The loss of biodiversity worldwide has prompted a close investigation of the link between diversity and ecosystem functions. Most experimental studies have looked at the relationship between grassland plant diversity and aboveground productivity. Less is known about other ecosystems or how diversity affects belowground processes. Our objective was to investigate the link between plant community (diversity and composition) and key belowground processes such as root biomass production and CH₄ dynamics in wetland ecosystems. We hypothesized that 1) root biomass would increase with functional group diversity due to complementarity and 2) the sediment pool of CH₄ would decrease with diversity due to increased CH₄ oxidation facilitated by root biomass. Four plant functional groups (facultative annuals, obligate annuals, reeds and tussocks) were planted in controlled mesocosms to represent five levels of functional diversity and every combination of functional groups at each diversity level. Unplanted mesocosms served as the zero diversity treatment. At peak biomass in 2007, porewater samplers were used to extract water at 5, 15, and 25 cm. The samples were frozen prior to headspace analysis of CH₄ and CO₂. Porewater was also analyzed for DOC. Afterward, we took soil cores from each mesocosm at 0-10 cm, 10-20 cm and 20-30 cm to determine the root biomass in each depth.
Results/Conclusions

Root biomass increased with functional diversity (F _{3, 71} = 2.78, P < 0.05), however only the lowest diversity treatment had significantly lower root biomass than the highest diversity level; 150.6 ± 24.94 (SE) g DW m^-2 and 307.2 ± 49.9 g DW m^-2, respectively. At each depth, root biomass increased with diversity (P < 0.01), and root biomass at 0-10 cm >10-20 cm >20-30 cm. The facultative annual and obligate annual functional groups had significantly less root biomass than the other functional groups and combinations (P < 0.001). Porewater concentrations of CH₄ did not significantly differ between functional group combinations or diversity levels (P > 0.05). However, CH₄ was positively correlated with depth (F _{1, 194} =19.75, P < 0.001), CO₂ concentration (F _{1, 194} = 42.94, P < 0.001) and DOC (F _1,194 = 4.98, P < 0.001) and was not correlated with root biomass (P > 0.05). Since this data is from the first year of sampling following mesocosm establishment, the insignificant relationship between root biomass and CH₄ in the sediment pool may indicate that the microbial processes were influenced more by starting soil conditions than plant-mediated conditions.