Wheat Response to Azospirillum Inoculation, Organic Amendments and Mineral Nitrogen Fertilizer with Application of 15N Stable Isotope Technique

Wheat is one of the most important strategic crops needed for food and feeding. Due to its importance, this work aimed to demonstrate the role of organic/mineral N sources in collaboration with bio-fertilizer inoculation in enhancement of wheat growth and yield under low fertile sand soil with special emphasis to economical and environmental impacts. Field experiment was conducted on sand clay loam soil using wheat (Triticum aestivum L., Sakha 93) exposed to different rates of mineral nitrogen fertilizer either solely or in combination with different rates of organic compost. Fertilization treatments were applied with or without Azospirillum brasilinese as a representative of plant growth promoting rhizobacteria (PGPR). Tested treatments were completely randomly distributed in statistical factorial block design. Application of nitrogen fertilizer at full dose (120 kg N fed^-1), induced higher grain yield and nutrient uptake (NPK) than those recorded with other low mineral-N levels. Both plants fully treated with organic compost or in combination with levels of mineral nitrogen achieved grain yield and nutrient acquisition nearly closed to those of moderate mineral-N level. Relatively, inoculation with Azospirillum brasilense resulted in higher grain yield by about 25% over an un-inoculated plants. Nitrogen uptake by grains and straw significantly increased by increasing nitrogen fertilizer rates and inoculation treatment. Phosphorus and potassium contents not responded to Azospirillum inoculation but enhanced with increasing nitrogen fertilizer rates and application of organic compost. Combined treatments had enhanced nitrogen portion derived by either grains or straw from mineral-N form. Inoculated plants compensated remarkable amounts of nitrogen via N₂-fixation. Low amounts of nitrogen released from compost were derived to grains of inoculated and uninoculated plants but it increases with increase of compost component in the combined treatments. Nitrogen use efficiency (NUE) by either grains or straw of the inoculated plants was tended to increase with decreasing N fertilizer rates either individually or in combination with compost. Application of medium to low mineral N in combination with organic compost lowered the portion of N losses and achieved positive N balance. This phenomenon was more vigorous with combined treatment of 75% mineral plus 25% organic compost under the uninoculated treatment. Negative N balances were observed with sole addition of mineral nitrogen and tended to decline with reduction in nitrogen fertilizer rates. These trends were lower in case of the uninoculated plants than inoculated one.