|ROBERTSON, G. PHILIP|
Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2007
Publication Date: N/A
Interpretive Summary: Increasing the number of different crops grown together in agricultural systems may reduce the need for intensive use of chemical fertilizers and pesticides. The goal of this study was to assess how, in the absence of fertilizer and pesticides, increasing the number of crop species grown in sequence (using crop rotation and cover crops) from one to six crop species over a three-year period impacted crop yields. Crop combinations included corn, soybean, winter wheat, and three different legume and grass cover crops. Results of the study showed that grain yields of corn and soybean were higher in cropping systems that included all six crops compared to systems with fewer crops. Also, corn yields in these systems were comparable to those in conventionally-managed systems for each of the years studied. The increased crop yields were likely due, in large part, to the beneficial effects that leguminous crops can have on soil fertility when included in a diverse rotation. This data suggests that the sustainability of cropping systems for food, fiber, and biofuel production may be improved by increasing the number of different crop species grown together.
Technical Abstract: 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 Michigan USA, in which we manipulated the number of crop species grown in rotation and as winter cover crops over a three year period and in the absence of fertilizer or pesticides, to determine the impact of crop diversity on grain yields. 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 two and three-year annual rotations with and without cover crops (zero, one, or two legume/small grain species). Crop yields and weed biomass were measured each year and plant available soil nitrogen was measured over the course of the growing season in the final year of the study. The effects of crop diversity were manifest most strongly in corn. In all three years there was a significant positive linear relationship between the number of crop species in the rotation and corn yield. Corn yields in the highest diversity treatments were over 100% higher than the continuous monoculture and were not significantly different from the county average for each of the three years despite the absence of chemical inputs. In soybean and winter wheat, yield differences among crop diversity treatments were also significant, but of lower magnitude (32 and 53 %, respectively). Crop diversity treatment effects on corn yield were likely due to the number of legume species (crops and cover crops) in the rotation and their effects on available inorganic soil nitrogen. Results demonstrate that ecological theory is relevant to the management of agricultural systems and that increasing diversity in cropping systems can lead to significant increases in ecosystem functions related to crop production. More importantly, these results suggest that yield penalties associated with reduced chemical use for food, fiber, and biofuel production may be offset by managing for higher crop species diversity.