Comparing 15N2 uptake and acetylene reduction methods for measuring stream N2-fixation: release of N2 fixed

Abstract

Nitrogen fixation can be a substantial flux of new biologically available nitrogen (N) into some aquatic ecosystems. The most commonly used method for measuring N2fixation is the acetylene reduction method, which measures nitrogenase activity. For this method, it is necessary to assume a theoretical ratio of ethylene produced to N2-fixed unless the estimates are calibrated using 15N2 as a tracer. We measured N2-fixation using both the acetylene reduction method and 15N2 uptake methods to examine the ratio of ethylene produced to N2-fixed (C2H4:N2), estimate controls on variation in this ratio, and to quantify the immediate release of N2-fixed as dissolved N. We estimated N2-fixation for benthic biofilms and separated Nostoc in 4 streams in Wyoming, Little Laramie River, Ditch Creek, Spread Creek, and Polecat Creek. Estimates of the C2H4:N2 ratio for any one stream and date ranged from 1.6-7.1 with a group-level mean of 3.8. Immediate release of N2-fixed as dissolved organic and inorganic N was < 1% of N2-fixation. Ethylene production increased 1.8 times faster than N2-fixation as temperature increased showing strong control of temperature in the C2H4:N2 ratio. Temperature contributed substantially to the variability in ratios of ethylene produced to N2-fixed.