Conversion of native ecosystems to row-crop agricultural systems typically involves the replacement of dozens of annual and perennial plant species by a small number of annual crops.  Monoculture, the production of only one crop over time, represents the most extreme reduction in plant diversity; however, even commonly used rotations such as corn-soybean dramatically reduce the potential for species interactions that serve a critical role in ecosystem-level regulation of processes such as soil organic matter turnover and nutrient cycling.  These ecosystem functions, which are largely provided through species interactions in wildlands, are provided through external inputs that are expensive and potentially harmful to the environment.  We examined the effects of cropping system diversity on soil biogeochemical functions in the context of recent advances in diversity-ecosystem function theory.  We examined whether increasing plant species diversity in Midwestern row-crop systems can enhance microbial diversity and function, nutrient cycling, and SOM retention.  Our study included replicated treatments ranging in plant species richness from 1 to 6 in any given 3-yr rotation cycle.  We found a shift in general microbial community diversity related to cropping system diversity. Further, field N2O flux and denitrifier enzyme activity increased with cropping system diversity.  Soil ecosystem health and function can be manipulated through increasing rotation length and including cover crops in rotations, both practices that increase cropping system diversity.