Variation Characteristics Of Twig Functional Traits Of Wild Apple Tree Seedlings Under Nitrogen And Phosphorus Addition

The Malus sieversii（wild apple tree）is a rare and endangered wild fruit tree species left in the Tertiary period.Due to the influence of many factors,the population areas of wild apple trees were gradually decreased and the seedling regeneration was difficult in natural condition.The addition of nitrogen（N）and phosphorus（P）is one of the important measures to promote the rapid growth of plant seedlings in production practice.In order to explore the effect of N and P addition on the growth of wild apple tree seedlings,based on field experiments in this study,four types of N and P addition treatments(N,P,N20Px（mixed addition treatment of fixed N concentration with varied P concentration）and Nx P4（mixed addition treatment of fixed P concentration with varied N concentration）were set up.Three concentration gradients were used for each nutrient treatment,namely N(N10,N20,and N40;the number is nutrient concentration,the unit is g m^-2 yr^-1;the same below),P（P2,P4,and P8）,N20Px（N20P2,N20P4,and N20P8）,and Nx P4（N10P4,N20P4,and N40P4）.Each nutrient concentration gradient had five repeated quadrats.After four continuous years of fertilization,the experimental samples were collected in the growing season in August 2020.The variation characteristics of twig functional traits of wild apple tree seedlings under different N and P treatments were systematically compared and analyzed.The main results were as follows:（1）The analysis of the morphological traits of stems and leaves showed that,stem traits were generally and significantly different under the mixed addition of N and P,while only several traits were significantly different under N or P addition alone（P<0.05）.There was significant difference in the allometric relationship between stem length and the other traits under N addition,but no significant difference under P addition was observed.Under the mixed treatment of N and P,the above allometric relationships showed a coaxial shift.Under the N20Px treatment,there was a significant linear fitting relationship between P addition concentrations and stem and leaf traits（P<0.05）.The plant trait network clarified that there were significant differences in the hub traits of stems and leaves under different N and P treatments,the stability of the trait networks under the mixed addition of N and P was higher than that under N or P addition alone（i.e.,high graphics density and clustering coefficient）.（2）Results of leaf physiological traits presented that,with the increase of N concentration,the contents of MDA in leaves increased under N or P addition alone（P<0.05）,while there was no significant change in the mixed addition of N and P.The content of non-structural carbohydrates increased significantly with increasing nutrient concentration（P<0.05）.The linear or nonlinear fitting relationships between leaf physiological traits and nutrient addition concentrations of wild applee seedlings also existed to an extent.In plant trait networks,the key physiological traits of leaves differed under different N and P treatments,and the network of leaf physiological traits was the closest（i.e.,high graphics density and clustering coefficient）under N addition.（3）In the aspect of stoichiometric traits of stems and leaves,the carbon contents of leaves reduced significantly under N or P addition alone,while the P contents of stems and leaves increased significantly with the increase of nutrient concentration under the mixed treatment of N and P（P<0.05）.Under the mixed addition of N and P,significant（P<0.05）linear or nonlinear fitting relationships between the P contents of stems and leaves and the added concentrations of N and P were determined.In trait networks,the key stoichiometric traits of stems and leaves were markedly different under different N and P treatments,and the networks were the most complex and stable（i.e.,the highest graphics density and clustering coefficient）under the mixed treatments of N and P.In addition,leaf P resorption efficiency of wild apple seedlings was higher than that of N.（4）The fitting analysis between soil physical and chemical properties and nutrient addition concentrations indicated that,soil total P and available P contents increased significantly under P and N20Px treatments.The correlation coefficients between soil physical and chemical properties and morphological of wild apple seedlings under the mixed addition of N and P were higher than that under N or P addition alone,and the twig stoichiometric indicators also presented the similar tendency.（5）Comprehensive analysis on all stem and leaf traits showed that,no matter what nutrient treatment,in the trait network composed of all traits,the morphological traits were regarded as the hub traits.Based on the membership function,the comprehensive performance of wild apple seedlings under the mixed treatment of N and P was higher than that under N or P addition alone;among them,the N10P4 showed the best comprehensive performance in all 12 treatment gradients,and the N20Px presented the best comprehensive performance among the four types of nutrient additions.In summary,there was the best fitting relationship between the stoichiometric traits of wild apple seedlings and the nutrient addition concentrations,the morphological traits could better reflect the response of wild apple tree seedlings to nutrient addition,and the promoting effect of mixed addition of N and P on the comprehensive performance of wild apple seedlings were greater than that of N or P addition alone.The results provide a scientific basis for exploring how to improve the survival rate of wild apple tree seedlings in the field.