Microplastic pollution in terrestrial ecosystems has attracted increasing concern regarding possible impacts on soil functionality. Microplastics can affect soil physicochemical properties, such as aggregation, bulk density, water holding capacity, porosity, pH, etc., and also soil microbial activity measured as respiration and enzymatic activities, with ensuing consequences on plant performance. This doctoral work firstly summarized the sources, migration, and distribution of microplastics in the soil, their effects on soil physicochemical properties, soil biota and plant performance based on previous studies. Then this work investigated the microplastic effects on soil physicochemical properties and microbial activity. The effects included both direct and indirect effects of microplastics as a single factor, as well as a study including the combined effects of microplastics with other global change factors. The review study (chapter 2) summarized the microplastic pollution in terrestrial ecosystems including the sources and distribution of microplastics in soil, and the potential migration pathways. Microplastic effects on soil physicochemical properties such as aggregation, water dynamics, pH, and organic matter contents were also included. Finally, this review provided a general understanding of the impacts of microplastics on soil biota including soil fauna and microbes, and their known consequences on plant performance. The first laboratory study (chapter 3) explored the direct impacts of microplastics with different shapes and polymer types on soil pH and microbial activity, and how these effects may change over incubation time. This work revealed the influences of twelve microplastics (four shapes made of three different polymer types) on soil pH and microbial activities. We specifically found that microplastics could affect soil pH, respiration, and enzymatic activities depending on their shape, polymer type, and incubation time. Specifically, soil pH increased with foams and fragments, and overall soil pH reduced initially and increased afterwards over time. Soil respiration increased with foams, and soil respiration declined with time. Enzymatic activities were impacted by microplastic shapes and polymer types, and fluctuated with incubation time. They were negatively correlated with soil pH, and the presence of microplastics weakened this correlation. The second laboratory work (chapter 4) revealed the indirect effects of microplastic-contaminated soil layers on water distribution, soil aggregation, and microbial activities of adjacent soil layers without microplastics. This research indicated that microplastic-contaminating soil layers could affect the water flow and distribution, the proportion of different-sized aggregates, and microbial activities in adjacent soil layers. Specifically, microplastic-contaminating soil layers impacted the vertical water flow along the soil profile surrounding soil layers, with consequences on water contents and distribution in adjacent soil layers. In addition, microplastic-contaminating soil layers changed the proportion of different-sized aggregates in different depths of the adjacent soil layers. These physical changes contributed to the alterations in soil respiration in adjacent soil layers, but not translated to soil enzymatic activities. Interestingly, microplastic fibers showed more pronounced effects than microplastic films on such soil properties. The third laboratory research project (chapter 5) examined the combined effects of microplastics and drought on a soil-plant system. This study evaluated the microplastics direct effect, and its interaction with drought on soil ecosystem functions and multifunctionality. We found that these effects varied with soil water conditions. That is, microplastic fibers (1) inhibited microbial activity (respiration and enzymatic activities) under well-water conditions, while enhanced microbial activities under drought conditions; (2) promoted litter decomposition under well-water conditions, whereas suppressed it under drought conditions; (4) diminished leachate SO42- irrespective of the soil water conditions, decreased leachate NO3- only when microplastics combined with drought, increased leachate PO43- under well-watered conditions; (5) and increased soil aggregation and soil pH regardless of water conditions; (6) microplastic fibers and drought negatively affected not only single ecosystem functions, but also soil ecosystem multifunctionality.
