Effects of combined nitrogen on nodule function of leguminous crops with special reference to tropical soils

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.