Dynamic Processes Of Water Replenishment And Transfer In The Soil-plant System

The dynamic processes of water replenishment and transfer in the soil-plant system（SPS）are not only associated with changes in precipitation inputs,but also affected by water status and transfer within soil and plants.These processes involve water storage and transfer in soil and plants,and the interaction between soil and plants.Different methods can reveal the dynamics of water transfer in different water cycle processes in the SPS,which provides a full understanding of the processes and mechanism of water replenishment and transfer in the SPS.In this study,soil water content and stable isotopes(?²H and?¹⁸O)were used to reveal water replenishment and transfer in soil and root zone,and their water sources at the site in a grove dominated by Cinnamomum camphora trees in a red soil hilly region in China with a subtropical monsoon climate.Meanwhile,electrical resistivity tomography was used to measure electrical resistivity of tree stems at the site in a Mediterranean climate area of south Australia.The measured electrical resistivities were used to estimate stem water content,which was used to reveal temporal and spatial changes of water replenishment and transfer within tree stems.The main results are summarized as follows:（1）The seasonal origin index（SOI）was improved based on stable isotopes data,which was used to trace water sources of water replenishment in soil and root water uptake.Two new methods were proposed in this study,the first one was the piecewise isotope balance（PIB）method,which calculates the proportion of plant-accessible water within the root zone replenished by precipitation,and can be used to assess the ecohydrological separation hypothesis;the second method was used to estimate temporal and spatial distribution of water content within tree trunks based on electrical resistivity tomography,and thus to reveal seasonal patterns of water replenishment within tree trunks.（2）High-frequency sampling of hydrogen and oxygen isotope composition of soil water reveals the processes of precipitation replenishment,infiltration depth,water movement and retention duration in the soil profile.Monitoring soil water content can only quantify the amounts and depths of water replenishment.During the period between March and June,precipitation water infiltrated more deeper but infiltration amount was small;during the period between July and November,infiltration depth was low while its amount was large.These indicate that infiltration water was likely to bypass the soil and recharged groundwater in wet season while it was easy to be stored in topsoil layer（0?40cm）.The?¹⁸O values for soil water at the depth of 0?40cm varied seasonally while they became relatively stable for deep soil water（>40cm）.In addition,summer precipitation was a main water source for soil water,which accounts for 51%of the replenished water in soil water at the depth of 0?40cm.In spring,winter precipitation was the dominated water source for soil water at the depth of 0?20cm.（3）The PIB method revealed more details of the occurrence time and duration of the ecohydrological separation.The ecohydrological separation was very likely to occur in spring at this subtropical humid study site,which maintains about two months.Further,moisture replenishment in root zone showed an obvious seasonal variation,summer precipitation accounted for about 42%of the root zone moisture while spring precipitation was likely to bypass root zone.（4）A temperature correction model for tree electrical resistivity was established for the first time based on the electrical resistivity tomography technology.We found that the exponential model performed the best,and the key parameter in the model,reflecting temperature sensitivity,was independent of tree species（keep relatively stable）.Additionally,the temporal and spatial distribution of trunk moisture was estimated according to the relationship between electrical resistivity and trunk moisture.The seasonal variations of the resistivity-estimated volumetric moisture content within sapwood fluctuated between 0.23 and 0.50 cm³/cm³.The variations in sapwood volumetric moisture content captured the processes of moisture replenishment in wet season,and moisture loss in dry season.（5）The slope of soil water line was found to be larger than that of the local meteoric water line for this forest soil in this subtropical humid area,this was mainly due to the reason that precipitation mixing is the most important factor controlling isotopic composition of soil water.The isotopic composition of precipitation generally varies in different seasons.What’s more,89%of lc-excess values of summer precipitation（main water source for soil water）was negative,and 79%of lc-excess values of autumn and winter precipitation was positive,which leads to a high slope of soil water line.Therefore,the caution should be taken when the lc-excess method is applied in examining the evaporation effect on a sampled water,because negative values of lc-excess for a water body do not necessary tell the effects of evaporation.The new findings in this study such as spring precipitation easily bypasses plant root zone,and summer precipitation is the main water source for soil and plant in this subtropical humid area,if these phenomena are universal,it has very important guiding significance for water management in agriculture and forestry.