**5. Conclusions**

The accumulation of humic substances (including humic acid and fulvic acid) and litter humification degree were jointly determined by soil fauna, species, and environmental factors. Soil fauna is favorable to the accumulation of humic substances in growing and leaf falling seasons for the litter of birch, fir, and cypress rather than willow litter. Moreover, the accumulations of humic acid and fulvic acid were promoted by soil fauna in fir litter, but these accumulations were inhibited in willow litter. Additionally, soil fauna showed positive effects on the humification degree in willow litter, but negative effects on the other three litter. Overall, our study gives a deep insight in the contribution of soil fauna on litter humification, which could provide useful data in soil carbon sequestration in the cold forests.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/f13081235/s1, Table S1: The average densities and eating patterns of soil fauna in the four studied litters under different stages.

**Author Contributions:** Formal analysis, Z.X. and L.Z.; Investigation, K.Y. and H.L.; Methodology, C.Y.; Supervision, B.T.; Writing—original draft, Y.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (31870602, 32071745, 31901295 and 32001165), the Program of Sichuan Excellent Youth Sci-Tech Foundation (2020JDJQ0052) and the Program of Sichuan Applied Basic Research Foundation (2021YJ0340).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The study did not report any data.

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