**3. Results**

### *3.1. Microclimate, Litter C and N, Soil Samples, and Fine Root Biomass*

Soil temperature and moisture showed remarkable seasonal variations. Soil temperature increased from January to August and then decreased from August to December (Table 1, Figure S1a) and soil moisture was low in summer and high in spring and fall (Table 1, Figure S1b). N addition did not affect the C:N ratio of the litter, SOC, TN, soil temperature or moisture (all *p* > 0.05, Tables 1 and 2). While N addition had no significant effect on live, dead, or total fine root biomass (all *p* > 0.05), root biomass in general decreased with increasing soil depth (all *p* < 0.01, Table 3, Figure S2).

**Table 1.** Results of repeated-measures ANOVA for seasonal responses of litter mass, soil temperature, and moisture to nitrogen (N) addition (0, 50, 100, 150, and 300 kg N ha−<sup>1</sup> year<sup>−</sup>1). Statistical analysis was performed on log-transformed data. *p* levels are indicated beside the *F* values by the following: \*: <0.05, \*\*: <0.01, \*\*\*: <0.001, and no asterisks indicate *p* > 0.05. *df* represents degree of freedom.


**Table 2.** Results of one-way ANOVA for responses of litter carbon (C): N ratio, soil N (TN), and soil organic C (SOC) to N addition. Statistical analysis was performed on log-transformed data. *p* levels are indicated beside the *F* values by the following: \*: <0.05, \*\*: <0.01, \*\*\*: <0.001, and no asterisks indicate *p* > 0.05.


**Table 3.** Results of two-way ANOVA for responses of the density (*D*), number of groups (*G)*, Shannon-Wiener diversity index (*H*) of soil fauna and fine root biomass (live root biomass, dead root biomass, and total root biomass) to nitrogen (N) addition and soil depth (0–10, 10–25, and 25–40 cm). Statistical analysis was performed on log-transformed data. *P* levels are indicated beside the *F* values by the following: \*: <0.05, \*\*: <0.01, \*\*\*: <0.001, and no asterisks indicate *p* > 0.05.



**Table 3.** *Cont*.

*3.2. Soil Fauna Community as Affected by N Addition*

In general, N addition and soil depth both played important roles in altering the density of the soil fauna. With increasing N addition rate, the density of the total soil fauna (Dtotal) significantly increased first, then declined to the levels of the controls (*p* < 0.01, Table 3, Figure 1a). In the 25–40 cm soil layer, for example, Dtotal increased from N0 to N3 and then declined from N3 to N4 (Figure 1a). In addition, Dtotal significantly decreased with increasing soil depth (*p* < 0.001, Table 3, Figure 1a). The interaction between N addition and soil depth had a significant impact on Dtotal (*p* < 0.05, Table 3). Dtotal increased from N0 to N1 and then declined with higher N addition (from N1 to N4) in the topsoil layer (0–10 cm). In the deep soil layers (10–25 and 25–40 cm), however, declines in Dtotal started at N2 and N3, respectively, compared to the topsoil layer. The density of phytophagous soil fauna (Dp) showed similar responses to N addition along the soil profile (Table 3, Figure 1b).

**Figure 1.** The density of total soil fauna (Dtotal, **a**) and phytophagous soil fauna (Dp, **b**) 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). N0, N1, N2, N3, N4 indicate the gradient of 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).

N addition had no significant impact on either the number of groups of total soil fauna (Gtotal) or the number of groups of phytophagous fauna (Gp) (all *p* > 0.05, Table 3, Figure 2). With increasing soil depth, in general, we found significant decreases in Gtotal and Gp (all *p* < 0.05, Table 3, Figure 2). Abrupt decreases in Gtotal and Gp at N2 were observed in the topsoil layer (0–10 cm). We also found interactive effects between N addition and soil layer on Gp (*p* < 0.05, Table 3, Figure 2b). Along the soil profile, for instance, Gp reached its highest values at N1, N2, and N3. For the Shannon-Wiener diversity index of the soil fauna, neither N addition nor soil depth had significant effects on the diversity of total soil fauna (Htotal) or the diversity of phytophagous soil fauna (Hp) (all *p* > 0.05, Table 3, Figure 3).

**Figure 2.** The number of groups of total soil fauna (Gtotal, **a**) and phytophagous soil fauna (Gp, **b**) along the N addition gradient and the soil profile. See Figure 1 for abbreviations.

**Figure 3.** The Shannon-Wiener diversity index of total soil fauna (Htotal, **a**) and phytophagous soil fauna (Hp, **b**) along the N addition gradient and the soil profile. See Figure 1 for abbreviations.

### *3.3. Fine Root Biomass Regulation of Soil Fauna Density*

Variations in soil fauna densities (both Dp and Dtotal) were consistently regulated by fine root biomass (Table 4). We found that variations in Dp and Dtotal across N addition treatments and soil profile were positively correlated with the amount of live, dead, and total fine roots (all *p* < 0.01, Figures 4 and 5). The positive relationships remained even without the "outliers" — the very small values (both in fine root biomass on the *X*-axis as well as in the soil fauna density on the *Y* axis, all *p* < 0.01, Figures S3 and S4). Multifactor linear regression showed that the dominant factors controlling variations in Dp and Dtotal were the total and the dead fine root biomass across N addition treatments (all *p* < 0.001, Table 4).

**Table 4.** Results of multi-regression analysis of the density of phytophagous soil fauna (Dp) and total soil fauna (Dtotal) with fine root biomass (live, dead, and total), SOC, TN, mean annual soil temperature and moisture, and the soil temperature and moisture in June across N addition treatments. \*\*\*: *p* < 0.001.


**Figure 4.** Log-log regulation of fine root biomass ((**a**) live and dead root biomass; (**b**) total root biomass) on the density of phytophagous soil fauna (Dp). \*\*: *p* < 0.01, \*\*\*: *p* < 0.001.

**Figure 5.** Log-log regulation of fine root biomass ((**a**) live and dead root biomass; (**b**) total root biomass) on the density of total soil fauna (Dtotal). \*\*: *p* < 0.01, \*\*\*: *p* < 0.001.
