Just as soil microorganisms influence plant community dynamics in unmanaged ecosystems, plant-microbe interactions can be used to control noxious weed species in agroecosystems. Here we hypothesize that spring cover cropping alters soil microbial community composition in ways that can establish weed-suppressive soil legacies and encourage soil microbes to attack germinating seedlings. We collected soil from a variety of cover-cropping trials in Illinois, and we used this soil to conducted 7-day seed germination bioassays to estimate the suppression potential of these soils against various weed and crop species. We sequenced bulk DNA of bacteria and fungi associated with these seedlings at the conclusion of the assays, and we used partial least squares regression analysis to determine which microbial taxa were associated with highly-diseased and stunted seedlings. Finally, we compared the seedling microbiome that developed with and without a red clover-based green manure to determine if this practice could stimulate different forms of microbial attack.
Results/Conclusions
Cover crop-induced seed suppression varied significantly over time, with some cover crops (red clover, mustard, spring oats) producing weed suppressive effects in the days immediately following termination, while others (field pea) played a stronger role only after two weeks. Weed suppression varied significantly according to the cover crop and to the target weed species. Nevertheless, mustard and mixes containing mustard frequently suppressed a number of different weed species (e.g. velvetleaf, field pennycress). we found a broad diversity of potential microbial antagonists on weed seedlings, and the seedling microbiome was affected by the addition of green manure. In particular, we found that green manure stimulated attack by Alphaproteobacteria, Sphingobacteriales, Sordariomycetes, and Agaricomycetes. These findings suggest that agricultural practices like green manuring could induce native soil microorganisms to act as natural agents of biocontrol against weeds. However, site-specific variation in cover crop performance, weed-suppressive effects, and microbial communities emphasizes the highly local nature of these plant-microbe interactions and the importance of the environmental context in determining how soil legacies affect seed germination.