Adaptation And Trade-Off Of Artificial Robinia Pseudoacacia Plantation To Soil Water And Nutrient On The Loess Plateau

The study on the response mechanism of plant growth on the Loess Plateau to the change of future environmental factors has important theoretical significance for vegetation restoration and ecological protection in the region.Therefore,in this study,Robinia pseudoacacia,a typical afforestation tree species on the Loess Plateau,was selected as the research object.Using the methods of field sampling,pot experiment and meta-analysis,the spatial distribution pattern and influencing factors of leaf water use efficiency and nitrogen use efficiency of R.pseudoacacia were systematically analyzed,the responses of leaf water use efficiency and plant-soil stoichiometry to stand quality in R.pseudoacacia were explicated,the plasticity responses of growth parameters,photosynthetic parameters,fluorescence parameters and antioxidant parameters of R.pseudoacacia to different water and nutrient supply levels were clarified,and the plant-soil-microbial ecological stoichiometry and its homeostasis characteristics under different water and nutrient supply levels were revealed.Furthermore,through the integration of literature data on tree growth and its influencing factors in this region,the relationship models of R.pseudoacacia growth with forest age,density,climatic factors and topographic factors were established,and the trade-off relationships between R.pseudoacacia growth and soil variables were quantitatively evaluated.The main results were showed as follows:（1）The leaf water use efficiency（WUE）of R.pseudoacacia on the Loess Plateau varied from 64.03 to 103.08μmol mmol^-1,and the nitrogen use efficiency（NUE）varied from 11.26 to 18.19 g C g^-1 N.Latitude played a substantial role in WUE,and soil ammonium nitrogen and total nitrogen content played a substantial role in NUE.In addition,NUE in R.pseudoacacia was lower than the average value of whole plants on the Loess Plateau,and the R.pseudoacacia growth was mainly restricted by phosphorus.The sampling sites were divided into three zones according to climate,vegetation and soil characteristics.The higher latitude and lower precipitation zone,the higher WUE was,while NUE was not significant among the three zones.（2）The species richness,Shannon-Weiner diversity index,Simpson diversity index and evenness index of undergrowth vegetation of R.pseudoacacia forest reached their peaks in the 35-year-old stands with increasing afforestation age.Leaf WUE reached its highest value in the 35-year-old stands but was only significantly higher than that in the 8-year-old stands.Different afforestation ages had different effects on carbon（C）,nitrogen（N）and phosphorus（P）ecological stoichiometric characteristics in leaves,fine roots and soil.The most obvious trend was that soil total P decreased significantly with increasing afforestation age,and the leaf N:P ratio was greater than16,which indicated that P was the main limiting factor in R.pseudoacacia plantations.Leaf C、N、P nutrients and stoichiometry were closely related to forest growth,while fine root C、N、P nutrients and stoichiometry were more related to species composition and diversity of understory vegetation.Importantly,the WUE had no significant change with the increase in the stand quality index,and the response of soil stoichiometry to stand quality was stronger than that of plant stoichiometry.（3）Drought stress significantly decreased plant height,stem diameter,root,stem and leaf biomass and total biomass of R.pseudoacacia seedlings,significantly decreased actual photochemical efficiency of photosystem II and increased non-photochemical quenching,significantly decreased the activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）and significantly increased the malondialdehyde（MDA）content.Excessive application of nitrogen fertilizer inhibited plant growth,and phosphorus deficiency significantly weakened the positive effects of nitrogen addition on plant height,stem diameter and biomass under drought stress.High nitrogen treatment effectively weakened the inhibitory effect of drought stress and phosphorus deficiency on chlorophyll formation of R.pseudoacacia seedlings.Proper nitrogen and phosphorus fertilizers increased the SOD,POD and CAT activities and decreased the MDA content under drought stress.The average plasticity index of antioxidant parameters was the highest,and the average plasticity index of POD and CAT activities in response to water,nitrogen and phosphorus were the highest,indicating that R.pseudoacacia seedlings adapted to the changes of water and nutrient habitat mainly by regulating their POD and CAT activities.（4）The C、N、P nutrients and stoichiometry of leaf,fine root,soil and soil microbial biomass showed a certain degree of plasticity to the changes in the water and nutrient habitat.Variation partitioning analysis showed that root stoichiometry accounted for a large part of the variance in microbial biomass stoichiometry.Structural equation modeling further revealed that root stoichiometry and leaf stoichiometry were two direct factors affecting microbial biomass C:N and C:P,and root stoichiometry had the greatest direct effect.In addition,the analysis of homeostasis showed that microbial biomass C and C:P were more sensitive to changes in soil nutrients,and other nutrients and stoichiometry displayed strict homeostasis.（5）The average tree height of artificial R.pseudoacacia forest on the Loess Plateau was 8.80 m,and the variation ranged from 0.96 m to 29.50 m.The average diameter at breast height（DBH）was 10.41 cm,and the variation ranged from 0.70 cm to 62.83 cm.The average crown diameter was 3.21 m,and the variation ranged from0.83 m to 7.20 m.Random forest was a fast and effective machine learning method for predicting the growth of R.pseudoacacia,which could account for 67%of tree height variability and 55%of DBH variability,and it was better than linear regression,support vector machine and gradient boosting machine.Forest age and stand density were the main factors to predict the tree height,DBH and crown diameter of R.pseudoacacia,followed by climate factors and topographic factors.The trade-off analysis between R.pseudoacacia growth and soil characteristics showed that the growth of R.pseudoacacia was accompanied by the consumption of soil water and at the cost of soil nutrient consumption.Among the selected soil variables,soil p H and soil texture were the main factors affecting the tree height and DBH,and soil texture was the main factor affecting the crown diameter of R.pseudoacacia.