Intensive pesticide application history leads to reduced root symbiont functioning but not to long-term adaptation

Frequent pesticide applications have led to their widespread accumulation in agricultural soils. Pesticides can negatively impact plant symbionts such as arbuscular mycorrhizal fungi (AMF) and rhizobia, which are recognized as key plant growth promoters. However, the long-term effects of intensive pesticide use on plant-microbe symbioses and the potential of symbionts to adapt are still poorly understood. To elucidate this, we collected soil from twelve conventional vineyards with intensive synthetic pesticide use and from twelve organic vineyards without synthetic pesticide use for at least ten years and conducted a greenhouse experiment with red clover. We assessed the impact of management history on plant growth, symbiont performance, and microbial adaptation to fresh application of a fungicide mixture across eight weeks including multiple sampling timepoints. Moreover, we tested the potential of AMF inoculation to mitigate pesticide effects. Soils with intensive pesticide use history yielded less plant biomass and lower AMF root colonization compared to organic soils. Fresh pesticide application temporarily reduced AMF root colonization and reduced nitrogen fixation at the final harvest. Consequently, plant biomass was also reduced. However, in soils with intensive pesticide use history, neither the AMF nor the rhizobia showed increased tolerance to the fresh application of pesticides, nor was dissipation of the pesticides accelerated. This implies that there was no long-term adaptation of plant symbionts and pesticide degraders to frequent pesticide applications. Inoculation with AMF generally had a positive effect on plant biomass, AMF root colonization, and nitrogen fixation. This was particularly evident in soils with poor performance, such as those with fresh pesticide application. The inoculated Rhizoglomus irregulare strain established better in soils with low natural AMF abundance and plant biomass. It mostly replaced other Rhizoglomus strains, as revealed by PacBio sequencing of the root AMF communities. Overall, our study indicates that pesticide use has both short-term and long-term impacts on the functioning of plant symbionts and that their long-term adaptation potential may be limited. While the reduction of pesticide exposure should be prioritized, inoculation of AMF could aid mitigating non-target effects from both historic and fresh pesticide use and restoring soil functioning.