Effects of N and P enrichment on plant photosynthetic traits in alpine steppe of the Qinghai-Tibetan Plateau

Background N (nitrogen) and P (phosphorus) play important roles in plant growth and fitness, and both are the most important limiting factors that affect grassland structure and function. However, we still know little about plant physiological responses to N and P enrichment in alpine grassland of the Qinghai-Tibetan Plateau. In our experiment, five dominant common herbaceous species were selected and their photosynthetic parameters, leaf N content, and aboveground biomass were measured. Results We found that species-specific responses to N and P enrichment were obvious at individual level. N addition (72 kg Nha(-1) yr(-1)), P addition (36 kg Pha(-1) yr(-1)) and NP addition (72 kg Nha(-1) yr(-1)and 36 kg P ha(-1) yr(-1), simultaneously) significantly promoted net photosynthetic rate of Leymus secalinus. Differential responses also existed in the same functional groups. Responses of forb species to the nutrients addition varied, Aconitum carmichaeli was more sensitive to nutrients addition including N addition (72 kg Nha(-1) yr(-1)), P addition (36 kg Pha(-1) yr(-1)) and NP addition (72 kg Nha(-1) yr(-1)and 36 kg P ha(-1) yr(-1)). Responses of plant community photosynthetic traits were not so sensitive as those of plant individuals under N and P enrichment. Conclusions Our findings highlighted that photosynthetic responses of alpine plants to N and P enrichment were species-specific. Grass species Leymus secalinus had a higher competitive advantage compared with other species under nutrient enrichment. Additionally, soil pH variation and nutrients imbalance induced by N and P enrichment is the main cause that affect photosynthetic traits of plant in alpine steppe of the Qinghai-Tibetan Plateau.

Automated summary

Our study reveals that different plant species have distinct physiological responses to the enrichment of nitrogen (N) and phosphorus (P) at the individual level in alpine grassland. Specifically, for Leymus secalinus, N addition (72 kg Nha(-1) yr(-1)), P addition (36 kg Pha(-1) yr(-1)), and NP addition (72 kg Nha(-1) yr(-1) and 36 kg P ha(-1) yr(-1)) significantly enhance the net photosynthetic rate. Additionally, Leymus secalinus demonstrates a higher competitive advantage compared to other species under nutrient enrichment. Soil pH variation and nutrients imbalance caused by N and P enrichment are identified as the main factors affecting plant photosynthetic traits in alpine steppe of the Qinghai-Tibetan Plateau.