The Depletion Characteristic Of Deep Soil Water In Orchard And Its Effect On Water Balance In The Loess Tableland

Due to the economic and ecological benefits,apple orchards develop rapidly on the Chinese Loess Plateau,but it also has a significant impact on the regional deep soil eco-hydrological process.With the increase of orchard age,the depletion of deep soil increases gradually.Deep soil water,one of the characteristics of ecosystem stability in arid and semi-arid areas,is of great significance for vegetation to resist drought and and recharge the groundwater.However,the understanding of deep soil water depletion is not sufficient,and previous studies are mainly focused on the top 0-5 m soil layer,and the long-term monitoring of deep soil water is lacking.Meanwhile,with the stand age increasing,the contribution of deep soil water to plant root water uptake and influence of deep soil water desiccation on migration of precipitation are still not clear.Thus,further quantitative research on the deep soil water depletion pattern in different stand age and how the water balance changed is needed.Based on the above scientific question,apple orchards with different planting years in Changwu Tableland on the the Chinese Loess Plateau were selected(young orchard,planted in 2008,named as AP2008;middle age orchards,planted in 2005 and 2001,named as AP2005and AP2001;old orchards,planted in 1998,1994,named as AP1998 and AP1994).In-situ soil water monitoring coupled with soil coring methods,and isotopes,root water uptake model with water balance methods were used to quantify deep soil water depletion pattern with orchard age.The main results were as follow:(1)The main influence depth of multi-year precipitation to soil water in orchard was mainly 0-5 m,so soil layer below 5 m depth was defined as the deep soil in the study area.The soil water content in 5-20 m in old orchard AP1994 can hardly be changed,so it could be regarded as the extraction-limit of deep soil water in orchards in the study area.At the same time,this extraction-limit was not constant with soil depth,and the soil clay and bulk density can explain its change of 64%and 12%,respectively.(2)The conversion of land use from farmland to orchard caused significant consumption of shallow soil water,but compared with the orchard in 19 years,the shallow soil water in 22years showed a recovery trend.Soil water showed a replenishment from July 2016 to November 2020 due to the recharge of heavy precipitation.The maximum replenishment depth was 7.6 m in orchard AP1994,while the minimum replenishment depth was 5.4 m in orchard AP1998.The maximum replenishment amount was 400 mm in orchard AP2001 due to a drier shallow soil water.The peak depth of ³H decreased by 1.5 m approximately in 10years,showing a decreasing movement of soil water since the orchard was planted.(3)Annual deep soil water depletion rate showed a bell shape with orchard age,and reached a peak value at around 13 years,with the amount of 177 mm yr^-1.In dry years and normal years,deep soil water depletion showed a signigicant depletion in middle aged orchards,but no deep soil water depletion was found in all orchards in the wet year.In dry year,the contribution of deep soil water to actual evapotranspiration in orchards AP2005 and AP2001 were 27.0%and 13.6%,respectively;In the normal year of 2017 and 2018,their contribution rates were 15.1%and 7.9%,respectively.(4)With the orchard age,the proportion of fine roots below 5 m can be up to more than60%,but apple trees still relied on shallow soil water.The contribution of deep soil water below 5 m to orchard root water uptake over the lifetime can be 9%-39%,and the contribution rate decreased to 6%-23%when the soil and plant water isotopes were corrected.The seasonal water uses pattern of apple trees showed a slow shift to the precipitation,but they used more deep soil water in low precipitation years.Although the deep soil water was consumed significantly,but the change of isotopes values for?²H and?¹⁸O were within 2‰and 0.4‰,respectively,showing a weak change.(5)Deep soil water below 5 m decreased with the orchard age,which resulted in a significant decrease of soil water potential and hydraulic conductivity.Compared with the pedotransfer function built on the Chinese Loess Plateau,Rosetta function overestimated the contribution of deep soil water as it overestimated its water potential and water conductivity.Using the root water uptake model,the heterogeneity of soil water potential and isotopes would result into a opposite diurnal variation trend of deep soil water contribution ratio and the xylem water isotope.(6)The introduce of ecomomical apple tree has caused great accumulation of soil NO₃-N at a depth of about 3 m,with a concentration of 500 mg kg^-1,which caused a potential threat to the regional soil and water environment;At the same time,orchards plantation also increased regional potential evapotranspiration;but in the old period,orchards transpiration and evapotranspiration were decreased due to less deep soil water supply,which contributes to avoid long-term deep soil water drought stress.In conclusion,deep soil water is a stable water source for deep-rooted vegetation in the Loess Plateau,and it plays an important role in root water uptake process in orchards.The characteristics of deep soil water consumption and utilization are not only related to tree age,but also related to precipitation.Meanwhile,the deep soil water dessication reduces the precipitation recharge and solute transport rates.This study is of scientific significance to reveal the interaction between vegetation and soil water and the sustainable development of afforestation on the Chinese Loess Plateau.