Critical threshold pot trial
Soil and crop health determined by plastic type, size, and concentration in a mesocosm study
The use of plastic in agriculture has been increasing globally over the last 50 years and it is estimated that 8-10 million tonnes are used each year. One common use of plastic is as mulch films laid over the soil surface and crops like maize, vegetables and fruits planted through the film to increase crop yields. However, plastic mulch films are difficult to retrieve from the soil after harvest and are costly to recycle due to contamination of plastic with soil particles. This results in the plastic films contaminating the soil each year creating a legacy of plastic in the soil with yet unknown long-term effects. We know that most plastics used in agriculture do not come as pure polymers, but with a mix of different additives attached to the main polymer chain. When negative effects on soil and plant health are observed due to plastic application to the soil, it is often undetermined if these effects are caused by the plastic polymer itself, or by additives that are leaching into the soil, or a combination of both.
This experiment is investigating the effect of pure plastic on soil and plant health in a mesocosm study. We used different plastic types (polyethylene (PE) and polypropylene (PP)), different sizes (macroplastic (1cm²) and microplastic (7-27µm)), and different concentrations (equivalent of 1-, 10-, 25-, and 50-years accumulation of plastic mulch film use). The aim was to determine a critical threshold of plastic addition to the soil.
The experiment was set up as a greenhouse mesocosm study with maize (Zea mays) as crop and ran over a time course of 8 weeks. We had 5 replicates for each treatment, including a control with the equivalent of 50-years silicon dioxide added to the soil to account for the displacement of soil and usable organic matter.
The key soil measurements for this experiment include soil hydrophobicity, soil physical properties (pH/EC/ammonium/nitrate/phosphate), carbon & nitrogen content, enzyme activity, and microbial diversity (16S & 18S/ITS & PLFA).
The key plant measurements include chlorophyll content, growth dynamics, carbon & nitrogen content, and shoot & root biomass.
Pot trial showing different treatments. Pieces of plastic are visible on the surface of the pot in the left foreground.