Fifty-five years of soil development in restored freshwater depressional wetlands

Wetland restoration is increasingly used as a strategy both to address historical wetland losses and to mitigate new wetland impacts. Research has examined the success of restored wetlands for avifaunal habitat, plant biodiversity, and plant cover; however, less is known about soil development in these systems. Soil processes are particularly important as soil organic matter (SOM), cation exchange capacity (CEC), and other properties are directly linked to wetland functions such as water quality improvement. This research compared soil development processes and properties of 30 palustrine depressional wetlands of four different age classes (approximately 5, 14, 35, and 55 years since restoration) located in central New York (USA). Five natural wetlands were used as references. This chronosequence included wetlands 27 years older than previously conducted studies, making it the longest reported database available. Replicated soil cores from each site were analyzed for SOM, bulk density (D(b)), CEC, and concentrations of nutrients and other chemical constituents. Decomposition rate and aboveground plant and litter biomass were measured as key contributors to soil development. The results indicate that some soil properties critical for water quality functions take decades or centuries to reach natural reference levels. Of particular importance, in the top five centimeters of soil, SOM, D(b), and CEC achieved <50% of reference levels 55 years after restoration. Soil development processes in these depressional wetlands appear to be driven by autochthonous inputs and by internal processes such as litter decomposition and are not accelerated in the initial phase of development by allochthonous inputs as has been documented in coastal salt marshes and riverine floodplains. While monitoring generally focuses on the initial establishment phase of restored ecosystems, our findings indicate that the later autogenic phase strongly influences development trajectories for important wetland soil properties. Therefore, the role of different successional phases in determining long-term trajectories of ecosystem development should be considered in restoration design, research, and monitoring. This research highlights areas for improving the field of restoration through understanding of successional processes, increased efforts to jump-start soil development, longer-term monitoring programs, and greater focus on soil components of restored wetlands.