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There is growing interest in generalizing the impact of hydrogeomorphology and weather variables on riparian functions. Here, we used RZ-TRADEOFF to estimate nitrogen, phosphorus, water table (WT) depth, and greenhouse gas (GHG: N2O, CO2, CH4) functions for 80 riparian zones typical of the North American Midwest, Northeast (including Southern Ontario, Canada), and Mid-Atlantic. Sensitivity to weather perturbations was calculated for temperature and precipitation-dependent functions (CO2, phosphate concentration, and water table), and multivariate statistical analysis on model outputs was conducted to determine trade-offs between riparian functions. Mean model estimates were 93.10 cm for WT depth, 8.45 mg N L−1 for field edge nitrate concentration, 51.57% for nitrate removal, 0.45 mg PO43− L−1 for field edge phosphate concentration, 1.5% for subsurface phosphate removal, 91.24% for total overland phosphorus removal, 0.51 mg N m−2 day−1 for N2O flux, 5.5 g C m−2 day−1 for CO2 fluxes, and − 0.41 mg C m−2 day−1 and 621.51 mg C m−2 day−1 for CH4 fluxes in non-peat sites and peat sites, respectively. Sites in colder climates were most sensitive to weather perturbations for CO2, sites with deep water tables estimates had the highest sensitivity for WT, and sites in warm climates and/or with deep confining layers had the lowest sensitivity for phosphate concentration. Slope, confining layer depth, and temperature were the primary characteristics influencing similarities and trade-offs between sites. This research contributes to understanding how to optimize riparian restoration and protection in watersheds based on both water (nitrogen, phosphorus) and air quality (GHG) goals. [ABSTRACT FROM AUTHOR]

Abstract: Low-cost and simple technologies are needed to reduce watershed export of excess nitrogen to sensitive aquatic ecosystems. Denitrifying bioreactors are an approach where solid carbon substrates are added into the flow path of contaminated water. These carbon (C) substrates (often fragmented wood-products) act as a C and energy source to support denitrification; the conversion of nitrate (NO3 −) to nitrogen gases. Here, we summarize the different designs of denitrifying bioreactors that use a solid C substrate, their hydrological connections, effectiveness, and factors that limit their performance. The main denitrifying bioreactors are: denitrification walls (intercepting shallow groundwater), denitrifying beds (intercepting concentrated discharges) and denitrifying layers (intercepting soil leachate). Both denitrifcation walls and beds have proven successful in appropriate field settings with NO3 − removal rates generally ranging from 0.01 to 3.6gNm−3 day−1 for walls and 2–22gNm−3 day−1 for beds, with the lower rates often associated with nitrate-limitations. Nitrate removal is also limited by the rate of C supply from degrading substrate and removal is operationally zero-order with respect to NO3 − concentration primarily because the inputs of NO3 − into studied bioreactors have been generally high. In bioreactors where NO3 − is not fully depleted, removal rates generally increase with increasing temperature. Nitrate removal has been supported for up to 15 years without further maintenance or C supplementation because wood chips degrade sufficiently slowly under anoxic conditions. There have been few field-based comparisons of alternative C substrates to increase NO3 − removal rates but laboratory trials suggest that some alternatives could support greater rates of NO3 − removal (e.g., corn cobs and wheat straw). Denitrifying bioreactors may have a number of adverse effects, such as production of nitrous oxide and leaching of dissolved organic matter (usually only for the first few months after construction and start-up). The relatively small amount of field data suggests that these problems can be adequately managed or minimized. An initial cost/benefit analysis demonstrates that denitrifying bioreactors are cost effective and complementary to other agricultural management practices aimed at decreasing nitrogen loads to surface waters. We conclude with recommendations for further research to enhance performance of denitrifying bioreactors. [Copyright &y& Elsevier]