Have we overemphasized the role of denitrification in aquatic ecosystems? A review of nitrate removal pathways

© The Ecological Society of America www.frontiersinecology.org E nitrogen (N) concentrations, often in the form of nitrate, present a water-quality problem of growing concern. Nitrate concentrations in groundwater and rivers in developed areas of the world have risen substantially as a result of the use of synthetic N fertilizers and cultivation of N-fixing crops (Turner and Rabalais 2003). Increasing N export from landscapes to coastal waters has been implicated in coastal eutrophication and the development of hypoxic zones (eg in the Gulf of Mexico; Rabalais et al. 2001) and harmful algal blooms (Paerl et al. 2002). There is still some debate over whether N alone is the main driver of these problems (Dodds 2006), but there is no question that the increases in N loading represent a major perturbation of streams, rivers, estuaries, and coastal marine waters. Although N loading to coastal zones has increased, regional watershed mass-balance studies (wherein all N inputs and outputs are accounted for) indicate that most of the anthropogenic N that enters watersheds is removed before reaching the oceans (Howarth et al. 1996; Alexander et al. 2000). As nitrate-rich water flows through landscapes, it enters riparian wetlands and headwater streams, which can efficiently remove nitrogen (Peterson et al. 2001; Zedler 2003). Thus, key interfaces along landscape flow paths control nitrate export to downstream surface waters, such as large rivers and lakes, and ultimately to estuaries and marine ecosystems. Here, we discuss the many possible fates for this removed nitrate, which include some grossly underestimated and understudied microbial pathways, many of which have only recently received attention from the scientific community. The importance and possible prevalence of these pathways have profound implications for the management of aquatic ecosystems to promote nitrate removal.