Nitrogen transformations in a small mountain stream

Abstract

Ammonium, urea, and nitrate were added to Bear Brook, a second and third order stream in the Hubbard Brook Experimental Forest, New Hampshire. Removal of ammonium and urea during downstream transport coincided with the release of nitrate. Nitrate removal did not occur when it was added alone or with dissolved organic carbon. Laboratory experiments showed that coarse particulate organic material (detritus) and bryophytes taken from the streambed were active in the removal of ammonium from enriched stream water, and in the release of nitrate upon the addition of ammonium. The patterns of removal and release observed in these experiments suggest a biologically mediated, oxidation process. Budgetary calculations show that the in-stream transformation of nitrogen inputs during summer and autumn could represent 12 to 25 percent of the nitrogen exported as nitrate during winter and spring from heterotrophic streams like Bear Brook. This type of internal cycling affects the timing and form of nitrogen export from small streams draining forested watersheds in the northeastern United States.