Ecological effects of microplastics on terrestrial plants

Abstract

Human impact on the environment is undeniable and anthropogenically driven processes of global change are becoming more and more apparent. While some aspects of global change such as global warming and its consequences can be abstract and hard to grasp at times, other parts are very tangible. One such obvious part is global plastic pollution. Global plastic production is continuously increasing, as is the amount of plastic waste entering the environment due to mismanagement and improper disposal. As a result, plastic debris is accumulating in the environment, slowly disintegrating into ever smaller particles including microplastics, i.e. particles smaller than 5 mm. The special characteristics associated with their small size, such as easy distribution and high relative surface area, make them prone to wide-ranging interactions with their abiotic and biotic surroundings. Hence, microplastics have become a topic of high interest in ecological research. While microplastic studies initially mainly focused on marine ecosystems, awareness and research efforts regarding terrestrial ecosystems are continuously increasing. As soils are a major sink for microplastics, plant-soil systems are especially prone to experience impacts of plastic pollution. Potential impacts encompass changes in biotic and abiotic soil properties such as the composition of soil communities or soil structure, which can affect plants indirectly, as well as direct phytotoxic effects. However, despite recent advances in the investigation of microplastic effects on soils and plants, our knowledge about potential ecological implications is still limited. Especially aspects about how microplastics affect individual plant growth apart from productivity, how this might change interactions of plants with their surroundings, and how such alterations might feedback on higher ecological levels such as plant communities remain poorly understood. To contribute to filling these gaps, I conducted three experimental studies, assessing the effects of microplastic pollution on (1) individual plants with a main focus on alterations in root growth and its implications on plant performance, (2) heterospecific plant-soil feedbacks, i.e. how microplastics modify the way plants affect other plants growing in the same soil by changing the soil environment, and (3) the characteristics of complex experimental grassland communities. Specifically, as substances normally do not occur in even distributions in the environment but rather in patches of high and low concentrations, in Chapter 1, I tested how patchy plastic distributions in the soil affect the root growth of 29 common Central European grassland species, and the resulting effects on plant performance. To do so, I grew individual plants under non-polluted soil conditions, as well as under homogeneous and heterogeneous microplastic distributions in the soil. Overall, plants showed a clear plastic-avoidance response, i.e. preference of unpolluted patches, by means of root foraging. Interestingly, both within and across species, a stronger avoidance response was associated with reduced plant performance, indicating that costs and limits of root foraging exceeded the potential benefits of avoiding microplastics. In addition to the direct impacts on individual plant performance, these findings indicate that microplastics might also affect interactions among plants, due to differing strengths of effects across and within species. In Chapter 2, I assessed how microplastics affect heterospecific plant-soil feedbacks (PSFs), and how soil biota might mediate such effects. I performed a two-phase feedback experiment by first conditioning soil containing living or sterilized soil biota and different types of artificial particles (glass as chemically inert control, next to different microplastics) by either growing Centaurea jacea L. plants in the soil or not. In the second phase, I separately grew individuals of the grass Eragrostis minor Host and the forb Crepis biennis L. in all possible combinations of soil conditioning, soil biota, and artificial particles. Artificial particles in general strengthened PSFs, and effects of particles tended to be stronger with the initial absence of soil biota. These results indicate that artificial particles, including microplastics, can affect plant interactions related to both soil and other plants, showing one potential pathway of how microplastics might affect relations among plants. In Chapter 3, based on the findings about individual-plant performance in Chapter 1, and plant interactions in Chapter 2, I tested how microplastics might affect plant communities. Additionally, I tested how arbuscular mycorrhizal fungi (AMF) as important plant associates influence such effects. Over two growing seasons, I grew two different types of experimental communities differing in their compositions, representing dry and mesic grassland communities. I found that artificial particles mainly changed the productivity and diversity of the mesic community type, but effect strength and direction depended on AMF inoculation as well as on particle type and concentration. While these findings show that microplastics can affect plant-community characteristics, they also highlight that such effects depend on several factors, including initial community composition, particle characteristics, and at least certain types of soil organisms. Plastic debris can persist in the environment for long periods of time. During this time, interactions of plastic particles with both their biotic and abiotic environment are likely to change as weathering alters particle properties. Hence, in the Addendum, I discuss such temporal aspects and why investigating them might be important to better understand potential long-term effects of plastic pollution. In summary, my thesis provides important evidence that microplastics can affect plants on various levels, from individual root growth linked to plant performance, over interactions with the soil they grow in and other plants, to community productivity and composition. Further, I identify three important drivers influencing microplastic effects on the plant-soil system, i.e. plant characteristics, soil biota, and particle properties. While the new insights about these drivers contribute to a better understanding of ecological effects of microplastics, at the same time, my findings also highlight the high complexity and context-dependency, making it difficult to draw general conclusions about ecological effects of microplastics. One potential way to deal with this complexity, helping us to better predict and understand general effects of plastic pollution and microplastics in the environment, would be to shift the focus towards more holistic approaches. Investigating more general plant-trait related responses and effects of combinations of different microplastic types, instead of focusing on specific combinations of individual species and particular microplastic types, could make experimental approaches more realistic whilst keeping complexity manageable. While such broader approaches might initially limit our understanding of detailed mechanisms, they might yield information more directly relevant to ecological implications on natural systems.