Study Of N Transformation Processes And The Microbial Mechanism In The Calcareous/Acid Soil-Wheat System Under Different N-Forms Fit

Nitrogen（N）is one of the essential elements for maintaining life in the ecosystem and its efficient utilization is the basis for improving crop yield.Traditional N management strategies only focus on optimizing the fertilization measures to improve the soil quality,which might lead to the larger application of N fertilizer and limited effect on improving N use efficiency（NUE）.Actually,the interaction of plant and soil can also affect plant N uptake and soil N transformation while a quantitative understanding based on the plant N acquisition and soil gross N transformations in the plant-soil systems is largely lacking.Moreover,because plant species have a preferential uptake of inorganic N-forms（e.g.wheat is the NO₃^--preferring crops）,the N uptake ability of specific plant species can be easily influenced by different N transformation characteristics of soil(such as a higher gross nitrification rate(n_gross rate)in the high p H soil and a lower gross nitrification rate in the acid soil).However,we have no idea about how crops affect N transformation in soils with different N transformation characteristics.Thus,the study based on the calcareous soil(p H=8.17,high n_gross rate)-wheat system and acid soil(p H=4.83,low n_gross rate)-wheat system,to conduct a pot experiment to investigate the gross N transformation rates of rhizosphere and bulk soil,the root exudate,soil extracellular enzyme and the communities of soil microbes in four growth stages（tillering,jointing,flowering,and ripening）.Besides,a ¹⁵N labeling experiment was also conducted to investigate the NH₄⁺-N and NO₃^--N uptake rate of wheat and soil microorganisms.The main results are as follows:（1）The gross N transformation rates of rhizosphere soil can be regulated by different soil-wheat systems,varied with different growth stages.The average gross mineralization rate(m_gross rate)of rhizosphere soil(2.94 mg N kg^-1day^-1)in the acid soil-wheat system was significantly higher than that of the calcareous soil-wheat system(0.95 mg N kg^-1day^-1).Compared with the bulk soil in the acid soil-system,the n_gross rates of rhizosphere soil at the jointing and flowering stages are significantly higher.In the jointing stage,the n_gross rates of rhizosphere soil(3.46 mg kg^-1 day^-1)in the acid soil-wheat system can beyond the rate of calcareous soil-wheat system(3.16 mg kg^-1 day^-1).Moreover,the N immobilization rate of acid soil-wheat system is higher than that of calcareous soil-wheat system(2.33 mg N kg^-1day^-1>1.31 mg N kg^-1day^-1).（2）The NH₄⁺-N and NO₃^--N uptake rates of wheat can be changed to adapt the different soil-wheat systems,but the NO₃^--preference of wheat was not changed.In the acid soil-wheat system,NO₃^--N uptake rate(2.06μg g^-1 day^-1)of wheat is slightly higher than NH₄⁺-N uptake rate(1.93μg g^-1 day^-1).Whereas,in the calcareous soil-wheat system,NO₃^--N uptake rate is 1.32μg g^-1 day^-1,significantly higher than 0.42μg g^-1 day^-1.Compared with the N uptake rate of soil microorganisms in the acid soil-wheat system,the N uptake rate of wheat is higher,which shows a greater competition ability of N.（3）Further,the study investigated the microbial mechanism of the difference in N transformation and N uptake rates in the two soil-wheat systems.The contents of root-derived organic acids were different between calcareous and acid soil-wheat systems,and varied with different growth stages.The extracellular enzyme activities,composition and function of bacteria and fungi community in the rhizosphere soil were changed with varying degrees,compared to bulk soil in the two soil-wheat systems.Mantel test showed that the root-derived organic acids were significantly related to fungi communities in the acid soil-wheat system,and the n_gross rates,enzyme activities of BG and LAP were remarkably affected by fungi communities,which indicated that the variation of root exudation in the acid soil-wheat system might affect the soil N transformation by changing its fungi communities in the rhizosphere.Besides,In the calcareous soil-wheat system,the bacteria communities showed the high relationship with m_gross rates,microbial biomass N and BG enzyme activity.In conclusion,the study showed that the N transformation processes in the calcareous/acid soil-wheat system were affected by the N preference of plant species,and in return regulated the N uptake rate of plant,especially in the acid soil-wheat system.The changes of root exudation and fungi community might be the main reason to affect N transformation in rhizosphere in the acid soil-wheat system.