Soil aggregates are clusters of soil particles, supporting the soil physical structure. Soil aggregation is key to the functions of ecosystem, including aeration, water infiltration and storage, nutrient cycling, and the pore system. This doctoral work investigates the effects of threats on the stability of soil aggregates. First, a laboratory study explores the effects of elevated temperature and microplastic on fungi-induced aggregation. This work opens the door for the study on effects of two top important global change factors, warming and microplastic pollution, on soil aggregation. Next, a laboratory work investigates the effects of organic matter and microplastic on soil aggregation, revealing the interactive effects of the two factors, which can influence the soil aggregation substantially. Third, a laboratory work was done to explore the interactive effects of microbial diversity and elevated temperature on soil aggregation. Our investigation of above-mentioned factors on aggregation revealed the interactive effects among these factors. Elevated temperature and microplastic jointly influenced soil aggregation, while the effects varied among fungal species. Then we conducted an experiment to investigate the effects of organic matter and microplastic on aggregation; we assumed that different types of organic matter could induce various levels of microbial activity, which could lead to aggregation to different degrees. We found that effects of microplastic on aggregation depended on the type of added organic matter. We then found that microbial diversity and elevated temperature had interactive effects on aggregation, that microbial community and elevated temperature could influence aggregation by mitigating rates of decomposition. Our results provide insight about how emerging threats (microplastic, warming and microbial diversity loss) can influence aggregation in the context of different conditions (different species of fungi and organic matter addition). Microplastic fibers affected soil aggregation by interfering with the formation of stable aggregates. Fungi-induced aggregation is sensitive to microplastic and temperature, while those effects will shift in terms of the species of fungi, which play a dominant role in the formation of aggregates. In the presence of fungi that are more able to form the aggregates at elevated temperature, microplastic could lead to more loss in aggregates at elevated temperature. The organic matter-induced aggregation is also vulnerable to microplastic, the magnitude of negative effects of microplastic on aggregate stability depended on the type of added organic matter. Both chapter 2 and chapter 3 indicate that greater soil aggregation activity could lead to increased opportunities for microplastic to be integrated into aggregates, leading to subsequent destabilization of these structures by as yet unknown mechanisms. These studies contribute to the context dependency of microplastic effects in terrestrial ecosystems. Chapter 4 reveals that elevated temperature decreased the aggregates probably by accelerating decomposition of binding-agents, and also decreased the allocation of soil organic matter into aggregates. Different microbial communities had different reactions to elevate temperature, resulting in interactive effects between elevated temperature and microbial diversity on aggregates and formation of aggregate-protected C.
