| Soil salinization is a major issue limiting crop production and threatening food security around the world,especially in arid and semi-arid areas.More than 9.3 million hectares of farmlands in China have been affected by soil salinization,which accounts for 7%of its total farmlands.The saline-alkaline soils are mainly distributed in North,Northwest and Northeast China,which are all important grain-producing areas.Among them,68.65%of the farmlands in Hetao Irrigation District,the largest irrigation District in Asia,is affected by soil salinization to varying degrees.Soil salinization has become an important limiting factor affecting the growth of crops in this region,which seriously restricts its agricultural sustainability.In addition,as it is geographically close to the Yellow River,Hetao Irrigation District is facing a severe problem of soil secondary salinization because of its shallow groundwater table due to the long-term use of flooding irrigation.Although the application of nitrogen fertilizer can alleviate the inhibition of salt stress on crop growth to a certain extent,the phenomenon of excessive fertilization is widely reported in this area.The loss of excess nitrogen fertilizer from the farmlands will not only result in economic losses,but also lead to nitrate pollution in groundwater,water eutrophication in surface water and greenhouse gas emissions,which seriously affect the ecological environment and human health.Therefore,considering the background conditions that Hetao Irrigation District will still be dominated by flooding irrigation in the short term and its low nitrogen use efficiency need to be improved urgently,this dissertation chose sunflower as the research object,carried out pot and field experiments from 2014 to 2016,and aimed to reveal the effects of different salt stress,nitrogen application and their interaction on sunflower shoot and root development at different growth stages.On this basis,a coupling model called HYDROFOST was developed to simulate soil water movement,salt transport and crop growth in saline fields,which could also consider the interactive effect of salt and nitrogen stress in its modelling process.The main research contents and conclusions are as follows:(1)Based on numerous experimental observations,the canopy structure,photosynthetic characteristics,radiation use efficiency and nitrogen use efficiency of sunflower under the coupling effect of salinity and nitrogen stress were qualitatively evaluated;the growth strategy of sunflower in saline fields was reasonably explained according to the optimal partitioning theory by analyzing the accumulation and allocation of sunflower shoot and root biomass.Quantitative functions of leaf area index(LAI)increase and seed yield production in response to different salinity and nitrogen application levels were developed.(2)Based on the in-situ monitoring data of dynamic development of sunflower roots obtained by Minirhizotron Imaging System,the development of maximum root depth and the variation characteristics of fine root length density(FRLD)distribution at different growth stages under coupled salt-nitrogen stress were studied.A logistic growth function was used to describe the time-varying root depth growth,and the FRLD distribution was described by a modified Vrugt's function with three fitted parameters(a,p_z,z~*).A Dynamic Root Distribution Function in Saline Fields(DRDF-SF)was built by combining these two functions.The defect that the original HYDRUS model could not consider the dynamic change of root distribution was solved by integrating DRDF-SF into the HYDRUS model as a new module.The established new version of model was called as HYDRUS-DR.this modification,and the simulation accuracy of soil moisture and salt content at a higher salinity level was improved.Our study had proved that the simulation accuracy of soil moisture and salt content could be generally improved by HYDRUS-DR,especially at S0 and S2 levels.(3)The new model of HYDROFOST was developed by coupling the improved HYDRUS-DR as the soil module and the WOFOST model as the crop module.The response relationship between the critical crop parameters(SLATB100,CVO and CVL)in WOFOST and different levels of salinity and nitrogen applications was integrated into HYDROFOST.The validation of this coupling model had proved that it could have a good performance in simulating soil water movement,salt transport,crop root distribution,canopy development,dry matter accumulation and yield formation simultaneously.This study not only revealed the complicated response mechanism of sunflower-salt-nitrogen in saline fields and its changing law in different growth stages,but also helped managing water and fertilizer efficiently and formulating the strategies of sustainable agricultural development in saline areas. |
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