Objective Robinia pseudoacacia has been introduced to the Loess Plateau for soil and water conservation and ecological restoration because of its strong nitrogen fixation ability, fast growth speed and barren resistance. This study focused on the nutrient utilization characteristics and stand quality of R. pseudoacacia plantation at different ages in the loess hilly region, to provide theoretical basis for the evaluation and rational management of this plantation.

Method The stand structure and diversity of understory vegetation were investigated for R. pseudoacacia in 13, 25, 33 and 43-years plantations in Zhifanggou Watershed of northwestern China. The contents of C, N and P in leaves, branches, roots of R. pseudoacacia and soil were determined. In addition, the leaf nutrient use and resorption efficiency of R. pseudoacacia and the stoichiometric internal stability of N and P in leaves, branches, and roots were calculated, to evaluate the stand quality index of R. pseudoacacia plantation at different stand ages.

Result (1) The N and P of content of R. pseudoacacia leaves and soil were significantly increased with increased stand age (P < 0.05), and the leaves N∶P were higher than 20 among all stand ages. The leaf nutrient use efficiency of N and P was significantly decreased; however, the leaf nutrient resorption efficiency of N and P significantly were significantly increased with increased stand age (P < 0.05). The leaf nutrient use and resorption efficiency of P were higher than those of N. (2) Both the internal stability of N, P elements and N∶P in different organs of R. pseudoacacia and the diversity index of understory vegetation reached the maximum value at 25-years plantations; (3) The stand quality index of 25-year-old R. pseudoacacia plantation was significantly higher than other stand ages (P < 0.05).

Conclusion The growth of R. pseudoacacia is mainly limited by P content, R. pseudoacacia changes N, P content and internal stability of various organs by adjusting nutrient utilization characteristics. Both the maximum N, P and N∶P internal stability values and the highest understory vegetation diversity are observed for R. pseudoacacia at 25-years plantations, result in the highest stand quality index of this plantation at that stand age.