Soils in restored wetlands, very slow to develop

The findings from a new study may disappoint those hoping to quickly restore wetlands back to reference conditions. Researchers looking back over 50 years have found that soil development after wetland restoration proceeds much slower than previously thought.  Even 55 years after restoration, inland freshwater wetlands may not perform the water quality functions of their natural counterparts due to this gradual development.

Profile of a gley soil which exhibits a greenish-blue-grey soil color due to wetland conditions.Millions of hectares of wetlands are restored every year under U.S. federal laws to compensate for impacts to existing wetlands.  Although it has been previously reported that restored wetlands reestablish some reference conditions such as vegetation density and elevation relatively quickly (within 10 years), little is known about soil development post restoration. Wetland soils carry out some of the most important wetland functions such as filtering contaminants from water, supplying the food web base, sequestering carbon, and providing habitat structure.

Researchers Katherine Ballantine and Rebecca Schneider analyzed soils, vegetation biomass, and vegetation decomposition of 30 restored and 5 natural inland, non-tidal wetlands (also known as palustrine depressional wetlands). Each soil core was analyzed for bulk density, organic matter composition, cation exchange capacity (a water quality indicator), common metals, and nutrients. Each restored site fell within an age class of 5, 14, 35, or 55 years (post restoration).

Results

The study results indicate that a majority of the soil characteristics  develop slowly after restoration. For example, soil organic matter and cation exchange capacity in restored wetlands were less than 50% of natural levels, even after 55 years had passed since restoration. One possible explanation for these low levels is that depressional wetlands, unlike costal wetlands, do not receive outside organic and mineral input from daily tides, which bring in organic matter that binds to water contaminants.  The study also found that soil was more compacted in the restored wetlands likely due to heavy equipment weighing on the surface during project implementation and  the lack of external organic matter to compensate for the disturbance.

As reported in previous studies, vegetation and litter accumulation were not as time sensitive as soil. Live plant and litter biomass both increased with age rather quickly showing 60% of natural levels after only 10-14 years.

Implications

According to the researchers, the design, implementation, and monitoring of wetland restoration projects should take into consideration slow soil development. Some wetlands are more isolated than others and may require special soil amendments, like the addition of organic material. The scientists also recommend that restoration planners only use heavy machinery when absolutely necessary, and if used, attempt to drive on the least amount of wetland surface as possible.

What's perhaps most interesting about this study is that it represents the longest reported database available for freshwater wetlands and has shed light on the needs for additional research. The authors recommend future investigation of methods to jump start soil development. Also, more research on the conversion of plant litter into soil organic matter would inform planting and soil amendment plans. 

--Reviewed by Evyan Borgnis

Source:	Ecological Applications
Title:	Fifty-five years of soil development in restored freshwater depressional wetlands
Authors:	Katherine Ballantine and Rebecca Schneider
	Cornell University, Ithaca, New York