Water Use Sources Of Typical Plant Species In Subtropical Humid Area Based On Stable Isotopic Analysis

Research on the water use of plant is a hot issue in the fields of ecohydrology.We can understand the internal water cycle process of the regional forest system by studying the characteristics of plant water use.This study takes Cinnamomum camphora,Cunninghamia lanceolate,Gardenia jasminoides in the needle-leaf and broad-leaf mixed forest and Castanea mollissima,Camellia japonica in the artificial economic forest as objects,the stable isotopic composition of precipitation,Soil water,plant stem water and leaf water during the March 2017 to September 2019 were analyzed.Secondly,the water uptake layers of five plant species was analyzed by the graphical inference,water-line inference,Iso-Source model and Mix SIR model.In addition,the advantages and disadvantages of four methods in determining plant water uptake were compared.Finally,combined with meteorological data(temperature,relative humidity,solar radiation,vapor pressure deficit),the main factors affecting the enrichment of stable isotope in leaf water of C.camphora and C.lanceolate under different time scales was analyzed.The results show that:(1)The ?¹⁸O in precipitation,soil water,xylem water five plant species in the study area showed higher in wet period(October–June of the following year)and lower in dry period(July-September).To be more specific,the range of?¹⁸O in soil water decreased with the increase of depth.The?¹⁸O in xylem water of five plant species and 0—60 cm soil water of two type forest was basically consistent with the?¹⁸O in the precipitation,and the?¹⁸O in 60—100 cm soil water lagged behind precipitation by 1—2 months.(2)If there is no obvious difference of the stable isotopic composition in soil water at each depth,compared with the graphical inference,Iso-Source model and Mix SIR model,the water-line inference could determine the water uptake layers of C.camphora.The main water uptake layers and utilization ratios of C.camphora calculated by Iso-Source model based on different isotopic compositions were both different.However,the main water uptake layers and utilization ratios of C.camphora calculated by the Mix SIR model based on different isotopic compositions were both laregly identical.In concllusion,Mix SIR model is more reliable than Iso-Source model in the quantitative research of plant water use.(3)Pronounced seasonal variation of water use of five plant species in study eara was observed,during the wet period of March to June 2017,October 2017 to June2018 and October 2018 to June 2019,C.camphora mainly used the soil water at0—20 cm,0—40 cm and 0—20 cm depth,respectively.In wet period of October 2018to May 2019,C.lanceolate,G.jasminoides and C.japonica largely used the soil water at 0—20 cm?0—10 cm and 0—10 cm depth.With the advent of the drought period,the water absorption depth of the 4 plant species gradually decreased,During the dry periods of 2017,2018 and 2019 July to September,C.camphora mostly uptook the soil water at 20—60 cm,0—60 cm and 60—100 cm depth,respectively.From June to July of 2019,C.lanceolate,G.jasminoides and C.japonica mostly uptook soil water at the depth of 20—100 cm,0—20 cm and 0—100 cm,respectively;however,the three plants generally used the soil water at the depth of 60–100 cm in August to September of 2019,due to severe drought.In addition,C.mollissima hardly used soil water during leaves fallen period(from December 2018 to March 2019).During the periods of October to November and April,it primarily used soil water within 0—10cm depths.From May to September,it mainly used soil water at the depth of 60—100cm.(4)On the diurnal scale,the kinetic fractionation of stable isotopes in leaf water of the two plant species in the wet period were stronger than those in the dry period,the?¹⁸O and?¹⁸O_L(the enrichment of ¹⁸O in leaf water)in leaf water of C.camphora and C.lanceolate in wet period were higher than in dry period,otherwise the slope of LWL during the wet period was smaller than in dry period.On the hourly scale,In typical sunny days of wet period,the?¹⁸O and?¹⁸O_Lin leaf water of C.camphora and C.lanceolate were significantly higher than dry period,but slope of the LWL was smaller than in dry period.(5)On the diurnal scale,the enrichment of?¹⁸O in the leaf water of C.camphora and C.lanceolata was mainly affected by RH,RS and VPD.On the hourly scale,the main meteorological factors affecting the enrichment of?¹⁸O in the leaf water of two plant species were T,RH,and VPD.Among the three meteorological factors that affect the enrichment of?¹⁸O in leaf water on the diurnal scale,RH has the largest impact.However,among the three meteorological factors that affect the enrichment of?¹⁸O in leaf water on the hourly scale,VPD and T have the largest impact.