Assessing controls of nitrogen dynamics in created and natural wetlands in the Virginia Piedmont, USA

Background/Question/Methods Natural wetlands have been widely noted for their ability to improve the water quality of downstream systems by retaining, transforming, and permanently removing nitrogen (N) from polluted water. While there have been numerous studies on N dynamics in natural riparian and coastal plain wetlands, there is insufficient information on the controls of N cycling in created wetlands; especially in the Virginia piedmont. This study examined the influences of three factors known to control N cycling: hydrologic connectivity (HC) to the surrounding hydrologic system, age, and microtopography (MT). Coupled N fluxes were measured along gradients of the three factors using sedimentation tiles to quantify inputs of particulate N, modified resin cores to quantify ammonification (AMMON) and nitrification (NTR) rates, and denitrification enzyme assays to quantify denitrification (DNT) rates. To investigate the HC gradient, soil and hydrologic variables were entered into a principle component analysis that identified three primary components: (1) bulk density/soil moisture/total organic carbon, referred to as soil condition (SC), (2) standing water/sedimentation/redox (HC), and (3) soil moisture variability (SMVAR).

Results/Conclusions

The SC component was a significant positive predictor of NTR, N mineralization (N MIN), and DNT. The HC component was a significant positive predictor of AMMON and a significant negative predictor of percent NTR. The SMVAR component was not related to any fluxes. These results indicate that HC controls some N processes, like AMMON, which is the rate-limiting step in inorganic N cycling. Soil conditions, however, such as higher SM, TOC, and associated lower BD, can stimulate other N processes, such as DNT and NTR, which produces the NO₃^- substrate for DNT. Investigation of age-related factors showed that 3 and 4 year old created wetlands had significantly lower TOC and higher BD than 7 and 10 year old wetlands. Denitrification followed the same trend as TOC (and opposite trend of BD) with 3 and 4 year old wetlands demonstrating significantly lower DNT than 7 and 10 year old wetlands. TOC was also a significant positive predictor of NTR, AMMON, and N MIN. Experimental manipulations of MT demonstrated that NTR and DNT increased with certain aspects of MT. This study highlights the importance of HC, soil maturation, and incorporation of MT in the development of N cycling and coupled N processes in wetlands – wetlands with greater HC, soil organic matter, and MT had greater coupled N cycles – findings that can inform design and management decisions in created wetland systems.