Seasonal and geomorphic controls on N and P removal in riparian zones of the US Midwest
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Riparian zones are an important strategy to mitigate N and P export to streams. However, their efficiency with respect to nitrate (NO3 −), ammonium (NH4 +), or soluble reactive phosphorus (SRP) in groundwater remains uncertain in the US Midwest. This study investigates water table fluctuations and NO3 −, NH4 +, and SRP concentration dynamics in two riparian zone types (outwash vs. glacial till) common in the upper US Midwest. During low water table periods, NO3 − removal was 93 % at WR (outwash site), and 75 % at LWD (glacial till site); but during high water table periods, NO3 − removal efficiencies dropped to 50 % at WR, and 14 % at LWD. Median seasonal mass fluxes of NO3 − removed at WR (9.4–21.7 mg N day−1 m−1 of stream length) and LWD (0.4–1.9 mg N day−1 m−1) were small compared to other riparian zones in glaciated landscapes. The WR site was a small SRP sink (0.114 and 0.118 mg day−1 m−1 during the dry period and wet period, respectively), while LWD acted as a small SRP source to the stream (0.004 mg day−1 m−1 during the dry period; 0.075 mg day−1 m−1 during the wet period). Both LWD and WR acted as sources of NH4 + to the stream with mass fluxes ranging from 0.17 to 7.75 mg N day−1 m−1. Although riparian zones in the US Midwest provide many ecosystem services, results suggest they are unlikely to efficiently mitigate N and P pollution in subsurface flow.
KeywordsNitrate Ammonium Phosphorus Fluxes Riparian zone Removal
This work was supported by a U.S. Department of Agriculture–National Institute of Food and Agriculture grant (USDA–NIFA Grant # 2009-35112-0524) to P. Vidon, P.A. Jacinthe and M.E. Baker. Thanks are also due to landowners, Mr. Douglas Johnston (WR site) and Mr. Jeffrey Phares (LWD site) for granting access to the study sites.
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