【作者】 张志强；

【导师】 司炳成；

【作者基本信息】 西北农林科技大学 ， 农业水土工程， 2020， 博士

【摘要】 退耕还林工程的实施以及为脱贫致富而对经济效益的追求,导致黄土高原出现大面积的农田转变为深根的生态林或经济林。林地更大的蒸腾水分需求使黄土高原开始出现广泛的土壤干燥化现象。降水穿透到深层土壤是一个困难和长期的过程,深层土壤水的更新速度极其缓慢,使得深层土壤的干燥化成为了一个持续性的问题。因此,为了实现对黄土高原深层土壤水的合理开发应用,需要建立一套研究深根植物对深层土壤水利用策略和评估深根林地的地下水潜在补给量的方法,同时量化深根经济林整个产业周期内深层土壤水的动态变化。本文在黄土高原出现大面积土地利用方式变化的背景下,以深根经济植物苹果作为主要研究对象,以深层土壤水分动态变化为主线,对苹果园下的土壤水深层渗漏和土壤储水量动态变化进行了研究,主要得到以下结果:（1）利用放射性同位素³H,第一次从植物吸收的土壤水年龄的角度来证明了生态水文分离现象,即“两个水世界”,发现高产期的苹果林地主要利用深层土壤水,并且深根苹果林在生长季内对深层土壤水的利用与潜在蒸散量成线性正相关关系。对土壤水和苹果果实组织水中的³H分析结果表明,处于高产期（17年树龄）的苹果树,在生长季内主要吸收利用的是多年以前入渗到深层土壤中的“老水”,进入土壤的平均年限为30年左右。本研究也证明³H在植物水分来源划分研究中具有应用前景,通过对苹果树枝条木质部水分中³H的测定结果分析,表明苹果树对深层土壤水的吸收利用是随着蒸腾用水需求的变化而变化的,蒸腾用水需求高时,苹果树会更多的从深层土壤中吸收水分。（2）通过将水量平衡和氯你离子质量平衡法结合起来,建立了一个量化深根林地对地下水潜在补给量影响的方法,结果显示,传统农田向深根苹果园的转变严重减少了地下水潜在补给量。通过氯离子质量平衡法计算得到渭北旱塬地区农田多年平均土壤水深层渗漏量为58 mm yr^-1;利用水量平衡法和农田深层渗漏量估算出苹果园地下水潜在补给量均值约为12 mm yr^-1,约占当地多年平均年降水量的2%。改种苹果后,地下水潜在补给量减少了约70-95%。（3）本研究利用利用氯离子（Cl^-）和土壤水分剖面,对土壤水分在干燥化的深厚黄土中的运移机理进行了定性和定量研究。在砍伐掉苹果后形成的农田中,利用Cl^-峰值的迁移计算活塞流补给量,通过Cl^-峰值以下土壤水分的增加来计算优先流补给量。结果表明,在干燥化的深层土壤中,Cl^-峰值在苹果树被砍伐后向土壤深处迁移,说明深根苹果树吸水形成的干燥化土层并没有完全抑制水分的移动。Cl^-峰值下方土壤水分明显增加,直接说明了田间条件下深层土壤存在优先流,降水能够绕过一部分土层（Cl^-峰值及以上土层）直接补给到土壤更深处。深层黄土孔隙水流速虽小,但优先流补给量仍占总补给量的34～65%。本研究通过野外观测,揭示了经济林砍伐后土壤水分在干燥化的土壤中的运移机制,促进了我们对深层土壤水文过程的理解。（4）利用空间换时间和配对取样法,跟踪了苹果整个种植周期内深层土壤储水的动态变化。结果表明:在苹果树幼龄时期土壤储水量曾出现过增加的现象,但这部分增加量随后因为苹果树需水量的提高而被消耗掉;当苹果树树龄约为11年时,深层土壤水是不存在亏缺的;当树龄达到22年以后,苹果树吸水造成的深层土壤水亏缺量到达了最大值。苹果树被砍伐掉以后,由于干燥化的土壤不利于作物生长,深层土壤水在前两年会得到快速的恢复,前2年3 m以下土壤储水能增加约200 mm;在砍伐15-16年后,深层土壤储水累积恢复量能达到512-646 mm,约占先前种植苹果造成的深层土壤水累积亏缺量的42.7-53.8%。根据深层土壤储水量的恢复趋势线,如果要恢复到种植苹果以前的原生农田水平,至少要26年以上。将深层土壤水的消耗过程和恢复过程衔接起来,得出在浅根作物的农田与深根苹果园的轮作过程中,深层土壤水分的利用周期约为56-61年,因此,建议在研究区域内每年新增的苹果园的面积应不超过总种植面积的1.64%。综上所述,当土地利用方式由传统的农田转变为深根林地以后,深层土壤水作为后者的主要水分来源,在造成深层土壤水分亏缺的同时,也会降低未来地下水的潜在补给量。降水在干燥化的深层土壤中以活塞流和优先流两种方式运移,两者在深层土壤水的补给量中的权重相当。深层土壤水资源的更新非常缓慢,以苹果树为例,其深层土壤储水量的恢复周期超过26年。因此,应当高度重视对深层土壤水资源的开发和利用,对人工生态林和经济林要进行更合理的规划。更多还原

【Abstract】 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.更多还原