| Hetao irrigation area is an important national production base of grain and oil,but the development of agriculture and animal husbandry is seriously restricted by water resources shortage,soil salinization and non-point source pollution.In order to respond positively to the national and regional policy of high-quality development of agriculture and animal husbandry,the industrial structure adjustment of stabilizing agriculture and increasing livestock was implemented,and the development of forage industry became the first choice for salinized land resources.Alfalfa,as a high quality legume forage,has high yield,good nutritional value and can quickly restore and improve the farmland ecosystem.However,the effect mechanism of water and salt stress on water and nitrogen utilization and physiological characteristics of alfalfa is not clear in salinized shallow groundwater area,which affects the promotion and high quality production of alfalfa in salinized area.To this end,field and pot experiments of water and nitrogen efficient utilization of alfalfa under water and salt stress were carried out in Shengmu Hi-tech Economic Park,Dengkou County,Inner Mongolia,from 2017 to 2019.The characteristics of water absorption and water utilization strategies of alfalfa roots under water and salt stress in shallow groundwater area were analyzed.The effect mechanism of water and salt stress on water and nitrogen utilization and physiological characteristics of alfalfa was revealed,and the reciprocal feed-back relationship between alfalfa root characteristics and soil carbon,nitrogen and enzymes under water and salt stress was discussed.The process of nitrogen accumulation and utilization of alfalfa as nitrogen-fixing forage was clarified,and the water and nitrogen management scheme was optimized.The results provide scientific basis for the development of alfalfa industry in salinized soil.The main conclusions are as follows:（1）The water source and utilization strategy of alfalfa were determined by using hydrogen and oxygen isotope technology.After irrigation,alfalfa roots mainly absorbed water from 0-40 cm soil layer,with an average contribution rate of 70.34%.With the extension of irrigation end time,the main water absorption layer of the roots moved downward,and the soil layer was mainly 60-100 cm under the low water(600 m3·hm-2)treatment,with an average contribution rate of 48.13%.40-80 cm soil layer was mainly used in medium water treatment(825 m3·hm-2)and high water treatment(1050 m3·hm-2),and the average contribution rate was 43.83%and 33.87%,respectively.Under low and medium salt treatments(≤3 g·kg-1)and soil salinity above 3g kg-1,the contribution rate of roots to soil water utilization in 60-100 cm depth decreased by 6.15%,and that of groundwater decreased by 2.23%.However,the average contribution rate of irrigation water increased by 1.4%and that of precipitation increased by 0.6%,indicating that roots under salt stress can preferentially absorb and utilize irrigation water and precipitation contribution rate as"fresh water".（2）The physiological mechanism of alfalfa under water and salt stress was revealed based on pot experiment.Compared with water or salt stress,water and salt stress significantly reduced biomass,root-shoot ratio（p<0.05）,but had no significant effect on plant height（p<0.05）.Under water and salt stress,the contents of proline and Na+,SOD and POD activities of alfalfa plants increased,while SOD activity decreased significantly when soil salt was more than 3 g kg-1.S2W1 treatment increased the activities of proline,Na+and POD,and decreased the activities of SOD,resulting in water consumption and aboveground biomass increased by 11.93%and 17.51%,respectively,compared with S0W1 treatment,indicating that adding appropriate salt(≤3 d S·m-1)could improve the biomass and water consumption of alfalfa compared with single water stress.Alfalfa has adaptability to water and salt stress.When salt content was more than 3 d S m-1,alfalfa could improve its stress resistance by increasing proline,Na+content and POD activity,and decreasing SOD activity,but the aboveground biomass and water consumption decreased.When soil salinity≤3 g·kg-1,soil moisture content was 70%-85%of field water holding rate,and the relative yield was stable at 0.7,which reduced the risk of yield reduction.（3）The relationship between root characteristics and soil carbon,nitrogen and enzyme activities,as well as soil organic matter and related enzymes was clarified.The root parameters of alfalfa decreased with the increase of soil salinity,and the root diameter decreased with the increase of soil salinity and cutting times,while the ratio of fine roots（1-2 mm）and very fine roots（≤1 mm）increased.Soil high salinity(>3.0g·kg-1)inhibited the activities of nitrogenase and nitrate reductase,which were 22.05%and 18.38%lower than those of low salinity(≤3 g·kg-1),respectively,leading to a decrease in root nitrogen absorption capacity and an increase in soil total nitrogen and organic matter.Using structural equation model,root biomass not only has significant direct influence on soil organic matter content,but also indirectly affects soil enzyme activity to affect soil total nitrogen and organic matter content.When soil salinity>3g·kg-1,the root biomass decreased,which directly significantly increased soil total nitrogen content,but also indirectly affected soil enzyme activity and root characteristics,leading to the decrease of soil nitrate reductase and the increase of soil total nitrogen content.（4）The change characteristics of enzyme activity during nitrogen metabolism and its response mechanism to water and salt stress were clarified.Under the same water and salt conditions,N application significantly increased the ratio of leaf to stem,decreased the ratio of root to shoot,and promoted the yield increase,in which CRF treatment had the highest yield increase effect,and the yield increased by 22.81%and 4.3%compared with CK and UN treatments,respectively.Compared with no nitrogen application,nitrogen application increased the activities of nitrate reductase and nitrogenase in plants,promoted the increase of soil nitrogenase activities,and alleviated the effects of water and salt stress.However,nitrogen application inhibited the activities of nitrate reductase in soil,which decreased by 18.95%on average compared with the control.According to the redundancy analysis of soil enzymes and their relationship with soil environmental factors,soil water content,p H,C/N and organic carbon were the main factors affecting soil enzyme activity under nitrogen application.When soil salt was 2-3 g·kg-1,the nitrate reductase activity in soil was inhibited by 200 kg·hm-2,and the contents of nitrate and ammonium nitrogen in soil increased,but the increase of nitrogenase activity alleviated the water and salt stress,promoted the activities of nitrate reductase and nitrogenase,improved the nitrogen use efficiency,and promoted the yield and protein synthesis.（5）The effects of water and salt on alfalfa yield and quality and the correlation with root characteristics were analyzed.Under the same salt condition,alfalfa yield increased with the increase of irrigation amount,but the increase amplitude decreased.Under the same irrigation condition,the yield decreased with the increase of salinity,with a decrease of 10.54%-13.68%.Water and salt stress had significant effects on plant height and yield,but alfalfa height and yield were more sensitive to water.When soil EC≤0.55 d S·m-1,the protein synthesis was promoted,while when soil EC>0.55 d S·m-1,the crude protein content decreased.The yield and crude protein decreased with the increase of stubble,and the sensitivity of crude protein and fiber to water and salt stress decreased.Root characteristics had significant effects on alfalfa yield and quality,and there was"growth redundancy"in deep roots.When soil salt content was less than 3 g·kg-1,adjusting irrigation depth and increasing the surface area of fine roots（≤2 mm）in 0-40cm soil layer were beneficial to increase yield,increase the proportion of fine roots and decrease the surface area density of coarse roots were beneficial to increase crude protein content and water use efficiency.（6）Alfalfa crop growth model was constructed based on Dssat-Forages-Alfalfa with high precision.Alfalfa yield and crude protein formation process were simulated,and the water and nitrogen management model of Alfalfa was optimized..When the soil salt is less than 3 g·kg-1,the optimal water and nitrogen management mode of alfalfa in different planting years is as follows:The maximum hay yield was 10025-10602 kg·hm-2,and crude protein yield was 210-278 kg·hm-2when the fertilizer rate was 150 kg·hm-2and irrigation quota was 340 mm.When the fertilizer rate was 100-125 kg·hm-2and irrigation quota was 325-340 mm in the 3rd to 5th years of alfalfa.And the maximum hay yield and crude protein yield were 10601-12028 kg·hm-2and 278-303 kg·hm-2,respectively,but the agronomic efficiency and nitrogen use efficiency decreased.The hay yield and crude protein yield of alfalfa reached the maximum in the fourth year of planting.In conclusion,the optimal planting period is 4 years,nitrogen application rate is100-125 kg·hm-2,irrigation quota is 325-340 mm,and the quality（crude protein）of alfalfa basically meets the second-level standard requirements of Alfalfa Hay Quality Grading. |
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