**5. Conclusions**

Our study in poplar plantations on the east coast of China suggests that N addition could have substantial effects on soil fauna along the soil profile. Across the N addition treatments, low N addition rates had positive effects on soil fauna densities, likely due to improved physical and chemical soil conditions. With increasing N addition rates, responses of soil fauna community returned to the control level. Soil fauna responded differently to N addition in different soil layers, which may largely relate to changes in the soil substrate content with N addition. Multifactor linear regressions showed that fine root biomass mediated N induced changes in the soil fauna density. The global N deposition rate is projected to continuously increase in the future [3,83], which will undoubtedly affect soil fauna and further the biogeochemical cycles. Therefore, it is imperative to explore impacts of N addition on soil fauna communities to understand the way an ecosystem functions under the global N deposition.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1999-4907/10/2/122/s1, Figure S1: Variations in soil temperature (a) and moisture (b) from January to December along the nitrogen (N) addition gradient in 2016. N0, N1, N2, N3, N4 indicate different N additions (N0: 0 kg N ha−<sup>1</sup> year<sup>−</sup>1; N1: 50 kg N ha−<sup>1</sup> year<sup>−</sup>1; N2: 100 kg N ha−<sup>1</sup> year<sup>−</sup>1; N3: 150 kg N ha−<sup>1</sup> year<sup>−</sup>1; N4: 300 kg N ha−<sup>1</sup> year<sup>−</sup>1), Figure S2: The live (a), dead (b), and total (c) fine root biomass along the N addition gradient and the soil profile (red for 0–10 cm, green for 10–25 cm, and blue for 25–40 cm). See Figure S1 for abbreviations, Figure S3: Regulation of fine root biomass (a: live and dead root biomass; b: total root biomass) on the density of phytophagous soil fauna (Dp), without the very small points (circled in the panels). \*\*: *p* < 0.01, \*\*\*: *P* < 0.001, Figure S4: Regulation of fine root biomass (a: live and dead root biomass; b: total root biomass) on the density of total soil fauna (Dtotal), without the very small points (circled in the panels). \*\*: *p* < 0.01, \*\*\*: *p* < 0.001, Figure S5: The proportion of each individual soil fauna over total soil fauna along the nitrogen (N) addition gradient and the soil profile. See Figure S1 for abbreviations, Figure S6: The proportions of macrofauna, mesofauna, and microfauna over total soil fauna along the nitrogen (N) addition gradient and the soil profile. See Figure S1 for abbreviations. Table S1: Results of two-way ANOVA for the responses of each individual soil fauna's proportion over total soil fauna to nitrogen (N) addition (0, 50, 100, 150, and 300 kg N ha−<sup>1</sup> year<sup>−</sup>1) and soil depth (0–10, 10–25, and 25–40 cm). *P* levels are indicated beside the *F* values by the following: \*: <0.05, \*\*: <0.01, \*\*\*: <0.001, and no asterisks indicate *p* > 0.05.

**Author Contributions:** Conceptualization, H.R. and X.X.; Data curation, H.B., Q.G. and H.X.; Formal analysis, H.B., Q.G. and X.X.; Funding acquisition, H.R. and X.X.; Investigation, H.B., Q.G. and H.X.; Resources, C.S., H.R. and X.X.; Supervision, H.R. and X.X.; Validation, H.X.; Writing–original draft, H.B. and X.X.; Writing–review & editing, Q.L., X.C., Y.L., H.R. and X.X.

**Funding:** This study is financially supported by the National Key Research and Development Program of China (2016YFD0600204), the Recruitment Program for Young Professionals, the Jiangsu Specially Appointed Professors Program, the Natural Science Key Fund for Colleges and Universities of Jiangsu Province of China (17KJA180006), the Six Talent Peaks Program of Jiangsu Province (JY-041 & TD-XYDXX-006), the Nanjing Forestry University Science Fund for Distinguished Young Scholars (the "5151" Talent Program), the Jiangsu Collegiate Science and Technology Fund for the Excellent Innovative Research Teams, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

**Acknowledgments:** We thank many lab members for their help with the field work. **Conflicts of Interest:** The authors declare no conflict of interest.
