Nitrogen fixation in the Rhizosphere

Rhizosphere and the Phyllosphere
Rhizosphere Introduction
Rhizospher Effect
Nitrogen Fixation in the Rhizosphere
Alteration of Rhizosphere Microflora
Associative and Antagonistic Activities in the Rhizosphere
Root Exudates
Organic Compounds Detected in Plant Root Exudates
Fungistasis
Techniques of Rhizosphere
The Soil Dilution and Plate Count Method
The Soil Plate
The Soil Box
The Contact Slide Technique
Artificial Rhizosphere
Model System

Nitrogen fixation in the Rhizosphere

Nitrogen Fixation in the Rhizosphere

With the advent of acetylene reduction technique, it has been possible to measure nitrogenase activity in the rhizosphere of many non-legumes. In terms of moles C₂H₄ per g. of dry root per hour, Brachiaria mutica showed nitrogenase activity of 150-750 in Brazil and maize seedlings showed activity in France in the range of 100-3000.

Similar values have been recorded for various grasses and weeds with differing degrees of variability. Published reports indicate that rhizosphere of these plants harbour nitrogen fixing bacteria of the families Azotobacteriaceae, Spirillaceae, Enterobacteriaceae, Bacillaceae, Pseudomonadaceae and Achromobacteriaceae.

Colonization by Azotobacter, when it occurred, has been shown to be limited in the rhizosphere and practically negligible on the root surface (rhizoplane), probably due to acidity caused by root exudates. Colonization of root surface by Azospirillum has been noticed with rather extensive intrusion within root tissues. Sporangia of Frankia have been observed in the rhizosphere of Casuarina seedlings, indicating the variety of N₂ fixing microorganisms that are capable of inhabiting the rhizosphere. Azotobacter chroococcum and A. paspali are known to elaborate gibberellins and cytokin-like substances and these species of nitrogen fixing bacteria happen to be typical colonizers of the rhizosphere of grasses.

In low land flooded rice cultivation, two major subenvironments have come to be recognized-submerged plant parts and the rhizosphere. Epiphytic bacteria and algae colonize the surface of aquatic weeds. The rhizosphere is a nonphotic environment where redox, conditions are determined by the balance of oxidizing and reducing capacities of rice roots and characterized by exuded carbon compounds from roots providing energy sources for microbial growth. Nitrogen fixing microorganisms in the waterlogged rice fields contribute about 40-50 kg N /ha.

This fixation is a cumulative effect of Rhodopseudomonas, blue-gree algae, both free living as well as symbiotic (photoautotrophs) and Azotobacter, Beijerinckia, Methylomonas, Clostridium, Desulfovibrio, Klebsiella, Enterobacter, Flavobacterium, Pseudomonas, Azospirillum and Rhizobium (Heterotrophs).

The bulk of symbiotic nitrogen fixation in rice rhizosphere of flooded soil comes from root and stem nodules of leguminous green manure species when they are ploughed into the soil.

It is also now known that nitrogen fixing bacteria occur in the rhizosphere, stalks and phyllosphere of sugarcane plants and such bacteria have also been reported inside the root cells.

The occurence of N₂ -fising microorganisns in the root region of plants

A-Sacning electron micrographs of Azsopirillum bacterial cells on that root surface	B- Sacning electron micrographs of Azsopirillum bacterial cells on that root surface

C- Sporangia and hyphae of Franka in the rhizosphere of Casuarina equisetifolia

The numbers of these bacteria vary among genotypes of sugarcane and the types of soils on which they are cultivated. The following nitrogen fixing bacteria have been isolated from the root region of sugarcane: Azotobacter vinelandii, Klebsiella pneumoniae, Bacillus polymyxa, Azospirillum brasilense, Derxia gummosa, Enterobacter cloacae and Erwinia herbicola besides many other unidentified isolates.

Digram of Environments and N₂ Fixing Components in a rice field Ecosystem N2 fixing bacteria.

Diagram of Environments and N2 Fixing Components in a Rice Field Ecosystem N2 Fixing Bacteria

Diagram of Environments and N2 Fixing Components in a Rice Field Ecosystem N2 Fixing Bacteria

1.	Asscoiated with the roots	2.	In the Soil
3.	Epiphytic on Rice	4.	Epiphytic on weeds. Blue-gree algae
5.	at soil-water interface	6.	free floating
7.	at air water interface	8.	epihytic on rice
9.	epiphytic on weeds	10	Azolla

In Brazil, virgin soils supporting the growth of sugarcane receive no applications of chemical nitrogenous fertilizers. Apparently, nitrogen fixing bacteria together with other helper microorganisms that may excrete growth factors or help in lowering oxygen tension, are perpetuated through the continuous practice or vegetative propagation involving planting of sugarcane setts in the organically rich Amazonian soil.

Populations of nitrogen fixing bacteria have been found in sugarcane rhizosphere even upto a depth of 120 cm. Conclusive evidences concerning the ability of sugarcane bacteria to fix molecular nitrogen have come forth from field measurements involving acetylene reduction as well as15N tracer.

Home | Site map | Submit Article | Directory | Search