An assessment of nitrogen removal from headwater streams in an agricultural watershed, northeast Ohio, U.S.A

Abstract

The objective of this project was to assess in-stream nitrogen removal capacity in a fragmented agricultural landscape and to compare removal capacities in streams with agricultural or residential (hereafter referred to as agricultural streams) and forested riparian land use. We also identified what stream characteristics control nitrogen removal in these systems. We examined paired reaches (one agricultural and one forested reach) along five headwater streams in an agricultural watershed (Upper Sugar Creek Watershed) in northeast Ohio. Although denitrification rates were high (,0.1-17.2 mg N m 22 h 21 ), annual nitrogen removal was most likely low because during spring and fall, when in-stream nitrogen loads were high, removal was low, and during summer when instream nitrogen loads were low, removal was high. Between the agricultural and forested reaches removal rates were similar in terms of loss rate and uptake velocity. Removal capacities were similar despite forested reaches having higher hydraulic residence times. Using a redundancy analysis we identified temperature, in-stream nitrate concentration, and relative transient storage as stream characteristics that affect nitrogen removal. Further analysis suggests that nitrogen removal via denitrification in these headwater streams was not limited by the availability of nitrate. In this fragmented agricultural watershed in-stream nitrogen removal was low and riparian land use had no effect on this process, most likely because of nitrate saturation.Since the middle of the 20th century in the United States, reactive nitrogen inputs to landscapes have tripled as the result of agricultural fertilizer applications and cultivation of N-fixing crops (Galloway et al. 2003). Reactive N is considered by some researchers to be the third largest threat to our planet after biodiversity loss and climate change (Giles 2005). Excess N degrades habitat and limits biodiversity in aquatic ecosystems (Vitousek et al. 1997;Howarth et al. 2000). In the midwestern United States, agricultural watersheds are the dominant exporters of reactive N, particularly inorganic N (Goolsby et al. 1999), and have been linked to the eutrophication of the Gulf of Mexico (Rabalais et al. 2001). Because of these negative effects, several studies have examined N removal capacity and removal strategies over large spatial scales (Mitsch et al. 2001;Seitzinger et al. 2002). A crucial process responsible for the removal of N is denitrification. Denitrification is an important biogeochemical process because it removes a mobile, reactive form of N (nitrate) from an ecosystem by converting it to an unreactive gas (dinitrogen). This reaction has been extensively studied since the early 1970s, yet questions still remain about its efficacy and the characteristics that control this process in stream ecosystems (Boyer et al. 2006).In streams and rivers, the transport of nutrients has been coupled to the downstream movement of water by the nutrient spiraling theory (Webster and Patton 1979;Newbold et al. ...

The dark side of the hyporheic zone: depth profiles of nitrogen and its processing in stream sediments

“…The seasonal differences in denitrification rates were not likely due to bulk sediment characteristics (% organic matter, % silt, bulk density) or nitrate and dissolved organic carbon concentrations of ground water used in the incubations, as the magnitude of these variables did not vary among seasons. Other investigators have shown seasonal variation in denitrification rates in stream sediments, including higher rates during the summer when comparisons were made within substratum or habitat types (Richardson et al, 2004;Schaller et al, 2004;Arango et al, 2007;Herrman et al, 2008).…”

Nitrate removal in deep sediments of a nitrogen‐rich river network: A test of a conceptual model

“…Many studies of nitrogen uptake and removal in streams, including those based on whole‐stream injections [ Mulholland et al , 2008], sediment cores [ Herrman et al , 2008], and nitrogen retention at the network scale [ Wollheim et al , 2008] have not considered nitrogen processing in deep sediments. However, as our results have shown deep stream sediments, like riparian zones [ Hedin et al , 1998], can be locations where oxic nitrate‐rich groundwater from upland areas encounters favorable redox conditions for reduction.…”

“…Nitrogen processing studies at reach scales are usually designed to measure nitrogen uptake or transformation in stream channels [ Mulholland et al , 2008; Heffernan et al , 2010] and those studies at smaller spatial scales typically emphasize shallow sediments. For example, most studies of denitrification in stream sediments only include denitrification measurements from surficial sediments (cores less than 5 cm deep) [ Arango et al , 2007; Herrman et al , 2008] (also see review in Stelzer et al [2011b]). In gaining streams groundwater often passes through substantial quantities of sediment before discharging to the stream.…”

Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales

“…Herrman et al, 2008;Nimick et al, 2011;Palmer-Felgate et al, 2008;Pellerin et al, 2012). Those studies that measure sub-daily instream NO 3 dynamics tend to be based on short-term monitoring schemes, spanning a few days or specific months (e.g.…”