| The relationship between aquatic plant (macrophyte) diversity, bacterial diversity, and the biochemical reduction of nitrate (denitrification) within wetlands was examined. Denitrification occurs under anoxic conditions when nitrate is reduced to either nitrous oxide (N2O), or dinitrogen (N2). Although previous studies have identified physical and chemical factors regulating the production of either gas in wetlands, the role that macrophyte diversity plays in this process is not known. The central hypothesis, based on the niche-complimentarity mechanism, was that an increase in macrophyte diversity would lead to increased bacterial diversity, increased denitrification, and decreased N2O flux. This hypothesis was investigated in two mesocosm studies to control environmental conditions while altering macrophyte functional groups (FG) and functional group diversity. In Study #1, five macrophyte functional groups (clonal dominants, tussocks, reeds, facultative annuals, and obligate annuals) were each represented by two species. Fifty-five mesocosms with 5--6 replicates of 0, 1, 2, 3, 4, or 5 macrophyte FG (0--10 species) were established in the spring of 2001 and sampled in August 2001, September 2001, and April 2002. In Study #2, the clonal dominants were removed and forty-eight mesocosms with 6 replicates of 0, 1, 2, 3, or 4 macrophyte FG (0--8 species) were established in May 2002 and sampled in August and September 2002, and April 2003. In both studies, in situ denitrification, denitrification potential, sediment and interstitial water C pools, and bacterial biomass were measured. In Study #2, bacterial gDNA diversity using terminal restriction fragment length polymorphism (TRFLP) was also analyzed. Results showed no evidence of altered detrital C pools, denitrification flux, bacterial diversity or bacterial community composition due to macrophyte functional group diversity. However, distinct differences between individual macrophyte functional groups occurred. The tussocks and reeds exhibited higher denitrification function while the obligate annuals emitted significantly higher in situ N2O after nitrate addition. These results occurred despite no evidence for differences in bacterial diversity or bacterial community composition.; These findings suggest that macrophyte community composition rather than diversity plays an important role in regulating denitrification and N 2O emissions, and therefore has potentially important implications for a number of environmental issues pertaining to wetland mitigation, water quality, and global climate change. |
