Characteristics Of Soil Moisture And Root Water Absorption Of Aged Plantation In Loess Hilly Region

With the implementation of a series of vegetation restoration projects such as the project of Grain for Green,the vegetation coverage area of the Loess Plateau has been significantly increased,and soil erosion has been greatly alleviated.While maintaining the water and soil,the artificial forests also caused some problems such as the deep soil drying because of their deep roots.Therefore,a comprehensive analysis of the changes in soil moisture and the characteristics of root water uptake in the artificial forests is helpful for evaluating the sustainability of ecological restoration.At present,many achievements have been made in the research on the variation trend of soil moisture in the deep profile of the artificial forests with the stand age.However,most research replace the continuous monitoring of soil moisture with the space-for-time substitution method,which is impossible to reflect the real situation of long-term changes in water content of artificial forests.Therefore,old-aged Platycladus orientalis,Pinus tabulaeformis and Caragana korshinskii located in the typical loess hilly area of Suide County,Shaanxi Province in this study were selected as research subjects,and long-term measured data were used to analyze the changes of deep soil moisture and water uptake characteristics of vegetation’s roots in the old-aged artificial forests.Moreover,the impact of the artificial forests on potential groundwater recharge was analyzed using the tritium peak tracer method based on grassland.The main results are as follows:（1）The deep soil moisture and influencing factors of old-aged artificial forests were analyzed.When the plantation was 35 years old,the soil water deficit of 2-20 m soil layers of Platycladu sorientalis,2-23 m soil layers of Pinus tabulaeformis and 2-23 m soil layers of Caragana korshinskii were 1108 mm,1430 mm and 1205 mm and the deficit ratios were 43.5%,47.0%and 40.7%;respectively.The effective water storage capacity is484 mm,415 mm,655 mm,accounting for 15.7%,11.4%and 18.2%of the maximum effective water storage capacity.The overall soil desiccation degrees of the three plantations in the rank were Pinus tabulaeformis>Platycladu sorientalis>Caragana korshinskii.The soil moisture and soil clay content in the old-aged artificial forests were significantly positively correlated while between root distribution and soil moisture was not significant.The soil moisture of the three artificial forests in the soil water stable layer had a similar corresponding relationship with the soil clay content,indicating that the deep soil moisture of the old-aged forests were mainly affected by the soil properties after a long time of root water absorption,and has nothing to do with the vegetation type.（2）Based on the long-term water change and root distribution of the old-aged artificial forests,the water use characteristics of the old-aged artificial forests were clarified.Artificial forests had a large root system in the surface layer to make full use of precipitation and shallow soil water,and in the deep soil,a small number of fine roots with higher specific root length were used to extract deep soil water.The soil moisture of the soil layer below 17 m of Pinus tabulaeformis,and Caragana korshinskii at the age of 35（2020）was significantly lower than that of the age of 22（2007）,indicating that the old-aged plantation can still use deep soil moisture to alleviate drought,rather than only relying on precipitation in the current year for survival.The results of regression analysis showed that the water consumption depth of 35-year-old artificial forest has exceeded 20m.（3）The dynamic process of water deficit in old-aged artificial forests and its impact on potential groundwater recharge were quantitatively analyzed.The annual average water consumption rates were 42 mm·a^-1,53 mm·a^-1,and 47 mm·a^-1 for 35-year-old Platycladus orientalis,Pinus tabulaeformis,and Caragana korshinskii;respectively,accounting for about 11%of the average annual precipitation.The potential groundwater recharge of the grassland was 41.4 mm·a^-1,accounting for about 9%of the annual precipitation.Tritium peak tracer results show that the root growth rate of the artificial forest is much higher than the rate of precipitation infiltration.In addition to using the precipitation in the current year,the old-aged artificial forest also consumes the precipitation from at least 57 years ago,which can indicate that there is almost no potential groundwater recharge at this stage.This study revealed the characteristics of deep soil water utilization after the artificial forest entered the aging stage,and quantified the deep soil water consumption rate of the artificial forest,which can provide theoretical guidance for the selection of suitable planting years for vegetation restoration and construction on the Loess Plateau.