Seasonal and geomorphic controls on N and P removal in riparian zones of the US Midwest

Abstract

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.