Soil Water Use Source Of Apple Trees With Full Productive Age In The Loess Hilly Region

The Loess Plateau is characterized by its loose soil depth,sufficient light and large temperature difference between day and night.It is consider as the most optimized region for apple industry by FAO and it is also the largest concentration of apple in the world.In 2016,the area of apple cultivation in Shaanxi province is 11 million hectares,and the yield is 11 million tons,and the yield and area are 1/4 of the whole country and 1/7 of the world's production.The loess hilly region is in semi-arid climate area and the average annual rainfall is mainly between 400 and 600 mm,with complex topography and difficult irrigation.Water management in apple orchard is mostly dry farming,and natural precipitation is the only source of fruit growth.Due to the intense evapotranspiration of the apple trees and the rapid development of the apple industry has changed the orginal water balance.With the increase of the age of apple trees,the soil water content in the profile is consuming year by year.Otherwise,the unevenly distributed in spatial and temporal and the limited rainfall is difficult to meet the water consumption of fruit.Especially,under the warming and drying of climate in the loess hilly region,the water deficit in apple orchard will be further aggrevated.Therefore,exploring the source of soil water use during the growth period of the apple tree in the loess hilly region,which is of great significance to water management and sustainable development of orchard.In this study,the dynamic changes of soil water use sources of apple trees with different age(10,15 and 22 years old)were studied by combining the natural deuterium isotope and artificial deuterium isotope tracer technique to discuss the water use strategy of apple tree.The aim of this study was to provide scientific basis for the conservation and sustainable development of soil water resources in the apple orchard under dry conditions in the loess hilly region.The main conclusions as followings:(1)The soil moisture content of three different age apple trees was vary greatly at different growth stages.The soil water content of three different age apple trees showed an approximately vertical distribution during fruit-bearing stage and coloring mature period,but in fruit enlargement period,the distribution of soil moisture was low in shallow,and was high in the middle layer and was low in the deep layer.The soil moisture content of apple orchard profile decreased with the increase of tree age,and the average soil moisture content in the soil layer of 10 and 15 ages apple orchard were significantly higher than that of 22 years old.(2)The deuterium isotopes of soil water in three different age apple trees showed similar changes with soil depth and growth period variation.The deuterium isotope of soil water height changes in shallow layer(0-40 cm);Three different age apple trees showed that fruit-bearing stage > coloring mature period > fruit enlargement period.The deuterium isotopes in middle and deep soil water were relatively stable,and there was no significant difference between the growth stages.The deuterium value of xylem water in three different age apple trees varied with the sampling time and the age of apple trees.(3)The results show that three different age apple trees vary greatly in the water use strategies.The main water source of the 10-year-old apple trees is constantly changing throughout the growing season.It mainly absorbed the soil water from the middle layer(40-160cm)and the deep layer(160-300cm)in the flowering fruit-bearing stage,and the utilization ratio was 42.5% and 46.4% respectively.The main source of water in the fruit enlargement period came from the deep layer(160-300cm)soil,which accounted for 66.3% of the total water,And in the coloring mature period,the soil water was used mainly from the shallow layer(0-40cm),with a ratio of 57.9%.The 15-year-old apple trees mainly absorbed the middle layer(40-160cm)soil water in the flowering fruit-bearing stage and fruit enlargement period,and the utilization ratio was 74.2% and 70.2% respectively.And 59.3% of the water in the coloring mature period came from the shallow(0-40cm)soil water.The water source of 22-year-old apple trees was mainly from the shallow layer(0-40cm)and middle layer(40-160cm)during the whole growth period.The utilization ratio fluctuates in different growth periods.(4)Artificial deuterium isotope labeling method can increase deuterium isotope abundance in deep soil.The deuterium isotopes in the xylem samples of apple trees provide direct evidence for the absorption and utilization of deep soil moisture in apple tree roots.The presence of artificial markers in the samples of the xylem of the apple tree with three different tree ages at the fruit-bearing stage and coloring mature period showed that the apple tree could absorb the deep soil water using a certain proportion of 4m soil layer at this time.It is likely that deep soil can provide relatively stable water for them.