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Flood pulse effects on nitrification in a floodplain forest impacted by herbivory, invasion, and restoration

De Jager, N. R., W. Swanson, E. A. Strauss, M. Thomsen, and Yao Yin.  2015. Flood pulse effects on nitrification in a floodplain forest impacted by herbivory, invasion, and restoration. Wetlands Ecol Manage DOI 10.1007/s11273-015-9445-z

Abstract

We tested the hypothesis that management actions that alter floodplain plant communities will modify the effects of flooding on gross nitrification, a key process regulating the flux of nitrate along river floodplains. Soils were collected from mature forests, patches of Phalaris arundinacea, an exotic grass, and areas restored to early successional forest (unbrowsed and browsed) in the Upper Mississippi River floodplain. Samples were collected across an elevation gradient and along the descending limb of the hydrograph to test for effects of flooding. In all three forest types, soil properties were less favorable for nitrification as elevation increased, due to decreasing organic matter, porosity, total nitrogen, and temperature. In contrast, Phalaris maintained high soil organic matter and porosity as floodplain elevation increased. Corresponding with the differences in soil properties found in forested plots, the highest rates of potential gross nitrification were found in the lower elevation sites immediately following the spring flood (1–8 days post inundation). These high rates were later followed by a rapid decline in both NH4+-N and nitrification with increasing time since inundation ([11 days post inundation). Nitrification rates were also highest following the flood in Phalaris sites, but rates did not depend on elevation, likely because of the lack of elevational differences in soil properties. Our results generally support the flood-pulse concept of river-floodplain connectivity, with the highest nitrification rates found in areas and during times immediately following inundation. Furthermore, restoration of forest cover in areas invaded by Phalaris appears likely to restore flood-pulse effects on abiotic soil properties and nitrification dynamics.

Keywords
Connectivity,  Disturbance, Flood pulse,  Herbivory,  Nitrogen,  Phalaris arundinacea, Restoration