The Dynamic Of Soil Water And Shelterbelt Plants Photosynthetic And Stem Flow Under Saline Drip Irrigation In Taklamakan Desert

Artificial vegetation planting is one of the most effective methods for ecological reconstruction and desertification control in sand areas.Water is a key factor affecting the structural stability and maintain normal function of desert ecosystems,and also the biggest limiting factor of restricting vegetation growth.It plays an important role in wind sheltering and sand fixation.In the soil-plant-atmosphere continuum?SPAC?,transpiration is the main route for plant water dissipation,participates in and affects the material cycle and energy flow of the ecosystem,and is influenced by biophysical and environmental processes at the interface between soil,vegetation and atmosphere.As a basic unit of physiological metabolism and substance accumulation,photosynthesis is sensitive to changes in the external environment and is an important link in analyzing the response of plant growth and metabolism to the environment.The Taklamakan Desert,located in northwest China,is the second most mobile desert in the world.The Desert Highway Shelterbelt through it can effectively prevent and control sandstorm erosion.This study was located in the hinterland of the Taklamakan desert.Field water control experiments were used to study 1)the soil moisture dynamics of the shelter forest under different irrigation treatment;2)Transpiration characteristics and water use characteristics of Calligonum mongolicum?C.?and Haloxylon ammodendron?H.?under different irrigation treatments;3)Photosynthetic adaptation mechanism of Calligonum mongolicum and Haloxylon ammodendron under different irrigation treatments;4)The suitability and feasibility of water-saving irrigation system for the shelterbelt in the Taklamakan desert.The results are:?1?The soil moisture dynamicsAt the vertical depth,the 0-100cm soil layer can be divided into three layers according to the change characteristics of soil water content,the soil water rapid layer?0-30cm?,the middle variable layer and the relatively stable layer.The lower limit of the middle variable layer and the upper limit of the stable layer are closely related to the irrigation amount,plant type and plant root growth condition,and should be distinguished according to the actual situation.In terms of time scale,the change of soil moisture in shallow layer?0-60 cm?after irrigation can be divided into rapid decline period?1-9 d?,slow decline period?9-19 d?and relatively stable period?19-39 d?.After irrigation,the soil moisture content in 60-200 cm decreased slowly,and the rate of decline were significantly lower than that in the depth of0-60 cm.Soil moisture under 200 cm had replenishment to the soil moisture in the 60-200cm range.The water storage capacities of 0-100 cm soil were in cubic polynomial relation with the number of days 1-9 days after irrigation,and the fitting results R² were all above0.88.From the average water content of 0-300 cm in the soil from May to September,the soil moisture content planting C.and H.under the 10-day irrigation cycle were W3>W1>W2,under the 20-day and 40-day irrigation cycle were W3>W2>W1.Irrigation amount,plant type,atmospheric factors,plant root growth status et al.all have an impact on soil water change.In May to July,plants grew rapidly and air temperature increased gradually,which belongs to the net consumption period of soil water;in July to September,plant growth rate slowed down and temperature decreased,which belongs to the net storage period of soil water.The difference of soil water content between August and September was not significant,but there were significant or extremely significant between that of July?P<0.05?.?2?Transpiration characteristicsCompared with dust days,stem flow at night in sunny days accounted for a smaller proportion of stem flow in all day.Under the same irrigation condition,the stem flow fluxes of C.in sunny days were above 1.5 times of that in dust days,and that values of H.were above 3.5 times.The stem flow fluxes of C.and H.at night were respectively accounting for 14.3%-24.9%and 7.3%-10.4%of the whole day.The stem flow of C.and H.started after sunrise,and reached the peak at 10:00-13:0 and 13:00-15:00 respectively,and then gradually decreased.The decline rates of C.were faster while H.were slow,and H.maintained a high stem flux in long days.The diurnal variation of stem fluxes of the two plants under different treatments showed two kinds of curves:"single peak"and"double peak".The"double peak"curve was not in the treatment with the minimum irrigation amount,and was more correlated with the soil moisture content of the plant absorbent layer.There is a time lag between the stem flow and environmental factors of the two plants,and the correlation were solar radiation>temperature>relative humidity>VPD.BP Neural Network were used to fitting of C.and H.stem flow,the models R² were greater than 0.96.BP Neural Network model can be used to plant stem flow fitting.?3?Photosynthetic characteristicsThe daily change curve of P_n of C.under the three irrigation amounts were bimodal curve,that of H.under W₂ was a bimodal curve,and under W1 and W3 were unimodal curves.The P_n daily mean of C.under W3 was 20.68±7.60?mol CO₂ m^-2 s^-1,it under W2was 21.70%lower than that of W3,but under W1 was 0.10%higher than that of W3.The T_rdaily mean of C.under W3 was 9.78±1.57 mmol H₂O m^-2 s^-1,and it decreased by 26.89%under W2 and increased by 0.09%under W1.The P_n daily mean of H.under W3 was 8.97±2.00?mol CO₂ m^-2 s^-1,it under W2 and W1 were 30.71%and 0.03%lower than that of W3 respectively.The T_r daily mean of H.under W3 was 9.78±1.57 mmol H₂O m^-2 s^-1,and it decreased by 44.02%under W2 and increased by 18.96%under W1.The net photosynthetic rate and transpiration rate of C.were higher than those of H..C.under W1 improved WUE mainly by increasing P_n in the morning and taking a midday rest through photosynthesis.H.adopted high transpiration at noon to avoid the damage to leaves caused by high temperature,which resulted in the decrease of WUE,the P_nand T_r of C.and H.decreased under W2,but the T_r decreased more than P_n,so as to improve WUE.Water stress reduced LSP,LCP,R_d of C.and H..Plants photosynthetic capacity were weakened while subjected to water stress and photoinhibition was more likely to occur.However,plants showed different performances in different seasons,atmospheric RH has a certain compensation effect on soil water stress,which was reducing the impact of soil water stress on plant photosynthesis.In combination with soil water storage,plant photosynthesis and stem flow conditions,the single 17.5 L irrigation amount and 10 d irrigation cycle can be adopted in the C.,while the 35 L and 40 d irrigation cycle can be adopted in the H.In conclusion,the shelterbelt still has a certain water-saving space under the existing irrigation system.The photosynthetic and stem flow characteristics of C.and H.differ under the same irrigation system,which should be distinguished in the development of water-saving system.