Wednesday, June 6, 2007 - 8:15 AM
191

The forms of nitrogen in Wisconsin streams

Emily Stanley, Center for Limnology, University of Wisconsin, 680 N. Park St, Madison, WI 53706

Human activities often increase nitrogen (N) concentrations in streams.  Sources of N are diverse, but biogeochemical paradigms and data suggest that streamwater N should be dominated by DON in pristine watersheds.  Further, with increasing anthropogenic influence, the NO3-N fraction becomes increasingly large, driving up TDN concentrations.  NH4-N is typically low in pristine streams, but can be extremely high in systems receiving point source discharges.  I tested these predictions via a survey of 78 Wisconsin streams that varied from low (<2%) to high (>95%) human land use (%agriculture + urban cover) and 0-39% wetland cover.  TDN ranged from 0.09-14.01 mg N/L and was dominated by NO3-N and DON fractions, which, combined, accounted for an average of 97% of TDN.  As expected, NO3-N was the dominant fraction in high N (TDN>2 mgN/L) streams.  Unexpectedly, NO2-N was also present in most streams.  Although it represented a small fraction (~1%), NO2-N was >0.05 mgN/L at some sites.  Laboratory assays indicated that NO2-N could be generated via reductive transformations.  Thus, in agricultural streams, NO2-N production may be facilitated by coupled inputs of NO3-N and fine sediments.  Regardless of its source, presence of NO2-N represents an unexpected and additional management challenge associated with excess N loading.   

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