Effect Of Irrigation And Nitrogen Regulation On Buckwheat Growth And Soil Nitrogen Transformation In Yellow River Irrigation Area Of Ningxia

Multiple cropping of buckwheat after wheat plays critical roles in solving the dilemma that insufficient double cropping,promoting the utilization of light,heat and water resources and improving grain production.N application and irrigation are important cultivation measures for buckwheat production.Facing the challenge of climatic warm-drying trend,reasonable irrigation and nitrogen management is conducive to resisting drought,increasing production,and reducing the environmental risks,thus promoting the green and efficient production of agriculture in irrigation areas.The split plot experiment of buckwheat designed with irrigation amount as the main factor and N rate as the auxiliary factor was carried out in Pingjipu Farm,Yinchuan,Ningxia in the summer of 2020 and 2021.Three irrigation amounts were set as 65%（I₁）,75%（I₂）and 85%（I₃）of the field capacity,and four pure N rates were set as 0（N₀）,120（N₁）,180（N₂）and 240 kg/ha（N₃）.The responses of buckwheat agronomic traits,yield,water and N utilization and N transformation to irrigation amount and N rate were studied.Outcomes are as follows:（1）Appropriate amount of N and irrigation promoted the growth of buckwheat aboveground.With the increase of N rate,plant branching ncreased first and then decreased.In the normal year（2020）,plant height and stem diameter increased first and then decreased and flower cluster number of plant increased.With the increase of irrigation amount,plant branching and stem diameter and flower cluster number of plant with N application increased.Compared with N₀,N₁ inhibited the growth of the roots with low diameter levels.After increasing N rate,the root system surface area,volume and fractal dimension are higher than those of the control.Compared with I₁,I₂inhibited root elongation but increased diameter,whereas I₃ promoted the growth of main roots.Compared with I₂,I₃ degraded surface area and mean diameter of root system in the normal year,which was opposed with the dry year.（2）Water and N regulation affected the accumulation and distribution of biomass and N,and changed the grain number,and finally regulated the yield.With the increase of N rate,dry matter of stems and leaves,roots,generative organs and plants increased first and then decreased;grain number and yield increased first and then decreased or showed increasing trends;irrigation water use efficiency（IWUE）increased first and then decreased or showed increased trends,and the N agronomic efficiency and N partial productivity decreased.Appropriate N rate enhanced N distribution in stems and leaves,and raised N physiological efficiency.In the normal year,N content and accumulation in generative organs increased first and then decreased or showed increasing trends,and N accumulation in other parts raised with the increase of N rate.With irrigation amount raised,dry matter of each part almost increased first and decreased and the root-shoot ratio decreased in N₂ in the normal year;N content of stems and leaves increased first and decreased or raised;grain numbers and yield increased in most of N rates;IWUE decreased and all N use efficiency parameters in in N₂ in the normal year decreased first and increased whereas N physiological efficiency and N agronomic efficiency in the dry year decreased.Increasing irrigation amount promoted dry matter accumulation but was adverse to N.In the normal year,increasing irrigation amount promoted more assimilates allotted to the underground under N₁,which was opposite when N was sufficient.In the normal year,the yield and net profit under I₃N₃reached the peaks which had no remarkable difference with I₁N₂.In the dry year,the yield and net profit under I₂N₃ reached the peaks which had no remarkable difference with I₂N₂.（3）Water and N regulation affects N leaching and soil N transformation.Appropriate N rates increased the activities of Nitrogenase,Protease,Urease（Ure）,Ammonia monooxygenase（AMO）,Nitrate reductase（Nar）and Nitrite reductase（Nir）in the bulk soil and the activities of Ure,AMO,Nar and Nir in the rhizosphere soil.Appropriate N rates increased soil N content in the dry year and nitrate N content in the normal year.Increasing irrigation amount was beneficial to increase soil enzyme activities in bulk soils.With the increase of irrigation amount,the content of N except ammonium N and soil enzyme activities in rhizosphere soil decreased or decreased first and then increased in the normal year,which were opposed in the dry year.The rhizosphere effect increased the nitrate N content in rhizosphere soil generally,but decreased under low irrigation amount with high N rate.When irrigation exceeded 65%of field capacity in the dry years,the rhizosphere effect enhanced the contents of ammonium N,organic N and total N.Six soil enzymes’activities were increased when irrigation amount was low.Excessive irrigation and N application increased N leaching,and N leaching loss reached the maximum at I₂N₃.Irrigation to 75%of field capacity in the normal year and 85%in the dry year may cause non-point source pollution.In conclusion,applying pure nitrogen 180 kg/ha with irrigating to 65%field capacity in the normal year is recommended,when yield may be 3312±165 kg/ha,whereas applying pure nitrogen 180 kg/ha with irrigating to 75%field capacity in the dry year is recommended,when yield may be 2587±149 kg/ha.