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Title:	Microbial incorporation of nitrogen in stream detritus
Author(s):	Sanzone, Diane M.; Tank, Jennifer L.; Meyer, Judy L.; Mulholland, Patrick J.; Findlay, Stuart E.G.
Date:	2001
Source:	Hydrobiologia, Vol. 464: 27-35
Description:	We adapted the chloroform fumigation method to determine microbial nitrogen (N) and microbial incorporation of 15N on three common substrates [leaves, wood and fine benthic organic matter (FBOM)] in three forest streams. We compared microbial N and 15 content of samples collected during a 6-week15N-NH4 tracer addition in each stream. The 15N added during late autumn to Upper Ball Creek, a second-order stream at the Coweeta Hydrologic Lab, North Carolina, U.S.A.; during spring to Walker Branch, a first-order stream on DOE'S Oak Ridge National Environmental Research Park, Tennessee; and during summer to Bear Brook, a first-order stream in the Hubbard Brook Experimental Forest, New Hampshire. FBOM was the largest component of organic matter and N standing stock in all streams. Microbial N represented the highest proportion of total N in leaves and least in FBOM in Walker Branch and Bear Brook. In Upper Ball Creek, the proportion of microbial N was higher in FBOM than in used biofilm or on leaves. Standing stock of microbial N on leaves and in FBOM ranged from 37 mg N m-2 in Bear Brook to 301 mg N m-2 in Walker Branch. Percent of dehital N in living microbial cells was directly related to total microbial biomass (fungal and bacterial biomass) determined from microscopic counts, a15N values for microbes were generally higher than for bulk detritus, which would result in higher a 15N values for animals preferentially consuming or assimilating microbial cells. The proportion of 15N taken up by detritus during the 15N experiments that remained in microbial cells by the end of the experiments was highest for wood biofilm in Upper Ball Creek (69%), leaves in Walker Branch (65%) and FBOM in Upper Ball Creek (31%). Lower retention proportions (t 1-258) were observed for other substrates. Our results suggest that microbial cells associated with leaves and wood biofilm were most active in 15N-NH4 immobilization, whereas microbial cells associated with FBOM immobilized little15N from stream water.
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