Effects of multiple pesticides and their interactions on the soil microbial community

Pesticides are regularly applied to agricultural fields with the aim to prevent crop loss and improve the quality and quantity of crops. In the process, they are intentionally released into the environment, where they can inadvertently spread to other areas. They reach the soil directly through application and spray drift, as well as other means of pesticide distribution, where non-target soil microorganisms are among the first organisms to interact with them. These are an essential part of the soil ecosystem performing key ecosystem services. Various interactions between soil microorganisms and pesticides can occur, driven by either the pesticides, the microorganisms or both. While microorganisms are able to degrade various pesticides and derive energy or nutrients from them, pesticides can also negatively affect them. Past studies have demonstrated interactions with varying results, revealing both positive and negative effects of the pesticides. Given the vast number of pesticides currently in use and found in the environment, identifying the processes behind these effects is challenging for scientists. Until recently, most of the studies focused on single pesticides, paying little attention to the fact that, in reality, more than one pesticide is present in soils. However, in order to understand the processes effected and the mechanisms behind these effects, it is essential to investigate the effects of multiple pesticides on soil microorganisms. Closing this knowledge gap will improve our understanding of the impact pesticides have on our soils and the ecosystem services performed by soil microorganisms. Therefore, this thesis aimed to reveal these interactions between soil microorganisms and pesticides, focusing on pesticide mixtures, the processes affected and the underlying mechanisms. Utilizing samples from the field sampling campaign of the SPRINT project, we first aimed to establish the missing link between the presence of multiple pesticides and their in situ effects on soil microorganisms. To further deepen our understanding of the affected processes and the mechanisms behind them, we set up two microcosm experiments in which soil was exposed to pesticide mixtures up to 56 days. There we focused on investigating two ecosystem functions performed by soil microorganisms: The degradation of a pesticide, and the degradation of organic substrate. With the latter we additionally aimed to simulate the incorporation of plant material as a pesticide carrier and investigate how substrate quality affects the pesticide effects. Based on the results of these studies, this thesis introduced concepts that describe and explain the interactions between pesticides and soil microorganisms, including the presence and impact of multiple pesticides. Separating the effects into direct-specific, direct-unspecific, and indirect effects allowed for classification of the effects and showed potential new interaction pathways. As hypothesized, pesticides with a mode of action that targets processes that are also present in non-target organisms, had a stronger effect on soil microorganisms. Among them especially fungicides affected the microbial community, as they intentionally target processes in pathogenic fungi, showing that their effect is not limited to those. Interestingly, the herbicide prosulfocarb, aiming to disrupt the synthesis of fatty acids in weeds, also affected the synthesis of membrane lipids in bacteria, triggering a cascade of reactions to compensate for the effects of the pesticide. The latter effects were classified as direct-unspecific as they are a direct consequence of the pesticide, in this case triggered by the mode of action. Similarly, a stress response was observed in the microbial community exposed to the herbicide MCPA. Since MCPA's mode of action likely does not target a process in soil microorganisms, this was classified as a direct-unspecific effect, representing a general reaction of soil microorganisms to pesticides. But MCPA also showed an interaction with the herbicide glyphosate. By competing for binding sites, MCPA increased the bioavailability of glyphosate, hence increasing the contact chance with microorganisms. This resulted in an increased degradation of glyphosate by soil microorganisms, attributed to an indirect effect of MCPA and an effect that only occurs when multiple pesticides are present. Contrary we observed a lag phase in the degradation of the fungicide boscalid when the herbicide prosulfocarb was present, which we attributed to the faster degradation potential of prosulfocarb. These two results showed that abiotic interactions of pesticides can be reflected in the biotic reaction. Another abiotic variable was the substrate quality of added maize leaves, where likely a larger proportion of sensitive microorganisms was present in the presence of fresh maize, that resulted in an increased effect of the pesticides. This thesis revealed various interactions between soil microorganisms, pesticides, and environmental conditions extending our knowledge of the effects pesticides can have on these organisms. The presented concepts are able to classify the effects and incorporate the complex interactions between the variables. This should profoundly impact how we view the effects of pesticides on soil microorganisms, and regulatory work must adapt to incorporate more than just "worst case scenarios" of single pesticides, as these definitely do not reflect the actual effects of pesticides and their vast interactions in our agricultural soils.