Objectives We explored the ecological stoichiometric characteristics of carbon (C), nitrogen (N), and phosphorus (P), the reutilization and utilization efficiency of nutrients in Pinus tabuliformis plantations in different ages in nitrogen deficient areas, to reveal the responses of various indicators to nitrogen addition gradient.

Methods The survey was conducted in Jinshatan artificial forest farm, Shuozhou City, Shanxi Province, the tested pine (Pinus tabulaeformis) plantations were at Ⅱ to Ⅴ age groups, respectively. Nitrogen fertilizer was applied at gradients of 0 g/m², 5 g/m², 10 g/m², 15 g/m², and 20 g/m², to assimilate control, and low, suitable, moderate high, and high soil N levels. At the full growing stage in July, the mature needle leaves on the newly grown branches were sampled for determination of total C, N, and P content, and the surface soil was sampled for the measurement of available N and P contents. The N and P reabsorption efficiency (NRE, PRE), nitrogen and phosphorus nutrient utilization efficiency (NNUE, PNUE), and nitrogen and phosphorus homeostasis index (H_N, H_P) of pine trees were calculated.

Results The total carbon (TC) and nitrogen (TN) content, C/N and C/P ratio of leaves in various age groups of Chinese pine plantations showed a unimodal trend with the increase of soil N levels, both the peak and foot values occurred between 0.8−1.0 g/kg of soil total N content. The leaf N/P ratio gradually increased from Ⅱ to Ⅴ age groups, and the leaf P content was positively correlated with soil total N. The NRE of new leaves of different age groups decreased, while the NNUE showed an overall upward trend with the increase of soil total N content. PRE was relatively stable in age group of Ⅱ to IV, but the PRE at age group V changed significantly with the increase of soil total N level; PNUE decreased slowly with the increase of soil total N level. The fitting effects of homeostasis indexes (H) of pine plantations at all age groups reached significant level (P<0.1), and H_N was all higher than 4, showing a stable state, however, the H_P value at age Ⅴ was 0.999, indicating a sensitive state. And the fitting result for the H_P of majority aged pine plantations was floating around 1.951, which was in a weak but steady state.

Conclusions Ⅱ age grade Pinus tabulaeformis is sensitive to soil N change, while the change of P content in the other age grades are more stronger. The N homeostasis indices in all age grades are higher than 4, being stable state, and sufficient nitrogen supply strengths the stability. Ⅲ, and Ⅳ age grade Pinus tabulaeformis are strong in regulating the reabsorption and reutilization of nutrients, leading to high homeostasis stability in both N and P. While Ⅱ age grade Pinus tabulaeformis shows lower P homeostasis stability due to high uptake of nitrogen, and Ⅴ age grade Pinus tabulaeformis shows low P homeostasis stability due to the weak reabsorption and reutilization of P. So, when the soil nitrogen level changes, more attention should be paid on the P nutrition of young and old Pinus tabulaeformis plantations.