Sources Of Soil Water And Nitrogen And Their Response To Land Use Change In Deep Deposits Of Northern Shaanxi,china

With limited water resources,substantial land use change,and thick unsaturated zones,the transport of soil water and nitrogen in the Loess Plateau has been significantly affected.Previous studies only focused on soil water deficit and nitrate-nitrogen（NO₃^--N）accumulation in the shallow layers;however,the characteristics and sources of soil water and nitrogen in the deep layers are poorly understood,which is harmful to the sustainable management of vegetation and water resources.As such,this study took the loess deposits of northern Shaanxi as the study region,and collected precipitation and soil samples within 0-20 m under different land use types.The contents,stable,and radioactive isotopes(δ²H,δ¹⁸O,and ³H)of soil water were determined to discuss the characteristics,sources,and recharge mechanism.Further,the contents and stable isotopes(δ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-)of nitrate were measured to explore the characteristics,sources,and potential leakage risks to groundwater.Finally,the classical statistics and wavelet analysis were combined to identify the individual and combined factor（s）influencing water and nitrogen transport.The main results are as follows:1.Land use change has significant impacts on the characteristics and sources of deep soil water.Compared with farmland and grassland,shrubland and forestland had lower soil moisture and larger water deficit,and the different degrees of soil desiccation were mostly related to the excessive water absorption of developed root system.Deep soil water was mainly recharged by wet events in the rainy season.Piston flow dominated the soil water movement.But they were significantly influenced by land use types.Specifically,the forestland and shrubland had lower evaporation effects and higher precipitation offset,and their soil water was recharged by wet events with higher intensities than those under farmland and grassland.Nevertheless,the deep-rooted plants substantially reduced recharge rates due to excessive root water uptake,and thus prevented rainfall from replenishing deep soil water.2.There are significant differences in NO₃^-characteristics and sources under different land use types.The NO₃^--N profiles exhibited a parabolic shape with peak depths at 2.6-9.2 m.Greater peak depths corresponded to greater NO₃^--N values,and the peak NO₃^--N level varied from 0-10 m.The total NO₃^--N accumulation ranged 3181-9018 kg N ha^-1,and the non-fertilizer profiles accounted for 35%-73%of the arable farmland.The NO₃^-sources varied with depths.According to the isotope mass balance,the NO₃^-above,near,and below the peak depths mainly originated from atmospheric NO₃^-（25%-34%）,synthetic fertilizer（49%-68%）,and soil organic nitrogen and manure（50%-73%）.The estimated residence time of NO₃^-in the unsaturated zones ranged from 270-620 a.As such,the NO₃^-legacy effects should be given considerable attention for the protection of the soil and groundwater environment.3.The individual and combined factor（s）influencing soil water and nitrate transport are quite different.High variances of soil water were mostly observed in the deep layers,especially on large-and middle-scale.Soil physicochemical properties rather than meteorological factors dominated the transport process of deep soil water.In specific,sand content and magnetic susceptibility were the most important influencing factors.And their combined effects were also related to soil moisture,especially on large-scale.The magnetic susceptibility and bulk density were strongly correlated within 3-13 m,which significantly affects the soil water transport.High variances of NO₃^-were primarily observed below 6 m.soil organic carbon（SOC）and p H were the most important influencing factors.The combined effects of soil moisture-p H-SOC had significant impacts on NO₃^-variability,mostly on large-scale.Soil moisture significantly affected NO₃^-transport,while p H and SOC significantly affected NO₃^-transformation.This study analyzes the sources of soil water and nitrogen and their response to land use change in the deep loess deposits of northern Shaanxi,China.Shrubland and forestland had large water deficit and were recharged by wet events with higher intensities;however,the recharge rates were lower because of large root water uptake.The NO₃^--N profiles were parabolic,with the major source of synthetic fertilizer.Further,soil NO₃^-can exist for a long time in the deep layers,posing a potential threat to groundwater.Sand content and magnetic susceptibility significantly affected soil water transport,while SOC and p H significantly affected soil NO₃^-transport.This study is of great significance for optimizing land use structure and protecting soil water and groundwater resources.