| Since the large scale implementation of the“Grain for Green”program in 1999,the vegetation coverage of the Loess Plateau has improved significantly,and there were significant changes in land use type,landscape patterns,hydrological processes,and biochemical processes.The control of soil erosion in China has achieved remarkable results.However,unsuitable vegetation restoration patterns would led to the imbalance between soil moisture and nutrients,causing problems such as the soil desiccation and ecosystem degradation in arid and semiarid region.The dried soil layer,in turn,has a series of negative effects on ecological restoration.To explore the soil drying process,the possibility of soil water restoration in the dried soil layer and the restoration patterns,as well as the change of soil nutrients,the vegetation conversion experiment was conducted in the Liudaogou watershed in the water-wind erosion crisscross region of the Loess Plateau to study the response of soil water nutrients.Through field in-situ monitoring the change process of soil water content during vegetation conversion,and through indoor experiment analyzing soil nutrient contents,to clarify the soil water depletion and restoration processes in water-wind erosion crisscross area of the Loess Plateau and corresponding nutrient changes.Soil samples were collected to measure soil physical index,combined the water absorption characteristics of vegetation,soil bulk density and particle composition,and using the HYDRUS-1D model to quantify the upper boundary water content of the dried soil layer,to estimate the severity of soil desiccation accurately.Finally,analyzing the interception effect of shallow-rooted pastures on slope runoff and sediment,seeking vegetation restoration patterns to alleviate soil desiccation,improve soil quality and reduce soil erosion.In this thesis,two suitable methods for vegetation restoration in arid and semi-arid regions are proposed by systematically studying the process of soil water depletion and restoration.The main results are listed as follows:（1）Studying the change process of soil water content after conversion of cropland（soybean,Glycine max L.）to artificial grassland（alfalfa,Medicago sativa L.）,it was found that soil water content was depleted quickly,and the average soil water content decreased by 24.5%during four-year experimental period.Artificial alfalfa land and Caragana korshinskii Kom.land existed a dried soil layer in 300-600cm soil layer.After three soybean lands converting to alfalfa lands(the average dry biomass was 248 g m-2,194 g m-2,156 g m-2,respectively),the soil water content of the fields’profile was reduced to many degrees.By the end of the growing season in2020,soil water contents of three fields were reduced by 21.1%,27.3%,and 25.1%,and the depletion depth reached 380,560,and 580 cm,respectively,from the beginning of the growing season in 2017.The depletion time is 6.1,7.5,and 8.4 years,respectively.Therefore,this study considered that the optimal age of alfalfa planting should not exceed 8.4 years.In addition,by studying the changes of soil water content in soil profile of arificial shrubland（Caragana）field after stubble,the soil water content in 0-200 cm soil layer restored rapidly in a short period,and the water content increased by 41.7%.However,the soil water content was quickly consumed at the beginning of the next year’s growing season,and the average soil water content was reduced to a low level of 0.09 cm3cm-3.The growth of artificial vegetation would seriously depleted soil water content whether in wet,normal or dry years.（2）The soil water content of dried soil layer was restored after the artificial grassland（alfalfa）was converted to cropland（soybean）,or the artificial shrubland（Caragana）was converted to natural grassland.Abundant precipitation in experimental period has accelerated the recovery rate.After the alfalfa land was converted to soybean land,the soil water content was restored quickly in three consecutive wet years（2016-2018）.The soil water content in 0-500 cm soil layer increased by 89.4%,the restoration depth reached 800 cm.Also the soil water content of the entire profile was higher than the upper boundary water content of dried soil layer.At the end of growing season in normal year（2019）and drought year（2020）,soil water content decreased by 9.6%and 6.5%,respectively,compared with that at the end of the growing season in 2018.After four Caragana fields（the average leaf area index was 4.32,3.43,2.01,1.77,respectively）were converted to natural grasslands（NG1,NG2,NG3,NG4）,the soil water content restored to many degrees.Specifically,the soil water content of NG2 and NG3 fields increased by 21.8%on average during three experimental years（2018-2020）,and the restoration depth of both has reached 800 cm,the soil water content of NG1 and NG4 fields increased by21.3%on average.The restoration depth reached 400 and 420 cm,respectively.The average recovery period of NG1 and NG4 fileds was 11 and 24 years,respectively.Therefore,this study considered that the conversion of alfalfa land to soybean land and Caragana land to natural grassland were beneficial to the soil water restoration of the dried soil layer.（3）Artificial shrub-grass land and abandoned grassland increase soil organic matter（SOM）and total nitrogen（TN）contents compared with farmland and bareland.The influence depth of alfalfa,Caragana,and abandonment lands about SOM and TN contents reached 40 and 60 cm,and the average content of SOM and TN in alfalfa,Caragana,and abandonment land within the influence depth was higher than 43.0%and 30.5%of that average contents of farmland and bareland.In abandoned grassland,the relative increase of soil organic carbon（SOC）content in the 0-50 cm soil layer was larger than that of TN.In alfalfa land,the relative increase of TN content in50-200 cm soil layer was larger than that of SOC.The content of SOM and TN in0-10 cm soil layer of abandoned grassland was increased significantly with vegetation restoration time（P<0.05）.The SOM content in 2016 increased by 34.4%compared that in 2013,and the TN content increased by 34.3%.Vegetation conversion has a significant effect on the SOM and TN contents.The SOM and TN contents in 0-200cm soil layer decreased to many degrees after five years of conversion from alfalfa land to soybean land.It decreased by 16.2%and 19.3%on average,respectively.Four or five years after the conversion of cropland to alfalfa land,the SOM and TN contents were decreased by 12.3%and 20.7%,as well.Therefore,vegetation change affects soil content,which also affects soil nutrient content accordingly.（4）Soil water content of 0-400 cm profile of the three shallow-rooted pasture land（Leymus chinensis Trin.Tzvel.,Agropyron cristatum L.Gaertn.,Festuca elata Keng ex E.Alexeev）decreased year by year,and the decrease ratio were 25.0%,20.8%,and 23.3%from 2018 to 2020,respectively.The average soil water content in0-400 cm profile of shallow-rooted grassland was 0.15 cm3 cm-3 until to the end of2020 growing season.During the experimental period,erosive precipitation within the range of 52.2-108.4 mm,runoff yield in range three shallow-rooted grasslands were in the range of 19.8-546.0 L,and the sediment yield were in the range of 0.8-29.4 kg.With the grassland vegetation growing,the benefits of runoff and sediment reduction gradually enhanced compared with that in bareland.Until to 2020,the benefits of yield and sediment reduction of slopes of three shallow-rooted pasture grassland were15.9%,3.4%,17.2%and 20.5%,15.5%and 51.6%,respectively.The flow yield of grassland has been reduced by 8.7%on average compared that of bareland,and the sediment yield of grassland has been reduced by 23.9%on average compared that of bareland.The sediment yield of grassland has been reduced by 43%compared with that of farmland.With the growth of vegetation,the benefits of sediment reduction of grassland were gradually better than that of farmland.Therefore,the grassland could be a better type to reduce runoff and soil erosion. |
PDF Full Text Request