Effects of pesticide seed treatments on soil food webs in agroecosystems

Background/Question/Methods

The practice of using crop seeds pretreated with pesticides is common in row crop agriculture; however, its impacts on the soil food web and resulting agroecosystem services are poorly understood.  We evaluated the effects of pesticide seed treatments on rhizosphere mesofaunal community composition, aboveground litterbag decomposition, and the dynamics of plant available nitrogen throughout the 2013 (maize) and 2014 (soybean) growing seasons in a field experiment conducted in Rock Springs, PA. Treatment plots were planted with seeds that were either pretreated with a fungicide-insecticide mixture or untreated (control). We assessed soil mesofaunal composition and litter mass loss three times during each growing season. Ion exchange resin strips were also deployed and assessed three times during the summer. Maize and soybean leaf nitrogen content, plant height, and grain yields were also measured. We hypothesized that pesticide seed treatments would alter soil community composition early in the growing season, resulting in an increase in the abundance of detritivores and higher rates of litter decomposition and plant available nitrogen.

Results/Conclusions

We observed no community-wide compositional differences in mesofauna in the presence of the pesticide seed treatments. Mesofaunal communities, however, varied by year and sampling period, illustrating the dynamic and complex nature of soil food web communities in agroecosystems.  The abundance of select taxa, including individuals in herbivore (pest) and omnivore trophic levels, were affected by seed treatments; however, these shifts in populations were inconsistent across time. The rate of surface litter decomposition did not differ by treatment, suggesting pesticide seed treatments did not affect mesofaunal community function. Interestingly, final crop biomass and grain yields also did not differ between treatments. These data suggest that pesticide seed treatments do not radically alter the soil mesofaunal community, but can impact non-targeted populations that provide important agroecosystem services such as pest suppression. Additional research is necessary to determine the mechanisms that underlie these taxon-specific changes to the mesofaunal community and their consequences for agroecosystem function.