Abstract: We investigated the effects of different nitrogen forms on biomass allocation and photosynthetic characteristics of dominant species in alpine artificial grasslands, aiming to select nitrogen forms suitable for maintaining productivity. A randomized block design was used to examine biomass allocation pattern, leaf photosynthetic characteristics, and stoichiometric characteristics of dominant species in a 4-year-old mixed grassland of “Festuca sinensis Qinghai + Poa pratensis Qinghai” under four treatments: control (CK), amide nitrogen (U), ammonium nitrogen (A), and nitrate nitrogen (N). The results showed that nitrogen addition decreased the relative biomass of stems, increased that of leaves and ears, and increased the total aboveground biomass by increasing the biomass of stems and leaves within the range of 15-45 cm in plant height. The positive effect of different nitrogen forms on aboveground biomass showed a trend of U>N>A. Different nitrogen forms improved the photosynthetic performance of dominant species to varying degrees, with the effect on net photosynthetic rate and water use efficiency showing a trend of A>N>U. The three nitrogen forms did not affect leaf carbon content, but significantly increased nitrogen and phosphorus content and decreased carbonnitrogen ratio and carbon-phosphorus ratio, with the strength of the effect being U>N>A. Results of redundancy analysis showed that intercellular CO₂ concentration, stomatal conductance, and net photosynthetic rate were key photosynthetic variables affecting plant productivity, while leaf nitrogen content, phosphorus content, carbon-nitrogen ratio, carbon-phosphorus ratio, and nitrogen-phosphorus ratio were key factors affecting photosynthesis and productivity. Overall, amide nitrogen may be more beneficial for maintaining the productivity of alpine artificial grassland.

Key words: alpine artificial grassland, nitrogen form, photosynthetic characteristics, stoichiometry