Crop diversity may mediate the intensity of weed-crop competition by altering soil nutrient availability and plant-soil microbe interactions. A greenhouse experiment was conducted to analyze weed-crop competition in soils with varying crop diversity legacies. Soil greenhouse treatments included field soils (i.e., soil nutrient and microbial legacies), a sterile greenhouse potting mix inoculated with microorganisms of the field soils (i.e., microbial legacies), and a sterile greenhouse potting mix. Soils for the greenhouse experiment were sampled and assessed after two-years of conditioning with annual and perennial cropping systems under four levels of intercrop diversity. The greenhouse experiment involved growing one sorghum sudangrass (Sorghum bicolor (L.) Moench × S. sudanese Piper) crop plant and zero to six common lambsquarters (Chenopodium album L.) weed plants in soil from each diversity and cropping system treatment. The weed density treatments created a weed-crop competition gradient, which was used to quantify legacy effects of crop diversity. Weed-crop competition increased with crop diversity in both the field soil and inoculated soil treatments in the annual system. In the perennial system, differences in weed-crop competition intensity were driven by crop yield potential. In the perennial field soil treatment, crop yield potential was greatest in the highest diversity treatment, whereas in the perennial inoculated soil treatment, crop yield potential was greatest in the lowest diversity treatment. Results show potential for negative effects from previous crop diversity on weed-crop competition, and the divergent impact of microbial and nutrient legacies on crop yield potential. Future research should aim to evaluate the consistency of legacy effects and identify principles that can guide soil and crop management, especially in conservation agriculture where soil tillage and its microbial legacy reducing effects are minimized.
