
• Rachel Hurley, Scientist, NIVA
• Hafeez Ur Rehman, Post Doc at NIVA
• Sam van Loon, PhD candidate, Vrije Universiteit Amsterdam
• Lotte de Jeu, technician, Vrije Universiteit Amsterdam