Abstract: The ecological stoichiometry of foliar carbon (C), nitrogen (N), and phosphorus (P) can reflect nutrient use efficiency and ecological strategy of plants. The concentrations of the nonstructural carbohydrates (NSCs) and their components of leaves provide important information for understanding plant growth, development, and defense strategies, which are related to nutrient availability and dependent on stand density. Nitrogen addition can improve soil nutrient status. However, carbon utilization strategies and growth or defense mechanisms in relation to changes in soil N availability remain unclear. In this study, the concentrations of C, N, P, and NSCs and their ratios in the leaves of Mongolian oak (Quercus mongolica) and soil nutrients were examined with a three-year N addition experiment. Leaf samples of Mongolian oaks were collected from two blocks with different stand densities (D₁: 1300 trees·hm^-2and D₂: 1700 trees·hm^-2) and each stand density with one of three N addition levels (0 g N·tree^-1·a^-1, CK; 100 g N·tree^-1·a^-1, N₁; 200 g N·tree^-1·a^-1, N₂). The results showed that N addition significantly increased leaf N concentration and decreased leaf C∶N ratio. Leaf P concentration increased slowly, C∶P decreased, and N∶P increased first and then decreased with N application in D₁, whereas leaf P concentration decreased first and then increased, C∶P and N∶P increased first and then decreased in D₂. Additionally, N addition elevated the concentrations of NSCs and soluble sugar (SS) and reduced the concentrations of leaf starch (ST) in D₁. On the contrary, the concentrations of leaf NSCs, SS and ST first increased and then decreased with N addition in D₂. Furthermore, the relationship between leaf P and SS concentrations was significantly positive in D₁ but negative in D₂ (P<0.05). Overall, our results indicated that the development of Mongolian oak stands may be controlled by N and P limitations, as well as by stand density. Nitrogen addition would aggravate P limitation in higher stand density and result in a shift of carbon use strategies from growth to defensive processes.

Key words: Quercus mongolica, nitrogen addition, stand density, soluble sugar, starch, carbon use strategy.