Control of Nitrogen Export from Watersheds by Headwater Streams

Abstract

A comparative (15)N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.

Quantifying watershed‐scale groundwater loading and in‐stream fate of nitrate using high‐frequency water quality data

“…While field-based studies [Burns, 1998;Peterson et al, 2001;Duff et al, 2008;Mulholland et al, 2008Mulholland et al, , 2009Tank et al, 2008;Hall et al, 2009;Mulholland and Webster, 2010] and modeling approaches [Jaworski et al, 1992;Boynton et al, 1995;Alexander et al, 2000Alexander et al, , 2009Seitzinger et al, 2002;Boyer et al, 2006;Runkel, 2007;Ator and Denver, 2012] have provided much needed information on reach and watershed-scale nitrate dynamics, the limited spatial extent and/or low temporal resolution of discrete data collection continues to be a challenge for quantifying loads and interpreting drivers of change in watersheds. Recent studies have demonstrated that the collection and interpretation of high-frequency nitrate data collected using water quality sensors can be used to better quantify nitrate loads to sensitive stream and coastal environments [Ferrant et al, 2013;Bieroza et al, 2014;Pellerin et al, 2014], and provide insights into temporal nitrate dynamics that would otherwise be difficult to obtain using traditional field-based mass balance, solute injection, and/or isotopic tracer studies [Pellerin et al, 2009[Pellerin et al, , 2012Heffernan and Cohen, 2010;Sandford et al, 2013;Carey et al, 2014;Hensley et al, 2014Hensley et al, , 2015Outram et al, 2014;Crawford et al, 2015].…”

Nutrient Uptake and Mineralization during Leaf Decay in Streams – a Model Simulation

“…Investigations of nutrient dynamics in streams have almost exclusively focused on uptake processes, while quantitative information on mineralization has been limited to whole-stream tracer studies (e.g., NEWBOLD et al, 1983;PETERSON et al, 2001) or estimates of nutrient regeneration rates by animal excretion (e.g. WANNI, 2002;HOOD et al, 2005;MCINTYRE et al, 2008).…”

Effectiveness of a Natural Headwater Wetland for Reducing Agricultural Nitrogen Loads

“…For example, Whitmire and Hamilton [45] using push-pull techniques found that nitrate added to groundwater and re-injected into wetland sediments disappeared rapidly relative to conservative tracers without any lag time, and was depleted to below detection limits (10-15 µg N L −1 ) within 5 to 20 h. In a 15 N denitrification study in a similar natural riparian wetland intercepting shallow groundwater, Burns and Nguyen [46] measured >90% removal of injected 15 NO 3 -N along a 100 cm flow path through the organic wetland soil, with essentially all of the 15 NO 3 -N-removed within 30 cm. Extensive 15 N tracer study performed on small headwater agricultural streams in the USA by Peterson [21] showed that NH 4 -N and NO x -N entering the streams was removed within tens to hundreds of meters and in one of the streams the denitrification rate was 0.045 µg N m −2 s −1 [47]. Denitrification consisted almost entirely of N 2 production (99%), with very little N 2 O production occurring.…”

“…Although they are individually small, they may represent a significant proportion of headwater catchments. The potential of these wetlands to attenuate upslope derived pollutants is well recognised [21]. However, they can also be a potential source of agricultural pollutants because of their direct connection to the stream network, and farmers see them as a suitable drinking water source for livestock [20].…”