Question

Which of the following is a non-symbiotic biofertilizers?
A. VAM
B. Azotobacter
C. Anabaena
D. Rhizobium

Answer 1

Hint: Organisms which enrich the nutrient quality of soil are called biofertilizers, usually biofertilizers are made of bacteria, fungi, cyanobacteria etc. biofertilizers of bacteria are grouped under two categories that are 1. free-living nitrogen-fixing bacteria and 2. symbiotic nitrogen-fixing bacteria. Again, symbiotic bacteria are divided into legumes and non-legumes.

Complete answer:Free-living nitrogen-fixing bacteria are non-symbiotic association, can live in the soil, even without associating with the plants and they include members like azotobacter, klebsiella, clostridium and methanococcus. Among this, clostridium is found to be anaerobic, while the remaining are considered as aerobic. Features of azotobacter: they are pleomorphic ranging from rods to coccoid shapes and form cysts as the culture ages. These are widespread in soil and water. Azotobacter can be capable of forming cysts in response to nutrient limitation. Although cysts are also very resistant to desiccation, they are less tolerant of other environmental stresses such as heat and UV light. They have a thick outer coat and store an abundance of poly - 13 - hydroxybutyrate.
 When the bacteria live in close association with the roots of grasses and fix nitrogen, the association is of free mutualism, called associative symbiosis and this nitrogen fixation is called associative symbiotic nitrogen fixation. In such association the bacteria live in the rhizosphere or may enter into the roots. These bacteria fix nitrogen and supplies to the roots. In replacement, they give carbohydrates for the nourishment of bacteria. This process is carried out by azotobacter, azospirillum etc.
Hence, the correct answer is option B- Azotobacter.

Note: Few evidence suggests that the main contribution may not be nitrogen fixation but the production of growth-promoting hormones because it increases the root hair development, thereby enhancing plant nutrient uptake. Azotobacter, paspali, beijerinckia are some of the examples for non-symbiotic biofertilizers.