Index >> Nitrogen Fixation Free Living and Associative Symbiotic Bacteria >> Nitrogen Fixation in the Root Zone of Rice

	Nitrogen Fixation in the Root Zone of Rice Using acetylene reduction method, a group of scientists at the International Rice Research Institute, Manila have demonstrated significant amount' of N2 fixation in the root soil system of rice plants. The studies done at IRRl further point out that submerged soils have a greater capacity to fix atmospheric nitrogen than: non-submerged soil.
The amounts of nitrogen fixed in 1 ha estimated by using theoretical conversion factor (C2H2 : N2 = 3 : 1) during the dry season were 79.8 kg/ha in planted, submerged fields; 42.5 kg/ha in unplanted, submerged fields; 5.4 kg/ha in the planted, upland fields and 2.7 kg/ha in the unplanted upland field A. Pellicle of Azospirillum formed two mm below the surface of semi solid sodium malate agar medium indicating the microacerophilic nature of the bacterium B. Azospirillium brasilense as seen under a phase contrast microscope showing the spirilar morphology of the bacterium C. Chrysopogon fulvus left to right uninoculated control carrier alone carrier inoculatedwith A. brasilense   D. Cenchrus ciliaris left to Right uninoculated control carrier alone carrier inoculatedwith A. brasilense E. Hordeum vulgare barley left to right uninoculated control incoculate with A.
The effect of Azotobacter chroococcum inoculation in field on the of three crops, 1978-79 (experimental results of S.T. Shende) Crop Location of field trials in India without Azotobacter With Azotobacter C.D. at 5% % increase due to Azotobacter Sorghum (kg/ha) Pali Dharwar 1280 2360 1400 3260 122 1056 9.3 38.1 Maize (kg/ha) I.A.R.I. Dharwar 780 320 1340 4370 480 990 71.1 36.5 Cotton (kg/ha) Surat Indore Khandwa 1254 366 559 1339 401 708 241 104 165 6.7 9.5 20.6
Acetylene reducing activities of intact soil plant system, plant removed and remaining soil measured by anaerobic C2H2C2H4 assay (from IRRI Annual Report, 1972) Incubation time (h) Cumulative ethylene formed (n mol/tube) Intact soil plant system Plant removed Remaining soil 3 8 29 11 5 29 42 13 10 406 70 19 24 2360 375 27
	In the field, most nitrogen is fixed in soils during the reproductive and ripening phases of the growth of rice plants. The absolute figures reported by IRRI scientists may not be as important as the indica­tion that large quantities of atmospheric nitrogen are being continuously fixed in the rhizosphere of rice plants.

The aerenchyma present in the rice plant transfers air from the atmos­phere to the rhizosphere. The root system of rice in a submerged field is located in the anaerobic soil zone.

The air transferred by the rice plant to the root zone may contain enough nitrogen for the N₂ fixing activity of bacteria associated in the rhizosphere which belong to the genera, Beijerinckia, Azotomonas, Pseudomonas, Flavobacterium, Azospirillum and Azotobacter.

Gas chromatographic analysis of the gases in IRRI rice soils under flooded conditions (sampled from several experimental plots which did not receive fertilizer N) showed that submerged soils planted to rice variety IR-20 contained more nitrogen gas than unplanted soil at all loca¬tions, at tillering (34 days after transplanting), during panicle initiation (48 days after transplanting) and heading (77 days after transplanting). Another important factor to be taken into consideration is the amount of carbohydrates available in the root zone for nitrogen fixation.

The results obtained at IRRI further point out that at the tillering stage, 96% of ¹⁴C fed to experimental plants was located in stem and leaves and 4% 66% of ¹⁴C was in the straw, 18% in panicles, and 15% in roots. The cumulative amount of ¹⁴C in the water culture solution was less than 1% of the total 14C wh9ich remained in plant tissues. These results point out that organic carbon provided by rice roots is an important factor contributing to N₂ fixation but the amount of carbon exuded in the root medium is comparatively smaller than that held by the plant tissue.

Availability of cellulosic substrates in flooded soil is yet another factor to be reckoned with in bacteria-mediated nitrogen fixation in rice fields. Application of rice straw enhances nitrogen fixation whereas addition of ammonium sulphate (combined nitrogen) retard nitrogen fixation. Interestingly, the fluctuations in nitrogen fixation are related to the number of Azotobacter colonies present in rice soil under different treatments. The waterlogged soil planted to rice provides an ideal habitat for facultative anaerobic bacteria other than Azotobacter to function both in presence and absence of oxygen and fix nitrogen.