Baltimore Ecosystem Study Institute of Ecosystem Studies

2014 BES Annual Meeting Presentation and Poster Abstracts

Riparian Links and Nitrogen Sinks; The effects of
Tomat-Kelly, Giovanna
Co-Authors: Giovanna Tomat-Kelly and Dr. Peter Groffman

Abstract: Riparian ecosystems are important nutrient sinks that are useful in preventing excessive nitrogen loading into aquatic ecosystems. The primary mechanism of N removal in riparian ecosystems is denitrification, an anaerobic microbial process that converts inorganic nitrate into N gas. The process is carried out primarily by heterotrophic bacteria, and is optimized in the oxygen poor and carbon rich environments that are characteristic of wetland and riparian ecosystems. Unfortunately, increasing urbanization has lead to a suite of degrading effects in riparian ecosystems including hydraulic disconnection, erosion, drier soils and increased vulnerability to exotic plant invasions. Two prominent invasive plants found in urban riparian zones are Aliliaria petiolata and Microstigium vimineum, herbaceous C3 and C4 plants, respectively, that are known to alter soil chemistry and composition through various secondary effects. The objective of this study was to determine how hydrologic disconnection and the presence of A. petiolata and M. vimineum affect the ability of urban riparian ecosystems to function as N sinks. Soil samples collected from urban riparian zones in Baltimore were analyzed for denitrification potential, potential net N mineralization and nitrification, soil moisture, and ion concentrations. Denitrification was found to be significantly higher in the hydraulically connected DR2 site compared to the more disconnected HHB1 site and the strongly eroded and incised DR5 site. Differences in denitrification potential were strongly correlated to differences in soil nitrate pools across sites. Differences in soil nitrate pools were not the result of differences in internal N-cycling across sites, and were likely the result of varying exposure to nitrate rich stream water. Thus, it appears that more connected riparian sites have a greater potential to function as N-sinks. Among plant species, denitrification potential was found to be significantly lower in soils beneath M. vimineum than in soils beneath A. petiolata and S. foetidus. The lower denitrification potential in M. viminum colonized soils may be the result of M. vimineum altering N cycling process within the soil through the release of allelopathic chemicals or high levels of labile carbon. However, it is also possible that M. vimineum simply colonizes soils which have inherently lower nitrogen cycling and denitrification potential.