Understanding the role of diversity in the functioning of ecosystems has important implications for agriculture. Previous agricultural research has shown that crop rotation and the use of cover crops can lead to increases in yield relative to monoculture; however, few studies have been performed within the broader context of diversity-ecosystem function theory and in the absence of chemical inputs. We performed a field experiment in SW Michigan, USA, in which we manipulated the number of crop species grown in rotation and as winter cover crops over a 3-year period to test if varying the number of species in a rotation affected grain yield, a critical metric of ecosystem function in row-crops. The experimental design was unique in that no fertilizer or pesticides were used, and the only management variable manipulated was number of species in the rotation, thus providing a strong comparison to grassland diversity-ecosystem function experiments. Treatments included continuous monocultures of three row-crops, corn Zea mays L., soybean Glycine max (L.) Merr., and winter wheat Triticum aestivum L., and 2- and 3-year annual rotations with and without cover crops (zero, one, or two legume/small grain species), encompassing a range of crop diversity from one to six species. Crop yields and weed biomass were measured annually for 3 years and plant available soil nitrogen was measured over the course of the growing season in the final year of the study. In all 3 years, corn grain yield increased linearly in response to the number of crops in the rotation. Corn yields in the highest diversity treatment (three crops, plus three cover crops) were over 100% higher than in continuous monoculture and were not significantly different from the county average for each of the 3 years despite the absence of chemical inputs. Corn yields in the diversity treatments were strongly correlated with the availability of inorganic soil nitrogen, which was likely influenced by the number of different legume species (crops and cover crops) present in the rotation. In soybean and winter wheat, yield differences among crop diversity treatments were also significant, but of lower magnitude (32 and 53%, respectively), and showed little direct relationship to the number of crop species grown in a rotation. Results demonstrate that agricultural research motivated by ecological theory can provide important insights into the functioning of agroecosystems and enhance our understating of the linkages between diversity and ecosystem function. Importantly, these results suggest that reduced chemical inputs do not necessarily result in yield penalties and provide support for incorporation of crop or species diversity when determining how ecosystem services can be included in food, fiber, and biofuel production.
