Particulate nitrogen exports in stream runoff exceed dissolved nitrogen forms during large tropical storms in a temperate, headwater, forested watershed

Abstract

AbstractAlthough lasting only a fraction of the year, large storms may represent a significant, but highly variable, control on watershed nitrogen (N) fluxes. We determined the exports of particulate N (PN) and total dissolved N including nitrate‐N (NO3‐N) and dissolved organic N (DON) in streamflow from a 12 ha temperate forested watershed. Sampling was performed for 15 storms over September 2010 to December 2012 and included four large tropical storms—Nicole (2010), Irene and Lee (2011), and Sandy (2012). PN composed a substantial portion (39–87%) of the storm event N export with storms constituting 65% of the 2011 PN export. Tropical storm Irene alone generated 1.76 kg N ha−1 of PN which was 27% of the annual watershed N (6.43 kg N ha−1) export for 2011. In contrast, tropical storm Sandy (October 2012) yielded low sediment and PN exports, likely due to low precipitation intensity and a freshly fallen leaf cover that may have reduced soil erosion. Stream water concentrations of PN, NO3‐N, and DON ranged from 0–17.5, 0–2.02, and 0.01–0.54 mg N L−1, respectively. Nitrate‐N concentrations displayed a dilution trajectory for peak stormflows suggesting supply limitation, a response that was not as strong for PN. These results underscore the importance of large storms for PN export which is significant given that climate‐change predictions indicate an increasing intensity of large tropical storms for the northeast U.S.. Elevated PN exports could further exacerbate water quality and eutrophication problems in sensitive aquatic ecosystems already subjected to excess dissolved nitrogen loads.