| The introduction of high water-consuming plants and high-density planting in the Loess Plateau resulted in deep(>1 m)soil water deficit,leading to the formation of a large area of dry soil layer.Dry soil layer can aggravate the soil water repellency to a certain extent,hinder soil water movement,endanger the biological cycle of terrestrial ecosystem and weaken the water cycle,which has a certain negative effect on the existing vegetation construction achievements.In-depth understanding of soil hydrological process and its response to land use change in the Loess Plateau is of great scientific significance for the sustainable utilization of soil water and reasonable vegetation construction.The prior study on soil water and dry soil layer mostly focused on the limited observation and model simulation of plot scale.However,there are few studies on the long-term changes of deep soil water in the Loess Plateau from the farming period to the vegetation restoration period for decades,which limits the in-depth understanding of the quantitative relationship between soil water and vegetation and the recovery and regulation of dry soil layer.In this paper,the typical loess zone of the Loess Plateau is taken as the study area.Firstly,based on the three soil data sets(deep profile,north-south transect and regional sampling),the pedotransfer functions of soil hydraulic properties(including van Genuchten model parameters,bulk density and saturated hydraulic conductivity)in the study area are established,and the spatial data of deep soil hydraulic properties at the regional scale are obtained.Then,based on the in-situ observation data,the Hydrus-1D model was calibrated to simulate the response process of deep soil water to land use change during nearly one hundred years(1981–2060).Finally,the long-term change process of soil water in the 0–5 m profile from the farming period to the vegetation restoration period was simulated at the regional scale(1981–2014),and the evolution characteristics of hydrological variables such as soil water storage,potential groundwater recharge and evapotranspiration were quantified.The feasibility of regulating dry soil layer based on land use change in the typical grassland zone of the Loess Plateau was quantitatively studied,and the recovery process and main control factors of dry soil layer after the conversion of artificial forest vegetation into natural grassland were discussed.The main research results are as follows:(1)The soil hydraulic properties of 0-8 m profile in the Loess Plateau are closely related to soil texture,climatic conditions and topographic factors.The pedotransfer function equation of soil hydraulic properties constructed by various environmental factors has better simulation ability and prediction accuracy than other pedotransfer functions.Based on the established pedotransfer function model of soil hydraulic properties,combined with the spatial interpolation method,the spatial continuous 0–5 m soil hydraulic parameters(including van Genuchten model parameters(α,n,θr andθs),bulk density and saturated hydraulic conductivity)in the typical loess zone were obtained.We found that the soil hydraulic properties in the Loess Plateau had zonal distribution characteristics,which mainly showed the transition from southeast to northwest,and the south and north showed different patterns.The parameterαdecreases from southwest to north,parameter n decreases from north to south,residual water contentθr decreases from southeast to northwest,saturated water contentθs decreases from west to east.The bulk density was highest in the north,moderate in the south area,and lowest in the middle and west area.The saturated hydraulic conductivity decreases from southwest to east.(2)The long-term simulation results of soil water under land use change at plot scale show that the soil water dynamics in response to land use changes can be divided into five stages in the process of farming period-vegetation restoration-natural grassland conversion:1)the stage of soil water fluctuated with precipitation(farming land);2)the stage of significant decrease of soil water(7–8 years after the conversion of farmland to artificial vegetation);3)soil water retained at a low level close to wilting point(artificial vegetation);4)soil water gradually restored stage(7-8 years after artificial vegetation conversion into natural grassland);5)soil water fluctuated with precipitation(natural grassland).After the farmland was converted into M.sativa and C.Korshinskii,the soil water was in an imbalance state due to excessive consumption of deep soil water by plant roots,and the dry soil layer began to occur and form after 3–4 years of artificial vegetation planting.After M.sativa and C.Korshinskii planted for 7-8 years,the lower boundary of dry soil layer has developed to below4 m depth.After the artificial vegetation was converted into natural grassland,the soil water in the profile gradually recovered.It took 7–8 years for the soil water in the 0–4 m profile of the artificial vegetation growing for 13 years to fully recover to the natural grassland level,indicating that the dry soil layer under the artificial vegetation could be recovered and regulated by changing the vegetation type.(3)The simulation results of long-term evolution of soil water at regional scale in the Loess Plateau showed that the leaf area index at regional scale increased gradually during 1981-2014.From 1981 to1999,annual mean precipitation,soil water storage,actual evapotranspiration and potential groundwater recharge showed a gradually decrease trend,while potential evapotranspiration showed an increase trend.During the vegetation restoration period from2000 to 2014,the annual mean precipitation and actual evapotranspiration increased gradually,while the potential evapotranspiration decreased gradually.However,the interannual variations of soil water storage and potential groundwater recharge were not significant.All factors have significant spatial zonal distribution law feature of decreasing from southeast to northwest.Compared with the farming period,during the 14 years of vegetation restoration,the annual precipitation at regional scale increased by 2%,the leaf area index increased by42%,the potential evapotranspiration increased by 3%,and the actual evapotranspiration increased by 1%,while the soil water storage decreased by 4%,and the potential groundwater recharge decreased by 46%.(4)81%of the artificial forestland in the zonal grassland area of the Loess Plateau hadformed dry soil layer,with the mean thickness of 297 cm,and the degree of soil desiccation was serious.The simulation results based on the regulation of dry soil layer by land use change show that the restoration of dry soil layer at 5 m depth in the whole region takes 2–13 years(mean 7 years)after the artificial forestland converted into natural grassland.The results of redundancy analysis,variation partition and structural equation model show that the degree of dry soil layer(soil water content in dry soil layer),soil hydraulic parameters and topographic factors jointly affect the recovery duration of dry soil layer.Based on this,an empirical model including soil hydraulic parametersαand n,field capacity,slope and soil water content in dry soil layer was established[duration=54.7-22.6×a-15.9×n+0.2×FC-19.4×cos(slope)-0.008×DSLSWC,R~2=0.76,RMSE=1.3 a].Then the dry soil layer recovery duration under land use change could be quickly estimated.In this paper,the spatial distribution of deep soil hydraulic properties in the typical loess zone was estimated,which provided basic data support for quantitative simulation and analysis of regional soil hydrological process and response to land use change.Artificial introduction of exotic high water-consuming vegetation can lead to soil water imbalance,cause deep soil desiccation,and threaten the restored ecosystem health and sustainability.Combined with observation and simulation results,we found that dry soil layer can be effectively restored by changing vegetation types or land use,and the recovery duration is affected by dry soil layer degree,soil hydraulic properties and topographic factors.The study results have important scientific significance for further understanding the soil hydrological process in the Loess Plateau and its long-term response mechanism to land use change,and also provide a scientific basis for the regulation of dry soil layer of artificial vegetation in the Loess Plateau. |
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