Whole-system estimates of nitrification and nitrate uptake in streams of the Hubbard Brook Experimental Forest

Although they drain remarkably similar forest types, streams of the Hubbard Brook Experimental Forest (HBEF) vary widely in their NO3- concentrations during the growing season. This variation may be caused by differences in the terrestrial systems they drain (for example, varying forest age or composition, hydrology, soil organic matter content, and so on) and/or by differences between the streams themselves (for example, contrasting geomorphology, biotic nitrogen [N] demand, rates of instream nitrogen transformations). We examined interstream variation in N processing by measuring NH4+ and NO3- uptake and estimating nitrification rates for 13 stream reaches in the HBEF during the summers of 1998 and 1999. We modeled nitrification rates using a best-fit model of the downstream change in NO3- concentrations following short-term NH4+ enrichments. Among the surveyed streams, the fraction of NH4+ uptake that was subsequently nitrified varied, and this variation was positively correlated with ambient streamwater NO3- concentrations. We examined whether this variation in instream nitrification rates contributed significantly to the observed variation in NO3- concentrations across streams. In some cases, instream nitrification provided a substantial portion of instream NO3- demand. However, because there was also substantial instream NO3 uptake, the net effect of instream processing was to reduce rather than supplement the total amount of NO3- exported from a watershed. Thus, instream rates of nitrification in conjunction with instream NO3- uptake were too low to account for the wide range of streamwater NO3-. The relationship between streamwater NO3- concentration and rates of instream nitrification may instead be due to a shift in the competitive balance between heterotrophic N uptake and nitrification when external inputs of NO3- are relatively high.