**3. Results**

#### *3.1. The Contributions of Soil Fauna on Humic Substances Accumulation*

Soil fauna showed significant effects on the accumulation of humic substance, but these strengths differed among sampling time and species (Figure 4 and Table 1). Over one year incubation, the contribution rates of soil fauna to humic substances were 109.06%, 71.48%, 11.22%, and −44.43% for fir, cypress, birch, and willow, respectively. Pearson correlation showed that soil fauna group density was significantly correlated with the contribution rates of soil fauna to humic substances at OF stage (*r* = −0.59, *p* < 0.05, Table 2). At the stage of OF, soil fauna in willow litter promoted the accumulation of humic substances, while it inhibited the accumulation of humic substances in fir and birch litters. Moreover, soil fauna improved the accumulation of humic substances at both the GS and LF stages in fir, birch, and cypress litters, but negative effects of soil fauna on the accumulation of humic substances in willow were observed during both stages.

**Figure 4.** The contribution rate of soil fauna to the accumulation of four litter humic substance in different stages (mean ± standard error, n = 3). OF: onset of freezing stage, DF: deep freezing stage, TS: thawing stage, GS: growing season, LF: leaf falling stage, WY: whole year. The letters a–c mean in the sampling stage with different letters differ significantly, *p* < 0.05.

**Table 1.** *F* values for multivariate analysis of variance for individual (Di) and group density (Dg) of soil fauna, the contribution rate of humic substances (HS), humic acid (HA), fulvic acid (FA), and humification degree (HD) by species and time.


 *p* < 0.01. *p* <

**Table 2.** Pearson correlation analysis of contribution rate of soil fauna to humic substances (HS), humic acid (HA), fulvic acid (FA) and humification degree (HD) and C, N, P, lignin (L), cellulose (CE), total accumulated temperature (TAT), daily average temperature (DAT), litter water content (LWC), soil fauna individual density (Di), and group density (Dg) at different stages. OF: onset of freezing stage, DF: deep freezing stage, TS: thawing stage, GS: growing season, LF: leaf falling stage.


\* *p* < 0.05; \*\* *p* < 0.01.

#### *3.2. The Contributions of Soil Fauna to Humic Acid Accumulation*

The contribution of soil fauna to accumulation of humic acid was significantly affected by species (*F* = 23.79), sampling time (*F* = 9.63), and their interaction (*F* = 27.53, all *p* < 0.001, Table 1). Overall, the contribution rate of soil fauna to the accumulation of humic acid showed the order as fir (111.87%) > birch (91.24%) > cypress (−34.83%) > willow (−49.9%). Soil fauna group density was significantly correlated with the contribution rate of soil fauna to humic acid at TS stage (*r* = 0.69, *p* < 0.05, Table 2). Significant increases in the accumulation of humic acid contributed by soil fauna were detected in willow litter rather than in other species at OF stage (Figure 5). In contrast, the contributions of soil fauna to humic acid exhibited the same trend with that soil fauna promoted the accumulation of humic acid in the litters of birch and fir but inhibited them in the litters of willow and cypress at DF and TS stages. Soil fauna promoted the accumulation of humic acid in birch litter at GS and LF stages, while limiting the accumulation in other litters.

**Figure 5.** The contribution rate of soil fauna to the accumulation of four litter humic acid in different stages (mean ± standard error, n = 3). OF: onset of freezing stage, DF: deep freezing stage, TS: thawing stage, GS: growing season, LF: leaf falling stage, WY: whole year. The letters a–c mean in the sampling stage with different letters differ significantly, *p* < 0.05.

#### *3.3. The Contributions of Soil Fauna to Fulvic Acid Accumulation*

The contribution rate of soil fauna to the accumulation of fulvic acid showed the order as, fir (220.24%) > cypress (35.43%) > willow (−28.24%) > birch (−55.64%). Soil fauna promoted the accumulation of fulvic acid in the litter of willow and cypress but limited the accumulations of fulvic acid in other two litters at OF stage (Figure 6), which showed the opposite side at OF stage. Soil fauna promoted the accumulation of fulvic acid in the litter of fir and willow and inhibited the accumulation in the litter of birch and cypress at TS stage. At the stage of GS, soil fauna promoted the accumulation of fulvic acid in the litters of fir and cypress but inhibited the accumulation in the litters of birch and willow. Soil fauna only inhibited the accumulation of fulvic acid in willow litter at LF stage, whereas soil fauna contributed 40.42% and 128.10% increase of fulvic acid in the litters of birch and fir, respectively.

**Figure 6.** The contribution rate of soil fauna to the accumulation of four litter fulvic acid in different stages (mean ± standard error, n = 3). OF: onset of freezing stage, DF: deep freezing stage, TS: thawing stage, GS: growing season, LF: leaf falling stage, WY: whole year. The letters a–d mean in the sampling stage with different letters differ significantly, *p* < 0.05.

#### *3.4. Contributions of Soil Fauna to Litter Humification Degree*

Sampling time and the interaction of species with sampling time significantly affected the contribution rate of soil fauna on litter humification degree (*F* = 10.04 and *F* = 8.09, all *p* < 0.001, Table 1). The whole year contribution rate of soil fauna to humification degree was 12.66% for willow litter, and positive contribution appeared at both OF and DF stages (Figure 7). Contrarily, soil fauna showed −13.27%, −49.20%, and −7.63% of contribution rates to humification degree in the litter of birch, fir, and cypress in the whole year. However, the contribution rates of soil fauna to humification degree were 14.67%, 6.55%, and 48.79% in the litter of birch, fir, and cypress at LF stage.

**Figure 7.** The contribution rate of soil fauna to four litter humification degree in different stages (mean ± standard error, n = 3). OF: onset of freezing stage, DF: deep freezing stage, TS: thawing stage, GS: growing season, LF: leave falling stage. OF: onset of freezing stage, DF: deep freezing stage, TS: thawing stage, GS: growing season, LF: leaf falling stage, WY: whole year. The letters a–c mean in the sampling stage with different letters differ significantly, *p* < 0.05.

The linear regressions show the correlation of individual and group density of soil fauna with cumulative contribution rate of soil fauna to the litter humification degree differed among species (Figure 8). Soil fauna significantly influenced the humification degree of the litter of birch, willow, and cypress, while it had no significant effect for fir litter. There was a significant correlation of individual density of soil fauna with humification degree for the litter of birch (*p* < 0.05, *R*<sup>2</sup> = 0.27, Figure 8a) and cypress (*p* < 0.01, *R*<sup>2</sup> = 0.37, Figure 8d). Additionally, there were significant correlations of group density for the litter of willow (*p* < 0.05, *R*<sup>2</sup> = 0.28, Figure 8c) and cypress (*p* < 0.05, *R*<sup>2</sup> = 0.28, Figure 7d).

**Figure 8.** Linear regression analysis between the cumulative contribution rate of soil fauna to the humification degree of four kinds of litters and the individual and group density of soil fauna.
