The Effects of Chronic Nitrogen Application on a Mature Tallgrass Prairie Restoration and Nitrogen Mobilization into Above-ground Biomass of \u3ci\u3eAndropogon gerardii\u3c/i\u3e

The Upland Prairie Restoration was initiated on a 25-acre old field in 1993 in Upland, Indiana. It has matured into a community dominated by Andropogon gerardii and forbs, Monarda fistulosa and Ratibida pinnata. Prior research on the site found that acute nitrogen enrichment promoted weed cover in early stages of establishment and inhibited the establishment of native prairie species. This current study examined the effects of nitrogen enrichment applied to a mature prairie restoration. In 2002, a complete randomized block design experiment was implemented with annual enrichment using nitrogen-urea fertilizer at 4 levels – 0g/m2, 5g/m2, 10g/m2, and 20g/m2. Density and percent cover of all plant species was recorded annually over a five year time period. In year five, leaf level nitrogen concentrations in Andropogon gerardii were measured. The objectives of this multi-year study were to: 1) quantify the effects of chronically applied nitrogen at various concentrations on the density and percent cover of native prairie species, and 2) quantify the fraction of nitrogen that moves into the above-ground biomass of the dominant grass Andropogon gerardii at various levels of applied nitrogen and its potential relationship to changes in community structure. Our results indicate that chronic nitrogen enrichment did significantly decrease prairie grass cover and increase weed cover in the 10g/m2 and 20g/m2 transects. However, chronic nitrogen enrichment did not impact prairie forb cover. Correspondingly, we found that there is a significantly higher amount of nitrogen in the above ground biomass of Andropogon gerardii in the 10g/m2 and 20g/m2 transects, than in the control and 5g/m2 transects. Our results suggest that Andropogon gerardii buffered the prairie restoration against the negative effects of chronic nitrogen enrichment at lower concentrations, but that community structure degraded when N enrichment was 10g/m2 or more