Ecotoxicity of pollutants on soil nitrifiers under a climate change scenario

Soil microorganisms are essential for soil functioning but face increasing stress due to e.g. pollution and climate change. Yet, a scientific basis for how to assess impacts of environmental stressors on soil microorganisms is lacking. Nitrification, the oxidation of ammonia via nitrite to nitrate, is a process commonly shared between ammonia oxidising archaea (AOA) and bacteria (AOB), and nitrite oxidising bacteria, primarily Nitrobacter (NIB) and Nitrospira (NIS). Although these guilds are suggested as indicators of soil functioning, little is known about their combined responses to stressors, especially to multiple stressors, and effects on associations between ammonia and nitrite oxidisers. This thesis aimed to determine how soil microorganisms, particularly nitrifiers, respond to single and multiple stressors, using a resistance and resilience framework. Soils were exposed to contamination of herbicides, polycyclic aromatic hydrocarbons (PAH), or copper (Cu), and to dryingrewetting cycles in meso- and microcosm experiments. The results show minimal herbicide effects, but PAH altered total prokaryotic, AOA and AOB community composition at high contamination levels, while Cu caused major decreases in ammonia oxidation and community shifts even at low contamination levels. Drying temporarily decelerated ammonia oxidation and altered AOA community composition, indicating low resistance. However, rewetting restored ammonia oxidation, indicating resilience, but caused persistent shifts in NIS community composition, suggesting low resistance and resilience. Network analysis revealed drought effects on co-associations between ammonia and nitrite oxidisers, which could suggest a destabilised interaction. Drought effects were influenced by soil properties and contamination legacy. While herbicides affected the subsequent responses of the nitrifier guilds to drought only marginally, PAH and Cu displayed moderate to strong legacy effects. Overall, the findings emphasise the importance to consider stressor effects on soil microorganisms and subsequent consequences for soil functioning and N fluxes under both single and multiple stressor scenarios.