1. Increasing cropping system diversity is one strategy for reducing reliance on external chemical inputs in agriculture and may have important implications for agro-ecosystem functions related to the regulation of weed populations and community assembly. However, the impacts of cropping system diversity on weed communities have not been evaluated formally in a study comparable with those performed in experimental grasslands, where much of the evidence regarding diversity-ecosystem function has been reported. We performed a field experiment in Michigan, USA, in which we manipulated the number of crop species grown in rotation and as winter cover crops over a 3-year period and in the absence of fertilizer and pesticides, to determine the impact of crop diversity on the abundance, composition and structure of the weed community. 2. Crop diversity treatments consisted of three row-crops, corn Zea mays L., soybean Glycine max (L.) Merr. and winter wheat Triticum aestivum L., grown in continuous monoculture and in 2- and 3-year annual rotations with and without cover crops (zero, one or two legume/small grain species). Weed communities were measured each year at peak biomass, with soil resources and light availability being measured over the course of the growing season in the final year of the study. 3. The effects on weed communities of the crop diversity treatments were dependent on rotation phase. In winter wheat, weed abundance and diversity (species richness, H' and D) were lowest in the two highest crop diversity treatments. Across all phases of the rotation, weed community structure was affected more by crop identity than crop diversity per se. 4. In general, the effects of crop diversity on weed communities were mainly the result of the presence of cover crops, which had strong effects on soil resource and light levels, particularly in winter wheat. 5. Synthesis and applications. Increasing crop diversity in the absence of external chemical inputs can result in changes in soil resource availability without a concomitant increase in the abundance of weeds or a shift to weed communities that are more difficult to manage.
