Effect And Mechanism Of Winter Irrigation On Cold Resistance Of Alfalfa Cultivated Grassland

The low-temperature stress in winter is the key factor affecting the cultivated grassland and the low yield and the reduction of production in agriculture and animal husbandry.With the aggravation of climate abnormal change,the temperature changes more frequently,and the cold and drought disasters in some areas are in the trend of frequent occurrence,especially in the northern seasonal frozen soil area.At the same time,in recent years,the construction of alfalfa cultivated grassland in northern China has developed rapidly.Therefore,it is urgent to strengthen the study on the cold resistance and overwintering rate of alfalfa and put forward scientific and effective field management measures.This study takes alfalfa,a typical forage,which is widely planted in the grassland area of northern China and is susceptible to cold damage,as an object.Through field control experiments,the response of soil water and heat environment to winter irrigation(including freezing irrigation and thawing irrigation)of alfalfa during the winter period,and analysis of water source of winter irrigation alfalfa is carried out,comprehensive evaluation of winter alfalfa cold resistance effect,the effects and mechanism of winter irrigation on alfalfa cultivated grassland were studied,and the field management measures to deal with cold damage were optimized.(1)Freezing irrigation increased the soil water content before winter while thawing irrigation increased the soil water content before turning green.The effect of high water volume winter irrigation on soil water content was higher than that of low water volume winter irrigation;after freezing irrigation,the soil temperature of each soil layer increased with the increase of irrigation water.The soil temperature under high water treatment was 0.5-2 ? higher than that under low water treatment.After thawing irrigation,the soil temperature under high water volume treatment was 1-2 ?higher than that under low water volume treatment.Both freezing irrigation and thawing irrigation have the effect of stabilizing soil temperature,and the effect on shallow soil shows an increasing trend.Freezing irrigation can delay soil freezing while thawing irrigation accelerated soil thawing.The results showed that winter irrigation created good soil water and heat environment,which was conducive to alfalfa overwintering and regreening.(2)Based on the stable isotope technology of hydrogen and oxygen,the water source of alfalfa under winter irrigation was analyzed.The results show that after freezing irrigation,alfalfa absorbed soil moisture only when the root-soil moisture was high enough;The contribution rate of soil water to alfalfa rhizome water was different in different depth soil layers,and the highest contribution rate was 40-50 cm,50-60 cm,and 0-10 cm,respectively,with a total contribution rate of 70%.Low water volume and medium water volume freezing irrigation only caused the change of soil water content,but alfalfa did not absorb and use soil water.The above results showed that high water volume freezing irrigation was conducive to alfalfa entering the overwintering period.On the third day after thawing irrigation,alfalfa began to absorb and utilize soil water.Under the low water volume treatment,the main water sources of alfalfa rhizome were middle and deep soil.Under medium water volume treatment,the contribution rate of soil water in different soil layers to alfalfa rhizome water was similar.Under high water volume treatment,the middle layer soil became the main source of water absorption of alfalfa.The above results showed that thawing irrigation alleviated the drought stress at the early stage of alfalfa regreening,and promoted the early regreening of alfalfa.In view of this,in the process of field management,we should pay attention to choose the appropriate time to implement thawing irrigation to avoid the possible chilling injury in early spring.(3)The activities of proline,soluble sugar,POD,SOD,MDA,and root activity were significantly changed in alfalfa roots after winter irrigation.Except for the increase of proline content under low water volume irrigation,the proline content of the alfalfa root system increased significantly after two winter irrigation,and the increased range increased with the increase of irrigation amount,and the highest proline content of alfalfa root under high water volume winter irrigation;After different water volume,the soluble sugar content of alfalfa root system increased obviously,and the increase of soluble sugar content was positively related to the water volume in winter irrigation;POD and SOD of alfalfa increased significantly under different treatments,and POD and SOD activities of alfalfa were the highest under the medium water thawing irrigation.The low and medium volume thawing irrigation caused the root MDA of alfalfa to change greatly,while the effect of high water volume thawing irrigation was small.The low water volume thawing irrigation increased the MDA content of the alfalfa root system significantly,but the effect of medium water volume and high water volume thawing irrigation on MDA was not significant.Except for the high water volume thawing irrigation,other winter irrigation treatments caused the great changes in alfalfa root activity,among which the high water volume and medium water volume thawing irrigation were the most effective to improve the root vigor;Furthermore,the comprehensive effect of winter irrigation alfalfa on cold resistance was analyzed and evaluated by AHP.The results showed that the physiological indexes of the alfalfa root system responded positively under winter irrigation,and winter irrigation improved the cold resistance of alfalfa and played an important role in ensuring the safety of alfalfa overwintering.(4)Winter irrigation significantly increased the overwintering rate and grass yield of alfalfa.Under the treatment of high water volume freezing irrigation + medium water volume thawing irrigation,the overwintering rate and fresh and dry grass yield of alfalfa reached the maximum.Based on the comprehensive analysis of the effects of winter irrigation on soil water and thermal environment,water uptake strategy of alfalfa,and physiological indexes and cold resistance of alfalfa,this study concluded that high water volume freezing irrigation + medium water volume thawing irrigation was the optimal irrigation mode for winter irrigation of alfalfa.Winter irrigation improved the soil water and heat environment in the early stage of overwintering and the early stage of returning to green in the next year,promoted the activity of physiological indexes of cold resistance of alfalfa,increased the water absorption and utilization of alfalfa roots,and increased the overwintering rate and forage yield.In the production practice,high water volume freezing irrigation +medium water volume thawing irrigation is the best irrigation mode for winter irrigation of alfalfa.The best time of freezing irrigation is about 20 days before the soil enters the stable freezing period,and the best time of thawing irrigation is that the average daily minimum temperature in northern China is stable above-4 ?.This study deepens the cognition of the winter irrigation effect and mechanism of alfalfa cultivated grassland from multiple perspectives,and the proposed optimized winter irrigation model has application and promotion value.