Objective  Carbon (C) and nitrogen (N) elements are crucial for plant growth and development, especially in extremely arid inland river basins. Plants in different functional groups would show varied stoichiometric characteristics due to the variations in requirements for soil water and nutrients. It is of great significance to explore stoichiometric characteristics among different plant functional groups in the lower reaches of the Heihe River of northwestern China, as well as their nutrient contents responsing to groundwater fluctuation, aiming to further understand desert ecosystems under the background of global climate change.

  Method  In the lower reaches of the Heihe River, a total of 22 sampling sites were set up according to the vertical distance between vegetation and river. Correlation analysis and variation partition analysis (VPA) were applied to determine the relationship between plant stoichiometry and environmental factors among plant functional groups, respectively.

  Result  The average C contents of leaves and fine roots of plants in the lower reaches of Heihe River were 408.53 and 16.30 mg/g, the average N contents were 500.34 and 11.81 mg/g, and the average C∶N ratios were 30.74 and 49.48, respectively. Compared with global and regional studies, it was found that plants in the lower reaches of the Heihe River had higher C content, lower N content and higher C∶N. Different from herbaceous plants, the C content, N content and C∶N of woody plants were significantly correlated with the changes of groundwater depth. We found that the stoichiometric characteristics of plant carbon and nitrogen in the lower reaches of the Heihe River were significantly correlated with soil properties. The groundwater and soil variables jointly explained 53%−75% of the variation in woody plant stoichiometry. Additionally, soil pH and soil electrical conductivity explained 20% of the variation in herbaceous plant stoichiometry.

  Conclusion  Our study finds that groundwater is the key factor influencing the carbon and nitrogen stoichiometry of woody plants under extreme drought and saline-alkali environment, and the carbon and nitrogen stoichiometry of herbaceous plants is mainly influenced by soil pH and soil electrical conductivity.