**5. Conclusions**

Along with the chronosequence of black locust plantations, the NRE and PRE both increased and then decreased. Imbalanced C, N, and P variation with the increase in stand ages may result in P limitation. The NRE was significantly correlated with the C of green leaves and the P content and C:N of senesced leaves, whereas the PRE was significantly negatively correlated with the C:P and N:P of soil and green leaves. The PRE:NRE decreased with increased stand ages. In summary, our result showed that the NRE and PRE were both changed as the stand aged. The middle-aged black locust trees (approximately 30 years) were more efficient at resorbing P and N from senesced leaves than younger and older stands, but the NRE and PRE responded differently in soil and plant nutrients. The black locust plantations would alter the conservation and use strategy of nutrients in the ecosystem through a plant-mediated pathway with increasing stand age.

**Author Contributions:** Conceptualization, J.D., X.L., and X.H.; Formal analysis, S.W. and F.Z.; Investigation, J.D., S.W., C.R., and W.Z.; Writing—original draft, J.D. and D.Z.; Writing—review and editing, C.R., F.Z., X.H., and G.Y.

**Funding:** The study was funded by the Natural Science Basic Research Program of Shaanxi Province, China (2018JQ3041); Young Talent fund of University Association for Science and Technology in Shaanxi; Scientific Research Program Funded by Shaanxi Provincial Education Department (18JK0871), and Innovation and entrepreneurship Training Program of Shaanxi Province (201820086).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
