Microplastics in soils and their associated effects on agricultural crop development.

Plastic pollution is a global problem and will increase as plastics continue to be produced, used and discarded. Microplastics (< 5 mm in size) are ubiquitous, being found in every environmental compartment. Plastics are thought to occur in terrestrial environments at rates 4 – 23-fold higher than in aquatic environments and have the potential to pose significant risks to flora and fauna. However, the majority of studies on microplastics have focused on the impacts in aquatic systems, with less research focused on the terrestrial system. Even less research has focused on the agricultural system, and the risks microplastics pose in this environment, despite agricultural soils being proposed as acting as a sink for microplastic pollution.

This thesis presents information from the two aspects of this work, firstly an assessment of the microplastic abundance, morphology and polymer types of microplastics in conventionally managed farmlands in soils from the Midlands, UK. To do this, a methodology was created to enable the extraction of microplastics from soils, which used Fenton’s reagent to digest organic matter prior to density separating plastics from the remaining soil using zinc bromide. This research found that Fenton’s reagent was the most appropriate methodology to extract microplastics from soil media. Subsequently, the method utilizing Fenton’s reagent was used to generate quantitative data on the microplastics present in eight farms throughout the Midlands. A total of 24 fields were sampled, with a range of pasture and arable fields sampled to assess the common polymer types, shapes, and concentrations found in these soils. The assessment of agricultural soils found that microplastics were present in all the fields sampled and that fields that had anthropogenic additions (such as fertilizers, sewage sludge, or composts) contained higher levels of microplastic pollution than fields that did not. There were no differences in the microplastics per kg between arable and grazing fields. In the arable fields, the number of microplastics found in the centrally farmed area, as opposed to the unfarmed margins, was significantly higher but this did not occur in the pasture fields. Fibres were the most common shape of microplastic accounting for 95 % of the total microplastic shapes found, and polyester was the most common type of plastic found accounting for 41 % of the total plastic types.

Based on findings from the agricultural fields, the second aspect of this work assessed the impact of the most commonly found fibre, polyester, in four agricultural crop species (Brassica napus (rapeseed), Sinapis alba (mustard), Triticum aestivum (wheat), and Hordeum vulgare (barley)) reviewing any changes to germination, development and reproduction as a result of microplastic polluted soils. This research utilized germination bioassays to review changes to the germination rates, alongside chronic toxicity mesocosm studies to assess any changes to the early development and reproduction of plant species. The research indicated that the addition of polyester fibres resulted in reductions to the germination rates for all species test (at 5 % w/w resulting in reductions between 5 & 17 % reduction), but that this also occurred when a natural fibre was added to Sinapis alba (9 % reduction for polyester and 13 % reduction for keratin) , suggesting a physical change in the soil matrix resulted in the changes to germination rates for the four species tested. However, changes were not demonstrated in the shoot and root development when testing with natural fibres but were with polyester fibres. An assessment of chemical effects from plastics was performed using leachate (produced using 5 % w/w polyester fibres), where no effects were demonstrated in germination, however, changes in root development were shown when polyester leachates were used.

For further insight into the impacts of microplastics on plant development, Sinapis alba were grown in soils containing two different concentrations of polyester fibres (0.1 % and 1 % w/w) to the fruiting stage. This research indicated that the Fv/Fm measurements (which can demonstrate changes to the PSII system – the first protein complex in light-dependent reactions of photosynthesis) were reduced in the 1 % treatment compared to the control. Additionally, the total flower numbers produced, and the pod-to-seed ratio were reduced when exposed to 0.1 % and 1 % w/w microplastics compared to the control. This suggests that the addition of microplastics to soils could result in reduced yield for agricultural plastics.

The results from this thesis provide strong evidence that microplastics in high concentrations can change the germination, development, and reproduction of crop species. The current concentration of microplastics in UK agricultural soils as of present are not high enough to cause these effects. However, if plastic continues to be added to agricultural soils at the current rate, it is expected that these effects will pose issues for agricultural crop development. Further research is needed into the effects of different polymer types and on a wider range of agricultural crops to ascertain whether the shape or type of polymer has an impact on the development of plants.