Effect Of Molybdenum On Nitrogen Efficiency,Soil N Transformation,and Nutrients Uptake In Wheat-Soil System Under Different Nitrogen Forms

Nitrogen（N）is an essential element required in large quantities for the proper growth,development,and improved grain yield of wheat（Triticum aestivum L.）.Meanwhile,the precise determination of N in plant-soil systems requires the use of specialized techniques,particularly for above-and belowground uptake or residue of N in wheat-soil systems.Isotopic labeling of plants using ¹⁵N is a powerful tool to trace N in the soil-wheat system,including the different plant organs and soil processes.Molybdenum（Mo）is an essential micronutrient for higher plants and is involved in many physiological and biochemical processes.It is involved in N metabolism including nitrate reduction,N assimilation,and N acquisition.But how does Mo improves N efficiency in winter wheat under different N forms and rates is still not clear.Here,a pot experiment was carried out to comprehensively investigate the effects of Mo supply(0（-Mo）and 0.15 mg kg^-1 soil（+Mo）)with different N forms and rates using ¹⁵N isotope tracer on wheat biomass,grain yield,total N uptake,N uptake efficiency,¹⁵N recovery from fertilizer,soil N transformation,¹⁵N recovery and losses from the wheat-soil system.N fertilizer was applied at low（L）and high rates（H）(0.05 and 0.25 g N kg^-1 soil)in the form of ¹⁵NH₄Cl,Ca(¹⁵NO₃)₂ or ¹⁵NH₄¹⁵NO₃,and the ¹⁵N atom%excess was 5.10,5.21,and 5.16%,respectively.Another pot experiment was conducted to explore the role of Mo application on N bioavailability capacities in the wheat-soil system by determining the activity of nitrogenase and nitrate reductase enzymes and following the differences in the natural ¹⁵N abundance in pots fertilized with Mo relative to Mo deficient pots.Additionally,we investigated the effect of Mo on macro and micro-nutrient allocation,uptake,and their translocation factors.Mo-inefficient“97014”winter wheat cultivar was used in the planting of the experiments.The main findings of these experiments are summarized as follows:（1）The magnitude of Mo induced effects on total N uptake,fertilizer ¹⁵N uptake,and soil N uptake as a function of N forms decreased in the following order:NH₄⁺>NO₃^->NH₄NO₃,under low N rate,and NO₃^->NH₄NO₃>NH₄⁺under high N rate.Mo application with low rates of NH₄⁺,NO₃^-and NH₄NO₃increased N uptake efficiency in wheat by 38%,68%,and 71%,respectively.While in the case of Mo application at a high N rate,N uptake efficiency was increased by 46%and 45%under NO₃^-and NH₄NO₃treatments,respectively,but it was slightly reduced under NH₄⁺form（～11%decrease）.Mo supply improved ¹⁵N recovery from fertilizer in grain organ under low N rate regardless of N forms i.e.,NH₄⁺,NO₃^-,and NH₄NO₃,while at high N rate,Mo enhanced ¹⁵N recovery in grains organ only under NO₃^- and NH₄NO₃ forms.（2）Mo supply significantly increased soil microbial NO3^-consumption with NO3^-and NH₄NO₃ forms at both Nr,resulting in a higher nitrification rate than with NH₄⁺supply.Mo supply also significantly affected N mineralization and immobilization rates with NH₄NO₃ form,demonstrating that the NH₄NO₃ form has more complementary effects on the N cycle of the wheat-soil system.Mo application also significantly decreased ¹⁵N losses from the soil-wheat system by 44%and 55%with NO3^-and NH₄NO₃ forms,respectively,at low N supply rate,and by 24%and 26%at high N supply rate.（3）In the second experiment,it was revealed that Mo supply increased plant biomass,grain yield,N concentration,total N uptake,net photosynthesis rate,intercellular CO₂ concentration,microbial biomass of N,nitrate reductase,and nitrogenase enzymes in the soil-wheat system as compared with Mo deficiency.¹⁵N natural abundance differences revealed that Mo application significantly increased ¹⁵N bioavailability（could be fixed）into the soil by 92%,further,¹⁵N acquisition was significantly increased by 625%,283%,166.7%,284.6%,and 465%,in the root,stem,leaves,glumes,and grain of wheat respectively,as compared to-Mo supply,indicating that Mo supply increases the level of nitrogenase enzyme which converts atmospheric N₂ into bioavailable NH₃,while it may reduce the accumulation of NO₃^-in Mo deficient soil.（4）Mo application increased the allocations of macronutrients（nitrogen（N）,phosphorus（P）,and potassium（K））,and micronutrients（copper（Cu）,iron（Fe）,manganese（Mn）,zinc（Zn）,and molybdenum（Mo））among different wheat organs except for Fe.Nutrient allocation to plant tissues also varied between different organs.A high allocation of N,P,and Zn was observed in grain,K,and Cu higher allocations were found in the stem,while,flag leaf and glumes accumulated higher content of Mn and Mo,under Mo application as compared with Mo deficiency treatment.Mo application increased the uptake of N,P,K,Cu,Fe,Mn,Zn,and Mo in wheat organs compared to-Mo treatment.The relative increase of N,P,K,Cu,Fe,Mn,Zn and Mo uptake in the wheat grains was 58%,13%,30%,20%,31%,13%,53%and 621%,while in the flag leaf was 199%,198%,203%,714%,281%,179%,128% and 431% respectively.