Abstract:
Wetlands are unique ecosystems that provide the essential ecosystem service of excess
nutrient retention, processing, and removal from the landscape. In the early 1990s the
U.S. adopted a “no net loss” policy towards wetlands in order protect these remaining
wetland resources from the impacts of development. Under this policy the practice of
wetland mitigation banking became the preferred method of compensating for
“unavoidable” structural and functional impacts to natural wetlands. Created and restored
wetlands, however, do not necessarily develop the same structural or functional capacity
of their natural counterparts in the required five to ten year monitoring period. This study
investigated four created non-tidal freshwater wetlands of varying ages and two natural
non-tidal freshwater reference wetlands in the northern Virginia Piedmont. The purpose
of this study was to identify how the design features of hydrologic connectivity and
microtopography influence nitrogen cycling in created wetlands and investigate whether
created wetland soils develop with age in order to support nitrogen processing and
removal comparable to that of natural wetlands. Ammonification, nitrification, and net
nitrogen mineralization (ammonification + nitrification) were determined using in situ
incubation of modified ion exchange resin cores and denitrification potential was
determined with denitrification enzyme activity. Principal component analyses were
conducted on hydrologic and soil variables to identify hydrologic connectivity, soil
moisture, and soil condition indices. Total nitrogen sedimentation and ammonification
rates increased with the hydrologic connectivity index. Nitrification and denitrification
potential increased with soil moisture, soil condition, and microtopographic indices. Net
nitrogen mineralization also increased with soil condition index. Nitrogen flux rates
demonstrated age-related patterns, with younger created wetlands having lower rates of
ammonification, nitrification, nitrogen mineralization, and denitrification potential than
older created wetlands, which had flux rates similar to natural reference wetlands. Results
demonstrated a clear, but variable age-related trajectory of coupled soil and nitrogen
cycling development in created wetlands that trend toward natural wetlands. Findings of
this study support the incorporation of hydrologic connectivity and microtopography into
the design and regulatory evaluation of created wetlands, as well the monitoring of soil
development indicators in these wetlands, in order to improve their ecosystem service of
water quality improvement through the functional development of nitrogen cycling.
