Nitrogen fixation and 15N2 calibration of the acetylene reduction assay in coastal marine sediments

Abstract

Simultaneous assays of di-nitrogen fixation were carried out in sediments from Narragansett Bay, Rhode Island (USA) using the acetylene reduction technique and I5N2 tracer. The ratio of moles of acetylene reduced to moles of I5N recovered in ammonium and labile organic-N pools in estuarine sediments under anaerobic conditions ranged from approximately 10:l to nearly 100:l. The ratio of acetylene reduced to "N fixed was always significantly greater than the theoretical 3:l ratio. Rates of both acetylene reduction and I5N2 fixation increased with the addition of labile organic carbon (as sucrose) to the sediments. Ratios of acetylene reduction:"N, fixation in the sucrose-addition sediments (estimated to range between 12:l and 27:l) were generally lower than those found in sediments without sucrose additions, but were always significantly greater than the theoretical 3:l ratio. We caution that the use of the theoretical 3:l ratio in assays of N2 fixation by the acetylene reduchon technique in anaerobic coastal marine sediments results in overestimates of N2 fixation rates.