Interactions of microplastic, soil and plant with a particular emphasis on phosphorus

Relatively little is known about the impacts of microplastics in the soil, particularly the interactions between the microplastics and soil components (such as nutrients, enzymes and aggregates). Many of these interactions impact on the biochemical processes by changing the soil properties, that may directly/ indirectly influence the growth of plants. Although microplastic appeared to impact plants in previous studies, the effects varied, and possible reasons behind these impacts are largely unknown. The present thesis examines microplastic adsorbed phosphate, and how this could affect soil properties and plant growth, through laboratory and greenhouse experiments. The data showed that the microplastic had the potentiality to adsorb phosphate and UV weathering increased the adsorption. Adsorption on the microplastics was far lower compared to the soils. Experiments measuring the phosphate adsorption observed increased adsorption with the increasing concentration of the
background electrolyte above and below the point of zero charge (PZC). According to the literature, phosphate adsorption was not expected to increase with the concentration of the background electrolyte below the PZC. Experiments investigating the soil properties and estimating plant parameters found that the microplastic did not show any negative effect on soil and plant when applied at a 0.05 % rate. Nevertheless, an application rate of 0.50 % and 5.00 % did cause negative responses to the soil and plant. We used a range of environmentally relevant to higher than environmentally relevant concentrations of microplastic in both incubation and greenhouse experiments. Our data indicated reductions in available phosphate which were consistent with the reduced plant growth. However, we could not confirm whether reduced phosphate availability was driving reduced plant growth since other factors also changed in response to the microplastics that might also be responsible for the reductions
in plant growth. The present thesis on the soil and plant responses vis-à-vis microplastic additions indicates
that the interaction between the microplastic and soil leads to reduced growth of plants, and pervasive microplastic impacts may have consequences for terrestrial environments.