Microbial assay of N2 fixation rate, a simple alternate for acetylene reduction assay

Abstract

Nitrogen is an essential element for living creatures in every ecosystem but nitrogen cannot be absorbed by the plant itself directly from the atmosphere, so for nitrogen, plant depends on both free living and symbiotic microbes present in the soil. Nitrogen fixation potentiality of the soil thus reveals its fertility with respect to nitrogen. Researchers developed and modified techniques for measuring nitrogen fixation rate of the soil and acetylene reduction assay (ARA) technique became the most popular till now. At the same time this technique has few limitations especially for the researchers from third world country due to lack of special infrastructure in the laboratory and the most required instrument for this technique, gas chromatograph machine, is very expensive. Any alternation of this technique is deserved highly for the researchers from the developing countries. The present work/method explained a new approach for determination of nitrogen fixation rate and this new method was named as “Microbial bio-assay”. In this technique nitrogen fixers were cultured in specific medium and condition and after required time of interval the amount of nitrogen fixed by them were calculated. Exploration of soil of the Sundarban mangrove ecosystem was performed regarding the microbial N2 fixing capacity of that particular ecosystem. • The nitrogen fixation rate measured by acetylene reduction assay (ARA) was 1.13 times lower than the N2 fixation rate measured by microbial bio-assay. • Microbial bio-assay can be used as an alternate of ARA method to measure N2 fixation rate. The rates of N2 fixation measured by both two methods were positively correlated with the population of N2 fixing bacteria present in the soil of that particular ecosystem (R2 = 0.85, p < 0.005, n = 85 for microbial bio-assay and R2 = 0.78, p < 0.005, n = 85 for ARA). Keywords: Acetylene reduction assay, N2-fixing bacteria, Microbial bio-assay, Sundarban ecosystem