Background/Question/Methods Created and restored wetlands potentially provide many ecosystem services but there are few sites where created freshwater wetland function has been investigated in the context of primary succession and biogeochemistry for more than a few years.  In fact the Federal policy in the USA generally requires only five years of simplistic monitoring of wetland structure in the case of mitigating wetland loss with new or restored wetlands. In this paper we compare two 1-ha wetlands with identical geomorphology and hydrologic conditions that were subjected to a planting experiment in 1994.  One wetland was planted with propagules of 13 species of wetland plants while the other was allowed to colonize naturally. Over the 15-year period (1994-2008) each wetland was subjected to continuous inflow of river water with a hydraulic loading rates of 9.9 cm/day or 36 m/yr, 36 times that of precipitation input.

Results/Conclusions

Wetland soil turned hydric within a few years and soil organic matter has been increasing by about 1% every 3 years since the wetlands were created.  Wetland plant richness is high with 97 species in the planted wetland and 92 species in the naturally colonizing wetland after 15 years, with Typha spp. more dominant in the naturally colonizing wetland than in the planted wetland.  In the 15^th year (2008), Typha cover, which includes Typha angustifolia, T. latifolia, and the hybrid T. x glauca, was 26% in the planted wetland and 32% in the naturally colonizing wetland. The planted wetland has always had a higher community diversity index (CDI). Nutrient retention data also show decreased effectiveness by the wetlands over 15 years with total phosphorus retention decreases from 60% to 10%, soluble reactive phosphorus retention decreases from 80 to 30% and nitrate-nitrogen decreases slightly from 35 to 25%. Denitrification was found to account for only about 3 to 8 % of the total amount of nitrogen retention in these wetlands. Greenhouse gases, particularly CH₄, have shown trends of increasing over time but with substantially different rates from the two wetlands, with higher methane emissions from the higher productivity in the naturally colonizing wetland.  The wetlands are serving as more effective carbon sinks (181 to 193 g-C m^-2 yr^-1) than comparable natural wetlands.