Document: EMI-3-63-23

Instream nitrification: Can it explain [NO₃] in streams of the Hubbard Brook Experimental Forest?

BERNHARDT, E.S.* ^1,2, R.O.HALL, JR. ³ and G.E.LIKENS ²

Cornell University, Ithaca, NY 14853 USA ¹
Institute of Ecosystem Studies, Millbrook, NY USA ²
University of Wyoming, Laramie, WY USA ³

Abstract:
Despite draining remarkably similar forest types, streams of the Hubbard Brook Experimental Forest vary widely in their growing season NO₃ concentrations (<2 to >300ug NO₃-N/L). This variation may be due to a number of causes: Differences between the terrestrial systems they drain (e.g. varying stand age or composition, watershed hydrology, soil organic matter content, etc.) or differences between the streams themselves (e.g. contrasting geomorphology, nitrogen demand, or rates of instream nitrification and denitrification). In a survey of fifteen streams conducted during the summers of both 1998 and 1999 we estimated nitrification rates using a best fit model of the downstream change in NO₃ concentrations following a short-term NH₄- enrichment. Within the surveyed streams the nitrification rate varied considerably (from 0 to 100% of the added NH₄ was nitrified) and this variation was weakly correlated with [NO₃]. We examine whether this variation in instream nitrification rates could contribute significantly to the observed variation in streamwater nitrate by calculating the rate of nitrification that would have to occur to lead to the nitrate levels measured for each stream. Based on these comparisons we find that instream rates of nitrification are too low to account for the wide range of streamwater nitrate. We propose that rather than stream nitrifiers creating differences in water column nitrate concentrations, nitrification rates are in part determined by the availability of nitrogen.

Keywords: Nitrification, nitrate, streams, Hubbard Brook Experimental Forest

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.