Contribution Rate Of Deep Soil Water To Apple Tripiration In Loess Plateau

Loess Plateau,which is located in the northwest of China,is the largest loess area in the world and has the deepest loess layer.The deep soil layer makes the roots of woody plants(apples,etc.)in this area far deeper than other regions,and not only can absorb and utilize the recent precipitation in shallow soil,the soil water stored for many years can also be absorbed and utilized through its deep root system when the water demand is high.After years of apple planting,the deep soil water in the Loess Plateau is seriously depleted.How root system effectively utilize precipitation and deep soil water is the key to the sustainability of apple industry and the exertion of eco-hydrological effect.Based on this,the water sources of deep root crops represented by apple trees in the Loess Plateau were studied by using stable hydrogen and oxygen isotopes.Using space-for-time substitution,we selected apple orchards with different ages but with the same species and management.Samples were taken in year of 2015 and 2016 at different times before and after precipitation and during a complete growth period respectively.By sampling 0-5 m or 0-10 m soil and corresponding plant branches,using stable hydrogen and oxygen isotope technique to investigated the rules of water consumption in different apple orchards.Also,the contribution ratio of different soil layers to apple trees in different growth periods can be quantified by using Bayesian model.The results are as follows:(1)The contribution ratio of deep soil water to plants are large.The absorption of deep soil water by roots decreased with the increase of stand ages.For young fruit trees which hasn't formed the dry layer,such as 6,9 and 11 years trees,most of plant water comes from the deep soil water below 200 cm.Apple trees keep growing,and the dry layer appeared in the deep layer of the soil.The degree of absorption and utilization of soil water above 200 cm by fruit trees was increased.For seeding apple trees at 6years old,in the period of stable growth(May 30),the contribution of shallow soil water was equal to that of deep soil water,while the other fruit trees absorbed deep soil water significantly.The contribution rate of shallow layer is 6%~17%,which is much smaller than that of deep layer.During the driest period of the year(July 3)and it is also has the largest water consumption,all trees absorbed deep soil water significantly.The contribution rate was 60% to 70%.But compared with the other time of fruit expansion(July 30),the contribution ratio of shallow soil water was increased to 80%~90%.When there is recharge of precipitation(July 21),the older 18 and 22 years old fruit trees that have formed a dry layer will increase their absorption of shallow soil water at this time,it even absorbs shallow water significantly.The contribution ratio of shallow layer is 80% in 18 years apple orchard and 45% in 22 years.The absorption of shallow soil water by roots will increase gradually at the end of growing season.(2)The response of water uptake pattern to precipitation increased as stand ages increased.For small trees,the change of contribution ratio of different soil layer to plants before or after precipitation is small,and precipitation can only recharge the water consumption of trees that hasn't bear fruits.For old trees,the contribution ratio of shallow soil water to plants has significant increase after precipitation.But in most cases,the water source of apple trees in the Loess Plateau is deeper soil water.This study quantified the change of water uptake pattern of deep-root plants with the change of stand ages and growth conditions,quantified the importance of deep soil water to plants on the Loess Plateau,which may be useful for a sustainable management of apple industry in Loess Plateau.At the same time,the mechanism of apple water uptake can provide a theoretical basis for the study of water use strategies for other deep-root plants in the Loess Plateau.