Wednesday, May 28, 2008 - 2:00 PM
429

15N2 calibration of the acetylene reduction method for measuring nitrogen fixation in streams

Lisa A. Kunza, Program in Ecology, Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071 and Robert O. Hall Jr., Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071.

The acetylene reduction method commonly used for N-fixation measurements assumes a ratio of three or four moles ethylene produced to one mole of N2 fixed.  The large deviations in recorded ratios, ranging from 0.0216:1 in an arctic salt marsh to 94:1 in subtidal sediments, depict the need for using 15N2 to calibrate the acetylene reduction method in streams.  Summer 2007, we measured N-fixation using both 15N2 methods and the acetylene reduction technique in four streams using both syringes and chambers.  The acetylene reduction method directly measures nitrogenase activity while 15N2 methods measure fixed nitrogen that is incorporated into biomass.  For syringes containing only Nostoc, colonial cyanobacteria, we measured the mean ratio of 5 ± 0.95 and for chambers containing rocks with intact biofilm the mean ratio 9 ± 0.74. Thus, less 15N was incorporated into the biomass than predicted by theory.  The variability in ratios was not explained by total N, chl a, AFDM, or stream ammonium concentration, but rather appears to be analytical variation.  An explanation for our measured ratios being higher than predicted values may be the amount of release of the dissolved, fixed N.


Web Page: N fixation, acetylene reduction, conversion ratio

See more of Biogeochemistry III
See more of Contributed Sessions

See more of The Nabs 56th Annual Meeting (25-28 May, 2008)