The Effect Of Deep Soil Desiccation On Water Consumption Characteristics Of Apple Tree On Loess Plateau

The implementation of the project of returning farmland to forest can effectively reduce soil erosion,increase regional vegetation coverage,and achieve good results in ecological construction.However,the continuous input of deep-rooted vegetation consumes a large amount of soil water,the limited precipitation is difficult to meet the evapotranspiration demand of regional vegetation,and the phenomenon of deep soil desiccation is becoming more and more severe.Therefore,in order to further clarify the water consumption characteristics of the tree species returning farmland to forest(deep rooted)and understand the response relationship of deep root plants to soil water.In this study,based on loess plateau,and select the economic stand-apple tree as a representant of deep-rooted vegetation,and explore the potential harm of deep soil desiccation on water consumption for deep-rooted system based on these data: sap flow density,leaf behavior characteristics,soil water content,soil water isotope and so on.The experiment was carried out from 2017,June to 2018,October in the typical loess gully region(old apple orchard in Changwu table-land).Artificial drilling holes were used to recover soil moisture in the deep layer(4-8m),and two treatments were set: one plot is implemented soil surface irrigation(SI)about 100 mm,the other is implemented surface and deep irrigation(SDI),the deep irrigation is about 300 mm at 8 m depth additionally.The main conclusions are as follows:(1)Revealed the spatial variation characteristics of soil moisture and the water source of soil dry layer in older-age orchards.In older-age apple orchards,the apple trees have an evenly root distribution pattern upon all profile,and relatively homogenous soil texture,which makes the soil moisture change little along the depth,the main change of soil moisture is concentrated in the shallow(0 to 4 m),which is affected by seasonal precipitation and surface meteorological conditions,the deep soil layers have an uniform soil moisture distribution(below 4 m).The degree of desiccation is basically the same,and reach to the lower limit of plant available water;the soil desiccant index is stable between 40% and 50%.Deep soil water isotope variability is small,and it is uniform distributed along the entire deep profile,deep soil water isotope receives shallow soil evaporation at the plot scale.The radioactive tritium isotope have a single peak in the whole profile,and the peak depth is just located at the soil dry layers,so the soil water of soil dry layer was from the precipitation of 50 years ago.(2)Revealed the response of old apple trees to deep soil water and the effect of deep soil desiccation on the plant water uptake.On loess tableland,the deep root system of old apple orchards has an obvious re-response process to the recovery of deep soil moisture.After the recovery of deep dry soil,plants use the deep root system to absorb more deep soil water,thus reducing the full utilization for shallow soil water.Deep soil desiccation promoted the absorption for shallow soil water and enhanced the plants dependence on seasonal precipitation.Deep soil desiccationg weakens the plant's transpiration rate(Sap flow density),and weakens the water consumption capacity of apple trees.The recovery of deep soil moisture is conducive to reducing the degree of water stress of deep dry soil on the plant's deep root system and improving the plant's transpiration capacity.(3)Illuminated the effects of deep soil desiccation on the physiological characteristics of apple trees.There is an obviously seasonal change for leaf stomatal resistance of apple tree leaves in older-age orchards,and which is highly sensitive to seasonal precipitation.Stomatal resistance of apple trees' leaves was significantly reduced when deep soil moisture is recoveried.Moreover,stomatal resistance of plant leaves was not significantly changed seasonally when deep water was sufficient.At the same time,compared with the mature leaves,new-born leaves have a relatively low stomatal resistance,but they are still regulated and controlled due to the water stress of mature leaves.Compared the single shallow soil water recovery with the simultaneous recovery both deep and shallow soil,the deep soil desiccation increased the water use efficiency of apple trees for shallow soil water,and the carbon isotopes in the leaves were enriched as a whole.(4)Analyzed the contribution proportion of deep and shallow soil water to apple trees based on water isotope.The water consumption of old apple trees is seasonally dependent on precipitation,and the shallow soil water is the main water source in the whole growth process of old apple trees.