Nitrogen acquisition strategies used by Leymus chinensis and Stipa grandis in temperate steppes

摘要

Leymus chinensis and Stipa grandis are two important plant species of temperate steppes in Inner Mongolia of North China. They differ in their life forms, e.g., L. chinensis is a type of rhizomatous clonal grass, whereas S. grandis is a type of tussock grass. Here we hypothesize that both plant species possess distinct nitrogen (N) acquisition strategies for their growth and survival. To test this hypothesis, we conducted a four-factor experimental field study using a short-term (three hours) N-15 labeling technique in two plant communities mono-dominated by L. chinensis and S. grandis of the temperate steppes over two months (July and August) and at two soil depths. In both of communities, L. chinensis and S. grandis directly absorbed all three of the common forms of N, including substantial portions of N-derived from glycine (organic and inorganic forms) ranged from 2.7 to 17.8 %, although they absorbed more inorganic N. Nitrogen uptake rates showed significant effects of communities, months, soil depths, and N forms. The uptake rate was higher in August than in July and at 0-5 cm than at 5-15 cm soil depths. L. chinensis and S. grandis showed different preference on N form across months. L. chinensis shifted its uptake pattern from more nitrate (NO3 (-)) in July to more ammonium (NH4 (+)) in August, whereas S. grandis took up comparable NH4 (+) and NO3 (-) in both months. In general, L. chinensis showed a more flexible N acquisition strategy and S. grandis performed a more concentrated and relatively more stable N acquisition strategy. The distinct N acquisition strategies used by L. chinensis and S. grandis varied greatly across different months and soil depths. These findings are more helpful in further understanding the plasticity of nutrient utilization issues of different plant species in response to N-limited conditions of grassland ecosystems.