Optimization Of Irrigation And Nitrogen Fertilization Schedules Based On Field Experiment And Dssat Model

Irrigation and nitrogen（N）fertilization are important assurances for high and stable grain yield.The extensive irrigation methods and schedule have led to significant waste of water resources in many parts of China,whose irrigation water use efficiency is therefore much lower than many developed countries.Unreasonably extensive application of N fertilizers not only reduce the farmers’economic income,but also increase the risk of greenhouse gas emissions and nitrate leaching.Therefore,reasonable irrigation and fertilization system is an important way to realize the high yield and efficiency and sustainable development of food production.An irrigation and N fertilization experiment was conducted at water-saving irrigation station of Northwest A&F University in 2011-2014 with three irrigation levels of I0（rainfed）,I1（irrigating at jointing stage）and I2（irrigating at both over-winter and jointing stages）and four N levels of N0（without N fertilizer input）,N1(105 kg N hm^-2),N2(210 kg N hm^-2)and N3(315 kg N hm^-2)to explore the effect of water and N management practices on winter wheat growth and yield and soil water and N dynamics.The field data were used to calibrate and validate DSSAT-CERES-Wheat model,which was used then to explore water-saving irrigation system and N-saving fertilization mode matching with the local soil and weather conditions.The response of wheat yield to three irrigation systems,i.e.growth-stage（GS）,available soil water（ASW）and crop water requirement（ET）based irrigation and N fertilization technologies,i.e.N rates,topdressing period and ratio of basal to topdressed N in order to provide scientific basis and technical support for high yield and efficiency and sustainable development of wheat production in Guanzhong Plain.The main results were as follows:（1）irrigation significantly prolonged the thermal time required for the entire plant height and dry matter accumulation process and the growth rate of LAI.The plant height was 4 cm higher,LAI 0.6 larger and dry matter 2981 kg hm^-2 more with irrigation at wintering and jointing stages than rainfed treatment.N fertilization reduced the thermal time entering the rapid growth stage of height,LAI and dry matter accumulation and improved the growth rate of LAI and dry matter accumulation.Application of 315 kg N hm^-2 increased height 8 cm,LAI 3.11 and dry matter 6077 kg hm^-2 than N0,respectively.In general,N fertilization had larger effect on wither wheat growth than irrigation,while LAI and dry matter accumulation were affected more than plant height by irrigation and N fertilization.Irrigation improved winter wheat yield by increasing grain number and weight and N fertilization by grain number.The yield increase by water and N management was affected by precipitation distribution and initial soil N accumulation during the winter wheat growing season.Application of 315 kg N hm^-2 did not significantly improve wheat growth and yield compared with 210 kg N hm^-2 in three growing seasons and irrigation at wintering and jointing stages had larger yield-increasing effect in drier year in 2012-2013.（2）Soil water fluctuated with changes of precipitation,irrigation and field evapotranspiration（ET）.The accumulated ET increased slowly in early stage but fast in mid and late stages.irrigation significantly increased gross ET.For example,gross ET of irrigation at wintering and jointing stages was 93 mm more than rainfed treatment.The total water consumption was affected by initial soil water storage,irrigation and precipitation.In2011-2012,the sufficient initial soil water storage provided 42%⁵6%of total water consumption,while irrigation at wintering and jointing stages reduced 28 mm of soil water consumption.Irrigation decreased water use efficiency（WUE）while N fertilization increased WUE.In the dry 2012-2013 growing season,Application of 315 kg N hm^-2 obtained the highest WUE of 2.17 kg m^-3.The irrigation water use efficiency（IWUE）increased with irrigation and N fertilization amount.（3）The soil nitrate content and accumulation increased with N fertilization amount and experimental years.Application of 315 kg N hm^-2 led to a large amount of nitrate accumulation in 0-180 cm soil profile,which was as much as 825 kg hm^-2 averaged in2012-2014;the nitrate accumulation in 100¹80 cm under 210 kg N hm^-2 was much lower than that under 315 kg N hm^-2,with as much nitrate accumulation in 0-180 cm soil profile as216 kg hm^-2.Irrigation and precipitation accelerated the downward movement of soil nitrate,especially under 315 kg N hm^-2.At harvest in 2014,the maximum nitrate content of N3treatment under rainfed,irrigation once and twice occurred at 60⁸0,80¹00 and 100¹20cm,respectively.N Partial fertilizer productivity（NPP）,agronomic efficiency（NAE）and marginal N agronomic efficiency（MNAE）were all negatively correlated with N fertilization amount.As experimental years increased,the N0 yield reduced gradually,resulting in lager NAE,which improved from 4.2 kg kg^-1 in 2011-2012 to 25.4 kg kg^-1 in 2013-2014 under the application of 105 kg N hm^-2.（4）The DSSAT-CERES-Wheat model was calibrated by experimental data in 2013-2014and then validated by independent data in 2011-2013.The simulation results showed that the normalized root mean squared error（nRMSE）of anthesis date,maturity date,yield,grain number and weight,and maximum LAI were all within 20%in three years.In validation year,the aboveground biomass was underestimated 21%with yield underestimated only 5%.The nRMSE of soil water content was within 20%,which was 10%for 0-100 cm soil water storage.Soil nitrate content was simulated with lager nRMSE of 56%¹37%,which was25%⁴3%for 0-100 cm soil nitrate accumulation.Soil nitrate content was affected by many factors such as soil microbes,temperature,water and crop,resulting in a great uncertainty in its simulation.（5）The GS-based irrigation simulation showed that wintering and jointing stages are key to winter wheat yield formation,at one of which irrigation may obtain high yield or IWUE.The ASW-based irrigation simulation showed that the high yield and high efficiency resulted from irrigation lower threshold of 20%³0%and upper threshold of 75%¹00%.The ET-based irrigation simulation showed that irrigating 60 or 80 mm when the net cumulative ET reached 75 or 100 mm were recommendable irrigation schedule for high yield and efficiency,water-saving and labor-saving.Farmers could choose reasonable irrigation system according to irrigation equipment and conditions.（6）The optimum N application rate for high yield and profit was affected by water supply and initial soil nitrate content during the growing season.When the 0-60 cm initial soil nitrate accumulation was over 150 kg hm^-2,high yield may be achieved by applying zero or a small quantity of N fertilizer.When the soil initial N accumulation was very low,application of 120 kg N hm^-2 could ensure relatively high yield and profit in 75%years under rainfed condition,while 150 kg N hm^-2 was needed under irrigation.The yield under nitrogen application as base fertilizer was not significantly different with that under ratio of basal to topdressed N as 7:3,whenever the N was topdressed at green-up,jointing or anthesis stages.while the yield under ratio of basal to topdressed N as 3:7 reduced as the topdressing date was postponed and the appropriate topdressing period was at or before jointing stage.Considering the easier handling of N application as base fertilizer before sowing than split application,appropriate N application as base fertilizer was recommended.