Nitrate Removal in Stream Riparian Zones

Abstract

This review considers the role of stream riparian zones in regulating the transport of nitrate (NO−3) in groundwater flow from uplands to streams. The current consensus is that most riparian zones effectively remove NO−3 from subsurface water. However, research has not focused on the relationship between hydrology and chemistry within the context of the riparian zone hydrogeologic setting. Most riparian zones that remove NO−3 occur in landscapes with impermeable layers near the ground surface. In this setting, small amounts of groundwater follow shallow horizontal flow paths that increase water residence time and contact with vegetation roots and organic‐rich riparian soils. Limited research suggests that riparian zones have less effect on NO−3 transport in hydrogeologic settings where groundwater has little interaction with vegetation and sediments because flow occurs mainly across the surface, or at depth beneath the riparian zone before discharging to the stream. Considerable uncertainty surrounds the relative importance of vegetation uptake and microbial denitrification in NO−3 removal from subsurface water in riparian zones. Plant NO−3 uptake requires the presence of the root zone below the water table. Information is lacking on the vertical distribution and seasonal dynamics of fine root biomass in relation to water table fluctuations. High denitrification rates have been reported in 0 to 10 cm surface soils of riparian zones in the USA, France, and New Zealand. However, rapid NO−3 removal from groundwater also occurs in riparian locations where the water table is always >0.5 m below the surface. Denitrification at depth within the saturated zone has been studied to a limited extent and has been found not to occur at some sites. An interdisciplinary approach in which patterns of NO−3 depletion and the role of NO−3 removal processes are related to groundwater flow paths is needed to provide a better understanding of NO−3 regulation in riparian zones.

Nitrous oxide generation, denitrification, and nitrate removal in a seepage wetland intercepting surface and subsurface flows from a grazed dairy catchment

“…The elevated rates we observed reflect environmental conditions conducive to denitrification, i.e. low DO (<1 mg O 2 /L; Table 3) plus a highly enriched organic matter sediment (Table 2) and optimum sediment-water contact time within the confined lysimeter environment (Hill 1996;Fennessy and Cronk 1997;Hoffmann et al 2000;Smith et al 2003). The observed high denitrification rates could be attributed to the elevated DEA values (Table 4) in the wetland soils (Groffman et al 1999).…”

“…The low level of NO 3 À in wetland waters (Table 3) suggests that any NO 3 À that enters the wetland via seepage springs and runoff is readily removed by denitrification (Zaman et al 2008b) and other biological processes (Seitzinger 1994;Hill 1996;Fennessy and Cronk 1997;Matheson et al 2002Matheson et al , 2003. Ammonium and organic N were the predominant N fractions in both surface and subsurface waters (Table 3), likely due to inputs from farmland runoff and/or the incomplete breakdown of organic matter originating from wetland organic sediments and/or plant vegetation under anaerobic conditions (Nguyen 2000).…”

Denitrification in surface soils of a riparian forest: Effects of water, nitrate and sucrose additions

“…Only 33% of the apparent nitrate uptake could be explained by tree growth (Peterjohn and Correll, 1984), suggesting that much of the intercepted nitrogen may be consumed by denitrification. Although many studies have suggested that riparian forests may be important sinks for N, the mechanisms of N uptake are not well understood (Hill, 1996).…”

“…Much of the N released from agricultural lands can be intercepted by adjacent downhill riparian forests (Lowrance et a/., 1984;Osborne and Kovacic, 1993;Hill, 1996: Correll, 1997. The effectiveness of riparian forests as N sinks may strongly influence watershed discharges of N (Jordan et ul., 1997).…”

“…Lowrance et al, 1995). However, the mechanisms of N uptake in riparian forests are not well understood (Hill, 1996). Riparian forests may convert intercepted N into gaseous forms or accumulate N in vegetation or soil (Weller et al, 1994).…”