Biological Nitrogen Fixation of Summer Legumes and Their Residual Effects on Subsequent Rainfed Wheat Yield

Biological nitrogen fixation is the most important biochemical reaction for life on earth. Phosphorus and rhizobium inoculation increased N2-fixation by legumes. Legumes in rotation with cereals contribute to the total N pool in soil and improved cereals yield. In view of importance of grain legumes and the role they can play in maintaining soil productivity and succeeding cereal yield, rotational field experiments were conducted on mung bean (Vigna radiata) and mash bean (Vigna mungo) during summer of 2002 and 2003 followed by wheat (Triticum aestivum) in each year at Research Farm of University of Arid Agriculture, Rawalpindi, to assess N2-fixation by beans and their residual effect on subsequent wheat yield. Bean seeds were inoculated at sowing with effective brady rhizobia and grown with and without Phosphorus fertilizer. Sorghum (Var.YSS-98) was sown as non-legume crop with 100kg N ha. Xylem ureide method has been employed for estimation of N2-fixation. Nodulation, shoot dry matter, grain yield and N concentration of both beans were increased by phosphorus fertilization. Both beans showed excellent nodulations i.e., 4, which showed excellent potential for nitrogen fixation. Estimates of nitrogen derived from atmosphere (%Pfix) ranged from 49-71% during 2 year and up to 60% increase was observed from 1 year. Average N2-fixed ranged between 33-55 kg ha during both years and mash bean proved better N2-fixer. Water use efficiency (WUE) based on grain yield were 23-33% higher with phosphorus fertilization. Values of WUE for N2-fixation ranged between 0.22 and 1.00 kg ha mm and declined with declining %Pfix. Total NO3-N was between 56-67 kg ha for legumes and between 40-45 kg ha for non-legume sorghum. Additional residual soil N under legumes, relative to adjacent sorghum was in the range of 16-22 kg ha. Beans with phosphorus fertilization increased grain yield of succeeding wheat by 20% over sorghum. It was concluded that phosphorus fertilizer with inoculation enhanced N2-fixation and rotational results confirmed that legume-cereal sequence increased biomass and grain yield of subsequent wheat.