I dont think the soil nitrogen content that plays much role in formation nodulation. Because even nodulation happens due to inoculation of nitrogen fixers in legume seedlings grown in inert media like vermiculite containers . The suitable is strain is important for inoculation as well as the inoculation time. After formation of feeder roots in the legume plants the rhiobia inoculum to be applied so that the bacteria will get a shelter for nodulation.

Dear Amaral, Take a look at the attached article. Rhizobium inoculation can be highly effective especially with peat based inoculum. It may not be available however. When inoculant is used there is no value of supplying the Nitrogen by fertilized because it is more expensive and adds no addition benefit. IITA has varieties of soybeans which inoculate well with African native Rhizobia cowpea types. It would good to use these varieties it would be best to get a set of IITA lines and do a varietal trial. Soybean will respond well to phosphorus but this can be expensive. If you ability to process cow manures through a use of worms castings will be an excellent starter for soybean and give better results than other nutrients. You are right to understand that Nitrogen applications reduce biological nitrogen fixation. At rates of 50 kg/ha or greater the soluble nitrogen will hamper the symbiotic biological nitrogen fixation.

Dear Amaral Chibeba,

The soil nitrogen is about 2 (or ppm N). At 2 mM N, nodulation occurs (Improving salt stress responses of the symbiosis in alfalfa using salt-tolerant cultivar and rhizobial strain).

For greenhouse experiments, we water legumes with nutritive solution at 10 mM N-NO3-(KNO3). At that level, there is almost no nodule. (Root colonization of faba bean ( Vicia faba L.) and pea ( Pisum sativum L.) by Rhizobium leguminosarum bv. viciae in the presence of nitrate-nitrogen)

Few years ago, we tested 297 soybean lines from IIAT (Nampula, Mozambique) fertilized with 10 mM N-NO3-or inoculated B. japonicum 532C. If you read French, go to

https://corpus.ulaval.ca/jspui/handle/20.500.11794/24343or http://hdl.handle.net/20.500.11794/24343.

Best regards,

Chantal Beauchamp

there is no direct relationship, but f the CN ratio should not be wide, otherwise there will be microbial immobolization of N and deficiency of native nitrogen which inturn affect the N availability to the plant

Thanks Chantal J Beauchamp for your input. I never thought I could read French until I read your thesis. Congratulations, by the way! Chibeba.

Dear Amaral Chibeba,

As both NO3– and ammonium can be utilized by plants, it seems likely that both N forms may therefore be effective in inhibiting nodule formation in the field. In the few experiments where NO3– and ammonium sources have been compared for the same species, both N forms were shown to be equally effective at inhibiting nodulation. For example an Acacia species Concentrations >5 mM of both NO3– and NH4+ inhibited nodulation in growing in aeroponic culture .on the other hand under optimum conditions for plant growth, soil nitrate is a major external factor controlling legume symbioses . High nitrate levels suppress the symbiosis, as shown by decreased nodule mass as well as decreased nitrogenase activity per unit of nodule mass. The mechanism of nitrate inhibition is not fully understood , but two hypotheses that have received the most support are (i) that decreased carbohydrate availability resulting from nitrate reduction and assimilation limits nodule growth and nitrogenase activity and (ii) that products of nitrate reduction, particularly nitrite, are responsible. Nitrite, the first intermediate in the nitrate assimilation pathway, is a presumptive candidate for the molecule responsible for nitrate inhibition of nitrogen fixation, since it binds to and inhibits nitrogenase and leghemoglobin.

Hello dear Amaral Chibeba

I brought this written from the attached file and I'm not sure of its correctness. There is complete information for it in the file:

"Nitrate and nitrite fertilizers inhibit legume nodulation, even at the low concentration of 6.5 ppm. Ammonia and urea fertilizers may not inhibit nodulation directly (Gibson and Nutman, 1960), but under field conditions nitrification may quickly convert ammonia and urea to nitrate and nitrite. There is another and extremely practical reason for limiting the use of nitrogenous fertilizers when establishing a pasture with legumes in the mixture: Readily available nitrogen in the soil encourages a strong growth of grasses and forbs, which compete with the seedling legumes."

When ammonium is found in soluble forms it will be converted by nitrosomas and nitrobacter bacteria which are normally found in agricultural soils to soluble nitrate. While ammonium can be directly absorbed and utilized by plants many plants perform preferentially utilizing nitrate. When nitrogen is available for plants the susceptible factors which trigger flavonoid nod factors are not activated. Without the signal of nitrogen need the stress compounds which message the motility of flagellated Rhizobium bacteria are not engaged. Besides adding the inoculant bacteria the addition of nod factor might stimulate highly colonization by rhizobium bacteria and allow greater utility from the symbiosis increasing legume crop performance and their benefits.

Dear all,

I have just come across to this article: "Thies JE, Singleton PW, Bohlool BB (1991) Modeling symbiotic performance of introduced rhizobia in the field by use of indices of indigenous population size and nitrogen status of the soil. Appl Environ Microbiol 57(1): 29–37". I found it very intersting. Thank you.

Dear all,

Another interesting article I come across to on this issue is: "Streeter J, Wong PP (1988) Inhibition of legume nodule formation and N2 fixation by nitrate. Critical Reviews in Plant Sciences 7(1): 1-23 DOI: 10.1080/07352688809382257". Thank you. Chibeba.