The effect of PE and biodegradable PHBV microplastics on soil & crop health – a long term field experimental study    

Microplastic contamination in agroecosystems is becoming more prevalent due to the direct use of plastics in agriculture (e.g., mulch films) and via contamination of amendments (e.g., compost, biosolids application). Long-term use of agricultural plastics and microplastic pollution could lead to soil degradation and reduced crop health due to the slow degradation of conventional plastics creating legacy plastic. Biodegradable plastics are more commonly being used, both domestically and in agriculture, to minimise plastic pollution due to their biodegradable nature. However, the influence of biodegradable plastics on soil function at the field scale is largely unknown. The accumulation of plastic in agricultural fields is now threatening to undermine delivery of many of the UN’s Sustainable Development Goals. We aim to assess the impact of plastics on agroecosystem health and use this knowledge to co-develop interventions to reduce this plastic legacy, while still maintaining sustainable farming practices and livelihoods. 

Objectives

This multi-year experiment (start: 2020) is investigating the effect of conventional (PE) and biodegradable (PHBV) microplastic in different concentrations on crop and soil health after repeated application over time. Here, we are growing barley (Hordeum vulgare) to monitor differences in nutrient uptake of the plants, plant performance & growth, and biomass at harvest. We are also interested in monitoring any changes to soil physical properties, biochemical processes, and microbial community structure. Additionally, we are investigating the vertical migration of microplastic particles through the soil profile, uptake of particles and/or additives into plants, and the degradation of biodegradable plastic over time under realistic field conditions. 

Experimental approach

The experiment is repeated annually, using the same plots as in the previous year. Each year, the plots are split to maintain one third of the original application of microplastic (100 kg/ha) and apply a further 100 kg/ha to the remaining plot space. After three years, the plots will have been split into three parts of equal size with the following microplastic application rates: 1) 100 kg/ha, 2) 200 kg/ha, and 3) 300 kg/ha. The control plots remain without any microplastic application throughout the entire experimental process.  

The key soil measurements for this experiment include soil moisture & temperature, soil physical properties (pH/EC/ammonium/nitrate), carbon & nitrogen content, greenhouse gas emissions, and microbial diversity (16S).  

The key plant measurements include chlorophyll content, growth dynamics, carbon & nitrogen content, microplastic and/or additive uptake, and biomass. 

The key microplastic measurements include chemical changes in the polymers caused by soil microbial breakdown, plastic migration through the soil profile, and microbial colonisation on particles. 

Figure 2. Aerial photograph of experimental area during the first year of plastic application (2021).