The quantitative relationship between nitrogen fixation and the acetylene-reduction assay

Abstract

Acetylene reduction activity of detached soybean nodules was positively correlated with the nitrogen content of the host plants grown in nitrogen-free media. Acetylene reduction by nodulated root systems was related to light intensity preceding sampling; activity of detached nodules was lower and less influenced by light. Comparison of acetylene reduction and 15N2 uptake by detached soybean nodules gave ratios of acetylene reduced to nitrogen fixed (C2H4: NH3 ratio) ranging from 2·7 to 4·2 in different experiments. The mean value for four experiments was 3·3±0·6 (S.E.). Different effects of oxygen were obtained with detached soybean nodules when 0·1 atm nitrogen and 0·1 atm acetylene were used as substrates. When the acetylene concentration was reduced to 0·005 atm the effects upon both assays were similar but the ratio of the two assays was still affected by pO2. The C2H4: NH3 ratio in a growing culture of Azotobacter vinelandii fell with time. It was concluded that this was probably due to the failure to match the oxygen-solution rates in the culture and in the acetylene-reduction assays. Growth and nitrogen fixation, in a culture of Klebsiella aerogenes, were linear with time for 8 hr following the logarithmic growth phase. During this 8-hr period, the ratio C2H4: NH3 was 1.5±0.4 (S.E.). During the following 8 hr, uptake of nitrogen declined but acetylene reduction did not. The acetylene-reducing activity of this culture was destroyed by brief exposure to air during sampling_ It is concluded that caution should be observed in the application of the acetylene-reduction assay to quantitative determinations of nitrogen fixation. Major errors are likely to result when the conditions in the assays are not carefully matched with the conditions under which nitrogen fixation is occurring. © 1970 CSIRO.