Study On Optimal Regulation Of Water And Nitrogen In Maize Production In Arid Area Of Northwest China Under Supplementary Irrigation Regulating Soil Moisture Based On RZWQM2 Model

In arid areas of northwest China,water and nitrogen management is the key to achieve high yield in maize production.The shortage of water resources and low utilization rate of water and nitrogen are important problems faced by maize production in the study area.In addition,climate change also brings challenges to local maize water and nitrogen management.The effects of water and nitrogen management on maize production is affected by climate change.In order to achieve the sustainable production of maize in arid areas of northwest China,it is very necessary to study the regulation of water and nitrogen considering the impact of climate change.The purpose of this study is to explore the optimal water and nitrogen regulation strategy of maize in arid areas of northwest China under the background of climate change based on field experiment and simulations by Root Zone Water Quality Model 2(RZWQM2).Supplementary irrigation based on soil moisture(SIM)was used to regulate soil moisture in maize field in the study.In SIM,irrigation is adopted when soil water content in the target soil layer(TSL)decreases to the allowable lowest soil water content(ALW)in TSL.Based on the two years of field experiment,the effects of TSL and ALW on soil water and nitrogen condition,root characteristics,maize growth and physiology were studied.The parameters of RZWQM2 were calibrated based on field experimental data,and the performances of the model in simulating water and nitrogen dynamic,and crop growth in maize farmland was evaluated.The simulation scenarios of water and nitrogen regulation were designed.The control factors(and factor levels)in the simulation scenarios include TSL(0-40 cm,0-60 cm,0-100 cm soil layer),ALW(5%-45%TAW,with 2%TAW as interval),nitrogen application rate(100,150,200,250 kg/ha),and the ratio of nitrogen base application rate to topdressing rate(0:100,20:80,40:60,60:40,80:20,100:0).TAW is the total available water content(difference between field water capacity and permanent wilting point)in TSL.The calibrated RZWQM2 was driven by the observed climate data from 1981 to 2020 and the climate data from 2021 to 2060(under RCP4.5 scenario)output by the regional climate model HadGEM3-RA.The simulation study analyzed quantitatively the water use characteristics of root zone under different SIM options;investigated the effects of water and nitrogen regulation on maize growth and the utilization of water and nitrogen;analyzed the responses of maize phenology,grain yield,water consumption(ET)and water use efficiency(WUE)to climate change in 1981-2020 and 2021-2060 under SIMs with different ALW levels and TSL of 0-60 cm soil layer at different nitrogen application rates.The main conclusions are as follows:(1)This study verified and analyzed the high-yield and water-saving of the SIM only replenishing soil water in part of root zone.In the maize production in arid areas of northwest China,only replenishing soil water in part of root zone(0-40 cm or 0-60 cm soil layer)can achieve the high grain yield and higher WUE,and has less risk of drainage water and nitrogen leaching than the conventional irrigation strategy.The same TSL or ALW level can be used at different growing stages of maize,which is convenient for irrigation management and does not prevent SIM from achieving the goal of high yield and water saving in maize production.Under the SIM only replenishing soil water in part of root zone,the soil water content in the soil layer below TSL usually shows a downward trend with time during growing season,but the decreasing soil water in the soil layer below TSL has no obvious negative impact on the root growth and development in the soil layer.By adjusting the ALW level,the soil water condition in the root zone can be effectively regulated,and then the soil nitrogen condition and the physiological characteristics of maize growth can be regulated.Appropriately deceasing the ALW level can create a soil water condition similar to the alternation of dry and wet,which is conducive to the improvement of WUE.(2)This study proposed the crop genetic parameters of maize variety(Xianyu 335)in RZWQM2,and confirmed the applicability of the model in simulating soil water and nitrogen dynamic,and crop growth in maize field in arid areas of northwest China.RZWQM2 can well simulate the soil water and nitrate content and their dynamic changes in 20-40 cm,40-60 cm,60-80 cm,80-100 cm and 0-100 cm soil layers under different water and nitrogen treatments.The standard mean square error(NRMSE)levels in the simulation of soil water content and nitrogen content in each soil layer were in the range of 6.21%-26.19%and 15.11%-29.65%,respectively.The simulation accuracy of the model for soil evaporation was poor(NRMSE=38.78%-61.53%).The simulation error of the model for maize phenology under different water and nitrogen treatments was less than 6 days.The simulation of leaf area index(LAI)was at a high accuracy(NRMSE=9.83%-23.01%).The simulation accuracy of LAI in leaf growth process was generally better than that in leaf senescence process.The aboveground biomass,grain yield and plant nitrogen uptake simulated by the model were slightly underestimated with a high accuracy(NRMSE<12.5%).In conclusion,the calibrated RZWQM2 can be used for the optimization of water and nitrogen management in maize production in arid areas of northwest China.(3)This study quantified the regulation effects of TSL and ALW in SIM on the water use characteristics in root zone.Under SIM with different TSL levels,the regulation effects of ALW on root water uptake were also different.Under SIM with TSL of 0-40 cm or 0-60 cm soil layer,adopting higher ALW level can effectively improve root water uptake in TSL,but had little effect on root water uptake in the soil layer below TSL.The ratio(R/TR)of root water uptake in each soil layer to total root water uptake in the root zone was little affected by the ALW.The difference in R/TR in each soil layer among SIMs with different ALW levels was less than 4%.Taking 0-100 cm soil layer as TSL,the higher ALW was conducive to improve the root water uptake and R/TR in 0-60 cm soil layer,while it had an inhibitory effect on root water uptake in 60-100 cm soil layer.Under SIM,the ALW had an obvious effect on soil evaporation and plant transpiration,but had little effects on the proportion of soil evaporation in ET(E/ET).The difference of E/ET among SIMs with different ALW levels was less than 5%.Under SIM,adopting smaller TSL will improve the R/TR in the upper root zone but slightly increase E/ET(<2%).(4)This study revealed the characteristics of water and nitrogen utilization in high-yield maize plants,and clarified the regulation role of control factors(TSL,ALW,nitrogen application rate,the ratio of nitrogen base application rate to topdressing rate)in water and nitrogen management to maize growth and the characteristics of water and nitrogen use.Plants with high water use efficiency usually have high nitrogen use efficiency.The reasonable water and nitrogen regulation strategy should primarily control transpiration,reduce transpiration as much as possible on the premise of satisfying the requirements of high yield for transpiration and nitrogen uptake.Under SIM,the key to achieve water-saving and high grain yield in maize production by regulating water and nitrogen is to determine the suitable nitrogen application rate and the ALW level,but it is not necessary to split nitrogen applications.The suitable increase of nitrogen fertilizer is helpful to improve maize yield and WUE,but when the amount of nitrogen application exceeds a certain limit(>200 kg/ha),the increase of nitrogen fertilizer has little effect on grain yield and WUE.A higher ALW in SIM is conducive to high yield,but it may cause the decrease in WUE.Adopting smaller TSL in SIM is conducive to reducing the nitrogen application rate,and reducing the risk of drainage water and nitrogen leaching,but it may increase soil evaporation and limit WUE.(5)This study revealed the responses of maize phenology,grain yield,ET and WUE to climate change in 1981-2020 and 2021-2060 under SIMs with different ALW levels and TSL of 0-60 cm soil layer at different nitrogen application rates.In the past and the next 40 years,climate change has an adverse impact on maize production.Maize yield and WUE show a downward trend,while ET shows an upward trend.The growth period of maize showed a shortening trend.The change trends of grain yield,ET and WUE of maize in the past 40 years are closely related to the significant increase of air temperature and wind speed and the significant decrease of relative humidity,while these trends in the next 40 years is only closely related to the significant increase of air temperature.By optimizing the ALW and nitrogen application rate,the trend of grain yield,WUE and ET of maize in the past and the next 40 years can not be reversed.However,the higher ALW level or less nitrogen application can help to slow down the change trend of maize yield,ET and WUE and the variation over time under the background of climate change.The effects of regulating ALW on the change trends of maize yield,ET and WUE with time is obvious,but the effects of regulating the nitrogen application rate is very small.Compared with SIM for the whole root zone,the SIM only replenishing soil water in part of root zone(0-60 cm soil layer)did not aggravate the adverse impact of climate change on maize production.(6)Based on the simulation results under historical and future climate scenarios,this study proposed the optimal SIM and nitrogen application strategy in maize production in arid area of northwest China under the background of climate change.The optimal water and nitrogen regulation strategy for maximizing WUE in the past 40 years is the same as that in the next 40 years.In the optimal water and nitrogen regulation strategy,the nitrogen application rate is 200 kg/ha and is encouraged to be applied once as a base fertilizer,TSL is 0-60 cm soil layer and ALW is 15%TAW.Compared with the conventional water and nitrogen management strategy(CK),the optimal water and nitrogen regulation improved WUE by about 8.8%(in the past 40 years)and 7.4%(in the next 40 years)and obtains similar yield(accounting for more than 90%of CK).In addition,in the past and the next 40 years,the stability over time of grain yield and WUE under the optimal water and nitrogen regulation strategy is similar to that under CK.Therefore,this study recommends the optimal water and nitrogen regulation strategy for maize production in arid areas of northwest China under the background of climate change.