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Review
. 2021 Feb 25:12:628379.
doi: 10.3389/fmicb.2021.628379. eCollection 2021.

Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers for Crop Nutrition and Yield Stability

Abderrahim Aasfar  1   2 , Adnane Bargaz  3 , Kaoutar Yaakoubi  1 , Abderraouf Hilali  2 , Iman Bennis  1 , Youssef Zeroual  4 , Issam Meftah Kadmiri  1   3
Affiliations
Review

Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers for Crop Nutrition and Yield Stability

Abderrahim Aasfar et al. Front Microbiol. .

Abstract

Biological nitrogen fixation (BNF) refers to a microbial mediated process based upon an enzymatic "Nitrogenase" conversion of atmospheric nitrogen (N2) into ammonium readily absorbable by roots. N2-fixing microorganisms collectively termed as "diazotrophs" are able to fix biologically N2 in association with plant roots. Specifically, the symbiotic rhizobacteria induce structural and physiological modifications of bacterial cells and plant roots into specialized structures called nodules. Other N2-fixing bacteria are free-living fixers that are highly diverse and globally widespread in cropland. They represent key natural source of nitrogen (N) in natural and agricultural ecosystems lacking symbiotic N fixation (SNF). In this review, the importance of Azotobacter species was highlighted as both important free-living N2-fixing bacteria and potential bacterial biofertilizer with proven efficacy for plant nutrition and biological soil fertility. In addition, we described Azotobacter beneficial plant promoting traits (e.g., nutrient use efficiency, protection against phytopathogens, phytohormone biosynthesis, etc.). We shed light also on the agronomic features of Azotobacter that are likely an effective component of integrated plant nutrition strategy, which contributes positively to sustainable agricultural production. We pointed out Azotobacter based-biofertilizers, which possess unique characteristics such as cyst formation conferring resistance to environmental stresses. Such beneficial traits can be explored profoundly for the utmost aim to research and develop specific formulations based on inoculant Azotobacter cysts. Furthermore, Azotobacter species still need to be wisely exploited in order to address specific agricultural challenges (e.g., nutrient deficiencies, biotic and abiotic constraints) taking into consideration several variables including their biological functions, synergies and multi-trophic interactions, and biogeography and abundance distribution.

Keywords: Azotobacter; biological nitrogen fixation; nitrogen; nitrogenase; phosphate; plant nutrition.

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Conflict of interest statement

YZ was employed by company Situation Innovation Group–OCP Group. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
(A) Exponential growth in the number of scientific publications related to Azotobacter from 1990 to 2020 using Azotobacter as a key word from scopus (https://www.scopus.com). (B) Abundance and thematic distribution of the literature available on Azotobacter. Bibliometric research made in scopus (https://www.scopus.com) using Azotobacter as a keyword in June 2020.
FIGURE 2
FIGURE 2
Number and distribution of major free N2-Fixing species identified in seven agricultural locations representative of Moroccan agricultural soils using 16S rDNA marker.
FIGURE 3
FIGURE 3
Mechanism of non-symbiotic fixation of atmospheric nitrogen by Azotobacter sp.
FIGURE 4
FIGURE 4
Formulation possibilities and large-scale production of inoculants from Azotobacter sp.
FIGURE 5
FIGURE 5
Technology investment and the evaluation dynamics of inventiveness of the studied of biofertilizers based on Azotobacter bacteria in the last 20 years.
See this image and copyright information in PMC

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