Smith, R. G. 2005. Agroecosystem diversity: Impacts on weed communities and crop yield. PhD Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Ecological theory regarding the role of species diversity in the functioning of terrestrial ecosystems may have important implications for the design and management of agricultural systems that are less reliant on inputs of synthetic chemical fertilizers and pesticides for fertility and pest control. However, much of the evidence supporting the importance of diversity in ecosystem function comes from grassland systems where biotic conditions and disturbance regimes differ from those in most managed systems. Furthermore, agricultural studies that purport to demonstrate the impacts of crop diversity on crop yields and pest regulation are often confounded with applications of chemical fertilizers and pesticides. My research aims to understand how ecological theory regarding the role of diversity in ecosystem function can be used to predict weed and crop yield response to increasing crop diversity.

I examined the relationships between row-crop diversity and ecosystem functions related to weed population regulation and crop productivity in two long-term experimental agroecosystems. The first system compared corn grown in continuous monoculture and in rotation with soybean and winter wheat under two input management systems: conventional and organic-based. The second system was an experimental manipulation of crop diversity with no external chemical inputs of fertilizer or herbicides. Crop diversity was manipulated by growing corn, soybean, and winter wheat continuously and in two and three-crop rotations and with either no, one, or two cover crops annually.

Weed communities and crop yield both varied in response to these treatments. In the first system, weed communities were most variable from year to year in the organic rotation, while corn yield variability was highest in the least diverse cropping system (conventional monoculture). In the diversity experiment, crop diversity had relatively little effect on the abundance or composition of weeds. Cover crops in wheat had strong suppressive effects on weeds in both continuous wheat grown in monocultures and in rotation with corn and soybean, while cover crops had little effect on weed abundance or species composition in corn and soybean.

In contrast to the effect on weed communities, crop diversity had large effects on crop productivity. Corn yields varied across the diversity treatments and in the most diverse treatments were comparable to those in conventionally managed systems in the region. The positive effect of crop diversity on yields in corn was driven by increased soil N that was related to the number of legume species in the rotation (red clover and soybean). Soybean yields were also higher in rotations with higher crop diversity, while winter wheat yields were unaffected by the number of crops in the rotation.

These results suggest that increasing crop diversity can have significant impacts on crop yields, particularly in corn, and may decrease the need for intensive chemical management by suppressing weeds during phases of the rotation that are conducive to high cover crop growth (i.e. winter wheat) and by increasing soil nitrogen and the competitive advantage of crops over weeds.

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