Characteristics Of Soil Water Utilization In Jujube Plantation Of Different Stand Ages On Loess Plateau Of China Based On Stable Oxygen Isotope

Since the Grain for Green project initiated by the Chinese central government in 1999,due to ecological and economic benefits,jujube forest has been widely cultivated in the loess hilly region.The interleaved distribution of jujube forest combined with different ages is the main characteristic of jujube forest.Soil moisture is a key factor affecting jujube growth.Clarifying water use strategy of jujube trees of different stand ages can provide useful indications for the water management of jujube forest in this area.In 2016,we selected four different stand age jujube forests?4?8?17 and 22 years?in a rain-fed jujube?Ziziphus Jujube Mill.cv.Lizao?plantation in Qingjian County,Shaanxi Province.In the study,the oxygen stable isotope tracer method was used to measure the oxygen isotope ratio(?¹⁸O)of precipitation,xylem water and soil water sources in jujube orchards with four different stand ages,and the contribution of soil water from different depths to water uptake were estimated by the direct comparison,IsoSource model,MixSIR model and SIAR model.Finally,identify the reliable sources of water for jujube trees of different stand ages by comparing the outputs of the four methods.The results show that:?1?Soil water content is influenced not only by rainfall and evaporation,but also influenced by crop growth.Therefore,there is a significant difference in soil moisture content between different stand age jujube trees in time and depth variation.The trend of soil moisture of different stand ages of jujube in time sequence intend to increase first and then decrease,the most significantly change of soil water content happened in shallow layer?0⁴0 cm?.In vertical direction,the change of soil water content was 4->8->17->22-year-old jujube tree.However,the total fine root length density and the fine root length density distribution in each soil layer display an inverse way.The fine roots of different tree ages are mostly distributed in the 0-40cm soil layer.?2?Using LSD's multiple comparison to analyze the?¹⁸O of soil water and xylem water with different stand ages of jujube at different sampling dates.The soil profile of 0-200 cm was discretized into three layers.The shallow layer?0⁴0 cm?soil water?¹⁸O fluctuated widely and decreased with the depth increased.At the same time soil water?¹⁸O showed obvious seasonal variations.The middle layer?40¹20 cm?soil water?¹⁸O fluctuation ranged steady.The deep layer?120²00 cm?soil water?¹⁸O basically unchanged.The xylem water?¹⁸O with different stand ages of jujube have significant differences in time variation.In germination frondesce stage xylem water?¹⁸O was significantly greater than the other growth stages,while was significantly lower than the other growth stages in flowering and fruit setting stage.And xylem water?¹⁸O in 4-year-old jujube were quite different compared with other tree-age jujube trees.?3?By comparing and analyzing four methods,we found that the direct comparison method is a relatively simple answer to the water source analysis method,which can only give the main source of moisture qualitatively and cannot describe the water contribution rate of each water source.The IsoSource model has a strong applicability and calculate more reliable.It can describe the water contribution rate of each water source.However,due to the spatial difference of the isotopes composition of each source is not considered,the uncertainty of the calculation result is greater.Although the Mix SIR model considers the spatial difference of the isotope composition of each source more than the IsoSource model,the calculation result of the model is more accurate,but the stability of the model is easily affected by the size of the spatial difference value,thus affecting the reliability of the model calculation result.The SIAR model,which compensates the shortcomings of the MixSIR model,increases the stability and reliability of the model,minimizes the discreteness of the calculation results of the model and reduces the uncertainty of the calculation results.?4?The water source results with different stand ages of jujube at different growth stages were consistent with the direct comparison method and another three models.The 4-and 8-year-old jujube mainly absorbed shallow layer?0⁴0 cm?soil water in germination frondesce and flowering setting stage.The ratio of water absorption in the medium-deep layer?40²00cm?of soil water increased gradually during fruit enlargement and fruit ripening.While 17-and22-year-old mainly absorb shallow?0⁴0 cm?soil water in germination frondesce stage.From flowering fruit setting stage to fruit ripening stage,jujube tree gradually reduced the use of shallow layer?0⁴0 cm?soil water,and gradually absorbed and utilized medium-deep layer?40²00 cm?soil water.It shows that with the increase of tree age,jujube tree had a stronger utilization of medium-deep?40²00 cm?soil water.In the long term,it would cause the soil moisture content in the middle and deep layers to be too low or even become desiccated.In order to replenish and restore soil moisture in loess hills,water management that based on soil water utilization characteristics of jujube trees with different stand ages should be carried out for reducing the non-productive water consumption of 4-,8-year-old jujube and the luxury water consumption of 17-and 22-year-old jujube trees,to ensure the sustainability of the local jujube forest and soil water use.