Non-target effects of pesticide and microbial seed treatments in maize and barley on the resident soil microbiota under conservation agriculture

In production of cereals like maize (Zea mays L.) and barley (Hordeum vulgare L.), seeds are often treated with pesticides and/or commercial products of plant beneficial microorganisms (PBM) to reduce possible root damage from insect pests and soil borne root diseases. In a field experiment with maize and barley under conservation agriculture, we examined how such seed treatments affected the resident root and soil microbiota. The seed treatments included a pesticide mixture and different commercial products of common PBM based on the biocontrol agents (BCA) Trichoderma harzianum and Metarhizium anisopliae alone and in combination and a mix of plant growth promoting rhizobacteria (PGPR), which were compared to a negative control without seed treatment. Soil and root samples were taken at two and three sampling times during the crop cycles for barley and maize, respectively, to measure root biomass, root colonization with mycorrhizal fungi and pathogens, soil microbial communities at a general taxonomic level using biomarker fatty acids, and ecological guilds of soil nematodes. Root health was monitored with observations of the presence of insect feeding larvae and root disease symptoms, which in general showed healthy roots during the full crop cycle. Overall, most of the root and soil biota variables measured changed during the crop cycle. However, for both crops, the seed treatments had no effects on the soil and root microbiota measured, except in the case of barley root infection with Polymyxa sp., which was reduced by all treatments. In conclusion, the pesticide and PBM seed treatments evaluated in the present study for maize and barley under conservation agriculture, in general, had limited effects on the resident root and soil microbiota. However, future studies should include complementary high-resolution sequencing methods when examining non-target effects of pesticides and microbial inoculants on the root and soil microbiota.