Study On Characteristics Of Absorption Of Fertilizer Nitrogen And Nodule Nitrogen Fixation In Soybean Dual Root Systems

Soybean is an important crop which could be used either as food or oil,the main contributor of the world oil production,the main source of high-quality protein for human,and also one of the largest imported agricultural products in China,so it plays an important role in our national economy.The application of nitrogen fertilizer can promote soybean growth and increase yield.Nodule is formed by the complex interaction of soybean and rhizobia,which can effectively fix nitrogen in the air for their own nitrogen nutrition.Although root nodules nitrogen-fixing can be occured in soybean,but soybean is still a crop need nitrogen.The application of nitrogen fertilizer too little can reduce soybean yield and too much will lead to nitrogen waste and environmental pollution,so the study of soybean nitrogen nutrition mechanism is crucial.In this study,the grafting method was used to generate soybean plants with dual root systems of two noduled soybeans and one noduled soybean and one non-noduled soybean.Three experimental treatments were conducted with ¹⁵NO₃^-and ¹⁵NH₄⁺ under sand culture conditions.Dry weight,nitrogen content,and ¹⁵N abundance of each parts of the soybean at R1 and R5stages were measured to explore the characteristics of absorption,distribution and redistribution of fertilizer nitrogen and nodule nitrogen fixation.The test results show that:In the dual root system of bilateral nodules,the decrease of nitrogen application in the later growth stage had a great influence on the dry weight of the shoot,while in the dual root system of unilateral nodules,the decrease of nitrogen application in the early growth stage had a great influence on the dry weight of the shoot.No matter in the dual root system of bilateral nodules or in the dual root system of unilateral nodules,local application of nitrogen does not affect the nitrogen content of each part.In the dual root system of unilateral nodules,local application of nitrogen would affect the nitrogen accumulation of the root in the early growth stage of soybean,but had a weaker effect on the nitrogen accumulation of the root in the later growth stage of soybean.No matter in the dual root system of bilateral nodules or in the dual root system of unilateral nodules,the nitrogen absorbed by the root system and fixed by the root nodules were mainly transported to the shoot for the shoot growth,and only a few of them were supplied the growth ofthe root system and the nodules.Fertilizer nitrogen absorbed by root system and nitrogen fixation by root nodules have the same nutritive effect on stem,leaves and petiole.No matter in the dual root system of bilateral nodules or in the dual root system of unilateral nodules,the fertilizer nitrogen was preferentially absorbed and used by soybeans when fertilizer nitrogen and root nodule nitrogen fixation exist at the same time.At the nitrogen concentration of50mg/L,soybean plants tend to absorb fertilizer nitrogen in the early growth stage and root nodule nitrogen fixation in the later growth stage.No matter in the dual root system of bilateral nodules or in the dual root system of unilateral nodules,the part of ¹⁵NO₃^-and ¹⁵NO₄⁺ absorbed by the roots is assimilated in the root and is used for root growth.NO₃^-and NO₄⁺ did not affect the absorption and distribution of nitrogen in plants under 50mg/L nitrogen addition.In the dual root system of bilateral nodules,the application of nitrogen fertilizers can inhibit the growth of root nodules on the nitrogen-feeding side and promote the growth of root nodules on the non-nitrogen-feeding side.Besides,the application of nitrogen fertilizers affects the dry weight of root nodules and the ability of root nodules to supply nitrogen to the shoot.The effect of NO₃^-treatment on the inhibition of root nodules growth is less than that of NO₄⁺ treatment.No matter in the dual root system of bilateral nodules or in the dual root system of unilateral nodules,no matter nitrogen feeding or non nitrogen feeding,not all the nitrogen required for nodules growth and development was derived from self-nitrogen fixation,but also from the root absorption,and with the increase of fertilizer nitrogen,more fertilizer nitrogen is transferred to the root nodules.The nitrogen absorbed from the root in the nodule is transferred from the shoot,but not directly supplied by the root,while the nitrogen fixed in the nodule is directly supplied to the root.No matter in the dual root system of bilateral nodules or in the dual root system of unilateral nodules,after the absorbed fertilizer nitrogen was transported to the shoots in a certain proportion,a portion of it was then redistributed to the roots and nodules.In observing the shoots,one side root and its nodules in the dual root systems as a nitrogen translocation system,we calculated the amount of nitrogen translocation from soybean shoots to roots and nodules during the R1–R5 stages based on the difference in the 15 N abundance.In the dual root system of bilateral nodules,adding nitrogen at 50 mg/L concentration,in experiment ? the nitrogen translocated from the shoots during the R1–R5 stages accounts for 28.52%-40.76% of the nitrogen accumulation in roots,14.14%-17.15% of the nitrogen accumulation in nodules,and21.49%-27.65% of the nitrogen accumulation in roots and nodules.In the dual root system of unilateral nodules,adding nitrogen at 50 mg/L concentration,in experiment ? the nitrogen translocated from the shoots during the R1–R5 stages accounts for 25.14%-25.51% of the nitrogen accumulation in roots and nodules,and in experiment ? accounts for 32.48%-37.76%.