Study On Water Use Of Fruit-grass Intercropping System In Guanzhong Area Based On Isotope Tracer

Water resources are one of the main limiting factors for the healthy development of the fruit industry in Guanzhong.The fruit-grass intercropping system can promote precipitation infiltration,reduce water evaporation,improve soil structure,and increase fruit orchard water and fertility.However,different plants consume water to different degrees,and there is overlap between the root space of fruit trees and forage grass,which can easily cause interspecific water competition.This study assessed plant water use efficiency（WUE）,plant water sources and their utilization rates in different fruit-grass intercropping systems（apple+white clover（AW）,apple+ryegrass（AR））,and single-apple（SA）)using stable isotope tracing techniques,combined with meteorological data and soil water content.The results of the study were as followed:（1）During the observation period,theδD andδ¹⁸O values of atmospheric precipitation in Guanzhong region ranged from 4.52‰～-57.40‰and-1.18‰～-8.60‰,and showed seasonal variations.The local atmospheric precipitation lineδD=8.36δ¹⁸O+14.84,showed the rainfall effect.The atmospheric precipitation line deuterium surplus is higher than the global deuterium surplus value,and precipitation isotope enrichment due to evaporation.The native soil water evaporation line wasδD=7.11δ¹⁸O-4.22,which was smaller than the atmospheric precipitation line,indicating that rainwater infiltrates into the soil and produces isotopic fractionation.The soil water evaporation line of the composite system is larger than that of the monocrop system,and the fruit-grass intercropping system can reduce the evaporation of soil water.（2）The average soil water content in the study area ranged from 13.64%～17.29%.Compared to SA,AR increased soil water content,while AW was significantly higher than SA only when precipitation was high（P<0.05）.And the water content of the 40～90 cm deep soil layer in AW decreased with decreasing precipitation.The soil moisture content of AR was positive during the observation period,while AW was positive only in July,indicating that white clover increases soil moisture depletion when monthly precipitation is less than 200 mm,and intercropping ryegrass can improve soil moisture.The soil water content of the intercropping system in the 0～10 cm layer was higher than that of the single crop,suggesting intercropping system can increase the infiltration of precipitation.The ability of the surface soil of AW to intercept rainwater was stronger than that of the AR,but the AR has a higher capacity for water storage in the deep soil layer.（3）The main source of water for fruit trees in SA and AR was from the 20～40 cm middle layer（43.10%,44.20%）and 40～100 cm deep layer（73.60%,51.50%）soil.The main source of water for AW fruit trees differed in different seasons,using all three layers of soil water.The principal source of water for white clover and ryegrass was from the surface layer（49.00%,46.30%）,while ryegrass was also able to use the middle layer（62.20%）soil water in the dry season.The intervention of forage grass shifted down the water absorption layer of fruit trees in the intercropping system and increased the use of deep water.During the fruit ripening and picking stage,the water demand of fruit trees decreased and the water absorption level of fruit trees trended to move up.（4）Theδ¹³C values of plant leaves in the study area ranged from-27.27‰～-30.81‰.Except for AW in October,leafδ¹³C values were higher in monocrop systems than in intercropping systems.From apple,ryegrass,white clover,plant leafδ¹³C values decreased gradually.Through the use of leafδ¹³C values to evaluate plant WUE,the fruit-grass intercropping system reduced the WUE of fruit trees within the fruiting season.When soil water content was low,fruit trees increased WUE resistance to drought stress.AW increased the WUE to cope with the water competition with white clover in October.In conclusion,the two intercropped forages had different effects on soil moisture,the water source of AR’s fruit trees was deeper than in AW.Forage grass would increase the use of soil water below 20 cm in the low rainfall season,while the surface soil water was used preferentially when the surface water content was high.The main water source soil layers of white clover and fruit trees were the same in October,and water competition was intense.In contrast,the main water source layers of ryegrass and fruit trees did not overlap,and the water source pattern was well configured without water competition.It is suggested that ryegrass should be planted in apple orchards in Guanzhong area during the interannual period with abundant rainfall,which is helpful to reduce soil water consumption in dry season.