Water Use Strategies And Soil Water Restoration Regulation Of Deep-rooted Plants In The Loess Plateau

The Loess Plateau is characterized by thick soil layers,serious soil erosion and limited precipitation resources,making water the most critical factor limiting plant growth.Therefore,deep-rooted plants with well-developed and drought-tolerant root systems have become the main plant species selected for vegetation restoration and development of economic forests on the Loess Plateau,such Robinia pseudoscacia,Medicago sativa,Caragana korshinskii,Hippophae rhamnoides and Malus pumila.However,planting plants over a large area and for a long period of time will excessively consume soil water,resulting in a negative soil water balance and thus a soil water deficit.Long-term soil water deficit will slow down or inhibit the absorption and utilization of soil water and nutrients by plant roots,which will affect plant growth and even cause death.Therefore,it is of great scientific significance to study the water utilization strategies of deep-rooted plants and soil water recovery control measures for soil water management and sustainable construction of ecosystems after vegetation restoration in the Loess Plateau.The paper took the artificial Robinia pseudoscacia and Medicago sativa planted in large areas on the Loess Plateau as research objects,and used depth-controlled artificial isotope labeling experiments to study the distribution of active water-absorbing roots of Robinia pseudoscacia and Medicago sativa in the soil profile of 0～1000 cm to explore the feasibility of depth-controlled artificial isotope labeling techniques in the study of root distribution;it also utilized the Hydrogen and Oxygen Stable Isotope technique in combination with the Bayesian Mixing Model（Mix SIAR）to quantify the contribution of soil moisture to the xylem water of Robinia pseudoscacia and Medicago sativa in the 0～1000 cm profile,and to explore the influence of seasonal changes in climate on the source of plant water use;meanwhile,we calibrated and examined the hydraulic parameters of different soil layers in the deep soil profile by utilizing the measured soil water content and isotope profile dynamics of acacia in the years of 2021～2022,and simulated the distribution of soil with different thinning intensities in the Robinia pseudoscacia.We simulated the dynamic changes of soil moisture in the soil profile of acacia with different thinning intensities,and analyzed the effects of inter-annual climatic variations and thinning intensities on the water storage capacity of the 0～1000 cm soil profile.Finally,we considered the effects of climatic variations and planting systems on the soil moisture recovery process in the deep profile of Medicago sativa fields,and proposed measures for the soil water recovery of the desiccated layer of soil on the Loess Plateau.The main research results are summarized as follows:（1）The roots of 10 and 20 years Robinia pseudoscacia and 5 and 18 years Medicago sativa in the Loess Plateau were mainly distributed in the 0～400 cm soil profile.The correlation coefficients of the root distribution ratios of 10 and 20 years Robinia pseudoscacia obtained by traditional excavation and depth-controlled isotope labeling techniques were 0.98 and 0.97,respectively,and those of 5 and 18 years Medicago sativa obtained by the corresponding methods were 0.83 and 0.87,respectively,which indicated that artificial isotope labeling techniques can provide a direct and reliable method for the study of root distribution of plants on the Loess Plateau.The correlation coefficients were0.83 and 0.87,respectively.（2）Under the influence of rainfall infiltration and evapotranspiration,the soil profile water content,δ²H,δ¹⁸O and lc-excess of Robinia pseudoscacia（20 years）and Medicago sativa（5 years）,which were in the peak growth period on the Loess Plateau,fluctuated seasonally in the 0～200 cm soil layer,and showed different water utilization strategies.Compared with Medicago sativa,the water utilization pattern of Robinia pseudoscacia was more significantly affected by rainfall.In June,the water absorption source of Robinia pseudoscacia was mainly in the shallow（0～50 cm）and deep（600～1000 cm）layers,in July,the main water source was located in the（0～50 cm）soil layer,in August,the water source of Robinia pseudoscacia shifted downward to the middle（200～600 cm）and deep（600～1000 cm）layers,and in September,due to the increase of rainfall Robinia pseudoscacia’s water utilization source shifted to the shallow（0～50 cm）and middle（600～1000 cm）layers once again.（0～50 cm）and mid-shallow（50～200 cm）soil profiles.Medicago sativa utilized shallow（0～50 cm）and medium-shallow（50～200 cm）soil water from July to August,and the source of Medicago sativa water use shifted further to the shallow（0～50 cm）soil in September.Overall,the main source of water utilization of Robinia pseudoscacia was from 0～50 cm and 200～600 cm soil profiles throughout the growing season,while the water utilized by Medicago sativa was mainly from 0～200 cm soil profile.（3）Hydrogen and oxygen isotopes exist in both liquid and gas phases in unsaturated soils,and thus their dispersion coefficients are affected by changes in soil water content.Through the collection of literature,we established three possible functional relationships between the dispersion coefficient and soil water saturation,simulated the dynamic changes of hydrogen isotopes in the 0～1000 cm profile of acacia woodland,and compared them with the measured hydrogen isotope profiles.The accuracy between the simulated and measured isotope profiles was high when the dispersion coefficient did not vary with soil water saturation;the simulation had the lowest accuracy when the relationship between the dispersion coefficient and saturation was a power function;and the simulation accuracy was located in between when the dispersion coefficient first increased and then decreased with decreasing saturation.Therefore,it can be simply assumed that the dispersion coefficient does not vary with soil water saturation when modeling hydrogen isotope dynamics in unsaturated soils.（4）Climate fluctuations between years and thinning intensity were important factors influencing soil water storage.High-intensity thinning was the main factor for the increase of soil water storage in 0～300 cm of Robinia pseudoscacia.The soil water storage capacity of300～700 cm of Robinia pseudoscacia was greatly affected by interannual climatic fluctuations,and the soil water storage capacity of the soil profile increased more obviously in the climatic sequence of high initial rainfall,and the soil water storage capacity of 300～700 cm of acacia increased with the increase of thinning intensity under different climatic fluctuations between years.Soil water storage in 700～1000 cm of Robinia pseudoscacia was less affected by climatic fluctuations between years and thinning intensity,and increased only under high-intensity and prolonged thinning conditions.The effect of interannual climate change was more obvious.Robinia pseudoscacia T_r decreased with the increase of thinning intensity,and E_s increased with the increase of thinning intensity.Compared with no thinning,the median annual E_s of thinning 1/3,thinning 1/2 and thinning 2/3 increased by28.8、54.1and 88.2 mm y^-1,respectively,and the median annual T_rdecreased by 37.4、71.2and 118.9 mm y^-1,respectively.（5）The 15-year-old Medicago sativa field on the Loess Plateau already had a severe soil dry layer,and simulated the effects of cropping system adjustment and climate change on soil moisture recovery,and found that the median time for the soil water of the 0～1500cm soil profile of the 15-year-old Medicago sativa field converted to winter wheat to recover to the normal water content of the farmland under the condition of considering the inter-annual variability of the historical climate was 27 years,and the median time for the moisture of the soil profile of the 15-year-old Medicago sativa field converted to corn to recover to the normal water content of the farmland was 18 years.The median time for soil water to return to the normal moisture content of the farmland after conversion to corn was18 years.Considering different scenarios of future climate change（RCP 4.5 and 8.5）,based on the 10 GCMs modeled to predict the future climate profiles,the median time for soil water to return to the normal moisture content of the farmland after conversion of Medicago sativa fields to winter wheat was 37.5 years under the RCP 4.5 scenario,and the median time for soil water to return to the normal moisture content of the farmland after conversion of Medicago sativa fields to corn was 30 years under the RCP 8.5 scenario.scenario,the median soil moisture recovery time after Medicago sativa conversion to winter wheat was42.5 years and the median soil moisture recovery time after Medicago sativa field conversion to corn was 31.5 years.Thus,relative to historical climatic conditions,climate change will slow soil moisture recovery in the deep profile of Medicago sativa fields under the same cropping regime.