Validation of the ecosystem services of created wetlands: Two decades of plant succession, nutrient retention, and carbon sequestration in experimental riverine marshes

Wetlands provide many ecosystem services to society, most notably the provision of habitat for important plants and animals, the improvement of water quality, and the sequestration of carbon. Nitrogen and phosphorus budgets, vegetation structure and function, and carbon fluxes and accumulation are described for a pair of 1-ha created riverine wetlands in central Ohio USA over 20 years (1994–2013) of primary succession. The primary inflow to these experimental wetlands was from water pumped from the adjacent fourth-order Olentangy River. The pumping rate maintained for most of the years (an exception was a two-year comparison of pulsing and non-pulsing hydrology in the two wetlands in 2004–2005) was according to a pre-determined formula based on river stage. The pumped inflow to the wetlands averaged 38.7 ± 1.5 m yr−1 with precipitation averaging 1.1 m yr−1 for the same years. Surface outflow averaged 27.1 ± 1.4 m yr−1 and subsurface seepage was estimated to be 13.2 ± 0.2 m yr−1 over that period. Both outflows returned water to the Olentangy River via surface and subsurface pathways respectively.Wetland plant richness increased from 13 species initially planted in one of the wetlands to 116 species overall after 17 years, with most of that richness (99 species) occurring in the first 5 years. The planted wetland had higher community diversity every year except one over 20 years while the naturally colonizing wetland was more productive especially in the first 7 years and overall still had 2000 kg more organic matter input by ANPP after 17 years than did the planted wetland.Nutrient mass inflows to the wetlands averaged 5.61 ± 0.30 g-P m−2 yr−1 (n = 30 wetland years) for total phosphorus, 2.17 ± 0.27 g-P m−2 yr−1 (n = 30 wetland years) for soluble reactive phosphorus, 122 ± 3 g-N m−2 yr−1 (n = 14 wetland years) for total nitrogen, and 100 ± 5 g-N m−2 yr−1 (n = 32 wetland years) for nitrate–nitrogen. Retention rates were 2.40 ± 0.23 g-P m−2 yr−1 for total phosphorus, 0.87 ± 0.10 g-P m−2 yr−1 for soluble reactive phosphorus, 38.8 ± 2.2 g-N m−2 yr−1 for total nitrogen, and 15.6 ± 2.7 g-N m−2 yr−1 for nitrate–nitrogen. Total phosphorus retention was higher in the planted wetland compared to the natural colonizing wetland (44.3 ± 4.4% vs. 38.8 ± 5.3% respectively; p = 0.059) while total nitrogen retention was significantly higher in the naturally colonizing wetland compared to the planted wetland (32.1 ± 2.0% vs. 28.4 ± 2.6% respectively; p = 0.000085). Investigation of trends of water quality improvement showed that, overall, nutrient retention decreased from the beginning of the study through the 17th year in 2010. More recent trends showed tendencies for water quality improvement in 9 out of 11 of the nutrient parameters and time periods investigated between 2003 and 2010.The wetlands were effective carbon sinks, with rates of carbon sequestration (219–267 g-C m−2 yr−1, higher than those measured in a reference natural flow-through wetland. Both carbon sequestration and methane emissions have been consistently higher in the naturally colonizing wetland, theorized as due to the greater productivity of this wetland over several years in the middle of this study.