Land use and the changing reactivity of organic nitrogen and phosphorus exported from urban watersheds
Sujay S. Kaushal, Peter M. Groffman, Stuart E.G. Findlay, and David T. Fischer
Organic N and P can comprise a substantial proportion of the total N and P present in undisturbed and disturbed streams. Yet, relatively little is known about the ecological significance of these organic forms. Changes in the amount and biological reactivity of organic N and P were investigated in streams of the Gwynn’s Falls watershed along a gradient of land use. Concentrations of inorganic N and P declined from suburbanized and agricultural headwaters to urbanized reaches further downstream. In contrast, concentrations of organic N and P increased along the river network and comprised 53% and 71% of the total N and P exported to Chesapeake Bay. The biological reactivity of organic C, N, and P varied with land use and season. Across sites, 0-47% of organic carbon, 0-41% of organic nitrogen, and 0-58% of organic P were biologically available over time scales of 2-3 days. Measurements of bacterial production and respiration efficiency suggested that the growth and metabolism of microbes in urban streams were less affected by organic matter quality and limitation by inorganic nutrients as compared to forested reference streams. Reactive fractions of organic N and P may accumulate along suburban and urban drainages. Although not typically quantified, these fractions have the potential to be an important supply of nutrients, particularly to coastal ecosystems experiencing rapid changes in land use.
Keywords: nitrogen, phosphorus, stream, microbes

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