Effects of simulated nitrogen deposition on photosynthetic characteristics of plants in mixed shrub-grass communities of the Changbai Mountain tundra

As herbaceous plants continue to invade the shrub tundra of the Changbai Mountains, strong interspecific competition among mixed shrub-grass communities has formed. In this study, Comm. Rhododendron aureum-Deyeuxia purpurea was investigated to monitor the differences and changes in photosynthetic characteristics of R. aureum and D. purpurea in mixed communities. We established four different quadrats (no, mild, moderate, or severe invasion) according to differences in D. purpurea coverage degree and three nitrogen application levels (natural, 11.8 kgN·hm^-2·a^-1, 23.6 kgN·hm^-2·a^-1) under natural conditions, and determined the physiological mechanism related to D. purpurea invasion of the tundra. Results showed that: (1) The net photosynthetic rate of D. purpurea was higher than that of R. aureum. The higher the coverage of D. purpurea, the higher its chlorophyll content, whereas R. aureum showed the opposite pattern. With the increase in invasion degree, the net photosynthetic rate of D. purpurea increased. (2) Nitrogen application increased the chlorophyll content and net photosynthetic rate of R. aureum and D. purpurea and promoted plant growth. However, the growth rate of D. purpurea was larger, which enhanced its competitiveness. (3) Nitrogen application and D. purpurea invasion had a compound effect. Specifically, the greater the coverage of D. purpurea, the greater the increase in net photosynthetic rate and chlorophyll content of D. purpurea caused by nitrogen application; in contrast, the increase in R. aureum declined. Therefore, successful invasion by D. purpurea may be related to its higher net photosynthetic rate, which was enhanced by nitrogen application. Thus, an increase in nitrogen deposition would be conducive to D. purpurea growth and competitiveness.