| Agriculture is the foundation of our national economic development. Field water and nitrogen use efficiency play vital roles for overcoming the challenges of water scarcity, water environment pollution, and food security of our country. According to statistics, in our country, irrigation water use coefficient is only 0.5, which is far lower than developed countries of 0.7-0.8. Furthermore, the average nitrogen use efficiency of major food crops is only 27.5%, which is also lower than the world average of about 50%. Therefore, it is urgent to improve water and fertilizer use efficiency, and promote agricultural production increase and water environmental protection. This demand is more prominent in arid and semi-arid areas. For one thing, these areas have limited water storage and relatively barren land, which need balance the relationship between irrigation and fertilization. For another, the soil salinization is usually a challenge problem that might limit crop production in these areas. Take Hetao Irrigation District (located in North China) as an example, due to the containing of salinity in soil parent material and high level of water table depth, it has faced soil salinization problem for a long time. Since the 1980s, flood irrigation was used at least twice per year for soil salt leaching. However, it not only wastes lots of water, but also has the potential risk for environmental pollution and secondary salinization because a considerable number of nutrient substance may also be leached into deep soil and even groundwater. There are many domestic and foreign scientists have conducted extensive fruitful studies on the coupling of water and fertilizer, however, their researches mainly focus on the area which is not salinization. When crops are planted in salt-affected soils, there are interactions of water, salt, and fertilizer, which have a very complex internal mechanism. But in the view of about 8 million hectares of arable land is suffering from different degrees of soil salinity threat, It is therefore of great significance to study the internal mechanism of water, salt and fertilizer and develop an appropriate method for water and fertilizer management in salt-affected soils.Our study was made possible with the support provided by the National "Twelfth Five-Year" Plan for Science & Technology Support Program (Technology integration of agricultural water saving. No.2011BAD25B07) and the State Natural Science Fund (Research on the Production Function of Water-fertilizer in Saline Soil Based on Crop Growth Simulation. NO.51379151). More exactly, we chose the main economic crop-sunflower of Hetao Irrigation District as the study object, carried out experiments including soil column leaching, soil nitrogen cultivation, field irrigation, micro-plots and lysimeter planting and so on, and used analysis of variance, analysis of variance, correlation analysis, regression analysis, partial least squares analysis, artificial neural network according to the experiment data and theory of soil physics, soil chemistry, phytophysiology, agronomy, meteorology and hydraulics to study the sunflower's response for soil water and salt content and nitrogen application rate, and reveal the mechanism of crop-water-salt-nitrogen interaction. The main research contents are as follows:(1) Give the quantitative description of the effect of irrigation amount, nitrogen application rate, and initial soil salt content on the growth indicators of sunflower such as emergence rate, leaf area index, plant height, dry matter, photosynthetic capacity, seed yield and so on. Then propose the appropriate combination of irrigation amount, nitrogen application rate, and initial soil salt content for different growth indicators.(2) Study the effect of soil salt content on the rate of soil nitrification and denitrification. Give the quantitative description of the transform from urea to ammonium and nitrate nitrogen.(3) Establish models for sunflower seed yield simulation under the interaction of soil water, salinity and nitrogen application rate.The results indicate:(1) Sunflower growth indicators were affected by soil salt, irrigation amount and nitrogen application rate. However, the response for these agricultural managements varied from indicators. When the water stress was moderate or high, irrigation amount was the main factors for sunflower growth, followed by soil salt and the nitrogen effect was the weakest among the 3 factors.(2) The thresholds for sunflower salt tolerance in seedling, bud, flowering, and mature stages were 6.93 dS m-1?14.46 dS m-1?14.46 dS m-1 and 17.26 dS m-1 respectively. Appropriate nitrogen application rate can alleviate the salt stress, but excess nitrogen might be harmful for sunflower. When considering the environmental protection and economic factors, the Appropriate nitrogen application rate for low (<9 dS m-1) and high salinity level (?9-18 dS m-1) were 135 kg ha-1 and 180 kg ha-1 respectively.(3) No matter acid and alkaline soils, the gross nitrification rate was affected by soil salinity and there is a threshold for soil salinity, when soil salinity level was larger or smaller than the threshold, the salinity effect on gross nitrification rate was opposite. More exactly, the threshold for acid and alkaline soils were 4.93 dS m-1 and 5.52 dS m-1, respectively; the coefficient of the first-order kinetics reaction for urea to ammonium nitrogen could be expressed as exponential function of soil salinity level; while the coefficient of the first-order kinetics reaction for the nitrogen denitrification fit the linear relationship of soil salinity level.(4) Sunflower seed yield models were established from 3 different aspects. SWSN—Jensen Model is a seed yield model which has soil physical and plant physiological meaning. Due to the easily obtained input parameters, it can be used in large scales for yield prediction, irrigation and fertilizer management. YTF model was based on the soil water and salt content in the root zone of different growth stages, it can give more accuracy yield prediction than SWSN—Jensen Model. However, because YTF model needs more input parameters, it is hard to apply in large scales. PLSY and YTF AC models are only based on leaf area index (LAI) and plant height (H) in bud and mature stage, and they could give accurate seed yield simulation. In addition, when use field observations for model validation, YTFAC was proved to be more accurate, stable, and suitable than PLSY and could be used in large scales.The results and models in our study make the irrigation and fertilization before sowing as a starting point, throughout the whole growth stages of sunflower. It can not only be used for giving a preliminary estimation of sunflower seed yield before sowing, but also doing water and fertilizer management based on soil salinity level during the sunflower growth period. Therefore, it provides the theory and powerful tools for increasing field water and nitrogen use efficiency, boosting food production and water environmental protection. |
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