The Variation In Leaf Nitrogen,Phosphorus And Phosphorus Fractions Of Two Shrubs Along Altitude In Middle And Eastern Parts Of The Qilian Mountains

Nitrogen and phosphorus are important elements in the global biogeochemical cycle and are also vital features that reflect a range of physiological and ecological processes in plants.At present,a great deal of research has been carried out on plant nitrogen and phosphorus characteristics,revealing soil-plant interactions,nutrient cycling characteristics and their distribution patterns,but relatively few focus on plant nitrogen,phosphorus and especially phosphorus fractions（including phosphate,lipid phosphorus,nucleic acid phosphorus,metabolites phosphorus and residue phosphorus）variation patterns along the altitude.In this study,two shrubs,Salix gilashanica and Caragana jubata,distributed along the altitudes in the Pailugou in the middle part of the Qilian Mountains and the Xiangshuigou in the eastern part of the Qilian Mountains,were selected for the research.The aim of this study is to reveal the variation patterns of the nitrogen,phosphorus and phosphorus fractions of the leaves,and exploring the relationships between leaf nitrogen,phosphorus,phosphorus fractions and net photosynthetic rate to explain the adaptation of the plants to the altitude.The main findings of the study are as follows:（1）With the increase of altitude from 3100 m to 3400 m in Xiangshuigou and from3100 m to 3700 m in Pailugou,the concentration of soil alkali-hydrolyzable nitrogen increased first and then decreased,and the highest concentration was 0.53 mg g^-1 and0.56 mg g^-1 at 3300 m in both sites.Soil available phosphorus increased from 5.69 mg kg^-1 to 9.37 mg kg^-1 in Xiangshuigou and from 9.41 mg kg^-1to 11.9 mg kg^-1in Pailugou.Soil p H decreased significantly from 6.68 to 6.40 in Xiangshuigou and from 7.08 to6.83 in Pailugou.（2）With the increase of altitude from 3100 m to 3400 m in Xiangshuigou and from3100 m to 3600～3700 m in Pailugou,the leaf total nitrogen concentration of the two shrubs decreased significantly,which decreased by 5.68%and 7.18%respectively in Xiangshuigou,and decreased by 8.75%and 8.31%respectively in Pailugou.The leaf total phosphorus concentration increased significantly,which increased by 12.7%and11.3%respectively in Xiangshuigou and increased by 18.7%and 17.3%respectively in Pailugou.（3）With the increase of altitude from 3100 m to 3400 m in Xiangshuigou and from3100 m to 3600～3700 m in Pailugou,the leaf nucleic acid phosphorus percentage of both species increased significantly,by 23.8%and 10.7%in Xiangshuigou and by9.75%and 16.8%in Pailugou of Salix gilashanica and Caragana jubata respectively.The metabolites phosphorus percentage in leaves of both species increased significantly,by 29.3%and 19.8%in Xiangshuigou and by 40.4%and 24.7%in Pailugou of Salix gilashanica and Caragana jubata respectively.The leaf phosphate percentage decreased significantly,by 27.3%and 17.5%in Xiangshuigou and by 29.4%and 33.6%in Pailugou of Salix gilashanica and Caragana jubata respectively.But the leaf lipid phosphorus percentage and leaf residues phosphorus percentage of Salix gilashanica and Caragana jubata did not change significantly.（4）With the increase of altitude from 3100 m to 3400 m in Xiangshuigou and from3100 m to 3600～3700 m in Pailugou,the stomatal conductance of both species in both sites decreased significantly,by 23.1%and 75.0%in Xiangshuigou and by 32.0%and58.0%in Pailugou of Salix gilashanica and Caragana jubata respectively.The net photosynthetic rate of Salix gilashanica in both sites did not change significantly,but the net photosynthetic rate of Caragana jubata in both sites decreased significantly,by49.0%and 30.0%in Xiangshuigou and Pailugou respectively.The water use efficiency（ratio of net photosynthetic rate to stomatal conductance）increased significantly in both sites of two shrubs,by 52.1%and 160%in Xiangshuigou and by 22.6%and 82.4%in Pailugou of Salix gilashanica and Caragana jubata respectively.（5）The results of the Pearson correlation analysis showed that although the net photosynthetic rate of both species was constant or decreased along the altitude at both sites,there was a significant positive correlation between leaf lipid phosphorus percentage and net photosynthetic rate,indicating that the distribution of leaf lipid phosphorus in total phosphorus is essential for species photosynthesis,and photosynthetic nitrogen use efficiency and photosynthetic phosphorus use efficiency of both species decreased significantly along the altitude.The photosynthetic nitrogen use efficiency and photosynthetic phosphorus use efficiency decreased by 19.2%and61.6%,and 32.4%and 67.9%,respectively,in Xiangshuigou of Salix gilashanica and Caragana jubata,while the photosynthetic nitrogen use efficiency and photosynthetic phosphorus use efficiency decreased by 27.6%and 51.4%,and 44.2%and 62.2%,respectively,in Pailugou.In summary,this study revealed the response patterns of nitrogen,phosphorus and phosphorus fractions to altitude and their relationship with net photosynthetic rate of Salix gilashanica and Caragana jubata in middle and eastern parts of the Qilian mountains.This suggests that plants adapt to high altitude habitats with more phosphorus input than nitrogen to avoid excessive decline photosynthesis and enhance water use efficiency.The study may provide theoretical support for the understanding of the adaptation mechanism of plants to altitude gradients.