| Due to the implementation of the Grain to Green Program,and the pursuit of economic benefits,the loess plateau has experienced extensive land use change,many croplands has been converted into ecological or economic forests.Unlike crops with shallow roots,trees with deep roots can absorb water from deep soil.However,it is difficult for the deep soil to be replenished by precipitation on the loess plateau.The planting of deep-rooted trees on the loess plateau caused the desiccation in the deep soil layers.Therefore,in order to make the development and utilization of the deep soil water on the loess plateau sustainable,we need to understand the process by which the deep-rooted trees utilize the deep soil water,and what will happen to the future groundwater recharge after croplands have been converted into apple orchards.In this study,we took the deep-rooted apple trees as the main research object,and tracked the dynamic changes of water in deep soil as the core of the research.We propose a method for water source partitioning of deep-rooted plants by using tritium as a tracer,in addition,we provide a solution for calculating the groundwater recharge of deep-rooted plants.The main conclusions are as follows:(1)The determination of3 H in soil water and apple fruit tissue water shows that apple trees in the period of high yield mainly absorb and utilize the water from deep soil layers.This water was infiltrated into the soil long ago,and the average time of its infiltration into the deep soil is about 30 years.This is the first time that the ecohydrological separation has been demonstrated from the view of the age of soil water absorbed by plants.In a growing season,the change of 3H concentration in the water of apple tree branches indicates that the absorption of deep soil water by apple trees changes with the change of transpiration intensity,which also indicates that the 3H has a promising application in the partitioning of plant water sources.(2)By combining the water balance and chloride mass balance method,we have established a method to quantify the groundwater recharge under the deep-rooted apple orchard.By using the method of chloride mass balance,we calculated that the groundwater recharge rate under the original cropland in the tableland of the loess plateau was 58 mm yr-1;further,we calculated that the average groundwater recharge rate under the apple orchard in this area was 12 mm yr-1 according to the water balance method.After the conversion of cropland to apple orchards,the potential recharge rate of groundwater was reduced by70-95%.(3)In this study,we take advantage of the afforestation induced desiccated deep soil,and directly quantify piston and preferential water flow using chloride ion(Cl-)and soil water profiles in four deforested apple orchards on the Loess Plateau.The deforestation time ranged from 3 to 15 years.In each of the four selected orchards,there was a standing orchard that planted at the same time with the deforested one,and therefore was used to benchmark the initial Cl-and soil water profiles of the deforested orchard.In the deforested orchards,piston flow was detected using the migration of Cl-front,and preferential flow was measured via soil water increase below Cl-front.Results showed that in the desiccated zone,Cl-migrated to deeper soil after deforestation,indicating that the desiccated soil layer formed by water absorption of deep-rooted apple trees did not completely inhibit the movement of water.Moreover,there was an evident increase of soil water below the downward Cl-front,directly demonstrating the existence of preferential flow in deep soil under field condition.Although pore water velocity was small in deep loess,preferential water flow still accounted for 34~65% of the total infiltrated water.This study presented the mechanisms that regulate movement of soil water following deforestation through field observations,advancing our understanding of the soil hydrologic process in the deep soil.(4)After cropland was converted into an apple orchard,soil water content increased before the apple trees reached 5 years of age.When the trees were older,they would use water from the deep soil,which led to deficits in soil water storage.The deficit can reach a maximum cumulative deficit of more than 1200 mm when the stand age of the apple trees reached 22 years.The annual precipitation cannot meet the water demand of the apple trees after several years of afforestation,thus stimulating trees to progressively mine deep soil water,causing intensive deep soil water deficits in old orchards.Two years after the apple trees were cut down,soil water storage quickly recovered by about 200 mm.After 15–16years of recovery,soil water storage increased by 512–646 mm,accounting for 43%–54% of the total cumulative deficit.According to the trend line of soil water recovery derived in this study,it would take more than 26 years for the soil water storage to return to the level of the original cropland.To develop economic orchards in arid and semi-arid areas and to maintain sustainability,a holistic understanding of soil water consumption and replenishment should be considered when making a planting plan.This study suggests that new apple orchards should only account for 1.64% of the total planted area each year.In conclusion,in the Tableland of the Loess Plateau,when land use type changed from traditional cropland to deep-rooted forest,deep soil water,as the main water source of the latter,will not only cause water deficit in deep soil,but also reduce the potential recharge ofgroundwater in the future.The renewal of deep soil water resources is very slow.For example,it will take more than 26 years for soil water storage to be replenished after apple trees are cut down.Therefore,attention should be paid to the development and utilization of deep soil water resources,and more appropriate planning should be made for artificial ecological forest and economic forest. |
PDF Full Text Request