Effects of pesticide seed treatments on rhizosphere soil food web composition and function in agroecosystems

Background/Question/Methods

It is widely recognized that we need to reduce our reliance on agrochemicals while at the same time maintain or increase crop productivity; yet, we do not fully understand the impacts our current management practices have on our efforts to farm more sustainably. The use of crop seeds pretreated with pesticides is a commonplace practice that often occurs in consecutive crops, but its impacts on the soil food web and resulting agroecosystem services are poorly understood.  We evaluated the effects of pesticide seed treatments on rhizosphere community composition and function, aboveground litterbag decomposition, and fluxes in plant available nitrogen throughout the 2013 growing season in Rock Springs, PA. Treatments were planted with maize seeds either pretreated with a fungicide-insecticide mixture or untreated and were assigned in a completely randomized design. We assessed soil community composition and litter mass loss three times during the growing season. Ion exchange resin strips were recovered every three weeks throughout the summer. Maize served as a phytometer for this study; thus, plant height, leaf nitrogen content, and grain yields were also measured by treatment.

Results/Conclusions

We observed shifts in the soil bacterial and fungal communities in the presence of pesticide seed treatments early in the growing season. Maize height was significantly lower in the untreated plots suggesting the presence of pesticide seed treatments can affect early plant growth. Plant leaf nitrogen content and rates of surface litter decomposition were similar between treatments early in the season. As the summer progressed, the observable differences in plant growth disappeared, resulting in no differences in final crop biomass and grain yields at harvest. The rate of surface litter decomposition did not differ by treatment throughout the summer suggesting the decomposer community was not adversely affected by the observed early season shift in the rhizosphere microbial community. In our study, rhizosphere soil food web composition was most affected early in the growing season when the pesticide seed treatment was most concentrated. The observed shift in community composition, however, did not adversely affect the decomposition of aboveground litter or crop yields