Elevated stream inorganic nitrogen impacts on a dominant riparian tree species: results from an experimental riparian stream system

[1] The release of inorganic nitrogen from intensive agricultural practices and urbanization has resulted in significant alterations of the aquatic nitrogen cycle in riparian ecosystems. Nevertheless, impacts of stream nitrogen inputs on the terrestrial nitrogen cycle and the water and carbon cycles are unclear. Information on terrestrial ecosystem responses to stream N loading is largely absent in part because of the difficulty in controlling for temporal and spatial variation in streamflow, geomorphology, climate, and vegetation. To address these issues, we constructed a dual-plot artificial stream riparian system within a 10-year-old plantation of a dominant riparian tree species, box elder (Acer negundo). The dual-plot design allowed for different concentrations of stream inorganic nitrogen between plots while controlling for ecohydrologic and geohydrologic variability. The system was used to investigate elevated inorganic stream nitrogen impacts on water use patterns, above-ground productivity, and leaf chemistry of streamside box elder trees over two consecutive growing seasons (2006 and 2007). One plot received inorganic soluble fertilizer that brought the NO 3 concentration of stream water from 5 mmol l À1 to about 100 mmol l À1 , while the second plot received no additional nitrogen. Relative stem sap flux density (J s ) did not vary between plots until near the conclusion of the 2006 growing season, when trees in the fertilized plot showed a steep upswing in J s relative to trees in the control plot. Sap flux in 2007 increased consistently by 0.4% day À1 in the fertilized plot relative to the control plot over a 75-day period, before leveling off near the conclusion of the growing season. At the onset of the experiment, leaf nitrogen per unit mass and leaf nitrogen per unit area were significantly higher in the control plot, and leaf C:N ratios were lower. In 2007, however, differences in leaf chemistry disappeared, suggesting that leaf nitrogen increased in the fertilized trees relative to the control trees. Stem diameter growth in 2007 was 15% greater in the fertilized trees, although there were no differences in either canopy radial or canopy height growth throughout the experiment. Results from this investigation suggest that increases in stream inorganic nitrogen affect water use, litter quality, and productivity of dominant riparian vegetation. These effects may have important feedbacks on several ecohydrological processes. Citation: Hultine, K. R., T. L. Jackson, K. G. Burtch, S. M. Schaeffer, and J. R. Ehleringer (2008), Elevated stream inorganic nitrogen impacts on a dominant riparian tree species: Results from an experimental riparian stream system