Synergic Relation Between Plant Growth And Soil Water And Evaluation Of Soil Water Availability On The Loess Plateau

Soil water availability attract the attention of researchers in ecology,soil and plant science,which is one of the main limitation of the terrestrial ecosystem productivity in arid region.The precipitation is the main source of soil water,unevenly distributed within the year and mainly concentrate in the period of July-September,which often cannot meet the requirement of the plants growth on the Loess Plateau.Therefore,it is necessary to study the synergic relation between plant growth and soil water,which can provide important information for vegetation recovery in this area.In the study,we conducted the continuous drought experiment,and four typical plants were selected,include tree plants(Robinia pseudoacacia L.),shrub palnts(Amorpha fruticosa L.),herbaceous plants(Medicago sativa L.)and crop plants(Zea mays L.),and the R.pseudoacacia and A.fruticosa experienced two drought cycles.The leaves water relations,gas exchanges,chlorophyll fluorescence parameters and soil respiration,etc.,were monitored in the experiment period.Besides,the carbon isotope and nutrients content of total 132 plant samples were measured in eight sites,and the factors that influence the plant water use efficiency(WUE)were analyzed on the regional scale.The results in the study could provide important information in determine the zonal soil water availability and the vegetation carrying capacity of soil water in this area.The main conclusions of this study were showed as follows:(1)During the drought,the physiological parameters response to drought could described as three stages in all four plants;firstly,the physiological parameters maintain stable,then decrease rapidly with the soil water decline,and decrease slightly with the further decline of soil water,and the sensibility of gas exchanges response to drought was higher than leaf relative water content and chlorophyll fluorescence parameters,and the woody plants maintained higher photosynthesis in lower soil water.The soil water content showed significant correlations with physiological parameters in all four species,which was the main reason of physiological parameters decrease.The soil respiration was sensitive to decrease of soil water,and decreased rapidly at first and decreased slightly with soil water decline except the A.fruticosa.(2)The response of A.fruticosa and R.pseudoacacia to recurrent drought cycles showed that drought caused significant effects on physiological parameters,which also showed some difference between the two species.The stomatal closure behavior in early drought may mainly regulated by abscisic acid(ABA)in R.pseudoacacia,and controlled by water potential in later drought,whereas it may be mainly controlled by water potential in A.fruticosa during the drought.Drought resulted in the shrinkage of both plants stem,which impeded the transport and distribution of photosynthate.The soil water content showed some differences when physiological parameters displayed significant change in two drought cycles,which were higher in the second drought except the respiration of aboveground,indicated that plants experienced drought enhanced the risk of drought related mortality.Although the soil respiration recovered after re-watered,the photosynthesis and respiration of aboveground could not recovered due to the leaves shedding.In addition,predawn leaf water potential showed significant correlation with gas exchanges in both species,and showed some differences in two drought cycles.The recovery of water potential was faster than gas exchange,and the pattern of transpiration recovery indicated the hydraulic factor was the main limitation of gas exchange recovery,which also may involve the non-hydraulic factor in R.pseudoacacia.(3)The study showed that it was equivalent to plants when the soil water content higher than the upper limitation of soil water availability,and declined rapidly with the decline of soil water.Different physiological parameters showed different thresholds of soil water,which was the highest in gas exchange and lowest in chlorophyll fluorescence,and the photosynthesis was suitable for evaluation of soil water availability in all four species,and leaf relative water content was also suitable in alfalfa and maize.It also showed different thresholds of soil water availability in four species,which showed lower threshold in woody plants.In addition,the thresholds of soil water availability were influenced by recurrent drought,which was higher when plants experienced drought.(4)The results showed that WUE had a negative correlation with annual precipitation,and was the lowest in herbaceous plants,in addition,N limitation also decreased the WUE.The results showed a significant correlation between WUE and nutrients/stoichiometry,which showed positive correlation between WUE and N:P ratio in trees and shrubs,but showed negative correlation with P and K contents in herbaceous plants;in addition,it also showed different correlations when plants under different nutrients limitation,which showed significant positive correlation between N:P and WUE when plants under non P-limitation,and significant correlation between WUE and K content was observed when plants under P-limitation.