The effects of species richness and elevated CO2 on community productivity under altered nutrient levels were studied in experimental herbaceous communities composed of species from the Midwestern United States annual community, which consists of three functional groups C3, C4 and N-fixers. Aboveground and belowground biomass were measured at flowering stage and at the end of the experiment when fruits of most plants were ripe. At the low nutrient level, species richness did not have a significant effect on community productivity. However, at the high nutrient level, the community biomass decreased with decreasing species richness at both ambient and elevated CO2 in the first harvest, and at elevated CO2 in the second harvest. At low nutrient level, CO2 slightly increased community biomass at medium and high species richness. At high nutrient level, CO2 significantly increased community biomass in all species-richness treatments in the first harvest, but a significant response was observed only in the high richness treatment in the second harvest. At the functional group level, biomass of C3 responded positively to CO2, and C4 responded very negatively to CO2. The N-fixers responded positively to CO2 at low and medium species richness, but negatively at high species richness, showing a CO2×richness interaction. CO2 increased species evenness in the communities, depending on nutrient level. Species varied in the responses of light-saturated net photosynthesis (Pmax) to elevated CO2, even within functional groups. Our findings suggest that (1) the relationship between productivity and species diversity was dependent on nutrient levels. (2) Species diversity enhances responses of communities to elevated CO2. (3) Harvest time can affect the results of diversity-productivity experiments. (4) Responses of C3, C4, and N-fixers to elevated CO2 in communities did not follow the prediction based on functional groups or plants grown individually, rather it depended on species richness.
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