**5. Conclusions**

We examined the changes in SOC, soil TN, and soil TP stocks and stoichiometry at depths of 0–100 cm following vegetation restoration in the Loess Hilly Region. Our results revealed that the SOC stocks appeared to increase and reach stable levels. The soil TN stocks first increased and then decreased with restoration age, but they did not reach the levels seen in natural grassland or natural forest in the Loess Hilly Region in the absence of appropriate management. Soil TP stocks failed to improve substantially with restoration age. The C:N, C:P, and N:P ratios gradually increased with restoration age. At the same time, the SOC, soil TN, and soil TP stocks and the C:N, C:P, and N:P ratios decreased with soil depth. Reforested land had the highest sequestration rate of SOC and TN at 0–10 cm soil depth. The results of this study provide data for the assessment of the long-term SOC, soil TN, and soil TP stocks and stoichiometry after vegetation restoration under different restoration types in the Loess Hilly Region.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1999-4907/10/1/27/s1, Figure S1: Changes in soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) content with restoration age. Figure S2: Changes in content and stoichiometry of soil organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) for different vegetation types at 5, 10, and 20 years. Table S1: The details of the sample sites selected for the study. Table S2: F and *sig* values for independent factors (soil depth, restoration age) and their interactions. Table S3: The slope parameters of the linear regression models for soil

*Forests* **2019**, *10*, 27

organic carbon (SOC), soil total nitrogen (TN), and soil total phosphorus (TP) contents and C:N, C:P, and N:P ratios with recovery year at different soil depths under four restoration types.

**Author Contributions:** The research design was completed by H.X. and S.X. The manuscript was written by H.X. and Q.Q. The collection and analysis of soil samples were performed by H.X., P.L., Q.Q., Z.G. and E.W.

**Funding:** This research was funded by National Science and Technology Support Program (2015BAC01B03), Natural Science Foundation of China (41771557) and Foundation for Western Young Scholars, Chinese Academy of Sciences (XAB2015A05).

**Acknowledgments:** Many thanks to Juan Li, Dou Zhang and Zhao Fang for their technical assistance in the laboratory work. We would like to thank Mancai Guo and Minggang Wang for providing statistics assistance.

**Conflicts of Interest:** All the authors declare no conflicts of interest.
