**5. Conclusions**

This is the first study that explores the divergent responses of foliar N:P stoichiometry during different seasons to N deposition. In this study, plant leaf stoichiometric characteristics showed different variations with seasonal changes, and responses to N deposition in three seasons also differed. Thus, significant variations in leaf stoichiometric characteristics across these three seasons over just four months may be worth considering. Our research emphasizes the importance of multiple sampling across seasons in a temperate forest. Broadleaved species vs. conifers and AM species vs. EM species showed significant differences in seasonal variation and in response to N addition. Arguing against the traditional consensus, the net primary productivity of this mixed broadleaved-Korean pine (*Pinus koraiensis*) forest, an old-growth temperate forest and a critical forest type in Northeast China, is under N and P colimitation. The level of P limitation is enhanced by N deposition, and this N-induced

P limitation may be alleviated by the adjustment of strategies for plant adaption such as nutrient retranslocation and mycorrhizal association.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1999-4907/10/3/257/s1, Table S1: Marginal means of main factors.

**Author Contributions:** G.J. conceived and designed the experiments; D.Y. and H.M. performed the experiments; D.Y. analyzed the data; D.Y. contributed reagents/materials/analysis tools; D.Y. and G.J. wrote the paper.

**Funding:** This work was funded by Fundamental Research Funds for Central Universities (2572017EA02).

**Acknowledgments:** We thank three anonymous reviewers and editor for constructive suggestions to improve the quality of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
