No-net-loss not met for nutrient function in freshwater marshes: recommendations for wetland mitigation policies

Wetlands provide many important services throughout the world, with an estimated economic value that, in comparison to other ecosystems, far exceeds their relatively small global extent. In recognition of their importance, both national and international regulations exist to protect the world's remaining wetlands. Of growing interest is the no-net-loss'' policy which permits unavoidable destruction of wetlands if compensated by restoration of degraded wetlands or creation of new wetlands. The fundamental assumption of no-net-loss is that wetlands can be created which function equivalently to natural wetlands. One integral function that wetlands perform is cycling of carbon, nitrogen and phosphorus. Here we demonstrate that loss of this nutrient-related function is not being mitigated by creation or restoration of wetlands. We compare indicators of plant-and microbial-mediated functions, as well as abiotic (e. g., soil character, hydrology) and biotic (e. g., plant community composition) structure, between 10 created or restored and 5 natural freshwater depressional wetlands in central Ohio, USA. Nutrient stocks were generally smaller and transformations slower in created wetlands than in natural wetlands, with little development over time. Of particular concern were differences in C-and N-related function. Created wetlands stored 90% less C within litter and 80% less C within soil and processed 60% less N through denitrification, on average compared to natural wetlands. Our study suggests that subversion of natural wetlands into restored or created wetlands could have large-scale environmental consequences such as reduced capacity for nitrate removal and C sequestration.