Characteristics Of Water-salt Nitrogen Distribution And Response Of Nitrogen Fertilizer Utilization In Salinized Farmland With Different Levels Of Nitrogen Application

Sunflower production in the Hetao irrigation area is severely constrained by soil salinity.Studies have shown that the application of nitrogen fertilizer can alleviate the stress caused by salinity on crop growth,but the utilization efficiency of nitrogen fertilizer varies depending on the salt content of the soil.In this paper,we conducted two consecutive years of field and micro-area experiments with sunflower as the test material for medium to severe salinization farmland in Wuyuan County of the river-loop irrigation area.Under four N fertilizer application levels（0,150,225,300 kg/hm²）,the soil water-salt and nitrogen migration patterns were analyzed,the coupling response of soil water-salt and nitrogen with salt ions was explored,and the main controlling salt ions affecting soil water-salt and nitrogen distribution were identified.Meanwhile,field micro-area experiments were conducted to explore the uptake and utilization of fertilizer nitrogen by sunflower under the interaction of salt and nitrogen after harvest,residual and loss characteristics,and analyze the utilization efficiency of residual nitrogen fertilizer by sunflower growth in the following year,so as to provide a theoretical basis for determining the appropriate amount of nitrogen application to salinized farmland in the river-loop irrigation area,and the main conclusions are as follows:（1）N fertilizer application to moderately and heavily salinized farmland increased the total soil salinity by 2.15%-5.06%in the 0-60 cm soil layer of moderately salinized farmland and by 8.37%-9.17%in the 0-40 cm soil layer of heavily salinized farmland.Controlling the appropriate amount of nitrogen fertilizer application in moderately and heavily salinized farmland can reduce the risk of secondary salinization in soil to some extent.（2）Soil nitrate-nitrogen content of heavily salinized farmland decreased by about27.22%-33.67%compared with moderate during the reproductive period,and the effect of salinity gradually diminished with crop growth,and the effect of nitrogen fertilizer application on nitrate-nitrogen content of 0-60 cm soil layer of moderately salinized farmland was greater.The difference in soil ammonium nitrogen content was the greatest at the beginning of fertility,and the ammonium nitrogen content in the 0-20 cm and 20-100cm soil layers of heavily salinized farmland was significantly higher by about 13.59%、17.88%compared to moderate,and the effect of soil salinity also gradually weakened as fertility progressed.（3）N fertilizer application rates of 150 kg/hm²and 225 kg/hm²on moderately salinized farmland had a small effect on the change in soil salt accumulation,which increased by about 29.41%（P>0.05）relative to the no-nitrogen treatment,while the change in soil salt accumulation on heavily salinized farmland with N fertilizer application rate of150 kg/hm²increased by about 15.16%（P>0.05）compared to the no-nitrogen treatment.The main controlling salt ions affecting the post-harvest soil total salt,nitrate and ammonium nitrogen in moderately salinized farmland were SO₄^2-and Cl^-,while the main controlling salt ions in heavily salinized farmland were mainly Cl^-and Ca^2+.（4）The nitrogen uptake of sunflower plants in moderately and heavily salinized farmland was highest under 300 kg/hm²and 150 kg/hm²treatments,respectively,and was significantly different mainly due to the different effects of soil salinity content.The contribution of fertilizer N and soil N to nitrogen uptake of sunflower ranged from about12.25%-44.69%and 55.20%-87.74%,indicating that soil N was the main source of nitrogen uptake in sunflower,and the increase in N application significantly increased the contribution of fertilizer N to crop N uptake.About 47.42%、50.83%of the residual N fertilizer in moderately and heavily salinized farmland was concentrated in the 0-20 cm soil layer after the harvest of sunflower in the season of marker N fertilizer application,and the level of N application was significantly and positively correlated with the total amount of residual N fertilizer in the soil.（5）The overall fertilizer N fate showed that N fertilizer loss rate>N fertilizer residual rate>N fertilizer utilization rate.In the season of N fertilizer application,the highest N fertilizer utilization rate and the lowest residual rate were found in 225 kg/hm²of moderately saline farmland,and the N fertilizer utilization rate,residual rate and loss rate were 27.35%,32.14%and 40.61%,respectively,while the N fertilizer utilization rate gradually decreased and the residual rate gradually increased with the increase of N application rate in heavily saline farmland,and the N fertilizer utilization rate,residual rate and loss rate were 21.12%,37.19%and 41.69%,respectively,for 150 kg/hm²of N application.After two years of experiment,the final utilization rate of N fertilizer in medium and heavily salinized farmland ranged from 27.83%-31.56%and 19.58%-25.25%.In conclusion,in order to meet the crop growth and development,and considering the effects of N application on soil salt accumulation,N fertilizer utilization rate and residual rate,it is suggested that the N fertilizer application rate can be controlled at 225 kg/hm²for moderate salinization farmland and 150 kg/hm²for heavy salinization farmland,which can obtain higher N fertilizer utilization rate,and at the same time reduce the N residual in soil,reduce environmental pollution and fertilizer waste,and the above results can provide a theoretical basis for reasonable N fertilizer application in salinized farmland.