Water Uptake Strategy Of Apple Trees And Its Impact On Water Balance In The Loess Tableland

The large-scale grain for green projects in the Loess Plateau,have resulted in the changes of eco-hydrological processes,further influencing the sustainability of water resource in this region.Therefore,it is of great significance to understand the evolution mechanism of water hydrological processes under changing environment,which is helpful for the sustainable management of water resources and vegetation.At present,the impacts of vegetation change on single hydrological variable such as soil water and surface runoff have been fully understood,but the quantification of the impact on each component of the water cycle and mechanism are less studied.In particular,shallow-rooted plants have been substantially converted into deep-rooted plants in the Loess region.It is thus urgent to clarify the effects of such vegetation changes on each component of soil water balance and further elucidate the change mechanism.As economic tree species,apple tree has higher ecological and economic benefits,and was substantially planted in the grain for green project.Therefore,this study selected farmland and apple orchards with different stand ages as the research objects.Soil water balance components were decomposed and quantified using hydrogen and oxygen stable and radioactive isotopes combined with water balance equation,through the paired-point sampling and dynamic monitoring experiment.The effects of land use change on soil water recharge mechanism and soil water balance were analyzed,and the water use strategies of apple trees were further quantified to reveal the mechanism of vegetation change affecting soil water balance components.Finally,we analyzed the effects of water consumption of apple orchards on regional water balance and sustainable water resources.The main results and conclusions of this study are as follows:1.The transformation of farmland into orchards in the loess tableland resulted in significant decreases in soil water content,and the deep soil water deficit significantly increased and influenced deep soil water recharge with the increase of tree age.In Daning and Changwu tableland,soil water contents decreased by 9%-40%and 4%-14%in the soil profile of 0-20 m,respectively,and decreased by 20%-42%and 10%in the soil profile below 4 m,respectively.In two study areas,the soil water was mainly recharged by precipitation in the form of piston flow.The soil water in the shallow 0-2 m soil layer was easily recharged by precipitation,but that in deep soils below 6 m can only be recharged by heavy precipitation events with the intensity greater than 40 mm day^-1.The conversion of farmland to apple orchard resulted in the decrease of soil water recharge.The soil water under farmland can be recharged by precipitation with intensity greater than 10 mm day^-1.However,soil water under apple orchard could only be recharged when the precipitation intensity greater than 30 mm day^-1,and the deep soil water recharge was not only affected by the seasonal distribution of precipitation,but also affected by the residence time of soil water.2.The conversion from farmland to apple orchard has significantly changed soil water balance.Apple orchards significantly increased transpiration but decreased other water fluxes.In Daning and Changwu,the conversion of farmland to orchards resulted in water storage decrease of 14%-41%in the soil profiles of 0-15 m and 5%-14%in 0-20 m soil profiles;especially in the soil layer below 6 m,soil water storage was serious deficit with 22%-45%in Daning and 6%-20%in Changwu,respectively.The deep drainage decreased from 27.9 year^-1to 1.5-14.3 mm year^-1 in the Changwu Tableland,and that decreased from 25.8 year^-1 to 0 in the Daning Tableland,which completely hindered the recharge of groundwater.In two study areas,compared with the farmland,the transpiration of apple orchards increased by 15%-40%,while the evaporation decreased by 12%-32%.Under different land uses in Daning area,the proportion of evaporation to evapotranspiration was 31%-41%,while the proportion of transpiration to evapotranspiration was 59%-69%.In Changwu area,evaporation accounted for20%-32%of evapotranspiration under different land uses,while transpiration accounted for68%-80%of evapotranspiration.In two study areas,the evapotranspiration under orchards accounted for 100%-114%of the annual precipitation.Precipitation could no longer meet the evapotranspiration requirements of fruit trees,resulting in changes of the soil water balance.3.The changes of soil water balance components were mainly influenced by the seasonal changes of root water uptake of apple trees.Over the growing season,the water source of root water uptake of apple tree changed from shallow soil to deep soil.In the early growing season,apple trees mainly used soil water in the shallow layer of 0-2 m,and the water absorption rate was 64%.In the late growing season,apple trees mainly absorbed soil water in the deep layers,and the contributions of water in the soils below 2 m was 60%.Even in normal or wet years,apple trees absorbed about 40%water from deep soils.Especially in normal years,about 66%of the deep soil water was absorbed in the late growing season,of which about 20%of the water may have come from precipitation 50 years ago.Apple trees with different ages had similar water use strategies,but there were differences in the water uptake proportion from different soil layers.Young trees mainly utilized shallow soil water,while old trees utilized more water from deep soils.The contributions of soil water in different soil layers to root water uptake of apple trees was not significantly correlated with soil water content over the growing season,but the water use strategy was affected by precipitation,tree age and physiological characteristics.4.The water consumption of apple orchard changed the hydraulic connectivity in the unsaturated and saturated zone and regional water balance in the loess tableland.The decrease of groundwater level in the Changwu Tableland is mainly due to the increase of groundwater exploitation and the decrease of potential groundwater recharge caused by apple orchards.From1995 to 2020,the area and yield of apple orchards increased continuously in the Changwu Tableland,and the water consumption of apple orchards increased from 7.56×10⁶m³ to84.5×10⁶m³.The water consumption of apple orchard has far exceeded the average annual precipitation,and the precipitation cannot meet the demand of water consumption for apple production.From 2005 to 2020,the average annual deficit of soil water storage caused by apple orchard planting was 2.46×10⁶m³.By 2020,the total deficit of soil water storage was36.9×10⁶m³.The water consumption of apple orchards resulted in the serious deficits of soil water storage and the reduction of potential groundwater recharge,which changed the regional water balance and affected the sustainability of groundwater resources.In the future,the balance between water consumption of vegetation growth and regional water resources sustainability should be tradeoff in vegetation restoration,especially in the areas with limited water resources.The results of this study indicated that the conversion of farmland into apple orchard resulted in significant decreases of deep soil water storage and deep drainage,but the evapotranspiration significantly increased.The changes of these water balance components were closely related to the water uptake strategy of apple trees,which continuously obtained water from deep soils through developing deeper roots to meet transpiration requirements,resulting in an increasing water deficit in deep soils,thereby changing soil water balance.With increasing apple orchard area,its water consumption significantly increased,which made precipitation unable to meet water consumption requirements,resulting in the continuous reduction of soil water storage,thus changing the regional water balance and the sustainability of groundwater resources.It is important to investigate the effects of land use change on soil water balance,and further elucidate the mechanism of water use strategy of deep-rooted fruit trees affecting the changes of soil water balance components,which is helpful to strengthen the understanding of regional water cycle evolution under environmental change.The analysis on the impacts of apple water consumption on regional water balance and water resources sustainability can provide a basis for the sustainable management of water resources and vegetation,and ecological construction in the Loess Plateau.