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CHANGES IN THE DISSOLVED NITROGEN POOL ACROSS LAND COVER GRADIENTS IN WISCONSIN STREAMS
Author(s) -
Stanley Emily H.,
Maxted Jeffrey T.
Publication year - 2008
Publication title -
ecological applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.864
H-Index - 213
eISSN - 1939-5582
pISSN - 1051-0761
DOI - 10.1890/07-1379.1
Subject(s) - streams , biogeochemical cycle , environmental science , ecosystem , wetland , nitrate , nitrogen cycle , ecology , nitrogen , nutrient , hydrology (agriculture) , aquatic ecosystem , land cover , environmental chemistry , land use , chemistry , biology , geology , computer network , geotechnical engineering , organic chemistry , computer science
Increases in anthropogenic nitrogen fixation have resulted in wide‐scale enrichment of aquatic ecosystems. Existing biogeochemical theory suggests that N enrichment is associated with increasing concentrations of nitrate; however, dissolved organic nitrogen (DON) is often a major component of the total dissolved nitrogen (TDN) pool in streams and rivers, and its concentration can be significantly elevated in human‐influenced basins. We examined N concentrations during summer base flow conditions in 324 Wisconsin streams to determine whether DON was a significant component of TDN and how its relative contribution changed across a gradient of increasing human (agriculture and urban) land use for 84 of these sites. Total dissolved nitrogen varied from 0.09 to 20.74 mg/L, and although DON was significantly higher in human‐dominated basins relative to forested and mixed‐cover basins, its concentration increased relatively slowly in response to increasing human land cover. This limited response reflected a replacement of wetland‐derived DON in low‐N streams by anthropogenic sources in human‐dominated sites, such that net changes in DON were small across the land use gradient. Nitrate‐N increased exponentially in response to greater human land cover, and NH 4 ‐N and NO 2 ‐N were present at low levels. Nitrite‐N exceeded NH 4 ‐N at 20% of sites and reached a maximum concentration of 0.10 mg/L. This examination suggests that basic mechanisms driving N losses from old‐growth forests subject to N saturation also shape the summertime N pool in Wisconsin streams, in addition to other processes dictated by landscape context. The overwhelming role of human land use in determining the relative and absolute composition of the summertime N pool included (1) rapid increases in NO 3 ‐N, (2) limited changes in DON, and (3) the unexpected occurrence of NO 2 ‐N. High (>3 mg/L) TDN conditions dominated by NO 3 ‐N, regardless of landscape context or forms of N inputs, indicate a state of “N hypersaturation,” which appears to be increasingly common in human‐influenced streams and rivers. Many sites in agriculturally rich areas had NO 2 ‐N and NO 3 ‐N concentrations that, if sustained, are at chronically toxic levels for sensitive aquatic biota, suggesting that N enrichment now has local consequences for resident stream biota in addition to contributing to coastal eutrophication.