The loss of natural wetlands to anthropogenic development has warranted the creation of wetlands to mitigate the reduction of valuable ecosystem functions and services. However, the complex interactions between the main drivers of wetland community structure – hydrology, nutrient availability and herbivory – makes creation of fully functional wetland replacements challenging. In this study, we examined the interactions among these drivers, and their impacts on plant diversity and nitrogen removal in two created wetlands with different land use histories: A1N (previous gravel depository) and A3 (previous cattle pasture). We established paired plots protected from – and open to – large wetland grazers and compared vegetation abundance and diversity, soil characteristics, and soil respiration and potential denitrification in each wetland. At A1N, a permanently flooded emergent marsh with high observed grazer densities (predominantly waterfowl) and low nutrient availability, grazing significantly reduced plant growth and diversity. In contrast, at A3, a seasonally flooded wetland with lower grazer densities and high nutrient availability, grazing enhanced overall plant diversity and decreased invasive species cover. The effects of grazers varied seasonally and increased over time, eventually leading to a reduction in soil organic matter at both sites. In light of significant site differences, potential denitrification was limited by differential hydrology (very wet, anaerobic versus very dry, aerobic), nitrogen or carbon availability, and grazing (low or high levels) at both sites. These results suggest the possibility of long-term grazer-induced shifts in community composition and delivery of key ecosystem services in young, vulnerable created wetlands. To improve created wetland design and function, we suggest that the impact of prior land use on present nutrient availability must be considered, and sites must be heterogeneous in both physical and bathymetric structure, to provide conditions for diverse plant communities, both aerobic and anaerobic biogeochemical processes, and balanced habitat use by wetland grazers
