Soybean biofertilization in sandy soils of Egypt using N-15 tracer technique

The effect of inoculating soybean plants with Bradyrhizobium japonicum and Azospirillum brasilense either solely or in mixture, and of using different N fertilizer levels was studied in pot experiments. The nodulation of soybean grown in a sandy soil was enhanced by the inoculation while the highest nodule numbers and fresh mass, recorded at a N-fertilizer dose of 20 kg N/hm(2), decreased when the fertilizer quantity increased to 40 kg N/hm(2). By contrast, the dry mass of above-ground parts and the N uptake was increased with increasing N fertilizer level. A similar effect was observed for inoculation as compared with the uninoculated variant. Although the nodulating and non-nodulating soybeans has nearly the same dry mass, the nodulating isoline accumulated more N than the non-nodulating. The percentage of nitrogen derived from air (%, Ndfa) and estimated by isotope dilution (ID) or N difference method (DM) dropped with increasing N level from 10 to 40 kg N/hm(2). Dual inoculation resulted in a high per cent of fixed N-2 (42.5%) at 10 kg N/hm(2). Correlation between the ID and DM methods was found to be dependent on inoculation treatments. The amount of nitrogen utilized by nodulating soybean (FUE %) was enhanced as a function of inoculation with B. japonicum.