Abstract:
The effect of combined nitrogen on nitrogen fixation has been discussed by several investigators
from their findings it could be concluded that inorganic nitrogen may have either stimulatory, neutral or negative effect on nitrogen fixation depending on the type of legume
soil condition. It was reported in some instances that low levels of combined nitrogen especially when applied as a "Starter" fertilizer at germination, stimulates plant growth
enhances nodule mass
nitrogen fixation. On the other hand when most of the legumes are grown in soil high in available nitrogen, the nitrogen fixation rate is severely affected. This phenomenon may become more critical when legumes are grown as cover crops where non legume counterpart is fertilized with nitrogenous fertilizer. The degree of inhibition appears to vary with several factors viz. concentration of combined nitrogen, host species, cultivar, strain of of root nodule bacteria, rate of inoculation, light intensity
temperature. However, the most critical factor is the concentration of combined nitrogen
it has been shown that very low concentrations such as 240 ppm N reduced the nitrogen fixation of bradyrhizobia - legume symbioses whereas 80-100 Kg of N ha-1 inhibited the nodulation
nitrogen fixation of Vigna spp. totally. It was further pointed out that extra caution should be taken when recommending nitrogenous fertilizer to mixed cropping systems in tropical soils as nodulation
nitrogen fixation of legume counter parts such as Pueraria, Desmodium
Mimosa were reduced to neglegible levels when 140 ppm N was added to Sri Lankan rubber soils.
